[信息与通信]电力专业英语11-电力系统通信基础.ppt
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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
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