Definition: A system that transmits information with electrical signals (or optical signals), also known as a telecommunications system.A system is usually an organic whole composed of several units with specific functions, interactions and interdependencies to achieve a unified goal.The simplest communication system allows users at two points to send and receive information to each other.In the general communication system, users can communicate with other users in the system through switching equipment.
communication system
communication systems
The general term of technical systems used to complete the information transmission process.Modern communication systems are mainly realized by the transmission mechanism of electromagnetic waves in free space or in guidance media. The former is called wireless communication system, and the latter is called wired communication system.When the wavelength of the electromagnetic wave reaches the optical wave range, such a telecommunication system is specifically called an optical communication system, while the communication system in other electromagnetic wave ranges is called an electromagnetic communication system, referred to as a telecommunication system for short.Because the guiding medium of light uses special glass fiber, the wired optical communication system is also called optical fiber communication system.Generally, the guiding medium of electromagnetic wave is conductor, which can be divided into cable communication system and open line communication system according to its specific structure;According to the wavelength of its electromagnetic wave, wireless telecommunication system can be divided into microwave communication system and short wave communication system.On the other hand, according to different communication services, communication systems can be divided into telephone communication systems, data communication systems, fax communication systems and image communication systems.Because people have higher and higher requirements for communication capacity and more diversified requirements for communication services, communication systems are rapidly developing towards broadband, and optical fiber communication systems will play an increasingly important role in communication networks.
Basic system
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It is generally composed of a source (sending equipment), a sink (receiving equipment) and a channel (transmission medium), and is called the three elements of communication.
mode
Basic communication system
The message (language, text, image or data) from the source is first sent by the terminal equipment (such as telephoneTeletypewriter, fax machine or data terminal equipment, etc.)signal, and then via the originating devicecodeAfter modulation, amplification or transmission, transform the baseband signal into a form suitable for transmission in the transmission medium;After transmission by transmission media, the message is restored to the receiver by reverse transformation at the receiver through the receiver equipment.This kind of point-to-point communication is mostly two-way transmission.Therefore, both ends of the communication object are equipped with sending and receiving devices.
classification
Communication systems can be divided into two categories according to the different transmission media used: ① using metal conductors as transmission media, such as commonly used communication cables, etc. This communication system using cables as transmission media is called wired communication system; ②utilizeradio wavesThis communication system is called radio communication system.Optical communication systems can also be divided into "wired" and "wireless". The transmission media they use are optical fiber and atmosphere, space or water, respectively.
The communication system can be divided into telephone, telegraphFax, data communication system, etc.The signal changes continuously in time, which is called analog signal (such as telephone);The signals that are discrete in time and whose amplitude values are also discrete are called digital signals (such astelegraph）。Analog signals can also be converted into digital signals through analog digital conversion (including sampling, quantization and coding processes).When the baseband signal transmitted in the communication system is analog signal, such system is calledAnalog communication system;The communication system that transmits baseband signals as digital signals is calledDigital communication system。
All communication systems work in a noisy environment (represented by noise sources in the figure).Design simulationcommunication system The minimum mean square error criterion is adopted, that is, the signal noise ratio output by the receiver is the largest.DesignDigital communication systemThe minimum error probability criterion is adopted, that is, according to the statistical characteristics of the selected transmission medium and noise, the best modulation system is selected, and the best signal and receiver are designed.
Analog digital
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Analog communication
Analog communicationIt refers to a communication mode that transmits analog signals from the source to the destination on the channel.Due to the resistance in the conductor, the distance of signal direct transmission cannot be too far. The solution is to transmit analog signal through carrier wave.Carrier wave refers to the waveform modulated to transmit signal, usually a sine wave with high-frequency oscillation.In this way, modulation of analog signals on the carrier wave can be far more than direct transmission.It is generally required that the frequency of the sine wave is much higher than the bandwidth of the modulation signal, otherwise, aliasing will occur and the transmission signal will be distorted.
Analog communication system is usually composed of source, modulator, channel, demodulator, sink and noise source.
The advantages of analog communication are intuitive and easy to implement, but poor confidentiality and weak anti-interference ability.Because the signal spectrum of analog communication transmitted in the channel is relatively narrow, the channel utilization can be improved through multiplexing.
Digital communication
Digital communication refers to a communication mode that transmits digital signals from source to destination on the channel.Compared with analog communication, it has the following advantages: strong anti-interference ability, no noise accumulation;Long distance transmission and quality assurance;It can adapt to various communication business requirements and facilitate comprehensive processing;The transmitted binary digital signal can be directly received and processed by the computer;It is easy to use large-scale integrated circuit to realize, and communication equipment is easy to integrate;It is easy to encrypt and ensure security.
Multiplex system
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communication system
In order to make full use ofcommunication channel To expand communication capacity and reduce communication costs, many communication systems use multiplexing, that is, to transmit multiple messages simultaneously on the same transmission path.Multiplexing is divided into frequency divisionTime divisionAnd code division multiplexing.In the analog communication system, the divided available frequency bands are allocated to each information and share a common transmission medium, which is calledFrequency division multiplexing。stayDigital communication systemOne time slot (short time period) is allocated to each message, and each channel occupies the time slot in turn, which is calledTime division multiplexing。Code division multiplexingAt the sending end, each input signal is multiplied by a code sequence waveform generated by the orthogonal code waveform generator, and then added to obtain multiple signals.The equipment that completes the multiplexing function is called multiplexing terminal equipment, or terminal equipment for short.The multiplex communication system is composed of terminal equipment, terminal equipment, transmitting equipment, receiving equipment and transmission media.
Wired system
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communication system
For long distancesTelephone communicationThe carrier communication system based on frequency division multiplexing is a communication system.It consists of carrier telephone terminal equipment, repeater, transmission line and auxiliary equipment.Carrier telephone terminal equipmentVoice frequency signalOr other group signals are moved to the line spectrum or the line spectrum transmitted by the other party is inversely transformed and can meet the line transmission requirements;The repeater can compensate the transmission attenuation and its change of the line, and one repeater shall be installed every certain distance along the line.
Microwave system
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Long distance and large capacity radio communication system occupies frequency due to transmission signalbandwidth, generally working in microwave or ultrashort wave band.In these bands, they generally have stable transmission characteristics only within the sight range, so relay (also known as relay) communication mode must be used for long-distance communication, that is, several adjacent microwave relay stations (also known as relay stations) are set up on the route through which signals are transmitted from one terminal station to another,The space distance between stations is about 20-50 km.Relay stations can also be divided into intermediate stations and sub transfer stations.Microwave relay communication systemThe signal transmitted by the terminal station ofbasebandIt can be connected with analog frequency division multiplex terminal equipment or digital time division multiplex terminal equipment.The former is called analog relay communication system;The latter is called digital relay communication system.With the advantages of easy encryption and good transmission quality,Digital microwave relay communication systemPeople pay more and more attention to it.In addition to the above line of sight relay communication system, the over the horizon scattering communication system using tropospheric scattering propagation can also be used as a long-distance medium capacity communication system through relay mode.
Satellite system
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In the microwave communication system, if the communication satellite located in the geostationary orbit is used as the repeater to transmit the signals of each earth station, it will constitute asatellite communication system。The characteristic of satellite communication system is that it has a large coverage area. It can flexibly organize communication and liaison according to needs within a large coverage area of satellite antenna beam. Some of them also have a certain transformation function, so it has become the main means of international communication, as well as an important means of domestic communication in many countries.Satellite communication system is mainly composed of communication satellite, earth station, measurement and control system and corresponding terminal equipment.The satellite communication system can be used as an independent means of communication (especially applicable to themobile communicationBusiness andPrivate communication network）It can also be combined with the land communication system and complement each other to form a more complete transmission system.
The above carrier, microwave relay, satellite and other communication systems are used as transmission subsystems, and can be combined with the switching subsystem to form a communication system for transmitting various communication services.
telephone system
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The characteristic of telephone communication is that both sides of the conversation require real-time dialogue, so a channel should be temporarily connected between the two sides in a relatively short time. Therefore, the telephone communication system should have two functions: transmission and exchange.This system is usually composed of subscriber lines, switching centers, inter office trunk lines and trunk lines.Telephone communication networkThe switching equipment ofCircuit switching mode, which is composed of connection network (also called switching network) and control part.The session connection network can temporarily connect the communication channel to the user as required, and the control part is used to complete the signal processing in the whole process of user communication establishment and control the connection network.When designing the telephone communication system, the call quality is mainly evaluated by the loudness of the received voice, and transmission attenuation can be allocated after the transmission, reception and full range reference equivalent are specified.On the other hand, according to the traffic and the specifiedService level(i.e. the probability that the user is not connected - call loss rate) to determine the capacity of the required machine and line equipment.
Due to the growing demand for mobile communication services, mobile communication has been developed rapidly.Mobile communication systemIt is composed of on-board radio station, radio center (also called base station) and wireless switching center.Car radioEnter the radio exchange center through the fixed configuration radio center to complete the communication between mobile users;The radio switching center can also be connected with the switching center (usually the local telephone office) in the fixed telephone communication system to realize the communication between mobile users and fixed users.
Telegraph system
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A communication system established to enable telegraph users to exchange telegrams.It mainly uses telephone channels to transmit telegraph signals.Public telegraphTelegram switching equipment in communication system adoptsStore Forward ExchangeMode (also called message exchange), the message to be received is stored firstBuffer memoryAnd then forward to the destination route, which can improve the utilization of circuits and switching equipment.When designing the telegraph communication system,Service qualityIt is measured by the average delay of transmitting a message through the system.aboutTelexThe communication business still adopts the circuit switching mode, that is, the circuit between the two parties is connected, and then both users directly report.
data system
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data communicationIt is developed with the rapid development of information processing technology.The data communication system consists ofData terminalAnd data transmission equipment, data exchange equipment andCommunication lineThey are connected with each other.Multiple independentcomputer systemConnected networks calledcomputer network, so that users can share resources.Packet switching (or packet switching) is often used in data communication system, which is a special message switching modeData segmentationIt is formed into several shorter packets (or packets) and then transmitted, and then combined at the receiver.Its main advantages are that it can reduce the delay and make full use of the transmission channel
System indicators
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How to measure a communication system?We measure it mainly through effectiveness and reliability.In other words, the more efficient and reliable a communication system is, the better it is.However, in fact, effectiveness and reliability are a pair of contradictory indicators, and they need a certain compromise.Validity refers to the rate of information transmission. The faster the rate of information transmission, the better the effectiveness.However, the faster the information transmission is, the higher the probability of error is, and the quality of information transmission cannot be guaranteed, that is, the reliability is reduced.It's like a car speeding on the highway. It's fast, but there's a great potential safety hazard.Therefore, we cannot simply pursue high speed without considering reliability, otherwise, if we really want speed, we will not achieve it.
What indicators are used to describe the effectiveness and reliability of the system?
For analog communication systems, effectiveness is measured by system bandwidth, and reliability is measured by signal to noise ratio.If the bandwidth occupied by one phone is certain, the larger the total bandwidth of the system, the more phones can be accommodated.When the bandwidth of the system is fixed, the capacity of the system can be increased by reducing the bandwidth occupied by a single channel telephone. Therefore, the narrower the bandwidth required by a single channel signal, the better the effectiveness.However, after reducing the occupied bandwidth of a single channel signal, due to the narrowing of the band isolation between two channels of signals, mutual interference will inevitably increase, that is, noise will increase, reducing the ratio of signal power to noise power, thus reducing the reliability of the system.
For digital communication systems, validity is represented by information transmission rate, and reliability is represented by bit error rate or message error rate.The bit error rate refers to the ratio of the number of error symbols received by the receiver to the total number of transmission symbols, that is, the probability of error symbols in transmission.The error rate refers to the ratio of the number of error bits received to the total number of transmission bits, that is, the probability of the amount of error information in transmission.
When digital signals are transmitted in the channel, error correction codes are often added to ensure the reliability of transmission. Error correction codes occupy the transmission rate.When the total number of code elements or bits that a channel can transmit per second is fixed, if error correction coding is not required, obviously more information bits will be transmitted per second, and the efficiency will be improved. But without error correction codes, reliability cannot be guaranteed.These increased codes for improving reliability are also called transmission overhead, because the purpose of transmitting these symbols or bits is to pick up and correct errors, and they do not carry information.
In the communication system, frequency is a characteristic of any signal.Even digital signals, without exception, occupy certain frequency resources to transmit them.The bandwidth is proportional to the transmission rate of digital signals.Ideally, the transmission rate divided by 2 is the bandwidth occupied by digital signals transmitted at this rate.Therefore, the higher the rate, the wider the frequency band will be. Therefore, high-speed communication is often called "broadband communication".
This book deeply analyzes the basic theory and key links of communication system, trying to make students understand the essence of communication in the discussion.The book first gives an overview of the communication system and the key technologies to be studied, and then discusses in detail in chaptersrandom process, CW modulation, pulse modulation, baseband pulse transmission, signal space analysis, band-pass digital transmission, spread spectrum modulation, multi-user wireless communicationFundamentals of Information TheoryandError control codingEach chapter is attached with a large number of exercises for students to practice.This book emphasizesCommunication TheoryOfStatistical BasisAnd gives 8 simulation results with MATLABcomputerExperiments, these experiments almost cover the main contents of each chapter, forming a unique communication theory "software laboratory".
This book can be used as a textbook for senior undergraduate students of communication, information, electronics, computer and other majors in colleges and universities, as well as a reference for relevant technology, research and management personnel.
About the author
Simon Haykin, IEEE Fellow, graduated fromUniversity of Birmingham Department of Electronic Engineering.CanadaProfessor, Department of Electronic and Computer Engineering, McMaster University, Director of Communication Research Laboratory.Research directions includenonlinear dynamics, neural networks andAdaptive filterAnd its application.
Bibliography
BACKGROUND AND PREVIEW
CHAPTER 1 Random Processes
CHAPTER 2 Continuous-Wave Modulation
CHAPTER 3 Pulse Modulation
CHAPTER 4 Baseband Pulse Transmission
CHAPTER 5 Signal-Space Analysis
CHAPTER 6 Passband Digital Transmission
CHAPTER 7 Spread-Spectrum Modulation
CHAPTER 8 Multiuser Radio Communications
CHAPTER 9 Fundamental Limits in Information Theory
CHAPTER 10 Error-Control Coding
APPENDIX 1 Probability Theory
APPENDIX 2 Representation of Signals and Systems
APPENDIX 3 Bessel Functions
APPENDIX 4 Confluent Hypergeometric Functions
APPENDIX 5 Cryptography
APPENDIX 6 Tables
GLOSSARY
BIBLIOGRAPHY
INDEX
Terms related to communication system:[1]
one
extended total access communications systems (E-TACS)
Extensible full accesscommunication system
two
local multipoint communications system (LMCS)
Local multipointcommunication system
three
personal access communications system (PACS)
Personal accesscommunication system
four
wireless access communications system (WACS)
wireless access communication system (Developed by Bellcore)
five
Wireless Access Communications System
wireless access communication system (Developed by Bellcore)
