What is telecommunication systems. Digital telecommunication system

2 Two Root Computer Networks Computational and Telecommunication Technologies Evolution of Telecommunications Evolution of Computer Engineering Evolution of Computer Networks Evolution of Computer Networks at the junction of computing equipment and telecommunication technologies

3 Telecommunication systems 1. Basic information about telecommunication systems The main function of telecommunication systems (TKS), or territorial communication networks (TSS), is to organize the operational and reliable exchange of information between subscribers, as well as in reducing data transmission costs. The concept of "territorial" means that the communication network is distributed in a significant area. It is created in the interests of the whole state, institutions, enterprises or firms that have branches in the area, region or throughout the country. The main indicator of the effectiveness of the functioning of telecommunication systems the delivery time of information. It depends on a number of factors: communication network structures, communication lines bandwidth, communication channels connecting between interacting subscribers, information exchange protocols, subscriber access methods to the transmitting medium, package routing methods, etc.

4 Telecommunication systems 1. Basic information about telecommunication systems The characteristic features of territorial communication networks: the diality of communication channels from the wire channels of the tone frequency (telephone) to fiber optic and satellite; The limited number of communication channels between remote subscribers for which it is necessary to ensure data exchange, telephone communication, video communication, the exchange of facsimile messages; The presence of such a critical resource as the bandwidth of communication channels. Consequently, the territorial communication network (TCC) is a geographically distributed network that combines the functions of traditional data transmission networks (SPD), telephone networks and intended for transmitting traffic of various nature, with different probabilistic-time characteristics.

5 Telecommunication systems 1. Basic information about telecommunication systems Types of networks, lines and communication channels. Communication networks are used in TVS telephone, telegraph, television, satellite. As communication lines, it is used: cable (telephone lines, twisted steam, coaxial cable, fiber optic lines), radio relay and radio. Among cable lines of communication, the best indicators have fiber optic (i.e. fiber optic lines). Their main advantages: high bandwidth (hundreds of megabits per second); insensitivity to external fields and the absence of their own emissions; Low laboriousness of optical cable laying; Spark, explosion and fire safety; increased resistance to aggressive media; small specific mass; Different applications. Disadvantages: signal transmission is carried out only in one direction; The connection of additional computers significantly loosens the signal; high-speed road modems required for light guides; Filters connecting computers must be supplied with electrical signal transducers in light and back.

6 Telecommunication systems 1. Basic information about telecommunication systems In telecommunication systems, the following types of communication channels are found: Simplex, when the transmitter and receiver are associated with one communication channel, according to which the information is transmitted only in one direction (this is characteristic of TV networks); half-duplex when two nodes of communication are also connected by one channel by which the information is transmitted alternately in one direction, then in the opposite (this is characteristic of information and reference, request-response systems); Duplex, when two nodes are connected by two channels (direct and reverse), for which the information is simultaneously transmitted in opposite directions. Duplex channels are used in systems with decisive and informational feedback.

7 Telecommunication systems 1. Basic information about telecommunication systems Switable and dedicated communication channels. In networks (TKS, TCC) distinguish between the selected (non-commutable) communication channels and switched channels for the time of transmission on them. When using selected communication channels, the transceiver equipment of the communication nodes is constantly connected to each other. This ensures a high degree of system readiness to transmit information, higher quality communication, support for large traffic. Due to relatively large spending on the operation of networks with dedicated communication channels, their profitability is achieved only if there is enough to fully load the channels. For switched communication channels created only at the time of transfer of fixed information, high flexibility and relatively small cost are characterized. Disadvantages of these channels: loss of switching time (establishing a connection between subscribers), the ability to block due to the employment of individual links of the communication line, lower quality of communication, the big value with a significant amount of traffic.

8 Telecommunication systems 1. Basic information about telecommunication systems Analog and digital coding of digital data. Sending data from one network node to another is carried out by sequential transmission of all bits of the message from the source to the destination item. Physically information bits are transmitted as analog or digital electrical signals. Analog is called signals that can represent countless values \u200b\u200bof some values \u200b\u200bwithin a limited range. Digital (discrete) signals can have one value or a finite set of values. When working with analog signals, an analog-carrier signal of the sinusoidal form is used to transmit the encoded data, and when working with digital signals of a two and multi-level discrete signal. Analog signals are less sensitive to distortion due to attenuation in the transmitting medium, but the coding and decoding of data is simpler for digital signals.

10 Telecommunication systems 1. Basic information about telecommunication systems Synchronization of network elements This is part of the communication protocol. During the synchronization process, a synchronous operation of the receiver hardware and the transmitter is ensured, in which the receiver selects the incoming information bits strictly at the moments of their arrival. There are synchronous transmission, asynchronous transmission and transmission with auto-tuning. Synchronous transmission is distinguished by the presence of an additional communication line (except for the main) for transmitting synchronizing pulses (C) stable frequency. The issuance of the data bits by the transmitter and the signal sample receiver are made at the moments of the appearance of C. It is reliable, but an additional line is needed. Asynchronous transmission does not require an additional line. Transmission is carried out by small fixed blocks, and start-bit is used to synchronize. In the transmission with auto-adjustment, synchronization is achieved through the use of self-synchronizing codes (SC). Encoding the transmitted data using the SC is to provide regular and frequent changes in the signal levels in the channel. Each transition is used to adjust the receiver.

11 Satellite communications networks (CSS). Space devices (ka) of communication are launched to the height of the CM and are located on a geostationary orbit, the plane of which is parallel to the equator plane. Three such ka provide coverage of almost the entire surface of the Earth. The interaction between subscribers of the CSS is carried out by chain: AC-sender of the information\u003e Transferring ground station \u003e\u003e Satellite\u003e Reception ground station\u003e AS recipient. One ground station serves a group of nearby speakers. The following methods are used to manage data transfer between satellite and ground stations. 1. Normal multiplexing with frequency and temporary separation. 2. Normal discipline "Primary / secondary" using or without using methods and polling tools. 3. Equal-standing control disciplines with equal access to the channel in conjunction for the channel. Telecommunication systems 1. Basic information about telecommunication systems Transferring Ground Station \u003e\u003e Satellite\u003e Reception Ground Station\u003e As-Recipient. One ground station serves a group of nearby speakers. The following methods are used to manage data transfer between satellite and ground stations. 1. Normal multiplexing with frequency and temporary separation. 2. Normal discipline "Primary / secondary" using or without using methods and polling tools. 3. Equal-standing control disciplines with equal access to the channel in conjunction for the channel. Telecommunication systems 1. Basic information about telecommunication systems "\u003e

12 Telecommunication Systems 1. Basic information about telecommunication systems The main advantages of communication satellite networks: a large bandwidth due to the work of satellites in a wide range of Gigahertz new frequencies. Satellite can support several thousand speech communication channels; Ensuring communication between stations located at very long distances, and the possibility of servicing subscribers in the most hard-to-reach points; independence of the cost of transferring information from distance between subscribers; The ability to build a network without physically implemented switching devices. Disadvantages of communication satellite networks: the need for funds and time to ensure the confidentiality of data transfer; the presence of a delay of radio signal reception by the ground station due to long distances between the satellite and the communication station; the possibility of mutual distortion of radio signals from ground stations operating in neighboring frequencies; Signal exposure to various atmospheric phenomena.

13 Telecommunication systems 2. Commuting in networks Switching is a vital element of communication of subscriber systems (AC) with each other and with control centers, processing and storing information in networks. Networks network are connected to some switching equipment, thus avoiding the need to create special communication lines. A commutable transport network is a network in which the end items are established between two (or more) end items on request. An example of such a network is a switched telephone network. There are the following switching methods: circuit switching (channels); Commuting with intermediate storage, sharing messages and switching packets.

15 Telecommunication systems 2. Communication in networks Switching channels (chains). When switching channels (circuits) between binding end items throughout the time interval, the connection is made in real time, and the bits are transmitted with constant speed over the channel with a constant bandwidth. The advantages of the circuit switching method: chain switching technology; work in dialogue and real time; ensuring transparency regardless of the number of compounds between the AU; Wide scope. Disadvantages of the chain switching method: for a long time to establish a through channel of communication due to the possible expectation of the release of individual sections; The need to re-transmit a call signal due to the employment of the switching device in the signal chain; the absence of the possibility of selecting information transfer rates; the possibility of channel monopolization by one source of information; the increase in the functions and capabilities of the network is limited; Uniformity of communication channels is not ensured.

17 Telecommunication systems 2. Communication in networks Switching messages - Early data transfer method (applied in email, news). Technology - "Remember and Send." The message entirely retains its integrity in the process of passing from one node to another until the destination, and the transit node cannot begin the further transmission of the message part if it is still accepted. The advantages of the method: no need to establish a channel; Formation of a route from sections with different bandwidth; implementation of query service systems taking into account their priorities; the possibility of smoothing peak loads by memorizing streams; Lack of service requests. Disadvantages: the need to implement serious memory capacity requirements in communication nodes for receiving large messages; insufficient possibilities for implementing the dialogue and real-time operation during data transfer; Channels are less efficient compared to other methods.

18 Telecommunication systems 2. Communication in networks Switching packets combines the benefits of switching channels and switching messages. Its main objectives: ensuring the full availability of the network and acceptable reaction time to the request for all users, smoothing the asymmetric flows between users, ensuring the multiplexing of the capabilities of the communication channels and the ports of the network computers, the dispersal of the critical network components. Data is divided into short fixed length packets. Each package is supplied with protocol information: the start and end of the package, the sender and recipient addresses, the packet number in the message, information to monitor the accuracy of the transmitted data. Independent packages of one message can be transmitted simultaneously on various routes as part of the datagram. Packages are delivered to the destination, where the initial message is formed from them. In contrast to switching messages, packet switch allows: to increase the number of connected stations; it is easier to overcome difficulties with connecting additional communication lines; carry out alternative routing, which creates elevated user amenities; Significantly reduce the time to transfer data, improve the bandwidth and efficiency of networking resources. Now batch switch is basic to transfer data.

20 Telecommunication Systems 2. Communication in Networks Conclusion Section The analysis of the considered switching technologies allows us to conclude the possibility of developing a combined switching method based on the use of messages, packages and providing more efficient management of heterogeneous traffic.

21 Telecommunication systems 3. Routing packages in networks. Essence, goals and routing methods. The routing task is to choose a route to transmit from the sender to the recipient. This is, first of all, on networks with arbitrary (cellular) topology, in which the packet switching is implemented. However, in modern networks with mixed topology (star-ring, star-tire, multi-transparent), the task of selecting a route for transmitting frames is actually solved, which uses appropriate means, such as routers. In virtual networks, the routing task when sending a message dissected on packages is solved by a single time when a virtual connection is established between the sender and the recipient. In datagram networks, where data is transmitted in the form of datagram, routing is performed for each individual package. The choice of routes in the communication nodes of telecommunication networks is made in accordance with the implemented algorithm (method) of routing.

24 Telecommunication Systems 3. Package Routing In Networks Routing Algorithm This is a rule of assignment of the output link for transmitting a packet based on the information contained in the packet header (sender and recipient address), information about the loading of this node (package queues) and networks in general . The main routing objectives are to provide: the minimum delay in the package when it is transferred from the sender to the recipient; maximum network bandwidth; maximum protection of the package from threats for the information contained in it; reliability package delivery addressee; The minimum cost of transferring a package address. The following routing methods distinguish: - centralized routing; - distributed (decentralized) routing; - Mixed routing

25 Telecommunication systems 3. Package routing in networks 1. Central routing is implemented in centralized control networks. The selection of the route for each package is carried out in the network management center, and communication network nodes only perceive and implement the results of solving the routing task. Such routing management is vulnerable to the denounces of the central node and does not differ in high flexibility. 2. Distributed (decentralized) routing is performed in networks with decentralized control. Routing control functions are distributed between network nodes, which have appropriate means for this. Distributed routing is more complicated by centralized, but differs in greater flexibility. 3. Mixed routing is characterized by the fact that it is implemented in a certain ratio of the principles of centralized and distributed routing. The routing task in networks is solved provided that the shortest route providing the packet transmission in the minimum time depends on the topology of the network, bandwidth and load on the communication line.

26 Telecommunication systems 3. Routing packets in networks Routing methods - simple, fixed and adaptive. The difference between them to the degree of accounting of changes in the topology and load of the network when the route is selected. 1. Simple routing is characterized by the fact that when choosing a route, neither the network topology change is not taken into account nor the change in its load. It does not provide directional packet transfer and has low efficiency. Its advantages are simplicity of implementation and ensuring the sustainable network operation when it fails to separate its elements. Practical application received: random routing - one random free direction is selected for the transfer of the package. The package "wanders" over the network and with the ultimate probability reaches the addressee. Avalanche routing provides for the transfer of a package from a node over all free output lines. There is a phenomenon of "reproduction" of the package. The main advantage of this method is guaranteed to ensure the optimal delivery time of the package address. The method can be used in unloaded networks when the requirements for minimizing the time and reliability of package delivery are quite high.

27 Telecommunication systems 3. Routing packets in networks 2. Fixed routing - when choosing a route, it takes into account the change in the network topology and the change in its load is not taken into account. For each destination node, the transfer direction is selected by the table of the shortest routes. The lack of adaptation to the load change leads to network packet delays. There are unintended and multiple fixed routing. The first is based on the only way to transfer packets between the two subscribers, which is associated with instability to failures and overloads, and the second based on several possible paths between the two subscribers from which the most preferred path is selected. Fixed routing is applied in networks with little changing topology and established packet streams. 3. Adaptive routing is characterized by the fact that the decision on the direction of transmission of packets is carried out taking into account the changes both topology and the network load. There are several adaptive routing modifications that differ in which information is used when choosing a route. Local, distributed, centralized and hybrid adaptive routing (is clear from the title) disappeared.

28 Telecommunication systems 4. Protection against errors in networks When transmitting data, one error per thousand transmitted signals can be seriously reflected in the quality of information. There are many methods for ensuring the reliability of information transfer (protection against errors), characterized: according to the funds used, on time spent on their use, to the degree of ensuring the reliability of information transfer. The practical embodiment of methods consists of two parts of the program and hardware. The ratio between them may be the most different, up to almost the absence of one of the parts. The main causes of errors when transferring in networks: Failures to some part of the network equipment or the occurrence of unfavorable events on the network. The data transfer system is ready for this and eliminates them using the plan provided by the plan; Interference caused by external sources and atmospheric phenomena.

29 Telecommunication systems 4. Protection against errors in networks among numerous methods Sewing from errors There are three groups of methods: group methods, noise-resistant coding and error protection methods in feedback transmission systems. From group methods, a large application of a majority method and the method of transmitting information blocks with a quantitative characteristic of the block were obtained. The essence of the majority method is that each message is transmitted several times (more often three times). Messages are remembered and compared, the correct chosen coincidence "2 out of 3". Another group method, which also does not require the transcoding of information, implies data transmission by blocks with a quantitative characteristic of the block (the number of units or zeros, the checksum of characters, etc.) at the receiving point, this characteristic is re-calculated and compared with the communication channel transmitted via the communication channel. If the characteristics coincide, it is believed that the block does not contain errors. Otherwise, a signal comes to the transmitting side with the requirement of re-transmission of the unit. In modern TVS, this method was widely distributed.

30 Telecommunication systems 4. Protection against errors in networks noise-resistant (excessive) encoding involves the development and use of corrective (noise-resistant) codes. Feedback transmission systems are divided: on systems with crucial feedback and information feedback system. A feature of systems with crucial feedback is that the decision on the need to re-transmit information receives the receiver. A noise-resistant encoding is applied by which the received information is checked at the receiving station. When an error is detected on the transmitting side over the feedback channel, the rewriting signal is sent to which the information is transmitted again. In information feedback systems, information transmission is carried out without interference coding. Receiver, accepting information on the direct channel and remembering, transfers it back, where it is compared. While matching the transmitter sends a confirmation signal, otherwise repeatedly transmitted all information, i.e. Transmission decision takes a transmitter.

Timely transmission of information is the basis of the stable functioning of many industries and agriculture.

Modern information society is actively used by various telecommunications systems for sharing a large number of information in a short time.

Modern telecommunication systems and networks

Telecommunication systems are technical means intended to transmit large amounts of information through fiber optic communication lines. As a rule, telecommunications systems are designed to maintain a large number of users: from several tens of thousands to millions. The use of such a system involves regular transmission of information in a digital form between all participants in the telecommunications network.

The main feature of modern equipment for networks is to ensure uninterrupted connection so that the information is transmitted continuously. At the same time, periodic deterioration in the quality of communication at the time of establishing the compound, as well as periodic technical problems caused by external factors is allowed.

Types and classification of telecommunication communication systems

Modern telecommunication systems are combined over several main features.

Depending on the purpose, television broadcasting systems, personal communications, as well as computer networks differ.

Depending on the technical support, which is used to transmit information, traditional cable communication systems are highlighted, more perfect - fiber optic, as well as essential and satellite.

Depending on the method of encoding an array of information, analog communication channels and digital are distinguished. The last type was widespread, while the analog communication channels are becoming less demanded today.

Computer systems

Computer systems are a combination of several PCs combined into a single information field through cables and specialized programs.

The combination of installed equipment and software is an autonomous self-regulating system that serves the enterprise in the complex.

Depending on its functions, the equipment of the computer system is divided into:

• service (for intermediate and backup storage of information);


• active (to ensure timely and high-quality signals;


• personal devices.

To ensure the work of the entire system, appropriate software is required, properly configured, based on user needs.

Radio engineering and television systems

At the heart of radio transmission systems, electromagnetic oscillations are connected, which are broadcast by special radio channel. The unit functioning is a signal that is converted to the transmitting device and then transformed into an informational message in the receiving.

The basis of the uninterrupted functioning of radio engineering systems is a communication line - a physical environment and hardware that ensure timely and complete transmission of information.

Television systems operate according to a similar principle of the receiver and transmitter. Most of them use a digital signal that allows you to transmit a message in higher quality.

Global telecommunication systems

Global telecommunication systems include hardware and software that connect users regardless of their physical position on the planet. The main feature of global networks is intellectualization, which allows you to easily use network power with optimal efficiency, while minimizing the cost of maintenance of equipment. Among global networks are distinguished by several basic species.

Digital networks with integrated modules use continuous channel switches, while data arrays are processed in digital form. Network users have access to only some functions, the interface does not allow you to independently change the technical parameters.

The X25 networks are the oldest, reliable and proven information transfer technologies between an unlimited number of users. The main difference of such networks is the presence of a device for "assembling" of individual blocks of transmitted information in "Packages" for the fastest transmission.

Asynchronous data transfer mode is a modern technology used for broadband networks that are based on fiber optic cables.

Optical telecommunication systems

The basis of optical telecommunication systems is a fiber-optic cable that connects individual devices into a single global network.

Signals are transmitted using an infrared radiation range, while the bandwidth of the fiber optic cable multiplying the indicators of other types of equipment.

The technical characteristics of the material provide a weak level of attenuation of the signal at large distances, which allows the use of a cable for communication between the continents. Passed along the bottom of the ocean, the fiber optic cable is protected from unauthorized access, as it is quite difficult to intercept the transmitted signals in the technical plan.

Multichannel telecommunication systems

A distinctive feature of such communication systems is to use multiple information signals transmission channels.

Modern telecommunication systems use cable, waveguide, radio relay, as well as space lines. The encrypted signal is transmitted at a speed of several gigabits per second for huge distances.

The main advantage of multichannel systems is to ensure stable operation. At the failure of one communication channel, the following is automatically connected.

Users are protected from a sudden coupling and loss of important information. The basis of such systems is structured structures from cables.

Multiservice telecommunications systems

Multiservice Telecommunication Systems are a hardware and software environment for transmitting data on packet switching technology - connecting separate blocks of information in a large message.

The feature of multiservice systems is the need to ensure the stable operation of all elements of the transport environment. As a rule, various technologies are used to transmit data, as well as speech and video information, but the infrastructure is one. Therefore, the basic principle of building multiservice networks is the universality of a technological solution, with which heterogeneous equipment is served, designed to perform various operations.

The multiservice system uses a single channel to transmit data from various types. Due to this, the means of maintenance and hardware of the system will save: a single design requires fewer personnel and costs.

Structure, equipment and components of telecommunication systems

The basis of any telecommunication system is based on servers on which the information needs and is processed by users.

Server are small rooms with industrial ventilation, ensuring the functioning of a plurality of high-volume hard disks.

Custom computers are a means of communication between the database and specific information users carrying out search queries.

The technical basis of telecommunication networks is lines, that is, data transmission medium, which use fiber optic, coaxial or wireless communication channels.

Network equipment providing data and receiving data:

• modems;
• adapters;
• routers;
• concentrators.

Such devices complement the telecommunications system and are necessary for stable operation.

Software allows you to effectively monitor the operation of installed equipment, which ensures timely transmission of information in the necessary volumes.

Methods and measuring instruments in telecommunication systems

Depending on the implementation phase, three varieties of measurements are distinguished:

1. Installation measurements are made after mounting the equipment to make sure that all the telecommunication system components are working.


2. During the work, it is necessary to conduct configuration measurements that allow you to adapt the functionality of the equipment to the changing conditions of the external environment. For example, if hardware or software or software are changed in the telecommunications system, you must make sure that it continues to fully function.


3. Control or preventive measurements are carried out regularly in order to prevent sudden breakdowns of the telecommunications network.

Basics of construction and installation of telecommunication systems and networks

The main principle of constructing a telecommunications system of any size and destination is its separation into separate functional sections. The maintenance time of each of them decreases, simplifies the procedure for finding the location of the breakdown in any technical fault.

In addition, when installing systems, it is necessary to take care of the insulation of the cable itself so that the data transmission is, as small as possible, depending on external factors. Modern fiber optic cables are located underground, at the bottom of the ocean or in special corrugations, which maximize protects them from harmful effects.

Ensuring information security of telecommunication systems

The main task in constructing a security system in telecommunications is to prevent information leakage through separate channels. The reason for such phenomena can be the hardware damage to the transmitting channel (fiber optic cable), and attack of intruders using software.

In the first case, information security is to provide high-quality cables capable of withstanding intensive loads and regular operation.

The second requires the development, implementation and maintenance of software tools that restrict access to the resources of the telecommunications system.

Telecommunication systems of hotels

The hotel business is a whole range of services that provide comfortable accommodation in the hotel. That is why the timely provision of full and reliable information about everything that may be interested in guests - customer retention guarantee.

As a rule, telecommunication systems in the hotel complexes consist of:

• video communication;
• computer systems;
• software.

Thus, each guest receives ease of accommodation in the room and all the necessary information.

Telecommunication systems and railway transport networks

Unlike the industry of hospitality, the main priority of telecommunications in the railway sphere is the accuracy of the information. Therefore, telecommunication networks in rail transport are designed in such a way that all the transmitted information can be quickly traced, with probable leaks, minimal attention is paid.

Companies serving telecommunication systems

Service telecommunication systems are engaged in equipment suppliers for communications and service companies.

Among the enterprises can be noted:

• Telecommunication Systems is one of the oldest specialized companies in St. Petersburg, providing customers with the current repair, configuration and maintenance of information transmission systems;


• "Stroykom-A" is a small company providing services for servicing and improving dilapidated telecommunication systems;


• "Cryptock" is a narrow profile company engaged in security in telecommunication systems of the defense complex.

Manufacturers and suppliers of equipment for telecommunication systems

Production and supply equipment for telecommunication systems companies are engaged in companies as:

• "Montair" - a supplier of ready-made solutions for telecommunication systems offering customers a large selection of server equipment.


• "RDCAM" is a full-cycle company, which offers customers not only the finished equipment, but also the development of engineering solutions for telecommunication systems.


• LAN-ART is a provider of network switching equipment and a communication cable manufacturer.

Modern telecommunication systems and specialized equipment for communication demonstrate at the annual exhibition "Communication".

Read the other our articles:

Modern diverse and cover, almost, all spheres of human vital activity.

Building any effective network and infrastructure for any destination, whether services for consumers or manufacturing enterprise, determines the tasks of providing the timely and reliable exchange of information to which all the more stringent requirements are presented.

An increase in the number of users of information systems leads to the ever-increasing volume of appeals, calculations and other operations requiring the data transmission systems for greater performance, scalability and with the implementation of more stringent safety and manageability conditions.
A wide variety of telecommunication systems are now surrounded by a person. In essence, the telecommunications system can be called almost any system of communications, which underlies companies providing ground and mobile services, a computer or cable television network built by providers of these services, corporate networks of various enterprises, regardless of their scale and profile. Even when two children play a primitive negotiation device, they also use the simplest telecommunications system.

In the nineteenth century, when telegraph and telephone was invented, all such systems consisted of telecommunication cables from subscribers to local switches, that is, local communication lines, a number of communal funds, which provided communication connections with subscribers, lines or communication channels that were transmitted Calls between switches and, ultimately, subscribers.

The invention of the radio at the end of the nineteenth century by Russian scientist Popov A.S. It became the starting point of the future technical coup in communication systems. From the beginning and by the middle of the twentieth century, the emergence of telephone exchange, electromechanical switter systems, cables, repeaters, carrier systems, microwave equipment, and further, in thick populated industrial areas, worldwide telecommunication systems began to get widespread.

After the middle of the last century, new technologies continue to develop in this industry. These include satellite and improved cable communication systems, appeared and obtained distribution in all spheres of human life digital and fiber-optic technologies, as well as video telephony. The telecommunications industry itself was fully computerized. All these positive changes and modernization played a decisive role in the distribution of telecommunication systems around the world.
The introduction of new technologies has significantly modified the telecommunications systems themselves. They became more difficult. They combine the totality of various methods of communication and require high-class specialists professionally prepared in different technical areas for their service. But, undoubtedly, much due to telecommunications our life has become dynamic and more interesting!

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1. Principles for building wireless telecommunication systems

1.1 Architecture of cellular communication systems.

1.2 Subscriber service network.

1.3 Methods for dividing subscribers in cellular communication

1.4 DECT standard.

1.5 Bluetooth standards, Wi-Fi (802.11, 802.16).

2. Systems of complex signals for telecommunication systems.

2.1 Signal spectra

2.2 Correlation Signal Properties

2.3 Types of complex signals

2.4 Derivative Signal Systems

3. Modulation of complex signals

3.1 Geometric Signal Presentation

3.2 Methods of phase manipulation of signals (FM2, FM4, OFM).

3.3 Modulation with a minimum frequency shift.

3.4 Quadrature modulation and its characteristics (QPSK, QAM).

3.5 Sales of quadrature modems.

4. Features of receiving signals in telecommunication systems.

4.1 Probability of errors of distinction of m known signals

4.2 Probability of errors of distinction of M fluctuating signals.

4.3 Calculation of errors of distinction of m signals with unknown

non-energy parameters.

4.4 Comparison of synchronous and asynchronous communication systems.

5. Conclusion.

6. List of references

1. Principles for building wireless telecommunication systems

1.1 Cellular communication architecture

The cellular system is a complex and flexible technical system that allows a lot of variety, both by options for configurations and the set of functions performed. An example of the complexity and flexibility of the system is that it can provide transmission as speech and other types of information, in particular text messages and computer data. In terms of transmission of speech, in turn, a regular bilateral telephone connection can be implemented, a multilateral telephone connection (the so-called conference foundation - with the participation of more than two subscribers in conversation at the same time), voice mail. When organizing a conventional two-sided telephone conversation, which starts with the call, there may be autodist modes, call waiting, call forwarding.

The cellular system is built in the form of a set of cells, or cells covering the serviced territory, for example, the territory of the city with suburbs. Cells are usually schematically depicted in the form of regular equilibrium hexagons (Fig. 1.1.), That in similarity with bee honeycombs and served as a reason to name the cellular system. The scholar, or cellular structure of the system is directly related to the principle of reuse of frequencies - the basic principle of the cellular system, which determines the effective use of the selected frequency range and the high capacity of the system.

Fig. 1.1. Cells (cells) systems covering the entire serviced territory.

In the center of each cell there is a base station serving all mobile stations (subscriber radiotelephone devices) within its cell (Fig. 1.2.). When the subscriber is moved from one cell to another, its maintenance is transmitted from one base station to another. All basic stations of the system, in turn, will be closed to the switching center from which there is an output into a mutually related network of communication (WCS) of Russia, in particular, if the case is happening in the city, is to enter the usual urban network of wired telephone.

Fig. 1.2. One cell with a base station in the center serving all mobile stations in the cell.

In fig. 1.3. A functional scheme corresponding to the structure described is shown.

Fig. 1.3. Simplified functional diagram of cellular system: BS - base station; PS - mobile station (subscriber radiotelephone apparatus).

In reality, cells are never a strict geometric shape. The real boundaries of the cells have the appearance of incorrect curves, depending on the conditions for the propagation and attenuation of radio waves, i.e. from the terrain, nature and density of vegetation and development and the like factors. Moreover, the boundaries of the cells are generally not clearly defined, since the border of the transfer of mobile station from one cell to another may in some limits shift with the change in the propagation conditions of radio waves and depending on the direction of movement of the mobile station. Similarly, the position of the base station only approximately coincides with the center of the cell, which is also not so easy to determine unambiguously, if the cell is irreversible. If at base stations are used directed (not isotropic in the horizontal plane) antenna, the base stations are actually on the boundaries of the cells. Further, the cellular communication system may include more than one switching center, which may be due to the evolution of the system development or limited switch capacity. It is possible, for example, the structure of the type system shown in Fig. 1.4. - With several switching centers, one of which can be called "head" or "leading".

Fig. 1.4. Cellular system with two switching centers.

Consider a mobile station - the most simple functional purpose and device element of a cellular communication system, besides this is the only element of the system that is really available to the user.

The block diagram of the mobile station is shown in Fig. 1.5. It includes:

Control block;

Transceiver unit;

Antenna block.

Fig. 1.5. Block diagram of a mobile station (subscriber radiotelephone apparatus).

The transceiver unit, in turn, includes a transmitter, receiver, frequency synthesizer and a logical block.

The most simple composition is an antenna block: it includes the actual antenna and the receiving switch. The latter for a digital station can be an electronic switch that connects an antenna or to the transmitter output, or to the receiver input, since the digital system's mobile station never works for reception and transmission at the same time.

The control unit includes a microthephon tube - microphone and speaker, keyboard and display. The keyboard (Digital and Function Key Pitch) is used to dial the phone number of the called subscriber, as well as commands that determine the mode of operation of the mobile station. The display is used to display the various information provided by the device and the mode of operation of the station.

The transceiver block is much more complicated.

The transmitter includes:

Analog-to-digital converter (ADC) - transforms a signal to a digital form from the microphone output, and all subsequent processing and transmission of the speech signal are produced in digital form, up to the reverse digital analog conversion;

The speech encoder coding the speech signal is the transformation of a signal having a digital form according to certain laws in order to reduce its redundancy, i.e. in order to reduce the amount of information transmitted via the communication channel;

Channel encoder - adds to a digital signal obtained from the speech encoder exit, additional (redundant) information intended for protection against errors when transmitting a communication signal; By the same purpose, information is subjected to a certain repacking (multiplication); In addition, the channel encoder instructs the control information received from the logical block into the transmitted signal;

Modulator - transfers the information of the encoded video signal to the carrier frequency.

The composition receiver mainly corresponds to the transmitter, but with the inverse functions of the blocks included in it:

The demodulator highlights a coded video signal from the modulated radio signal, which brings information;

The channel decoder highlights control information from the input stream and directs it to a logical block; Adopted information is checked for errors, and dedicated errors are correctly corrected; Before subsequent processing, the received information is subjected to reverse (relative to the encoder) repacking;

The speech decoder restores the speech signal on it from the channel decoder, translating it in a natural form, with the redundancy characteristic, but in digital form;

The digital-analog converter (DAC) converts the received speech signal into an analog form and feeds it to the output of the speaker;

The equalizer serves to partially compensate for signal distortions due to multipath distribution; Essentially, it is an adaptive filter configured by a training sequence of symbols included in the information transmitted; The equalizer unit is not, generally speaking, it is functionally necessary and in some cases may be absent.

To combine the encoder and decoder, sometimes use the name of the codec.

In addition to the transmitter and receiver, the transceiver block includes a logical block and frequency synthesizer. The logical block is, in fact, a microcomputer with its operational and constant memory that controls the operation of the mobile station. The synthesizer is a source of bearing frequency oscillations used to transmit information on radio channel. The presence of a heterodyne and frequency converter is due to the fact that various sections of the spectrum are used for transmission and reception.

The block diagram of the base station is shown in Fig. 1.6.

Fig. 1.6. Balary block diagram.

The presence of several receivers and the same number of transmitters allows you to simultaneously work on multiple channels with different frequencies.

Receivers and transmitters have common reinsured supporting generators, providing them with a consistent restructuring when switching from one channel to another. To ensure simultaneous operation of n receivers to one receiving and n transmitters to one transmitting antenna between the receiving antenna and receivers, the power divider is set to N outputs, and between the transmitters and the transmitting antenna - the power adder on n inputs.

The receiver and the transmitter have the same structure as in the mobile station, except that there are no DAC and ADCs in them, since the transmitter input signal and the output signal receiver have a digital form.

The conjugation unit with the communication line provides a packaging of information transmitted by line to the switching center, and the unpacking of information received from it.

The base station controller, which is a fairly powerful and perfect computer, ensures the management of the station, as well as monitoring the performance of all blocks and nodes.

The switching center is a brainstant and at the same time the dispatching point of the cellular communication system, which is closed by information flows from all base stations and through which access to other communication networks is a stationary telephone network, a long-distance network, satellite communications, other cellular networks.

The switching center flowchart is presented in Fig. 1.7. The switch shifts information flows between the corresponding communication lines. It can, in particular, send the flow of information from one base station to another, or from the base station to the stationary communications network, or vice versa.

The switch connects to the communication lines through the appropriate communication controllers carrying out intermediate processing (packing / unpacking, buffer storage) information flows. The overall management of the work center and the system is generally made from the central controller, which has powerful mathematical support. The operation of the switching center involves the active participation of operators, so the center includes the relevant terminals, as well as means of displaying and registering information. The operator includes data on subscribers and conditions for their maintenance, source data on system operation modes.

Fig. 1.7. Switching center block diagram.

Important elements of the system are databases - home register, guest register, authentication center, equipment register. The home register contains information about all subscribers registered in this system and the types of services that may be provided to them. It also records the location of the subscriber to organize its call, and actually rendered services are recorded. Guest register contains approximately the same information about subscribers - guests (reamery), i.e. On subscribers registered in another system, but currently using cellular communication services in this system. The authentication center provides subscriber authentication procedures and message encryption. The hardware register, if it exists, contains information about the operated moving stations for their health and sanctioned use.

1.2 Subscriber maintenance Network

The interface is a signal system by which the cellular communication system is connected to each other. Each cellular standard uses several interfaces (different in different standards).

From all interfaces used in cellular communication, one occupies a special place - this is the interface of the exchange between mobile and base stations. It is called the etheric interface. The etheric interface is necessarily used in any cellular communication system, with any configuration and in the only possible version for its standard cellular.

The Essential Interface of the D-AMPS system of the IS-54 standard is comparative simplicity (Fig. 1.8.).

Traffic Channel is a speech or data transmission channel. The transmission of information in the traffic channel is organized by the following one by other frames with a duration of 40 ms. Each frame consists of six time intervals - slots; The duration of the slot (6.67 ms) corresponds to 324 bits. With a complete coding on one speech channel, two slots are given in each frame, i.e. The 20-millisecond segment of speech is packaged in one slot, the duration of which is three times less. When half a coding, one speech channel is given one slot in the frame, i.e. Packaging Signal Speech is twice as thick than with full-speed coding.

Fig.1.8. Frame structure and D-AMPS system slot (traffic channel; IS-54 standard): Data - Speech information; SYNC (SC) - synchronizing (training) sequence; SACCH - information slow combination of the control channel; CDVCC (CC) is an encoded digital color confirmation code; G is a protective form; R - transmitter pulse front interval; V, W, X, Y - hexadecimal zeros; RES - reserve.

The slot has a slightly different structure in the direct channel of traffic - from the base station to mobile and in the reverse channel of traffic - from the mobile station to the base. In both cases, 260 bits are given to the transmission of speech. Another 52 bits occupy the managing and auxiliary information. It includes: a 28-bit training sequence used to identify a slot within the frame, synchronization of the time slot and the equalizer setting; 12-bit signaling (control and control and control) SACCH channel; The 12-bit pattern of the coded digital color code (CDVCC) serving to identify the mobile station when receiving its base station (the code is assigned to the base station individually for each channel, i.e. for each mobile station and retransmitted the last back to the basic).

The remaining 12 bits in the direct channel are not used (reserve), and in the reverse channel, the function of the protective interval is performed during which no useful information is transmitted.

At the initial phase of communication establishment, a shortened slot is used, in which the synchronizing sequence and CDVCC code are repeated, separated by zero numbers of various lengths. At the end of the shortened slot there is an additional protective form. The mobile station transmits short-circuit slots until the base station selects the necessary time delay determined by the removal of the mobile station from the base.

There are several communication channels: frequency, physical and logical.

The frequency channel is a frequency band assigned to transmit one communication channel information. In one frequency channel, several physical can be placed, for example, in the TDMA method.

The physical channel in a temporary separation-based multiple-based system (TDMA) is a temporary slot with a specific number in the frame sequence of the ether interface.

Logical channels are separated by type of information transmitted in the physical channel to the traffic channel and the control channel. A signaling information is transmitted over the control channel, which includes control information and the status control information, and speech and data are transmitted via traffic channel.

(Traffic is a set of messages transmitted over the link).

Consider the work of a mobile station within one cell of its ("home") system, without handover. In this case, in the operation of the mobile station, four stages can be distinguished, which correspond to four modes of operation:

Inclusion and initialization;

Standby mode;

Communication establishment mode (call);

Mode of communication (telephone conversation).

After turning on the mobile station, initialization is performed - the initial start. During this stage, the mobile station is configured to work as part of the system - by signals regularly transmitted by base stations on the appropriate control channels, after which the mobile station goes into standby mode.

Being in standby mode, the mobile station monitors:

Changes in system information - these changes may be associated both with changes in the operation mode of the system and with the movements of the mobile station itself;

System commands - for example, a command to confirm its performance;

Receiving a call from the system;

Call initialization by own subscriber.

In addition, the mobile station can periodically, for example, every 10 ... 15 minutes, confirm its performance, transmitting the corresponding signals to the base station. In the center of switching for each of the included movable stations, a cell is recorded in which it is "registered", which facilitates the organization of the procedure for calling a mobile subscriber.

If the number of the mobile subscriber is received from the system, the switching center sends this challenge to the base station of the cell in which the mobile station is registered, or several base stations in the neighborhood of this cell - taking into account the possible movement of the subscriber during the time that has passed since The last "registration", and the base stations transmit it to the corresponding call channels. A moving station in standby receives a call and responds to it through its base station, transmitting the data necessary for the authentication procedure at the same time. With a positive result of authentication, traffic channel is assigned, and the mobile station is reported to the number of the corresponding frequency channel. The mobile station is configured to the dedicated channel and, together with the base station, performs the necessary steps to prepare a communication session. At this stage, the mobile station is configured to the specified slot number in the frame, specifies the delay in time, adjusts the level of radiated power, etc. The selection of the time delay is made in order to temporarily harmonize the slots in the frame in the organization of communication with mobile stations located on different distances from the base. In this case, the temporary delay in the transmitted rolling station packs is adjusted by the commands of the base station.

Then the base station issues a call signal feed message (call), which is confirmed by a mobile station, and the caller receives the ability to hear the call signal. When the called subscriber responds to a call, the mobile station provides a request for completing the connection. With the completion of the connection, a communication session begins.

During the conversation process, the mobile station is processing transmitted and received speech signals, as well as transmitted simultaneously with the speech of control signals. At the end of the conversation, the service messages are exchanged between the mobile and base station, after which the mobile station transmitter turns off and the station goes into standby mode.

If the call is initiated by the mobile station, i.e. The subscriber dials the number of the called subscriber and clicks the "Call" button on the control panel, the mobile station transmits through its base station a message indicating the called number and data to authenticate the mobile subscriber. After authentication, the base station assigns traffic channel, and subsequent steps to prepare a communication session are the same as when the call is received from the system.

The base station then reports to the Mobile Station Preparation Center, the switching center transmits a call to the network, and the mobile station subscriber is able to hear the call signals or "occupied." The connection is completed on the network side.

Each time you establish communication, authentication and identification procedures are performed.

Authentication - the procedure for confirming the authenticity (reality, legality, the availability of rights to use cellular services) of the subscriber of the mobile communication system. The need to introduce this procedure is caused by the inevitable seduction of obtaining unauthorized access to cellular services.

Identification is the procedure for establishing a mobile station accessory to one of the groups with certain properties or features. This procedure is used to identify lost, stolen or defective devices.

The idea of \u200b\u200bthe authentication procedure in the digital cellular system is encrypted by some passwords identifiers using quasalound numbers, periodically transmitted to the movable station from the switching center, and individual for each mobile encryption algorithm. Such encryption, using the same source data and algorithms, is made both on the mobile station and in the switching center, and authentication is considered to be successful if both results coincide.

The identification procedure consists in comparing the identifier of the subscriber apparatus with the numbers contained in the corresponding "black lists" of the equipment register, in order to withdraw from the treatment of stolen and technically defective devices. The identifier of the device is done so that its change or fake is difficult and economically disadvantageous.

When moving a mobile station from one cell to another, its maintenance is transmitted from the first cell base station to the second base station (Fig. 1.9.). This process is called service transfer. It takes place only when the mobile station crosses the boundaries of the cells during the communication session and the connection is not interrupted. If the mobile station is in standby mode, it simply tracks these movements on the system information transmitted via the control channel, and at the right moment is rebuilt into a stronger signal of another base station.

Fig. 1.9. Transmission of maintenance from cells A into cell B When crossing a mobile station of the boundaries.

The need for maintenance transmission occurs when the quality of the communication channel measured by signal level and / or frequency of the bit error falls below the permissible limit. In the D-AMPS standard, the mobile station measures these characteristics only for the working cell, but with a deterioration in the quality of communication, it reports it through the base station to the switching center, and on the latter team, similar dimensions are performed by mobile stations in neighboring cells. According to the results of these measurements, the switching center selects the cell to which the maintenance must be transmitted.

The service is transmitted from the cell with the worst quality of the communication channel to the cell with the best quality, the specified difference must be at least some given value. If you do not require the execution of this condition, then, for example, when moving the mobile station is moving along the boundaries of the cells, a multiple handover is possible from the first cell to the second and back, leading to the loading of the system with meaningless work and to reduce the quality of communication.

By making a decision on the transfer of service, and selecting a new cell, the switching center reports this basic station of the new cell, and the mobile station through the base station of the old cell provides the necessary commands with the new frequency channel, the operating slot number, etc. The mobile station is rebuilt into a new channel and configures on collaboration with a new base station, performing about the same steps as when preparing a communication session, after which the connection continues through the base station of the new cell. At the same time, the break in the telephone conversation does not exceed the fraction of a second and remains invisible to the subscriber.

The cellular system can have roaming function - this is the procedure for providing cellular services to the subscriber of one operator in the system of another operator.

The idealized and simplified scheme of roaming organization is: a subscriber of a cellular communication, which turned out to be in the territory of a foreign system that implements roaming, initiates a challenge as if he was located on the territory of his "his" system. The switching center, making sure that this subscriber does not mean this subscriber in his homecard, perceives it as a reampem and enters the guest register. At the same time, it asks for the home register of the "native" system of the reamer relating to it for it necessary for the organization of service, and reports in which system the ROmer is currently currently; The latest information is fixed in the home register of the "native" system of the ROmer. After that, the reamer enjoys cellular communication like at home.

1.3 Methods for dividing subscribers in cellular communication

Communication resource represents the time and width of the strip available for signal transmission in a specific system. To create an effective communication system, it is necessary to plan the resource allocation between the system users, so that time / frequency is used as efficiently as possible. The result of such a planning should be equal access of users to the resource. There are three basic methods for dividing subscribers in the communication system.

1. Frequency separation. Certain subbands of the frequency band used are distributed.

2. Temporary separation. Subscribers are distinguished periodic time intervals. In some systems, users are provided with limited communication time. In other cases, users' access time to the resource is determined dynamically.

3. Code separation. Certain elements of the set of orthogonal (or almost orthogonal) distributed spectral codes are distinguished, each of which uses the entire frequency range.

With frequency separation (FDMA), the communication resource is distributed according to fig. 1.10. Here, the distribution of signals or users by frequency range is long-term or permanent. Communication resource can simultaneously contain several signals separated in the spectrum.

The primary frequency range contains signals that use the frequency gap between F 0 and F 1, the second - between F 2 and F 3, etc. The range of spectrum between the ranges used is called frequency protective strips. Protective bands perform a buffer role, which reduces the interference between adjacent (in frequency) channels.

Fig. 1.10. Frequency separation seal.

In order for an unmodulated signal to use a higher frequency range, it is converted by overlaying or mixing (modulation) of this signal and a sinusoidal Fixed Frequency Signal.

With a temporary separation (TDMA), the communication resource is distributed by providing each of M signals (users) of the entire spectrum for a small period of time called by a time interval (Fig. 1.11.). Time intervals separating the used intervals are called protective intervals.

The protective interval creates some temporary uncertainty between adjacent signals and acts as a buffer, thereby reducing the interference. Typically, time is broken at intervals, called frames. Each frame is divided into time intervals that can be distributed among users. The overall frame structure is periodically repeated, so that data transmission according to the TDMA scheme is one or more time intervals that are periodically repeated throughout each frame.

Fig. 1.11. Seal with temporary separation.

Multiple code separation access (CDMA) is a practical application of the spectrum expansion methods that can be divided into two main categories: expanding the spectrum by the method of direct sequence and expanding the spectrum by the method of jumping frequency to restructure.

Consider the expansion of the spectrum by the method of direct sequence. The spectrum expansion method has received its name due to the fact that the band used to transmit a signal is much wider than the minimum required for data transmission. So, N users receive an individual code G i (t), where i \u003d 1.2, ..., n. Codes are approximately orthogonal.

The block diagram of the standard CDMA system is shown in Fig. 1.12.

Fig. 1.12. Multiple access coded separation.

The first block of the circuit corresponds to the modulation by the carrier wave of the ACOSω 0 T. The output of the modulator belonging to the user from group 1 can be written in the following form: S 1 (T) \u003d A 1 (T) COS (ω 0 t + φ 1 (t)).

The view of the received signal may be arbitrary. The modulated signal is multiplied by an expanding signal G 1 (T), fixed by group 1; The result G 1 (T) S 1 (T) is transmitted via the channel. Similarly, for users of groups from 2 to N, the product of the code function and signal is taken. Quite often, access to the code is limited to a clearly defined group of users. The resulting signal in the channel is a linear combination of all transmitted signals. Neglecting delays in the transmission of signals, the specified linear combination can be written as follows: G 1 (T) S 1 (T) + G 2 (T) S 2 (T) + ... + g n (t) s n (t).

The multiplication of S 1 (T) and G 1 (T) results in the function, the spectrum of which is a convolution of S 1 (T) and G 1 (T). Since the signal S 1 (T) can be considered narrowband (compared to G 1 (T)), the bands G 1 (T) S 1 (T) and G 1 (T) can be considered approximately equal. Consider a receiver configured to receive messages from a group of users 1. Suppose that the resulting signal and code G 1 (T) generated by the receiver are fully synchronized with each other. The first step of the receiver will be multiplying the resulting signal on G 1 (T). As a result, a function G 1 2 (T) S 1 (T) and a set of side signals G 1 (T) G 2 (T) S 2 (T) + G 1 (T) G 3 (T) S 3 (T ) + ... + g 1 (t) g n (t) s n (t). If the code functions G i (T) are mutually orthogonal, the resulting signal can be perfectly removed in the absence of noise, because

.

Sided signals are easily eaten by the system, since

.

The main advantages of CDMA are confidentiality and noise immunity.

1. Privacy. If the user group code is known only to the permitted members of this group, CDMA ensures the confidentiality of communication, since unauthorized persons who do not have a code cannot access the transmitted information.

2. Noise immunity. The signal modulation by a transmission sequence requires its re-modulation by the same sequence when receiving (which is equivalent to the demodulation of the signal), as a result of which the original narrowband signal is restored. If a narrow-band interference, the demodulant direct sequence when receiving affects it as modulating, i.e. The "sinks" its spectrum on a wide band W SS, as a result of which only 1 / g part of the interference power falls into a narrow band of the signal W S, so that the narrowband interference will be weakened in G times, where G \u003d W SS / W S (W SS - the strip of the extended spectrum, W S is the source spectrum). If there is a broadband interference - with a strip of order W SS or wider, then demodulation will not change the width of its spectrum, and in the bar of the signal will fall aslend at as many times as its strip is wider than the start of the source signal.

1.4 Standard DECT for communication

DECT systems and devices are distributed in more than 30 countries on all continents of the planet. In fact, DECT is a set of specifications that determine radio interfaces for various types of communication networks and equipment. DECT combines requirements, protocols and messages that ensure the interaction of communication networks and terminal equipment. The organization of the networks themselves and the equipment device are not included in the standard. The most important DECT task is to ensure the compatibility of equipment of various manufacturers.

Initially, DECT was focused on telephony - radio suites, wireless institutional PBXs, providing radio access to public telephone networks. But the standard was so successful that it began to use it in data transmission systems, wireless subscriber access to public communication networks. DECT has found application in multimedia applications and home radio networks, to access the Internet and facsimile.

What is the DECT radio interface? In the range of 20 MHz (1880-1900 MHz), 10 carrier frequencies were isolated with an interval of 1.728 MHz. The DECT uses a temporary separation of channels - TDMA. The time spectrum is divided into separate frames of 10ms (Fig. 1.13.). Each frame is divided into 24 time slots: 12 slots for reception (from the point of view of the wearable terminal) and 12 for transmission. Thus, on each of the 10 carrier frequencies, 12 duplex channels is formed - only 120. Duplex is provided by a temporary separation (with an interval of 5 ms) receiving / transmission. For synchronization, the 32-bit sequence "101010 ..." is used. DECT provides compression of speech in accordance with the technology of adaptive differential pulse-code modulation at a speed of 32 kbps. Therefore, the information part of each slot is 320 bits. When transmitting data it is possible to combine the time slots. In the radio collateral used Gaussian frequency modulation.

Basic stations (BS) and subscriber terminals (AT) DECT constantly scan all available channels (up to 120). In this case, the signal power is measured on each of the channels, which is entered into the RSSI list. If the channel is busy or hurt, RSSI is high for it. BS Selects the channel with the lowest RSSI value for constant transfer of service information on subscriber calls, station identifier, system capabilities, etc. This information plays the role of reference signals for AT - on them subscriber devices determine if there is the right to access a particular BS, whether it provides the services required by the subscriber whether the system has a free container and choose the BS with the highest quality signal.

In DECT, the communication channel always defines at. When requesting a connection from the BS (incoming connection) AT receives a notification and selects the radio channel. Service information is transmitted by the base station and analyzed by the subscriber terminal constantly, therefore, AT is always synchronized with the closest of the available BS. When establishing a new connection, the AT selects the channel with the lowest RSSI value - this ensures that the new connection occurs on the "clean" channel from the available. This procedure for dynamic channel distribution allows you to get rid of frequency planning - the most important property of the DECT.

Fig. 1.13. Spectrum DECT.

Since AT is constantly, even when the connection is established, it analyzes the available channels, their dynamic switching during the communication session can occur. Such switching is possible both on another channel of the same BS and on another BS. This procedure is called Handover. When Handner AT sets a new connection, and some time communication is supported by both channels. Then the best is selected. Automatic switching between channels of different BS occurs almost imperceptibly for the user and completely initiated by AT.

It is essential that in the radio paint equipment DECT signal the signal is quite small - from 10 to 250 MW. Moreover, 10 MW is almost rated power for microspheric systems with a radius of honeycomb 30 - 50 m inside the building and up to 300 - 400 m in the open space. Transmitters with a capacity of up to 250 MW are used to radiocry the large territories (up to 5 km).

With the power of 10 MW, it is possible to place the base stations at a distance of 25 m. As a result, a record density of simultaneous compounds (about 100 thousand subscribers) is achieved under the condition of the BS according to the hexagon scheme in one plane (on the same floor).

To protect against unauthorized access in DECT systems, the BS and AT authentication procedure is used. AT is recorded in the system or on separate base stations to which the tolerance has. Each connection is authenticated: BS sends an AT request - a random number (64 bits). AT and BS on the basis of this number and key of authentication on a given algorithm calculate the authentication response (32 bits), which AT transmits to the BS. BS compares the calculated answer with the accepted and in their coincidence permits the connection of the AT. In DECT, there is a standard DSAA authentication algorithm.

As a rule, the authentication key is calculated based on the UAK subscriber authentication key of 128 bits or an AC authentication code (16 - 32 bits). UAK is stored in the AT ROM or in the DAM card - an analogue of a SIM card. AC can be manually recorded in the ROM AT or enter when authentication. Together with UAK, a personal UPI user identifier 16- 32 bits entered only manually applied. In addition, unauthorized entries in systems with TDMA is extremely complex and is available only to those skilled in the art.

1.5 Standards Bluetooth , Wi - Fi (802.11, 802.16)

The Bluetooth specification describes a batch method of transmitting information with temporary multiplexing. Radio exchange occurs in the frequency band 2400-2483.5 MHz. The radio collateral is used by the spectrum expansion method with frequency jumps and two-level Gaussian frequency modulation.

The frequency jump method implies that the entire bandwidth is subdivided into a certain amount of subchannels of 1 MHz width each. The channel is a pseudo-random sequence of jumps of 79 or 23 radio frequency subchannels. Each channel is divided into temporary segments with a duration of 625 μs, and each segment corresponds to a certain subchannel. The transmitter at each moment of time uses only one subchannel. Racing occurs synchronously in the transmitter and receiver in a predetermined pseudo-random sequence. During a second, up to 1600 frequency jumps can occur. This method provides confidentiality and some noise immunity. Noise immunity is ensured by the fact that if the transmitted packet cannot be accepted on any subchannel, the receiver reports this and the transfer of the package is repeated on one of the following subchannels, already at another frequency.

The Bluetooth protocol is delicate as a point-to-point connections and a multipoint point. Two or more used the same channel of the device form a picone. One of the devices works as the main one, and the rest are like subordinates. In one picotter, there may be up to seven active subordinate devices, while the remaining subordinate devices are in the "Parking" state, remaining synchronized with the main device. Interactive picosetics form a "distributed network".

In each picoseti there is only one main device, but the slave devices can enter various picosetics. In addition, the main device of one picoseti may be subordinate to another (Fig.1.14.). Picotions are not synchronized with each other in time and frequency - each of them uses its sequence of frequency jumps. In the same picoseti, all devices are synchronized over time and frequencies. The sequence of jumps is unique for each peak and is determined by the address of its main device. The length of the pseudo-random sequence cycle is 2 27 items.

Fig. 1. 14. Picone with one subordinate device A), several b) and distributed network B).

Bluetooth standard provides duplex transmission based on time separation. The main device transmits packets into odd time segments, and the subordinate device is in even (Fig. 1.15). Packages depending on the length can occupy up to five time segments. In this case, the channel frequency does not change until the end of the packet transmission (Fig. 1.16.).

Fig. 1. 15. Temporary channel operation diagram.

The Bluetooth protocol can support an asynchronous data channel, up to three synchronous (at a constant speed) of voice channels or a channel with simultaneous asynchronous data transfer and synchronous voice transmission.

When synchronous connection, the main device reserves temporary segments, following the so-called synchronous intervals. Even if the packet is accepted with an error, it is not transmitted during synchronous connection. Asynchronous communication uses temporary segments that are not reserved for synchronous connection. If the address is not specified in the address field of the asynchronous package, the package is considered "broadcast" - all devices can read it. Asynchronous connection allows you to re-send packets taken with errors.

Fig. 1. 16. Transmission of packages of various lengths.

The standard Bluetooth package contains an access code 72 bits, a 54-bit title and an information field with a length of no more than 2745 bits. Access code identifies packages belonging to one picoseti, and also used to synchronize and query procedures. It turns on the preamble (4 bits), the synchronization word (64 bits) and the trailer - 4 bits of the checksum.

The title contains information for managing a link and consists of six fields: AM_ADDR- 3-bit address of the active element; Type - 4-bit data type code; Flow is 1 data flow control bit showing the readiness of the device to receive; ARQN - 1 bits of confirmation of the correct reception; SEQN - 1 bit, serving to determine the sequence of packets; Hec- 8-bit checksum.

The information field, depending on the type of packages, may contain either the field fields, or the data field, or both types of fields simultaneously.

Consider the IEEE 802.11 standard used in local data networks - i.e. In Ethernet-like wireless networks, fundamentally asynchronous in nature.

IEEE 802.11 considers the two lower levels of the interaction model of open systems - physical (the method of working with the transmission medium, speed and modulation methods are determined) and the level of data link, and at the last level, the lower believer is considered - Mac, i.e. Control of access to the channel (transmission medium). IEEE 802.11 uses a range of 2,400 - 2.4835 GHz with a strip width of 83.5 MHz and provides packet transmission with 48-bit address packages.

The standard provides two main methods of organizing a local network - according to the "each with each" principle (the connection is set directly between two stations, all devices must be in the zone of radio abuse, no administration occurs) and in the form of a structured network (an additional device appears - an access point appears As a rule, stationary and acting on a fixed channel; the relationship between devices occurs only through the access points, through them it is possible to exit into external wired networks).

As a rule, control functions are distributed between all IEEE 802.11 network devices - DCF mode. However, PCF mode is possible for structured networks when control is transmitted by one specific access point. The need for PCF mode occurs when transmitting sensitive to information delay. After all, the IEEE 802.11 network operate on the principle of competitive access to the channel - there is no priorities. To set them if necessary, the PCF mode is entered. However, work in this mode can only occur in certain periodically repeated intervals.

For data security, the MAC level provides stations authentication and encryption of transmitted data.

IEEE 802.11 performs multiple access to the communication channel with the control and conflict detection. The station can start the transmission only if the channel is free. If the stations are found that several stations are trying on one channel, they all stop the transmission and try to resume it at a random period of time. Thus, even during transmission, the device must control the channel, i.e. Work at the reception.

Before the first attempt to access the channel, the device loads the duration of the random expectation interval into a special counter. Its value is decreventy with a given frequency until the channel is free. As soon as the counter is reset, the device can occupy the channel. If the channel takes another device before resetting the meter, the account stops, while maintaining the value achieved. At the next attempt, the countdown begins with the preserved value. As a result, did not have time for last time gets more chances to take the channel next time. In the wired Ethernet networks there is no such.

Packages by which the transmission takes place is actually formed on the MAC level, the physical layer header (PLCP) consisting of the preamble and the PLCP header itself is added to them. Mac packets can be three types - data packages, control and control packets. Their structure is the same. Each package includes MAC header, information field and checksum.

In broadband urban wireless data networks with fixed access, the IEEE 802.16 standard is used.

The IEEE 802.16 standard describes the operation in the range of 10 - 66 GHz systems with the "point-multiple" architecture (from the center - many). This is a bidirectional system, i.e. There are downwards (from the base station to subscribers) and ascending (to the base station) flows. At the same time, the channels are meant by broadband (about 25 MHz), and transmission rates are high (for example, 120 Mbps).

The IEEE 802.16 standard provides a scheme with a modulation of one carrier (in each frequency channel) and allows three types of quadrature amplitude modulation: four-position QPSK and 16-position 16-QAM (required for all devices), as well as 64-QAM (optional).

The data at the physical level is transmitted as a continuous sequence of frames. Each frame has a fixed duration - 0.5; 1 and 2 ms. The frame consists of a preamble (syncrequence of a 32 QPSK symbol length), control section, sequence of packages with data. Since the Bidirectional system is defined by the IEEE 802.16 standard, a duplex mechanism is needed. It provides both frequency and temporary separation of ascending and downward channels. With temporary duplexing channels, the frame is divided into downward and upward subframes separated by a special interval. With frequency duplexization, the upstream and downstream channels are broadcast by each on its carrier.

The IEEE 802.16 MAC level is divided into three sublevels - the service conversion fabric (services are different applications), the main sublevel and the fan of protection. On imprint protection, authentication mechanisms and data encryption are implemented. On imprint, the service conversion is transformation of upper levels data streams for data transmission via IEEE 802.16. For each type of upper levels, the standard provides its conversion mechanism. On the main MAC subproduction, data packets are generated, which are then transmitted to the physical layer and are broadcast via the communication channel. The packet closes the title and the data field, followed by the checksum.

The key point in the IEEE 802.16 standard is the concept of the service flow and the concepts associated with it and the connection identifier (CID). The service stream in the IEEE 802.16 standard is called a data stream associated with a specific application. In this context, the connection is to establish a logical connection on the MAC levels on the transmitting and receiving side for transmitting the service stream. Each connection is assigned a 16-bit CID identifier with which the type and characteristics of the connection are unambiguously connected. The service stream is characterized by a set of requirements for the transmission channel of the information transmission (by the time delay time, the level of delays and the guaranteed bandwidth). Each service stream is assigned the SFID identifier, based on which the BS defines the necessary parameters of the specific connection of a particular connection associated with this service stream.

The basic principle of providing access to the channel in the IEEE 802.16 standard is access on request. None of the AU (subscriber station) can transmit anything except queries for registration and the provision of the channel until the BS is allowed for it, i.e. Takes the time interval in the ascending channel and will indicate its location. AU can, as requested a certain size of the band in the channel, and ask for a change in the channel resource already provided to it. The IEEE 802.16 standard provides two access provision modes for each individual connection and for all compounds of a specific speaker. Obviously, the first mechanism provides greater flexibility, but the second significantly reduces the amount of service messages and requires less productivity from the equipment.

2. Systems of complex signals for telecommunication systems

2.1 Signal spectra

Signal spectrum S (T) is determined by the Fourier transformation

In general, the spectrum is a complex frequency function Ω. The spectrum can be represented as

,

where | S (Ω) | - amplitude, and φ (ω) - a phase spectrum of the signal S (T).

The spectrum of the signal has the following properties:

1. Linearity: If there is a set of signals S 1 (T), S 2 (T), ..., with S 1 (T) S 1 (Ω), S 2 (T) S 2 (Ω), ..., then the sum of signals Transformed by Fourier as follows:

where A i is arbitrary numerical coefficients.

2. If the S (T) signal corresponds to the spectrum S (Ω), the same signal shown to T 0 corresponds to the spectrum S (Ω) multiplied by E - JωT 0 S (TT 0) S (Ω) E - JωT 0 .

3. If S (T) S (Ω), then

4. If S (T) S (Ω) and F (T) \u003d DS / DT, then f (t) f (ω) \u003d jωs (ω).

5. If S (T) S (Ω) and G (T) \u003d ∫S (T) DT, then G (T) G (Ω) \u003d S (Ω) / Jω.

6. If U (T) U (Ω), V (T) V (Ω) and S (T) \u003d U (T) V (T), then

.

The signal is on the spectrum using the Fourier reverse transformation

.

Consider the spectra of some signals.

1. Rectangular impetus.

Fig.2.1. The spectrum of a rectangular pulse.

2. Gaussian impetus.

s (T) \u003d UEXP (-βT 2)

Fig.2.2. Spectrum of the Gaussian impulse.

3. Smoothed impulse

With the help of numerical integration, we find the spectrum S (Ω).

S (0) \u003d 2.052 s (6) \u003d - 0.056

S (1) \u003d 1.66 s (7) \u003d 0.057

S (2) \u003d 0.803 s (8) \u003d 0.072

S (3) \u003d 0.06 S (9) \u003d 0.033

S (4) \u003d - 0.259 s (10) \u003d - 0.0072

S (5) \u003d - 0.221 s (Ω) \u003d s (-ω)

Fig. 2.3. Spectrum of a smoothed pulse.

2.2 Correlation Signal Properties

To compare the signals shifted in time, an autocorrelation function (ACF) of the signal is introduced. It quantitatively determines the degree of distinction of the signal U (T) and its shifted copy of U (T - τ) and is equal to the scalar product and copy:

It is directly seen that at τ \u003d 0, the autocorrelation function becomes equal to the signal of the signal: B U (0) \u003d E U.

Autocorrelation function is even: B U (τ) \u003d B U (-τ).

With any value of the time shift τ, the ACF module does not exceed the signal energy | in U (τ) | ≤B u (0) \u003d E U.

ACF is associated with a signal spectrum by the following ratio:

.

Right and reverse:

.

For a discrete signal ACF is determined in the following form:

and possesses the following properties.

Discrete ACF is even: b u (n) \u003d b U (-n).

At a zero shift, the ACF determines the energy of the discrete signal:

.

Sometimes an intercorrlation function (VKF) signals are introduced, which describes not only the signal shift relative to each other in time, but also the difference in the form of signals.

VKF is defined as follows.

for continuous signals and

for discrete signals.

Consider the ACF some signals.

1. Sequence of rectangular pulses

Fig. 2.4. ACF sequence of rectangular pulses.

2. 7-position Barcker

B U (0) \u003d 7, B U (1) \u003d B U (-1) \u003d 0, B U (2) \u003d B U (-2) \u003d - 1, B U (3) \u003d B U (-3 ) \u003d 0, b U (4) \u003d b u (-4) \u003d - 1, b u (5) \u003d b u (-5) \u003d 0, b u (6) \u003d b u (-6) \u003d - 1 , B U (7) \u003d B U (-7) \u003d 0.

Fig. 2.5. ACF of the 7-position signal of the barker.

3. 8-position Walsh functions

Walsh Function 2nd Order

B U (0) \u003d 8, B U (1) \u003d B U (-1) \u003d 3, B U (2) \u003d B U (-2) \u003d - 2, B U (3) \u003d B U (-3 ) \u003d - 3, b U (4) \u003d b U (-4) \u003d - 4, b U (5) \u003d B U (-5) \u003d - 1, b U (6) \u003d B U (-6) \u003d 2, B U (7) \u003d B U (-7) \u003d 1, B U (8) \u003d B U (-8) \u003d 0.

Fig. 2.6. ACF functions Walsh 2nd order.

Walsh 7th order function

B U (0) \u003d 8, B U (1) \u003d B U (-1) \u003d - 7, b U (2) \u003d B U (-2) \u003d 6, B U (3) \u003d B U (-3 ) \u003d - 5, b U (4) \u003d b U (-4) \u003d 4, b U (5) \u003d b U (-5) \u003d - 3, b u (6) \u003d b U (-6) \u003d 2 , B U (7) \u003d B U (-7) \u003d - 1, B U (8) \u003d B U (-8) \u003d 0.

Fig. 2.7. ACF function Walsh 7th order.

2.3 Types of complex signals

A signal is a physical process that can carry useful information and spread through line. Under the signal S (T), we will understand the function of the time displaying a physical process that has a finite duration of T.

Signals in which the base in equal to the product of the signal of the signal T to the width of its spectrum is close to one, are called "simple" or "ordinary". The distinction of such signals can be carried out in frequency, time (delay) and phase.

Complex, multidimensional, noise-like signals are formed by complex law. During the duration of the signal T, it is subjected to additional manipulation (or modulation) in frequency or phase. Additional amplitude modulation is rarely used. Due to the additional modulation, the spectrum of the signal Δf (while maintaining its duration T) expands. Therefore, for such a signal B \u003d T ΔF \u003e\u003e 1.

Under some laws of forming a complex signal, its spectrum turns out to be solid and almost uniform, i.e. Close to the noise spectrum with limited bandwidth. In this case, the signal of autocorrelation signal has one main emission, the width of which is not determined by the signal duration, but its width of its spectrum, i.e. It has the form, similar functions of noise autocorrelation with limited frequency band. In this regard, such complex signals are called noise-like.

Noise-like signals were used in broadband communications systems, since: ensure high noise immunity of communication systems; allow you to organize the simultaneous work of many subscribers in the overall frequency band; allow you to successfully combat the multipath propagation of radio waves by separating the rays; Provide better use of frequency spectrum in limited territory compared to narrowband communication systems.

A large number of different noise-like signals (SPS) are known. However, the following main SPSs are distinguished: frequency-modulated signals; Multiplass signals; phasenapulated signals; discrete frequency signals; Discrete compound frequency signals.

Frequency-modulated signals (FM) are continuous signals whose frequency changes according to a given law (Fig. 2.8.).

Fig. 2.8. FM signal.

In communication systems, you must have many signals. At the same time, the need to quickly change signals and switching the formation and processing equipment lead to the fact that the frequency change law becomes discrete. At the same time, from the World Cup signals transfers to the HDC signals.

Multiplass (MCH) Signals are the sum N Harmonic U 1 (T) ... U N (T), the amplitudes and phases of which are determined in accordance with the laws of the formation of signals (Fig. 2.9.).

Fig. 2.9. MCh signal.

MCH signals are continuous and for their formation and processing difficult to adapt the methods of digital technology.

Fazoomanipulated (FM) Signals represent a sequence of radio pulses, the phases of which are changed according to a given law (Fig. 2.10., A). Typically, the phase takes two values \u200b\u200b(0 or π). In this case, the radio frequency FM signal corresponds to the video FM signal (Fig. 2.10., B).

Fig. 2.10. FM signal.

FM signals are very common, because They allow you to widely use digital methods when forming and processing, and you can implement such signals with relatively large bases.

Discrete frequency (DCH) Signals represent a radio pulse sequence (Fig. 2.11.), Which carrier frequencies are changed according to a given law.

Fig. 2.11. DC signal.

Discrete composite frequency (DSH) Signals are HDC signals in which each pulse is replaced by a noiseless signal.

In fig. 2.12. Pictures a video frequency FM signal, separate parts of which are transmitted at various carrier frequencies.

Fig. 2.12. DSH signal.

2.4 Derivative Signal Systems

The derivative signal is called a signal that is obtained as a result of multiplying two signals. In the case of FM signals, multiplies should be aligned alternately or, as more often, they are called, prevail. The system composed of derived signals is called a derivative. Among derivative systems, systems built as follows are of particular importance. As a basis, some system of signals are used, the correlation properties of which do not quite meet the requirements for KF, but which has certain advantages in terms of ease of formation and processing. Such a system is called source. Then the signal is selected, which has certain properties. Such a signal is called producing. Multipably generating a signal for each source system signal, we obtain a derivative system. The generating signal should be selected so that the derivative system is really better than the original, i.e. So that it possesses good correlation properties. The complex envelope of the derivative signal S μ M (T) is equal to the product of complex envelopes of the initial signals U M (T) and the generating signal V μ (T), i.e. S μ M (T) \u003d U M (T) V μ (T). If indices change within m \u003d 1..m, μ \u003d 1..h, then the volume of the derivative of the signal system L \u003d MH.

The choice of production signals is determined by a number of factors, including the source system. If the signals of the source system are broadband, then the signal can be broadband and have small levels of lateral peaks of the uncertainty function close to the rms value. If the signals of the source system are narrowband, then it is enough to perform an inequality F V \u003e\u003e F U (F V - the width of the spectrum of the generating signals, F U is the width of the source spectrum) and the requirements of the smallness of the side peaks of ACF.

Take as source - Walsh system. In this case, the generating signals must be broadband and have good ACF. In addition, the generating signal must have as many elements as the source signals, i.e. N \u003d 2 K elements, where k is an integer. These conditions are generally satisfying nonlinear sequences. Since the main activity of the ACF side peaks is basic, the best signals with the number of elements n \u003d 16, 32, 64 were presented in the class of nonlinear sequences. These signals are shown in Fig. 2.13. In fig. 2.13. The values \u200b\u200bof the number of blocks μ are also indicated for each generating signal. They are close to the optimal value of μ 0 \u003d (n + 1) / 2. This is a prerequisite for obtaining a good ACF with small side peaks.

Fig. 2.13. FM generating signals.

The volume of the derivative system is equal to the volume of the system of Walsh N. Derivative systems have better correlation properties than Walsh systems.

3. Modulation of complex signals

3.1 Geometric Signal Presentation

Consider the geometric or vector representation of the signals. We define the N-dimensional orthogonal space as a space defined by a set N of linearly independent functions (ψ j (t)), called basic. Any function of this space can be expressed through a linear combination of basic functions that must satisfy the condition

,

where the operator is called a symbol of a macketer. With nonzero constants K j, the space is referred to as orthogonal. If the basic functions are normalized so that all K j \u003d 1, the space is called orthonormal. The basic condition of orthogonality can be formulated as follows: each function ψ j (t) of the set of basic functions should be independent of the other dialing functions. Each function ψ j (t) should not be interferred with other functions during the detection process. From a geometrical point of view, all functions ψ j (t) are mutually perpendicular.

In orthogonal signaling space, the Euclidean measure of the distance used in the detection process is determined. If waves carrying signals do not form a similar space, they can be transformed into a linear combination of orthogonal signals. It can be shown that an arbitrary finite set of signals (Si (T)) (i \u003d 1 ... M), where each element of the set is physically realized and has a duration T, it can be expressed as a linear combination of n orthogonal signals ψ 1 (t), ψ 2 ( t), ..., ψ n (t), where nm, so

where

The type of basis (ψ j (t)) is not set; These signals are selected from the point of view of convenience and depend on the waveform of the signal transmission. A set of such waves (s i (t)) can be considered as a set of vectors (S i) \u003d (a i 1, a i 2, ..., a in). The mutual orientation of the signal vectors describes the link between the signals (relative to their phases or frequencies), and the amplitude of each set of the set (S i) is a signal of the signal energy transferred during the symbol transmission time. In general, after selecting a set of N orthogonal functions, each of the transmitted signals s i (t) is fully determined by the vector of its coefficients s i \u003d (a i 1, a i 2, ..., a in) i \u003d 1 ... m.

3.2 Phase manipulation methods (FM2, FM4, OFM)

Phase manipulation (PSK) was developed at the beginning of the development of a long-range research program; Now the PSK scheme is widely used in commercial and military communication systems. The signal in modulation PSK has the following form:

Here, the phase φ i (t) can take M discrete values, usually defined as follows:

The simplest example of phase manipulation is binary phase manipulation (FM2). The E parameter is the symbol energy, T is the symbol transmission time. The operation of the modulation scheme consists in displacement of the phase of the modulated signal S I (T) to one of the two values, zero or π (180 0). The typical view of the FM2 signal is shown in Fig. 3.1.a), where characteristic sharp phase changes are clearly visible during the transition between characters; If the modulated data stream consists of alternating zeros and units, such sharp changes will occur at each transition. The modulated signal can be represented as a vector on the graph in the polar coordinate system; The length of the vector corresponds to the amplitude of the signal, and its orientation in the overall M-Arn case - the signal phase relative to the other M - 1 of the set signals. When modulated FM2 (Fig. 3.1.b)), the vector representation gives two antiphase (180 0) vector. Signal sets that can be represented by similar antiphase vectors are called antipodes.

Fig. 3.1. Binary phase manipulation.

Another example of phase manipulation is the modulation of FM4 (M \u003d 4). When modulated the FM4 parameter E is the energy of two characters, time T is the transfer time of two characters. The phase of the modulated signal takes one of the four possible values: 0, π / 2, π, 3π / 2. In vector view, the FM4 signal is view shown in Fig. 3.2.

Fig. 3.2. FM4 signal in vector representation.

Consider another type of phase manipulation - relative phase manipulation (OFM) or differential phase manipulation (DPSK). The name Differential Phase manipulation requires some explanation, since two different aspects of the modulation / demodulation process are associated with the word "differential": the encoding procedure and the detection procedure. The term "differential coding" is used when the encoding of binary characters is determined not by their value (i.e., zero or unit), but whether the symbol coincides with the previous or differs from it. The term "differential coherent detection" of signals in differential modulation PSK (This value is usually used by the name DPSK) is associated with the detection scheme, which often refers to non-hotter schemes, because it does not require harmonization by phase with the adopted carrier.

In incoherent systems, attempts are not made to determine the actual value of the phase of the incoming signal. Consequently, if the transmitted signal is viewed

the received signal can be described as follows.

Here α is an arbitrary constant, usually intended by a random variable, evenly distributed between zero and 2π, and n (t) - noise.

For coherent detection, consistent filters are used; For non-coherent detection, this is impossible, since in this case the output of the agreed filter will depend on an unknown angle α. But if we assume that α changes slowly relative to the interval in two periods (2t), the phase difference between two consecutive signals will not depend on α.

The basis of the differential coherent signal detection in the DPSK modulation is as follows. In the process of demodulation, the phase of the previous symbol transmission interval can be used as a support phase. Its use requires differential coding of the message sequence in the transmitter, since the information is encoded by the phase difference between two consecutive pulses. To transfer the i-th message (i \u003d 1.2, ..., m) the phase of the current signal should be shifted to φ i \u003d 2πi / m radians relative to the phase of the previous signal. In general, the detector calculates the coordinates of the incoming signal by determining its correlation with locally generated signals Cosω 0 T and SINΩ 0 T. Then, as shown in Fig. 3.3., The detector measures the angle between the vector of the received received signal and the previous signal vector.

Fig. 3.3. Signal space for DPSK scheme.

The DPSK scheme is less effective than PSK, since in the first case, due to the correlation between signals, errors tend to distribute (for adjacent symbol transmission times). It is worth remembering that the PSK and DPSK schemes are distinguished by the fact that in the first case, the received signal is compared with the ideal reference, and in the second - two roaring signals. Note that the DPSK modulation gives a greater noise than the PSK modulation. Consequently, when using DPSK, you should expect a greater likelihood of an error than in the case of PSK. The advantage of the DPSK scheme can be called a smaller complexity of the system.

3.3 Modulation with a minimum frequency shift.

One of the modulation schemes without a phase break is a minimum frequency shift manipulation (MSK). MSK can be considered as a special case of frequency manipulation without a phase break. The MSK signal can be represented as follows.

Here F 0 is the carrier frequency, D k \u003d ± 1 represents bipolar data, which are transmitted at a speed R \u003d 1 / T, and X K is a phase constant for the K-th interval of binary data transmission. Note that when D k \u003d 1, the transmitted frequency is F 0 + 1 / 4T, and when D k \u003d -1 is F 0 -1 / 4T. During each T-second data interval, the value of X K is constantly, i.e. X k \u003d 0 or π, which is dictated by the requirement of the continuity of the signal phase at the moments T \u003d KT. This requirement imposes a phase restriction, which can be represented by the following recursive relation for X K.

The equation for S (T) can be rewritten in a quadrature representation.

The syphanger component is indicated as a k cos (πT / 2t) COS2πF 0 T, where Cos2πF 0 T is carrier, COS (πT / 2T) - sinusoidal weighing of the symbols, A K is an information-dependent member. Similarly, the quadrature component is b K sin (πT / 2T) sin2πf 0 T, where Sin2πF 0 T is the quadrature carrier, SIN (πT / 2T) is the same sinusoidal weighing of the symbols, B K is an information-dependent member. It may seem that the values \u200b\u200bof a k and b k can change their value every T seconds. However, due to the requirements of the phase continuity, the value A k may change only when switching the COS (πT / 2t) function via zero, A B k - only when switching through zero sin (πT / 2t). Therefore, weighing characters in a syphase or quadrature channel is a sinusoidal impulse with a period of 2t and variable sign. The syphanit and quadrature components are shifted relative to each other on T seconds.

The expression for S (T) can be rewritten in a different form.

Here d i (t) and d q (t) have the same meaning of the syphase and quadrature data streams. The MSK scheme recorded in this form is sometimes called MSK with pre-coding. The graphic representation S (T) is given in Fig. 3.4. In fig. 3.4. a) and c) shown the sinusoidal weighing of the pulses of the syphase and quadrature channels, here the multiplication of the sinusoid gives more smooth phase transitions than in the initial representation of the data. In fig. 3.4. b) and g) The modulation of orthogonal components Cos2πF 0 T and SIN2πF 0 T with sinusoidal data streams is shown. In fig. 3.4. e) the summation of orthogonal components depicted in Fig. 3.4. b) and d). From the expression for S (T) and Fig.3.4. You can conclude the following: 1) Signal S (T) has a permanent envelope; 2) the phase of the radio frequency carrier is continuous at bit transitions; 3) Signal S (T) can be considered as a signal modulated FSK, with frequencies of transmission F 0 + 1 / 4T and F 0 -1 / 4T. Thus, the minimum separation of the tones required by modulating MSK can be written as follows:

what is equal to half the rate of bits. Note that the separation of the tones required for MSK is half (1 / t) the separation required with incoherent detection of signals modulated by FSK. This is explained by the fact that the carrier phase is known and continuous, which allows the coherent demodulation of the signal.

Fig. 3.4. Manipulation with minimal shift: a) modified syphanit bits; b) the product of the syphase bits and carrier stream; c) modified quadrature bits; d) the product of the quadrature bits and carrier stream; e) MSK signal.

3.4 quadrature modulation and its characteristics ( Q. PSK. , QAM. )

Consider quadrature phase manipulation (QPSK). The initial flow of data d k (t) \u003d d 0, d 1, d 2, ... consists of bipolar pulses, i.e. D k take values \u200b\u200b+1 or -1 (Fig. 3.5.A)) representing a binary unit and binary zero. This pulse stream is divided into a syphan flux D i (T) and the quadratus - D q (t), as shown in Fig. 3.5.B).

d i (t) \u003d d 0, d 2, d 4, ... (even bits)

d q (t) \u003d d 1, d 3, d 5, ... (odd bits)

The convenient orthogonal implementation of the QPSK signal can be obtained using the amplitude modulation of the syphase and quadrature flows on the sinus and cosine carrier functions.

Using trigonometric identities S (T), it can be represented in the following form: S (T) \u003d COS (2πF 0 T + θ (T)). QPSK modulator shown in Fig. 3.5.In), uses the sum of the sinusoidal and cosine-shaped components. The pulse stream d i (t) is used for amplitude modulation (with amplitude +1 or -1) cosine. It is equal to the phase shift of the cosine phase by 0 or π; Therefore, as a result, we obtain the BPSK signal. Similarly, the pulse stream D Q (T) modulates the sinusoid, which gives the BPSK signal, the orthogonal previous one. When summing these two orthogonal carrier components, the QPSK signal is obtained. The value of θ (t) will correspond to one of the four possible combinations D I (T) and D Q (T) in the expression for S (T): θ (T) \u003d 0 0, ± 90 0 or 180 0; The resulting vector vectors are shown in the signal space in Fig. 3.6. Since COS (2πF 0 T) and SIN (2πF 0 T) are orthogonal, two BPSK signals can be detected separately. QPSK has a number of advantages over BPSK: Because When modulated QPSK, one pulse transmits two bits, the data transfer rate or at the same data transmission rate, as in the BPSK scheme, is used twice as long as the frequency band; and also increases noise immunity, because The pulses are two times longer, and therefore more power than BPSK pulses.

Fig. 3.5. QPSK modulation.

Fig. 3.6. Signal space for QPSK scheme.

Quadrature amplitude modulation (KAM, QAM) can be considered a logical continuation of QPSK, since the QAM signal also consists of two independent amplitude-modulated carriers.

With quadrature amplitude modulation, both the phase and the amplitude of the signal is changed, which makes it possible to increase the amount of bits encoded and at the same time significantly increase noise immunity. The quadrature representation of the signals is a convenient and fairly universal means of their description. The quadrature representation is to express fluctuations in a linear combination of two orthogonal components - sinusoidal and cosine (syphase and quadrature):

s (t) \u003d a (t) cos (ωt + φ (t)) \u003d x (t) sinωt + y (t) cosωt, where

x (t) \u003d a (t) (- sinφ (t)), y (t) \u003d a (t) cosφ (t)

Such discrete modulation (manipulation) is carried out on two channels, on the carriers shifted by 90 0 each relative to each other, i.e. in quadrature (hence the name).

Let us explain the operation of the quadrature scheme on the example of the formation of the signals of the four-phase FM (FM-4) (Fig. 3.7).

Fig. 3.7. Scheme of the quadrature modulator.

Fig. 3.8. 16-Wonder Signal Space (QAM-16).

The initial sequence of binary symbols duration t using a shear register is divided into odd pulses Y, which are supplied to the quadratus channel (COSωt), and even - x entering the SinΩT (SINΩT). Both pulse sequences are entered on the inputs of the corresponding manipulated pulses, on the outputs of which the sequences of bipolar pulses x (t) and y (t) are formed with an amplitude ± u m and a duration of 2t. The pulses x (t) and y (t) enter the inputs of channel multiplers, on the outputs of which two-phase (0, π) fm oscillations are formed. After summation, they form the FM-4 signal.

In fig. 3.8. The two-dimensional signal space is shown and a set of vectors of signals modulated by the 16-riche qam and depicted points, which are located in the form of a rectangular aggregate.

From fig. 3.8. It can be seen that the distance between the signals in the signal space with QAM is greater than with QPSK, therefore, QAM is more noise-resistant compared to QPSK,

3.5 Sales of quadrature modems

The modem is designed to transmit / receive information on ordinary telephone wires. In this sense, the modem performs the role of interface between the computer and the telephone network. Its main task is to convert the transmitted information to the form acceptable to transmission over telephone communication channels, and in converting information received to the form acceptable to the computer. As you know, the computer is able to process and transmit information in binary code, that is, in the form of a sequence of logical zeros and units called bits. A logical unit can be put into line with a high voltage level, and the logical zero is low. When transferring information on telephone wires, it is necessary that the characteristics of the transmitted electrical signals (power, spectral composition, etc.) correspond to the requirements of the ATS reception equipment. One of the basic requirements is that the signal spectrum lay in the range from 300 to 3400 Hz, that is, there has been no more than 3100 Hz width. In order to satisfy this and many other requirements, the data is subjected to appropriate coding, which, in fact, is engaged in a modem. There are several ways of possible encoding in which data can be transmitted by subscriber switched channels. These methods differ from each other as a transfer rate and noise immunity. At the same time, regardless of the encoding method, the data is transmitted by subscriber channels only in analog form. This means that a sinusoidal bearing signal that is subjected to analogue modulation is used to transmit information. The use of analog modulation leads to a spectrum of a much smaller width at a constant information transmission rate. Analog modulation is a method of physical coding in which the information is encoded by changing the amplitude, frequency and phase of the sinusoidal carrier frequency signal. There are several basic methods of analog modulation: amplitude, frequency and relative phase. In modems, listed modulation methods are used, but not separately, but all together. For example, amplitude modulation can be used in conjunction with phase modulation (amplitude-phase modulation). The main problem arising from the transfer of information on subscriber channels is to increase the speed. The speed is limited by the spectral bandwidth bandwidth. However, there is a way to significantly increase the speed of information transmission without increasing the width of the signal spectrum. The main idea of \u200b\u200bthis method is to use multi-position coding. The sequence of data bit is divided into groups (characters), each of which is put in accordance with some discrete state of the signal. For example, using 16 different states of the signal (they may differ from each other, both amplitude and phase), you can encode all possible combinations for sequences of 4 bits. Accordingly, 32 discrete states will encode the group of five bits in one state. In practice, a multi-position amplitude-phase modulation with several possible values \u200b\u200bof the amplitude levels and a signal phase shift is mainly used to increase information. This type of modulation was called a quadrature amplitude modulation (CAM). In the case of a signal status, it is convenient to depict the signal plane. Each point of the signal plane has two coordinates: amplitude and signal phase and is an encoded combination of the sequence of the bit. To increase the noise immunity of quadrature amplitude modulation, the so-called TELLIS-modulation (TRELLIS Code Modulation, TCM) or, otherwise, lattice coding can be used. When drill modulation to each group of bits transmitted in one discrete signal status, another excess trillis bit is added. If, for example, the information bits are divided into groups of 4 bits (only 16 different combinations are possible), then 16 signal points are located in the signal plane. Adding the fifth trillis bit will lead to the fact that possible combinations will be 32, that is, the number of signal points will double. However, not all combinations of bits are permitted, that is, having meaning. This is the idea of \u200b\u200btrillis coding. The value of the added trillis bit is determined by a special algorithm. A special encoder is engaged in the calculation of the added TELLIS-bit. On the receiving modemia, a special decoder is intended for analysis of incoming bits sequences - the so-called Witerby decoder. If the received sequences are allowed, it is considered that the transmission occurs without errors and the trillis bits is simply deleted. If there are prohibited sequences among the received sequences, then with the help of a special algorithm, Viterbi decoder finds the most appropriate allowed sequence, thus correcting the transmission errors. So, the meaning of the lattice coding is the price of relatively small redundancy to increase the noise immunity of the transfer. The use of trillis coding makes it mainly to protect against the tuning point in the signal space, which are just most of all susceptible to "murmur" under the action of interference.

4. Features of receiving signals in telecommunication systems

4.1 probabilities of errors of distinction M. famous signals

Under the detection of the signal in the radio electronics, the analysis of the received oscillation y (t) is understood, concluded by the decision on the presence or absence of a certain useful component in it, which is called the signal. The distinction of M signals is defined as an analysis of the received oscillation Y (T), ending with the decision on which one of the signals belonging to the specified in advance set S (S 0 (T), S 1 (T), ..., S M -1 ( t)) is present in y (t). Signal detection There is a special case of distinguishing two signals, one of which is zero over the entire observation interval.

Let the observed oscillation Y (T) be the implementation of a random process that has a distribution W y, i.e. N-dimensional probability density (PV) W (y) [either PV functional W (y (T))] belonging to one of the unseasonable classes w i (W i ∩w k \u003d Ø, i ≠ k, i, k \u003d 0, 1, ..., m-1). It is necessary, by passing the implementation of Y (T), to decide which from classes belongs to W y. The assumption that W y w i is called the hypothesis H i: W y w i. Decisions that are the result of testing hypotheses will be denoted where I (0, 1, ..., M-1) is the number of the hypothesis, the truth of which is declared by the decision being declared. The analyzed oscillation y (t) is the result of the interaction of the signal s i (T) present in it with a hindered process (hindrance, noise) x (t): y (t) \u003d f. From the one of which of the possible signals is present in y (t), it depends on the ensemble to which Y (T) belongs, so that each S I (T) corresponds to some class W I of the distribution of the ensemble, represented by y (t). Thus, hypothesis H i are treated as assumptions about the presence of the i-th (and only i-th) signal in Y (T). At the same time, solutions, one of which serves as a result of the procedure for distinguishing, there are allegations that the I-th signal is contained in the accepted oscillation. Hypothesians H i correspond to classes w i. The hypothesis of H I is called simple if the class W i contains one and only one distribution. Any other hypothesis is called complex. M of complex hypotheses is called parametric if the corresponding classes differ from each other only by the values \u200b\u200bof the final number of parameters of the same distribution described by the well-known law. Otherwise, the hypothesis is called parametric.

Consider the distinction of M deterministic non-zero signals of the same energy. At the same time, the basis will be made the rule of maximum believing (MP)

optimal in the case when the quality criterion is the sum of the conditional probabilities of errors, or the full probability of error with equal reciprocal probabilities of all signals p i \u003d 1 / m.

With an arbitrary M, the distinctor adheres to the MP rule, considers the signal to be present in y (T), the least remote from Y (T) in the sense of the Euclidean distance or that with the same energies of signals is equivalent to the maximum correlation with Y (T) . If we consider signals S 0 (t), s 1 (t), ..., s m -1 (t) as a bundle of vectors, located in the m-dimensional space, then in order to reduce the likelihood of confusion of the i-th signal to -M, it follows the maximum "push" the i-th and k-th vectors. Thus, the optimal selection of M deterministic signals is reduced to the search for such a beam configuration of m, in which the minimum Euclidean distance between the pair of vectors would be maximal: Mind Ik \u003d Max (I ≠ K). Since with equality of energy, i.e. Vector lengths

where ρ ok is the correlation coefficient of the i-th and k-th signals, e is the signal energy, then the requirement of the maximum minimum distance is identical to the minimum condition of the maximum correlation coefficient in the set of signals S (S 0 (T), S 1 (T), ... S M -1 (T)). The maximum achievable minimum of the maximum correlation coefficient is set pretty easily. Having lifting ρ ik for all I and K, we get

where the inequality follows from non-negativity of the square under the integral. In addition, in the amount of the onion M of the terms at i \u003d k are equal to one, and the remaining M (M-1) is not greater than ρ max \u003d max ρ Ik (I ≠ K). Therefore, M + M (M-1) ρ Max ≥0 and ρ Max ≥-1 / (M-1).

The configuration of the vectors in which the cosine of the angle between any pair of vectors is -1 / (M-1), is called the correct simplex. If these vectors take as M signals, the resulting deterministic ensemble with an equilibiousness of all s i (t) will provide a minimum of the full probability of the er error P OS, which solves the question of the optimal choice of M signals. When M \u003e\u003e 1, a ratio of -1 / (M-1) ≈0 is performed, and therefore, with a large number of different signals, the orthogonal ensemble almost does not lose the simplex in the value of P Osh.

The sequence of output of an accurate expression for the probability of error distinguishing M signals with arbitrary ρ ick is such. The probability density (PV) of the random variables z 0, z 1, ..., z m -1 is a M-dimensional normal law, for whose task is enough to know the average of all Z I and their correlation matrix. For medium, with the truth of the hypothesis H l we have. The correlation moment of the i-th and k-th correlations is N 0 Eρ IK / 2. After the M-dimensional PV was found, its M-multiple integral in the region z l ≥z i, i \u003d 0, 1, ..., m - 1, makes it possible to obtain the probability of the correct solution under the condition of the truth of H l. The sum of such probabilities, divided by M (taking into account the equilibrium of signals), will be the complete probability of the correct solution of P of the PR, associated with p Osh obvious equality P OSH \u003d 1-P Ave. The M-multiple integral obtained in a number of important cases Single. So, for any unlocked (equidistant) signals (ρ ok \u003d ρ, I ≠ K)

In practical calculations, this expression is rarely used due to the need for numerical integration. Its estimate is useful on top, for the withdrawal of which we will assume that the hypothesis of H l is true. In this case, the error occurs always when it is true at least one of the events z i\u003e z l, i ≠ l. The probability of its P OS L, equal to the likelihood of combining the events z i\u003e z l, I ≠ L, by the probability addition theorem,

and due to the inequality of Bul, no more than the first amount on the right. Since each term of this amount is the likelihood of the confusion of two signals, then for equidistant signals

Here is the signal-to-noise ratio at the output of the filter, consistent with S I (T) with the hypothesis of H i, - The probability of confused two signals. With equilibious signals (p i \u003d 1 / m) we come to the so-called additive border of the full probability of error

The use of this expression is justified, on the one hand, asymptotic convergence of its right part and P OS as the requirements for the quality of differences of distinction (P Osh → 0), and on the other, the fact that, choosing the necessary signal energy (the minimum value Q) based on The right part of the expression, the developer always operates with the known reinsurance, ensuring that the actual probability of the error is lower than the figure adopted by it when calculating.

4.2 probabilities of errors of distinction M. fluctuating signals

Not always the observer in detail a priori is aware of the distinguished signals. More often, not only the number of the signal present in the analyzed signal is not only known, but also the values \u200b\u200bof any parameters (amplitudes, frequencies, phases, etc.) of each of the possible signals. The signals themselves are no longer deterministic, since the parameters are not specified; The corresponding task of distinction is called distinction of signals with unknown parameters.

Consider the solution to this task on the example of the differences in signals with random initial phases. Such signals are described by the model.

s i (t; φ) \u003d RE (I (T) EXP),

where F 0 is a known central frequency; φ is the random initial phase with a priori PV W 0 (φ); (T) \u003d S (T) E Jγ (T) - the complex envelope of the signal S (T), which is the implementation of S (T; φ) at φ \u003d 0: s (t) \u003d s (t; 0); S (T) and γ (T) are well-known laws of amplitude and angular modulation. The application of the MP rule should precede the calculation of the function (functional) of the likelihood (FP) W (y (T) | H i), i.e. Averaging FP W (Y (T) | h i, φ) constructed for deterministic signals with a fixed phase φ in all possible values, taking into account a priori PV W 0 (φ). With the uniform PV phase W 0 (φ) \u003d 1 / (2π), | φ | ≤π, taking into account the equality of the energies of all distinguished signals W (y (T) | H i) is a modified function of the breaker of the zero order:

where C is a coefficient containing factors independent of I, and - the correlation module of complex envelopes of the received oscillation y (t) and the i-th signal. The monotony of the function I 0 (·) on the positive semi-axis allows you to go to sufficient statistics Z I and write the MP rule in the form of

Thus, the optimal distinctor of the signals of equal energy with random initial phases should calculate all M values \u200b\u200bof Z I and, if the maximum of which is Z K, decide on the presence of the K-th signal in Y (T). This means that the signal, the complex envelope of which has the largest correlation with the complex envelope y (t) is considered to be considered in the observed oscillation Y (T).

The exact formulas for the probabilities of errors of distinguishing M arbitrary signals are sufficiently cumbersome even at m \u003d 2, but in the applications more often than other signals are encambled, orthogonal in a strengthened sense. The latter means that any two inconsistent signals S i (t; φ i), S k (t; φ k) are orthogonal at any values \u200b\u200bof the initial phases:

∫s i (t; φ i) s k (t; φ k) dt \u003d 0 for any φ i, φ k and i ≠ k,

or, equivalent, orthogonal deterministic complex envelopes of these signals:

.

The condition of orthogonality in the enhanced sense is the tougher conventional orthogonality requirement that appeared earlier in the application to deterministic signals. Thus, two segments of cosineids shifted at an angle ± π / 2, being orthogonal in the usual sense, not orthogonal when changing the phase shift, i.e. In the enhanced sense. At the same time, signals not overlapping or on the spectrum, orthogonal and in a strengthened sense.

If you appeal first to distinguish between two signals, it is not difficult to understand that the opposite pair, minimizing P OS in the class of deterministic signals, in tasks where the initial phases of signals random, unacceptable. Indeed, the only sign by which opposite signals differ is a sign, i.e. Presence or absence in the initial phase of the category π. However, when each of the signals arrive at the distinth, each of the signals acquires a random phase shift, attempts to use the initial phase, and in the characteristic sign of the signal, meaningless, and in the distinctor on the non-informative value φ have to get rid of. Thus, it can be concluded that in the class M≥2 signals with random phases, simplex ensembles optimal properties do not possess. It is the optimal ensembles of signals, orthogonal in a strengthened sense: each of such signals causes a response to the output of only one of the filters of the receiving circuit, and therefore the confusion of the i-th signal with the K-M will occur only if the noise envelope occurs "The agreed filter (SF) will have a value that exceeds the value of the increment of the sum of the signal at the output of the i-th. The violation of the condition of orthogonality in a strengthened sense will lead to the appearance of a reaction to the i-th signal at the output of not only the i-th, but also other SF, for example, the K-th, as a result of which the emission of the envelope at the y outlet of the K-th sf, greater values \u200b\u200bof Z I, It will become more likely.

To find the probability of confusion p 01 s 0 (t; φ) with S 1 (T; φ) in distinguishing two signals, it is necessary to integrate the joint PV Z 0, Z 1 with a hypothesis H 0 W (z 0, z 1 | H 0) on the region z 1\u003e z 0. For orthogonal in the enhanced sense of signals Z 0 and Z 1, therefore w (z 0, z 1 | h 0) \u003d w (z 0 | H 0) W (z 1 | H 0). The one-dimensional PV Z 0 and Z 1 are known: with the truth of H 0 Z 0 as the envelope of the sum of the noise signal has a generalized Rayleigh PV; Z 1 How the envelope only noise is a Rayleigh random variable. Alternating these PVs after the integration of the obtained PV W (Z 0, Z 1 | H 0) and, taking into account the obvious equality P 01 \u003d P 10, for the complete probability of the error of distinguishing two equivalent orthogonal in the enhanced sense of signals with random phases, we obtain

Repetition of the arguments of paragraph 4.2. (for deterministic signals) leads to an additive boundary

which, as a rule, is used to estimate the probability of an error if the number of equivalent orthogonal in the enhanced sense of signals M≥2.

4.3 Calculation of errors of distinction M. Signals with unknown non-energy parameters

Consider the task of distinguishing "M" of orthogonal signals with an unknown temporary position in asynchronous communication systems with coded channels. The decision on the presence of a signal in the channel is made according to the method of maximum likelihood. We will find the probability of error of distinction, taking into account noise emissions at the interval of possible time delay of signals.

Suppose that there are "M" of the subscribers of the communication system, each of which uses its signal. The highest noise immunity in transmitting information in such conditions provide simplex signals. When M \u003e\u003e 1, the noise resistance of such a system of signals almost coincides with the noise immunity of the system of orthogonal signals for which

Here e kf is the signal energy F K. The condition of orthogonality, which can be called "orthogonality at the point", in practice requires a system of a single time to organize a synchronous connection. In asynchronous systems, orthogonal in the enhanced sense of signals for which with all values \u200b\u200bτ k and τ m

If R km (τ k, τ m)<0.25 – 0.3, то можно считать ансамбль сигналов практически удовлетворяющим условию ортогональности.

We will consider a system of complex signals (F k (t)), k \u003d 1 ... m orthogonal during an arbitrary shift. Among complex signals are very widely used phase-bypass (FM) signals with a complex envelope

where A i is the sequence code, U 0 (T) - the shape of the envelope of the elementary parcel, δ is its duration. In the case of a rectangular shape of the envelope of the elementary parcel, the autocorrelation function (ACF) has the form:

Here R 0 (τ) \u003d (1- | τ | / δ). In the neighborhood of the maximum ACF R (τ) \u003d R 0 (τ) \u003d (1- | τ | / δ). At the receiver input after passing a multipath channel, a useful signal can be recorded as

Δ N is the relative signal delay over the beam with N, τ - an unknown arrival time, which is inside the interval. ε n \u003d a n / a 0 is the relative amplitude "n" -th beam, the parameter ν makes sense of the number of additional rays of distribution. Relative delays δ n\u003e δ, i.e. Rays are separated when processing a complex signal. When ν \u003d 0, the signal has the form S (T) \u003d A 0 F (T-τ 0).

Consider the processing algorithm. The mixture comes to the receiver

x (t) \u003d s k (t-τ 0k) + η (t), (t),

where S k (t) is one of the possible signals, k \u003d 1 ... m, τ 0 k - the time delay of the signal, η (t) is a white Gaussian noise with a zero average value and power spectral power density N 0/2. It is necessary to make a solution, which of the possible signals is present at the receiver input. Consider the receiver without compensation for multipath. The linear part of such a receiver contains C channels in which the statistics of the form are formed

The expression for L K (τ k) can be rewritten in more comfortable for analysis

Here and in the following formulas, the index k for brevity is descended if the characteristics of the same channel, z 0 2 \u003d 2a 0 2 E F / N 0 are investigated - the power ratio of the signal / noise, S (τ-τ 0) \u003d ∫f (t-τ ) F (T-τ 0) DT / E F - normalized signaling function, n (τ) \u003d ∫n (T) F (T-τ) DT is a normalized noise function with a zero medium value, a single dispersion and a correlation function \u003d S (τ "-τ"). The envelope of the signal function S (τ-τ 0) is an ACF.

According to the maximum truth-like algorithm, the solution in favor of the signal with the number M is carried out if SUPL M (τ M) ≥SUPL K (τ k). To find the probabilities of the correct and incorrect solutions, according to this rule, it is necessary to calculate the distribution of absolute maxima of the processes L (τ) on the interval [t 1, t 2].

Consider the method of calculating the probability of error of distinguishing M signals with unknown parameters for single-rolled signals (or in the optimal arrangement scheme). Denote by H k \u003d supl k (τ k) - the value of the absolute maximum of statistics at the output of the K-th channel of the receiver. Joint distribution of random variables (H 1, H 2, .. H M) Write as W (U 1, U 2, .. U M). The condition of orthogonality for signals F K (T) in the statistical sense means the independence of random variables H k, k \u003d 1..m. Then the likelihood of the correct decision on the maximum truthfulness algorithm can be recorded

If we take into account the condition of the orthogonality of the signal system (S k (t)), then

Suppose that the signal system (S k (t)) has the same energy, that is, z 0 m \u003d z 0 k \u003d z 0. Then the formulas for H M and H K can be rewritten as

The distribution function of the absolute maximum H K implementation of the Gaussian process with the correlation function R (τ) can be approximated by the formula

ξ \u003d (T 2 -T 1) / δ is the length of the a priori interval [t 1, T 2], having the meaning of the number of permission of the FM signals at this interval. Approximation asymptotically accurate at ξ → ∞, u → ∞. At finite values \u200b\u200bξ and u can be used more accurate approximation

Integral probability. For ξ \u003e\u003e 1 and z 0 \u003e\u003e 1 The distribution function of the absolute maximum H M can be recorded as f m (u) \u003d F s (u) F n (u) ≈φ (u-z 0) f n (u). Substituting the expressions f n (u) and f m (u) to the ratio for P rights, we obtain after the corresponding transformations

The first term corresponds to the a priori probability of the correct solution for M equal events. The second term determines the change in the likelihood by making a decision. At z 0 → ∞, the integral in the expression for P rights tends to 1 and, respectively, p rights → 1.

The full probability of error of distinguishing M signals with unknown parameters is equal

It can be seen from the formula that with increasing the number of differential signals, the probability of a decision error P E (z 0) increases. With an increase in the a priori interval of time delay of signals ξ, the probability of the error of distinction P e (z 0) increases significantly.

4.4 Comparison of synchronous and asynchronous communication systems

As a rule, when considering the performance of the receiver or demodulator, it is assumed to have some level synchronization level. For example, when coherent phase demodulation (PSK scheme), it is assumed that the receiver can generate reference signals that are identical to (possibly up to constant displacement) phase of the elements of the transmitter signal alphabet. Then, in the decision-making process relative to the value of the accepted symbol (on the principle of maximum truth), the reference signals are compared with incoming.

When generating such reference signals, the receiver must be synchronized with the bearing received. This means that the phase of the incoming carrier and its copies in the receiver must be coordinated. In other words, if the incoming carrier is not encoded information that incorporates the carrier and its copy in the receiver will pass through zero at the same time. This process is called the phase auto-lifting frequency (this is a condition that should be satisfied as close as possible if we want to demonstrate coherently modulated signals in the receiver). As a result of the frequency phase, the local heterodyne receiver is synchronized in frequency and phase with the received signal. If the information carrier is modulted directly not carrying, and the subcarrier, it is required to determine both the carrier phase and the phase of the subcarrier. If the transmitter does not perform the phase synchronization of the carrier and the subcarrier (usually it happens), the receiver will require generating a copy of the subcarrier, and the phase control of the subcarrier is made separately from the phase control of the carrier phase. This allows the receiver to receive phase synchronization both by carrier and under the subcarrier.

In addition, it is assumed that the receiver knows exactly where the incoming symbol begins and where it ends. This information is necessary to know the appropriate interval of the symbol integration - the energy integration interval before deciding on the symbol value. Obviously, if the receiver integrates on the interval of inappropriate length or by an interval that addresses two characters, the ability to make an accurate solution will decrease.

It can be seen that the symbolic and phase synchronization combines that both include the creation of a copy of the portion of the dedicated signal in the receiver. For phase sync, it will be an exact copy of the carrier. For symbolic - this is a meander with a transition through zero simultaneously with the transition of the incoming signal between the characters. It is said that the receiver capable of doing this has symbolic synchronization. Since one period of transmission of the symbol usually accounts for a very large number of carrier periods, this second synchronization level is significantly rude phase synchronization and is usually performed using another schema different from the phase synchronization used.

Many communication systems require an even higher level of synchronization, which is commonly called personnel synchronization. Personnel synchronization is required when the information is supplied by blocks, or messages containing a fixed number of characters. This happens, for example, when using a block code for implementing a direct protection against errors or if the communication channel has a temporary separation and is used by several users (TDMA technology). When block coding, the decoder should know the location of the boundaries between the code words, which is necessary for the right decoding of the message. When using a temporary separation channel, you need to know the location of the boundaries between the channel users, which is necessary for the right direction of information. Like symbol synchronization, the personnel is equivalent to the possibility of generating a meander at the transfer rate with zero transitions, which coincide with the transitions from one frame to another.

Most digital communication systems that use coherent modulation require all three synchronization levels: phase, symbolic and frame. Systems with incoherent modulation typically require only symbolic and frame synchronization; Since the modulation is incoherent, accurate phase synchronization is not required. In addition, frequency synchronization is needed incoherent systems. Frequency synchronization differs from the phase that a copy of the carrier generated by the receiver may have arbitrary phase shifts from the received carrier. The structure of the receiver can be simplified, if you do not make a requirement for determining the exact value of the incoming carrier phase. Unfortunately, this simplification entails the deterioration of the dependence of the transmission from the signal-to-noise ratio.

Until now, there was a receiving part of the communication channel. However, sometimes the transmitter plays a more active role in synchronization - it changes the time report and frequency of its gears to match the expectations of the receiver. An example of that is a satellite communication network, where the set of ground terminals guide signals to a single satellite receiver. In most such cases, the transmitter to determine the synchronization accuracy uses the reverse channel of communication from the receiver. Consequently, the success of the transmitter synchronization often requires a two-way communication or network. For this reason, the transmitter synchronization is often called network.

The need to synchronize the receiver is associated with certain costs. Each additional synchronization level implies a large cost of the system. The most obvious investment of money is the need for additional software or hardware for the receiver, which ensures and maintain synchronization. In addition, it is less obvious, sometimes we pay time spent on synchronization before the bond, or the energy required for the transmission of signals that will be used in the receiver to receive and maintain synchronization. In this case, it may be possible why the communication system developer must generally consider the draft system requiring a high degree of synchronization. Answer: Improved performance and versatility.

Consider the usual commercial analog AM radio, which can be an important part of the broadcast system, which includes the central transmitter and many receivers. This communication system is not synchronized. At the same time, the receiver bandwidth must be wide enough to include not only an information signal, but also any carrier fluctuations arising due to the Doppler effect or the transmitter reference frequency drift. This requirement for the transmitter bandwidth means that the detector comes with an additional noise energy that exceeds the energy that is theoretically required to transmit information. Some more complex receivers containing the carrier frequency tracking system may include a narrow strip filter, centered on the carrier, which will significantly reduce noise energy and increase the received signal / noise ratio. Consequently, although ordinary radio receivers are fully suitable for receiving signals from large transmitters at a distance of several tens of kilometers, they may be incapacitated with less qualitative conditions.

For digital communication, the compromises between the performance and complexity of the receiver are often discussed when modulating is selected. The simplest digital receivers includes receivers designed for use with the FSK binary scheme with incoherent detection. The only requirement is bit synchronization and frequency support. However, if you select the BPSK coherent scheme as a modulation, then you can get the same probability of the bit error, but with a smaller signal / noise (approximately 4 dB). The disadvantage of the BPSK modulation is that the receiver requires accurate phase tracking, which can represent a complex constructive problem if the signals have high Doppler speeds or for them is characterized by fading.

Another compromise between price and performance affects encoding with error correction. When using suitable error protection methods, a significant improvement in performance is possible. At the same time, the price expressed in the complexity of the receiver can be high. For proper operation of the block decoder, the receiver reached block synchronization, personnel or message synchronization. This procedure is an addition to the usual decoding procedure, although there are certain error correction codes that have built-in block synchronization. Cutting codes also require some additional synchronization to obtain optimal performance. Although when analyzing the performance of convolutional codes, it is often assumed about the infinite length of the input sequence, in practice it is not so. Therefore, to ensure the minimum probability of the error, the decoder should know the initial state (usually all zero) from which the information sequence, the final state and the time to achieve the end state begins. Knowing the end of the end of the initial state and the achievement of the final state is equivalent to the presence of personnel synchronization. In addition, the decoder should know how to group the channel symbols for making a solution when branching. This requirement also relates to synchronization.

The above-mentioned discussion of compromises was carried out in terms of the ratio between the performance and complexity of individual channels and receivers. It is worth noting that the ability to synchronize also has significant potential consequences associated with the efficiency and universality of the system. Personnel synchronization allows you to use advanced, universal multiple access methods similar to multiple access circuits on request on request (DAMA). In addition, the use of spectrum expansion methods - both multiple access schemes and interference transmission schemes - requires a high level of system synchronization. These technologies offer the possibility of creating highly versatile systems, which is a very important property when changing the system or when exposed to intentional or unintentional interference from various external sources.

Conclusion

The first section of my work describes the principles for constructing wireless telecommunication communication systems: a diagram of building a cellular communication system is shown, the methods of separating subscribers in cellular communications and the advantages (confidentiality and noise immunity) of the code separation are noted compared to temporary and frequency, and the common wireless standards are also considered Communication DECT, Bluetooth and Wi-Fi (802.11, 802.16).

The correlation and spectral properties of signals and, for example, the calculations of the spectra of some signals (rectangular pulse, Gaussian bell, smoothed pulse) and autocorrelation functions common in digital border signals and Walsh functions, as well as the types of complex signals for telecommunication systems are also indicated.

The third chapter provides modulation methods for complex signals: phase manipulation methods, modulation with a minimum frequency shift (one of the modulation methods with a continuous phase), quadrature amplitude modulation; And their advantages and disadvantages are indicated.

The last part of the work contains consideration of the probabilities of errors of distinguishing M-known and m fluctuating signals on the background of interference, as well as an algorithm for calculating errors of dispensing M orthogonal signals with an unknown time position in asynchronous communication systems with code division.

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