通信系统绪论Background and Preview

1、1,Communication Systems （Fourth Edition） Simon Haykin,2,Contents,CH0: Background and Preview CH1: Random Processes CH2: Continuous-Wave Modulation CH3: Pulse Modulation CH4: Baseband Pulse Transmission CH5: Signal-Space Analysis CH6: Passband Data Transmission CH7: Spread-Spectrum Modulation CH8: Mu

2、ltiuser Radio Communications CH9: Fundamental Limits in Information Theory CH10: Error-Control Coding,3,Ch.0 Background and Preview,The Communication Process Primary Communication Resources Sources of Information Communication Networks Communication Channels Modulation Process Analog and Digital Typ

3、es of Communication Shannons Information Capacity Theorem A Digital Communications Problem Historical Notes,4,0.1 The Communication Process,Fig. 1. Elements of a communication system.,Converting the message signal produced by the source of information into a form suitable for transmission over the c

4、hannel,Introducing distortion and noise,Reconstructing the original message signal from the received signal,5,The Communication Process (contd),Three basic elements to every communication system Transmitter: to convert the message signal produced by the source of information into a form suitable for

5、 transmission over the channel Channel: introduces distortion and noise Receiver: to reconstruct the original message signal from the received signal,6,The Communication Process (contd),Two basic modes of communication Broadcasting One-to-many communication Usually one directional flow of signals E.

6、g. radio and television Point-to-point communication One-to-one communication Usually bidirectional flow of signals E.g. telephone and satellite communication One common feature of communication process: Statistical in nature,7,0.2 Primary Communication Resources,Transmitted power: the average power

7、 of the transmitted signal Channel bandwidth: the band of frequencies allocated for the transmission of the message signal Two types of communication channels: Power limited: e.g. satellite channel Band limited: e.g. telephone circuit Noise Unwanted and unavoidable The signal-to-noise ratio (SNR) Us

8、ually expressed in decibels (dBs), SNRdB = 10log10(SNR),8,0.3 Sources of Information,Four important sources of information: Speech: Using the 300 3100 Hz band Music: up to 15 kHz Pictures Can be dynamic (e.g. television) or static (e.g. facsimile) The scanning process Wideband signals Computer data

9、The American Standard Code for Information Interchange (ASCII) Wideband signals Burstiness Compression (lossless and lossy),9,0.4 Communication Networks,Fig. 4. Communication network.,10,Communication Networks Switching Mode,Circuit switching E.g. the telephone network Suitable to relatively stable

10、and long duration signals, such as voice Packet switching E.g. the Internet “Store and forward” Suitable to computer communication where data is bursty in nature Requires careful control from end users,11,Communication Networks Layered Architecture,Fig. 5. OSI model.,12,Communication Networks Intern

11、et,Fig. 6. An interconnected network of subnets.,Network of networks Underlying network technology is decoupled from the applications Best effort service,13,Communication Networks Internet (Contd),Fig. 7. Illustrating the network architecture of the Internet.,14,Communication Networks Broadband Netw

12、orks,B-ISDN Stands for “broadband integrated services digital networks” Evolves from telephone network whose major service is voice Supports multimedia communications The underlying technology is asynchronous transfer mode (ATM),15,ATM A cell-switching technology Basic transmission unit is cells (sm

13、all, fixed-size packets) Preserves cell sequencing in transmission Is not synchronous High bandwidth and low delay Provide quality of service (QoS) guarantees QoS is measured in terms of loss ratio, delay, and delay variation,Communication Networks Broadband Networks (Contd),16,SONET Stands for “syn

14、chronous optical network” Using time-division multiplexing (TDM) Need synchronization A hierarchy of data rates The basic rate is 51.84 Mb/s Byte-interleaved to generate higher rates,Communication Networks Broadband Networks (Contd),17,0.5 Communication Channels,Guided propagation Telephone channels

15、 Coaxial cables Optical fibers Free propagation Broadcast channels Mobile radio channels Satellite channels Different ways of classification Linear or nonlinear Time invariant or time varying Band limited or power limited,18,Communication Channels Telephone Channels,Bandwidth limited (300 3100 Hz) U

16、sing twisted pairs Susceptible to electromagnetic interference (EMI),Fig. 8. Characteristics of typical telephone connection: (a) Insertion loss. (b) Envelope delay. (Adapted from Bellamy, 1991.),19,Communication Channels Coaxial Cables,Better immunity to EMI than twisted pairs Has a characteristic

17、impedance of 50 or 75 ohms Higher bandwidth than twisted pairs (up to 20 Mb/s) Can be used as a multiple-access medium Applications Local area networks (LAN) Cable-television systems,20,Communication Channels Optical Fibers,Dielectric wave guide transporting light signals Enormous potential bandwidt

18、h (21013 Hz) Low transmission losses (0.1 dB/km) Immunity to EMI (dielectric) Small size and weight (thinner than a hair) Ruggedness and flexibility Low cost of the materials,21,Communication Channels Wireless Broadcast Channels,Information-bearing signal is usually modulated for transmission Antenn

19、a is used to efficiently transmit and receive electromagnetic waves Typically line-of-sight (LOS) propagation The super-heterodyne type receiver,22,Communication Channels Mobile Radio Channels,Typically non-line-of-sight (NLOS) propagation The multipath phenomenon The energy reaches the receiving an

20、tenna via more than one path Resulting from scattering of signals during propagation Linear and time-varying,23,Communication Channels Satellite Channels,Communicating through geostationary satellites Broad-area coverage Reliable transmission links Wide transmission bandwidth (6 GHz for the uplink a

21、nd 4 GHz for the downlink),24,0.6 Modulation Process,The modulation process varies some parameter of a carrier wave in accordance with the message signal, so that the resulting signal is suitable for transmission over the channel. Demodulation is the reverse of modulation.,25,Modulation Process Clas

22、sification,Continuous-wave modulation The carrier is a sinusoidal wave Amplitude modulation (AM) Angle modulation Frequency modulation (FM) Phase modulation (PM) Pulse modulation The carries is a periodic sequence of pulses Analog pulse modulation (PAM, PDM, and PPM) Digital pulse modulation (PCM),2

23、6,Modulation Process Multiplexing,Multiplexing is the process of combining several message signals for their simultaneous transmission over the same channel. Frequency-division multiplexing (FDM) Time-division multiplexing (TDM) Code-division multiplexing (CDM) Wavelength-division multiplexing (WDM)

24、 (a form of FDM),27,0.7 Analog and Digital Types of Communication,Fig. 9. Block diagram of digital communication system.,28,Analog Digital Types of Communication Digital Communication,System is complex, including Source encoder-decoder Channel encoder-decoder Modulator-demodulator Easy to build Effi

25、cient and reliable With the ever-increasing availability of very-large-scale integrated (VLSI) circuits, digital communications become popular today.,29,Analog Digital Types of Communication Analog Communication,System is simple Difficult to build Stringent requirements on linearity Difficult on sys

26、tem adjustment Very high-speed and very low-power communication systems may dictate the use of an analog approach.,30,0.8 Shannons Information Capacity Theory,Bit error rate (BER) reliability metric The information capacity theory A message signal can be transmitted through the system without error provided that the actual signaling rate is less than the information capacity C, C = B log2 (1+