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1、电力系统通信基础,2019/1/30,HUI REN NCEPU,2,Digital and Analog Communication System,Communication system m(t) : electrical signal representing messages m(t) : electrical signal sent to the user. (m prime t), may be not the same as m(t) due to noise or system unlinearity.,2019/1/30,HUI REN NCEPU,3,message: wo
2、rds, pictures, voices, value of voltage, current Electric signal: representing message. Signals parameters change according to message. Sine Wave(正弦波): amplitude(幅值), frequency, phase offset(相移). Pulse wave(脉冲波): amplitude, period, mark space ratio(占空比) = mark time/space time duty cycle = (high time
3、/period)*100%. continuous message: voices discrete message: words, status of circuit breakers Analog signal digital signal.,amplitude,2019/1/30,HUI REN NCEPU,4,Transmitter: dealing with the source m(t) for more efficient transmission. For example: signal processing: analog low pass filter(低通滤波器) in
4、analog communication system to restrict the bandwidth(带宽) of m(t). analog-to-digital converter(模数转换器), used to transmit an analog information through a digital communication system- source coding(信源编码) error correct coding(差错控制编码) to the digital word to achieve error detection and correction caused
5、by noise-channel coding(信道编码) carrier(载波) circuits: converts the baseband signal(基带信号) into a frequency band signal (频带信号) that is appropriate for the transmission medium(传输媒质). -modulation(调制) and demodulation(解调).,2019/1/30,HUI REN NCEPU,5,Transmission medium (channel): twisted-pair telephone line
6、s: 双绞线 coaxial cables: 同轴电缆 fiber optic cables: 光纤 wireless 无线 Noise: natural electrical disturbances (e.g. lightning) man-made noise (e.g. electromagnetic interference),2019/1/30,HUI REN NCEPU,6,Definition: a Digital Communication System transfers information from a digital source to the sink(user)
7、. an Analog Communication System transfers information from an analog source to the sink. Notes: an electronic digital communication system can also transmit analog source to the sink.,2019/1/30,HUI REN NCEPU,7,General digital communication system,Encoder: including source coding and channel coding,
8、2019/1/30,HUI REN NCEPU,8,Comparison between Digital and Analog Communication system,Advantages of digital Communication: In long-distance systems, noise does not accumulate from repeater(中继器) to repeater.,2019/1/30,HUI REN NCEPU,9,Channel + noise,repeater,2019/1/30,HUI REN NCEPU,10,Errors can be de
9、tected and corrected by the use of coding (编码). Data from voice, video, and data sources may be merged and transmitted over a common digital transmission system. Privacy is preserved by using data encryption(加密). Relatively inexpensive digital circuits may be used. Disadvantages: Generally, more ban
10、dwidth(带宽) is required than that for analog systems. Synchronization (同步) is required.,2019/1/30,HUI REN NCEPU,11,Communication Modes,Simplex (单工) data can be transmitted in one direction only Half duplex (半双工) two interconnected devices can exchange data in both direction alternately, e.g. a walkie
11、-talkie(步话机) Duplex (全双工) full duplex, data are exchanged between the two connected devices in both directions simultaneously,2019/1/30,HUI REN NCEPU,12,2019/1/30,HUI REN NCEPU,13,Information Measure,What is information exactly? message? information? Information is the valuable part a message contai
12、ns Read the following sentences: Tomorrow, the sun will rise from the East. The phone will ring in the next one hour. It will snow in Hainan this winter.,2019/1/30,HUI REN NCEPU,14,How many information does a message have? Definition: The Self-Information of the message j is given by where, Pj is th
13、e probability of transmitting the jth message. Ij is called Self-Information of event j,2019/1/30,HUI REN NCEPU,15,The unit of “I” : if the base of the logarithm is 2, the units are “bits”; 比特 if the base is e, the units are “nats”; if the base is 10, the units are “hartley”.,2019/1/30,HUI REN NCEPU
14、,16,Communication systems performance,Realiability Equipment reliability Data transmission reliability Accuracy Real-time,2019/1/30,HUI REN NCEPU,17,Equipment reliability: the probability that a system will perform its intended function under specified conditions during a specified period of time. A
15、vailability(可用率): the reference period of time includes operation time and corrective and preventive maintenance(维修) time. Ap = MTBF / ( MTBF + MTTR ) where, Ap is predicted availability MTBF: mean-time-between-failure(平均故障间隔时间) MTTR: mean-time-to-repair(修复的平均时间) (including administrative, transport
16、, and repair time ),2019/1/30,HUI REN NCEPU,18,Data transmission reliability: Bit Error Rate(比特差错率): to value the quality of communication. It is given by: Accuracy overall accuracy(总准确度): is the offset between the data source and data sink through transform and processing ( transformer, analog-to-d
17、igital converter, digital-to-analog converter, ), is given by: offset: 偏差; full scale: 满量程,2019/1/30,HUI REN NCEPU,19,Real-time: to be valued by overall transmission time(总传输时间). For example: for tele-metering or tele-indication, it refers to the period of time from a event happened or a value chang
18、ed at substation or power plant to this information displayed on the screen of operators computer. for important analog inputs, 3s for less important analog inputs, 10s or longer for status, less than 3s. for control or adjusting command, less than 3s. for others, such as dispatching plans, logs, mi
19、nutes or longer,2019/1/30,HUI REN NCEPU,20,Signal and spectra - Line Codes,Bipolar code(双极性码): using positive pulse and negative pulse to represent logical 1 and 0. Non-Return-to-Zero (NRZ, 不归零码): signal doesnt drop to zero between each pulse. Return-to-Zero (RZ, 归零码): signal drops to zero between e
20、ach pulse.,NRZ Bipolar code,RZ Bipolar code,2019/1/30,HUI REN NCEPU,21,Unipolar code(单极性码): using positive pulse and grounded pulse to represent digit 1 and 0. Advantage: easy to create. Disadvantage: Unipolar code will create a significant DC-value, which is not possible to transport a DC-component
21、 through long wires. Solution: RZ unipolar code could be used to reduce the DC-component. Or using modulation and demodulation techniques. This code is used in telecontrol system.,2019/1/30,HUI REN NCEPU,22,Alternate Mark Inversion(交替极性码)：using positive, negative and grounded pulses to represent log
22、ical values. Logical 0 is represented with a grounded pulse, and a logical 1 by pulses of alternating polarity.,2019/1/30,HUI REN NCEPU,23,Manchester code(裂相码):,2019/1/30,HUI REN NCEPU,24,Signal and Spectrum frequency analysis,white light can be break up by a prism into the colors of the rainbow. Sp
23、ectrum is first used to describe the continuous bands of colors produced by a prism. The analysis of light into colors is actually a form of frequency analysis.,Fourier analysis tools Fourier series / Fourier transform,2019/1/30,HUI REN NCEPU,25,“spectrum” is used when referring to the frequency con
24、tent of a signal. Frequency or spectrum analysis: the process of obtaining the spectrum of a given signal using the basic mathematical tools.,2019/1/30,HUI REN NCEPU,26,2019/1/30,HUI REN NCEPU,27,Features: Envelope has a shape of sinx/x. The spectrum of period signal is composed of discrete spectral
25、 components. Ck decays to zero when f , is zero at multiples of 1/.,2019/1/30,HUI REN NCEPU,28,Bandwidth: Generally, in engineering definitions, the bandwidth would be f2-f1, for baseband waveform, f1 is usually taken to be zero, and f2 is the first null in the envelope of the magnitude spectrum alo
26、ng the positive frequency axis. So, for this periodic rectangular waveform, Bandwidth B=1/.,2019/1/30,HUI REN NCEPU,29,Channels and noise,What is a channel? The propagating medium or electromagnetic path connecting the transmitter and receiver is called the channel. Open wire, seldom used nowadays t
27、wisted Pair Cable(双绞线): Two insulated wires are twisted around each other, and combined with others into a cable . Two forms: UTP (unshielded Twisted Pair), STP (shielded twisted pair),2019/1/30,HUI REN NCEPU,30,coaxial cables(同轴电缆): has a single core, with an outer conductor acting as a shield(屏蔽).
28、 The signal is transmitted on the inner core. The inner core and the outer shield are separated by an insulator (normally plastic). It has a larger bandwidth than twisted cable, used in television, radio, and Ethernet LANs.,2019/1/30,HUI REN NCEPU,31,The atmosphere micro-wave system(微波): a radio sys
29、tem which uses very high frequencies to send and receive data. Because of the high frequencies involved, stations are located about 30 kilometers apart and in line of sight (目视距离). microwave systems are used in a number of areas, such as linking local area networks together between campus buildings,
30、 sending radio signals from a radio station to its transmitter site, and the sending of video or audio signals from an outside sports event back to a TV broadcasting studio.,2019/1/30,HUI REN NCEPU,32,fiber optic cables(光纤): made from fine strands of silica (glass), coated with a plastic sheath. ,20
31、19/1/30,HUI REN NCEPU,33,Communication systems Engineering,Organization of the book Before we embark on the analysis and design of communication systems, we provide a brief review of basic frequency-domain characteristics of signals and linear systems in Chapter 2. Emphasis is placed on the Fourier
32、series and the Fourier transform representation of signals and the use of transforms in linear systems analysis. The process of sampling a bandlimited analog signal is also considered.,2019/1/30,HUI REN NCEPU,34,In Chapter 3, we treat the modulation and demodulation of analog signals. This chapter p
33、rovides detailed description of amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM). As examples of analog signal transmission and reception, we consider radio and television broadcasting, and mobile radio cellular communication systems.,2019/1/30,HUI REN NCEPU,35,In Chap
34、ter 4, we present a review of the basic definitions and concepts in probability and random processes. These topics are particularly important in our study of electrical communications, because information sources produce random signals at their output and communication channels generally corrupt the transmitted signals in a random manner, through the addition of noise and other channel distortion. Special emphasis is placed on Gaussian random processes, which provide mathematically tractable models for additive noise disturbances.,