Communication System(通信系统).doc

09通信本科班 刘京燕 学号：0612090105Communication System【Abstract】 Communication industry is one of the oldest industry. In the new technology revolution, driven by information highway construction wave, communication technology has been developing rapidly.【Key Words】 communication system; composition; model; classification; development trend1.1 Composition of Communication SystemFrom ancient to modern times, human social activities always can not leave news transmission and exchange.The ancient of the news tree, beacon-fire tower and modern social text, letters, telegraph, telephone, radio, television, remote control, remote sensing etc, these are the means of information communication.People can use language, words, data, or images of different forms to express information.But these languages, text, data, or image themselves are not information but news.Therefore, the basic purpose of communication is to transmit the useful information. Based on this understanding, communication is information transmission or news transmission.1.2 General Model of Communication System1.Source of InformationThe source of information is the generation of news.Its effect is to convert all the news to the original electrical signals, called news signal or baseband signal.All kinds of digital terminal equipment for example, telephone, television camera, the telex machine, computer is the source.The former are analog source and the output is analog signal.The latter is digital source and the output is the discrete digital signal.2.Send EquipmentThe basic function of send equipment is matching up the source and channel. Communication means is varied.Modulation is the most common way of transformation.For digital communication system,send equipment can be divided into the source coding and channel coding.3.Information ChannelChannel refers to the physical media of the transmission signal.In a wireless channel,it can be atmosphere (free space). In the cable channel, it can be cable or optical fiber.Wire and wireless channel all have a variety of physical media.The inherent characteristics of the medium and the interference directly related to the quality of the communication.According to the difference of the study, it need to set up different mathematical model of the actual physical media.So as to reflect on the influence of the signal transmission medium.4.Source of NoiseNoise is not the thing that human want to add,but channel of communication system inherent in.The source of the noise is various.It can be divided into the interior noise and external noise. The external noise often introduces from the channel.5.Receive DeviceThe basic function of the receiving device is complete inverse transform of sending equipment.That is demodulating, decoding, decoding, etc.6.InformationsinkInformationsink is home to the information that will be transformed. It can convert the original signal into the news.1.3 Classification of Communication System1.According to communication business classificationAccording to communication business, communication system classify telephone communication and other communication.Telephone service is indispensable in the telecommunication field. It belongs to the communication between people.In recent years, with the rapid development of telephone communication, the telephone communication include data service, computer communication, database retrieval, E-mail, electronic data exchange,visual graphic and meeting TV, image communication, etc.The phone communication is the most developed. So other communication ofen draw support from public phone communication system.In addition, there are control communications business for example remote measure, remote control.2.According to Modulation Method ClassificationAccording to modulation method ,communication system can be divided into baseband transmission and frequency transmission.Baseband transmission will be transmitted directly without modulation signal, such as audio local call.Frequency transmission is the floorboard of the transmission of signal that had modulated.3. According to Signal Features ClassificationAccording to the transmission of channel is analog signal or digital signal, the communication system can classify analog communication system and digital communication system.4. According to The Transmission Medium ClassificationAccording to the transmission medium,communication system can be divided into wire communication system and wireless communication system.Wire communication uses lead as transmission medium to finish communication, such as the telephone, cable TV, submarine cable communication.Wireless communication depends on electromagnetic wave propagation in space to achieve the purpose of sending messages, such as short wave ionosphere spread, microwave spread.5. According to Workband ClassificationAccording to the working frequency of communication equipment ,communication can be divided into long wave communication, medium wave communication, short wave communication, far infrared communication, etc.6. According to Signal Multiplex Method ClassificationTransmission multiple signal have three multiplex method, namely frequency division multiplex, time division multiplex and code division multiplex.Frequency division multiplex uses the method of spectrum moved to occupy different frequency of different signal.Time division multiplex uses the method of modulation by using a pulse signal to occupy different time interval.Code division multiplex uses orthogonal pulse sequences carried different signals.Traditional analog communication uses frequency division multiplex.With the development of digital communication, time division multiplex communication system applys more and more extensively. Code division multiplex mainly used in the spread spectrum communication of the space communication.1.4 Development Trend of Communication System1.Satellite Communication SystemSatellite communication system uses communication satellites as air relay station. It can transmit wireless signal from earth station to another station.So as to realize the communication of different regions.According to the relation between communication satellite and the ground,it can be divided into stationary communication satellite and mobile communications satellite.Stationary communication satellite is in the orbit of the equator surface.It leaves the ground in height of 35 780 km.Satellite communication system is compose of communication satellite, earth station, uplink line and downlink line.Uplink and downlink line is the transmission path of radio that earth station to communication satellite and communication satellite to earth station. Communication equipment focused on earth station and communications satellite.Satellite communication has the features of long distance communication, wide geographical coverage, large communication capacity, high reliability.The first satellite sended up successfully in October 1957. Satellite communication, as a kind of important means of communication, used widely in international,domestic and regional communication.Satellite communication will develop higher frequency range and larger capacity in 21 century.Satellite communications will use laser communication with high speed, wide frequency band, the good secrecy. 2.Optical Fiber Communication SystemOptical fiber communication uses optical fiber as transmission mediumand light waves as means of delivery.Optical fiber communication has lots of advantages such as great capacity, wide frequency band, small transmission loss, electromagnetic interference ability, high quality communication, low cost.It can save metal and energy compared with coaxial cable.Since the worlds first optical fiber communication system has put into operation in 1977.Optical fiber communication develop rapidly.It has become the main transmission method of communication.At present, the single length of wave optical communication system velocity rate has reached 10 Gb/s. In recent years, DWDM technology has made great progress.American optical transmission network is the future direction.It can handle light signal of the high rate.It can also get rid of electronic bottleneck to realize flexible and dynamic network.So as to support signal and realize fast network recovery.Therefore, many of the worlds countries to study, experiment ,demonstrate the feasibility of communication network. 3.Digital Cellular Mobile Communication SystemDigital cellular mobile communication system divide communication distance into several range.Mobile users can move from one district to another district.It rely on tracking the base station to make the communication not interrupt.Mobile users can also roam from one city to another and even to another country.Digital cellular mobile communication system mainly consists of three parts: the control exchange center, several base stations, mobile terminal.Getting into public cable network through the control exchange center to achieve communication between mobile phone and fixed phone, mobile phone and mobile phone.At present, the second generation of mobile communication system used widely.It adopt TDMA and narrowband code division digital access technology.It has developed many countries and regions standard such as European GSM system, American IS-95 system, Japans PDC system.Our country mainly uses the European GSM system.The second generation of mobile communication system realize the unity of area, covering the major,medium and small city.Therefore it provides great convenience for people exchanging information.With the popularization of mobile communications terminal, the number of mobile us