无线网络技术

1. 无线网络技术 张宝贤 2006年9月9日 Email: bxzhang@gucas.ac.cn

2. 回顾 • 上一次课中我们讲述了无线网络的概况、 现状、应用和发展，主要包括 • 无线局域网、移动蜂窝网络(3G)、无线自组织网络、无线传感器网络、WiMax、蓝牙等 • 我们的课程将主要集中在以下几个方面 • 基础知识：传输技术、天线与传播、调制、扩频编码与差错控制 • WLAN、卫星通信、移动IP、MANET、WMN、Celluar & Ad Hoc网络、无线 TCP 、无线传感器网络、BLUETOOTH 、WiMax、未来无线网络， • 主要包括网络体系结构、协议架构、组网问题、MAC、可靠通信、路由问题 • 涉及到一些移动蜂窝网络方面的架构和方案，但不会太多

3. 课件位置 • FTP site : ccce.gucas.ac.cn • Account : student • Password: student 具体位置 ./incoming/无线网络技术/…

4. Transmission Fundamentals(传输基础） Chapter 2

5. Electromagnetic Signal(电磁波) • Function of time • Can also be expressed as a function of frequency • Signal consists of components of different frequencies

6. Time-Domain Concepts • Analog signal - signal intensity varies in a smooth fashion over time • No breaks or discontinuities in the signal • Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level • Periodic signal - analog or digital signal pattern that repeats over time • s(t +T ) = s(t ) -< t < +  • where T is the period of the signal

8. Time-Domain Concepts • Aperiodic signal - analog or digital signal pattern that doesn't repeat over time • Peak amplitude (A) - maximum value or strength of the signal over time; typically measured in volts • Frequency (f ) • Rate, in cycles per second, or Hertz (Hz) at which the signal repeats

9. Time-Domain Concepts • Period (T ) - amount of time it takes for one repetition of the signal • T = 1/f • Phase () - measure of the relative position in time within a single period of a signal • Wavelength () - distance occupied by a single cycle of the signal • Or, the distance between two points of corresponding phase of two consecutive cycles

10. Sine Wave Parameters • General sine wave • s(t ) = A sin(2ft + ) • Figure 2.3 shows the effect of varying each of the three parameters • (a) A = 1, f = 1 Hz,  = 0; thus T = 1s • (b) Reduced peak amplitude; A=0.5 • (c) Increased frequency; f = 2, thus T = ½ • (d) Phase shift;  = /4 radians (45 degrees) • note: 2 radians = 360° = 1 period

11. Sine Wave Parameters

12. Time vs. Distance • When the horizontal axis is time, as in Figure 2.3, graphs display the value of a signal at a given point in space as a function of time 图2.3中当前显示横轴为时间，因此图中展示的是在空间上某一给定点的信号值随时间而变的函数 • With the horizontal axis in space, graphs display the value of a signal at a given point in time as a function of distance • At a particular instant of time, the intensity of the signal varies as a function of distance from the source 如果横轴为距离的话，那么此图显示的则是在某一给定时刻的信号值随距离发送节点距离变化的曲线

13. Frequency-Domain Concepts • Fundamental frequency（基频） - when all frequency components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency • Spectrum （频谱）- range of frequencies that a signal contains • Absolute bandwidth（绝对带宽） - width of the spectrum of a signal • Effective bandwidth (or just bandwidth)（有效带宽） - narrow band of frequencies that most of the signal’s energy is contained in

14. Frequency-Domain Concepts • Any electromagnetic signal can be shown to consist of a collection of periodic analog signals (sine waves) at different amplitudes, frequencies, and phases 任何电磁信号都可以都可以表示成不同振幅、不同频率和相位的模拟信号（正弦波）组成 • The period of the total signal is equal to the period of the fundamental frequency

16. Relationship between Data Rate and Bandwidth • The greater the bandwidth, the higher the information-carrying capacity • Conclusions • Any digital waveform（数字波形） will have infinite bandwidth • BUT the transmission system will limit the bandwidth that can be transmitted • AND, for any given medium, the greater the bandwidth transmitted, the greater the cost • HOWEVER, limiting the bandwidth creates distortions

17. Data Communication Terms

18. Examples of Analog and Digital Data • Analog • Video • Audio • Digital • Text • Integers

19. Analog Signals • A continuously varying electromagnetic wave that may be propagated over a variety of media, depending on frequency • Examples of media: • Copper wire media (twisted pair and coaxial cable) • Fiber optic cable • Atmosphere or space propagation • Analog signals can propagate analog and digital data

20. Digital Signals • A sequence of voltage pulses that may be transmitted over a copper wire medium • Generally cheaper than analog signaling • Less susceptible to noise interference • Suffer more from attenuation • Digital signals can propagate analog and digital data

21. Analog Signaling 模拟信号 模拟声波

22. Digital Signaling 比如,模拟话音 编解码器

23. Reasons for Choosing Data and Signal Combinations • Digital data, digital signal • 将数字数据编码为数字信号比将数字信号转换为模拟信号要便宜 • Analog data, digital signal • Conversion permits use of modern digital transmission and switching equipment • Digital data, analog signal • Some transmission media will only propagate analog signals • Examples include optical fiber and satellite • Analog data, analog signal • Analog data easily converted to analog signal

24. Analog Transmission • Transmit analog signals without regard to content • Attenuation limits length of transmission link • Cascaded amplifiers boost signal’s energy for longer distances but cause distortion • Analog data can tolerate distortion • Introduces errors in digital data

25. Digital Transmission • Concerned with the content of the signal • Attenuation endangers integrity of data • Digital Signal • Repeaters achieve greater distance • Repeaters recover the signal and retransmit • Analog signal carrying digital data • Retransmission device recovers the digital data from analog signal • Generates new, clean analog signal…noise will not accumulated

26. About Channel Capacity • Impairments, such as noise, limit data rate that can be achieved • For digital data, to what extent do impairments limit data rate? • Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions

27. Concepts Related to Channel Capacity • Data rate（数据率） - rate at which data can be communicated (bps) • Bandwidth（带宽） - the bandwidth of the transmitted signal as constrained by the transmitter and the nature of the transmission medium (Hertz) 传输信号的带宽,指由发送器和传输媒体的特性限制的带宽,用周数每秒或赫兹(Hz)表示 • Noise（噪声） - average level of noise over the communications path (传输线路上的平均噪声) • Error rate（误码率） - rate at which errors occur • Error = transmit 1 and receive 0; transmit 0 and receive 1

28. Nyquist Bandwidth • For binary signals (two voltage levels) • C = 2B • 如果信号传输的速率为2B，那么频率不大于B的信号就可以达到此信号传输率；或，如果带宽为B，则最大信号传输速率为2B • 因为码间干扰导致！比如时延失真 • 时延失真: 在信号的频率范围内,传播媒体的传播时延不是常数 • With multilevel signaling • C = 2B log2M • M = number of discrete signal or voltage levels (离散信号或电压电平的个数） • 理想状况－无噪声信道！ • 如果M=8, B=3100Hz, 那么C=? • C=2 3100  log2 8 =18,600 bps

29. Signal-to-Noise Ratio • Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission 信号功率与在传输的某一特定点处(如:距离发射点5km处)呈现的噪声中包含的功率的比率 • Typically measured at a receiver • Signal-to-noise ratio (SNR, or S/N) • A high SNR means a high-quality signal, low number of required intermediate repeaters • SNR sets upper bound on achievable data rate

30. Shannon Capacity Formula • Equation: • Represents theoretical maximum that can be achieved • In practice, only much lower rates achieved • Formula assumes white noise (thermal noise) • Impulse noise is not accounted for • Attenuation distortion（衰减失真） or delay distortion （时延失真）not accounted for

31. Example of Nyquist and Shannon Formulations • Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB • Using Shannon’s formula

32. Example of Nyquist and Shannon Formulations • How many signaling levels are required? M=16意味每个信元可以编为一个４比特的码字

33. Classifications of Transmission Media • Transmission Medium • Physical path between transmitter and receiver • Guided Media　（导向媒体） • Waves are guided along a solid medium • E.g., copper twisted pair(铜双绞线), copper coaxial cable (同轴电缆), optical fiber • Unguided Media（非导向媒体） • Provides means of transmission but does not guide electromagnetic signals • Usually referred to as wireless transmission • E.g., atmosphere, outer space

34. Unguided Media • Transmission and reception are achieved by means of an antenna • Configurations for wireless transmission • Directional • Omnidirectional

35. General Frequency Ranges • Microwave frequency range • 1 GHz to 40 GHz • Directional beams（定向波束） possible • Suitable for point-to-point transmission • Used for satellite communications1－10 GHz • Radio （无线电广播）frequency range • 30 MHz to 1 GHz • Suitable for omnidirectional applications • Infrared frequency range • Roughly, 3x1011 to 2x1014 Hz • 无法穿透墙体　 • Useful in local point-to-point and multipoint applications within confined areas (e.g., in a room)

36. Terrestrial Microwave • Description of common microwave antenna • Parabolic “dish”（抛物面碟型）, 3 m in diameter • Fixed rigidly and focuses a narrow beam • Achieves line-of-sight transmission to receiving antenna • Located at substantial heights above ground level • Raining has a great impact on the performance of microwave systems, in particular around 10GHZ • Applications • Long haul telecommunications service • Short point-to-point links between buildings • 频率越高,衰减越大 • 频率越高,天线越小越便宜

37. Satellite Microwave • Description of communication satellite • Microwave relay station • Used to link two or more ground-based microwave transmitter/receivers • Receives transmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on another frequency (downlink) • Applications • Television distribution • Long-distance telephone transmission • Private business networks • 问题 • 频段的拥挤 (4/6G (downlink/uplink) => 12/14G… =>… 20/30GHz) • 衰减增加, 可用频段加大,天线变小 • <1G….噪声太大..包括银河系,太阳,大气噪声等 • 较大的延迟(从一个地面站到另外一个地面站: ¼秒) • 拥塞控制/差错控制/,如:卫星通信中的TCP性能

38. Broadcast Radio（广播无线电） • Description of broadcast radio antennas • Omnidirectional • Antennas not required to be dish-shaped • Antennas need NOT be rigidly mounted to a precise alignment • Applications • Broadcast radio • VHF (甚高频) and part of the UHF (特高频) band • 30 MHZ to 1GHz • Covers FM radio and UHF and VHF television

39. Multiplexing • Capacity of transmission medium usually exceeds capacity required for transmission of a single signal • Multiplexing - carrying multiple signals on a single medium • More efficient use of transmission medium

40. Multiplexing MUX: 多路复用器multiplexer DEMUX: 多路信号分离器demultiplexer

41. Reasons for Widespread Use of Multiplexing • Cost per kbps of transmission facility declines with an increase in the data rate • Cost of transmission and receiving equipment declines with increased data rate • Most individual data communicating devices require relatively modest data rate support • 大多数专用的数据通信设备要求比较适中的数据率支持，比如，多数c/s应用，数据率达到64kbps就可以了

42. Multiplexing Techniques • Frequency-division multiplexing (FDM) • Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal • Time-division multiplexing (TDM) • Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal

43. Frequency-division Multiplexing

44. Time-division Multiplexing

45. 附：分贝的计算 • 我们用分贝表示增益、损耗及相对值 • 分贝是两个信号电平之间的比值的一种度量 • dB的计算 • Pin: 输入功率 • Pout:输出功率 • dB是一个相对值 • 例2.2：如果输入线上假如一个功率为10mW的信号，且一定距离外测得其功率为5mW,那么损耗就可以表示为 LdB = 10 lg (10/5)=3 dB

46. 附：分贝的计算(续） • 如果分贝用电压方面的比值表示的话，考虑P=V2/R….其中P为功率，V为电压，R为阻值， • 那么

47. 分贝的计算（续） • dBm（分贝-毫瓦）的计算 • 分贝-瓦 例2.4: 1000W的功率是30dBW, 1mW的功率是-30dBW

48. 作业 • 解释一下微波传输的一些主要优缺点 • 要求一个数字传输系统工作在9600b/s. • 如果一个信元(signal element)表示一个4比特的字,那么所需最小带宽是多少 • 在8比特字的情况下又是什么? • 某一信道所要求的容量是20Mb/s,信道的带宽为3MHz.为了达到这一信道容量, 要求信噪比是多少?