Antennas and Propagation ( 天线与传播 )

1. Antennas and Propagation(天线与传播) Chapter 5

2. Introduction • An antenna is an electrical conductor　（电子导线） or system of conductors（导线系统） • 发送 - radiates electromagnetic energy into space • 接收 - collects electromagnetic energy from space • In two-way communication, the same antenna can be used for transmission and reception • 假定：两个方向上使用同一频率

3. Radiation Patterns（辐射模式） • Radiation pattern • Graphical representation of radiation properties of an antenna • Depicted as two-dimensional cross section • Beam width (or half-power beam width) （光束宽度或称半功率光束宽度） • Measure of directivity of antenna （计算天线方向性的计量值） 半功率光束宽度表示一个角度，在这个角度内，天线辐射功率至少是该天线在最佳方向上辐射功率的一半 • Reception pattern • Receiving antenna’s equivalent to radiation pattern

5. Types of Antennas • Isotropic antenna (idealized) （各向同性天线） • Radiates power equally in all directions • 辐射模式：以天线为中心的一个球体 • 通常表示为三维模式的一个二维剖面 • Dipole antennas （偶极天线） • Half-wave dipole antenna (or Hertz antenna) （半波偶极天线或称赫兹天线） • Quarter-wave vertical antenna (or Marconi antenna) （1/4波垂直天线或称马可尼天线） • Parabolic Reflective Antenna（抛物反射天线）

6. 半波偶极 ¼波垂直天线

8. Antenna Gain （天线增益） • Antenna gain • Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) • 天线增益不是为了似的输出功率大于输入功率，而主要是为了方向性 • Effective area • Related to physical size and shape of antenna • 针对不同天线的经验计算公式见表5.2, 如：抛物线，抛物面面积为A，则其有效面积为0.56A.

9. Antenna Gain • Relationship between antenna gain and effective area • G = antenna gain • Ae= effective area • f = carrier frequency • c = speed of light (» 3 ´ 108 m/s) •  = carrier wavelength

11. Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation

12. Ground Wave Propagation

13. Ground Wave Propagation • Follows contour of the earth (沿地表传播) • Can Propagate considerable distances • Frequencies up to 2 MHz…>1km • Example • AM radio (调幅无线电)

14. Sky Wave Propagation

15. Sky Wave Propagation • Signal reflected from ionized layer of atmosphere back down to earth • Signal can travel a number of hops, back and forth between ionosphere and earth’s surface • Reflection effect caused by refraction (折射)。。。尽管实际效果看起来象反射 • Examples • Amateur radio • CB radio （民用波段无线电） • 国际广播 ( BBC, VOA)

16. Line-of-Sight Propagation

17. Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight • Satellite communication – signal above 30 MHz not reflected by ionosphere • Ground communication – antennas within effective line of sight (有效的直线范围) due to refraction。。因为微波会被大气弯曲或折射，大多数微波弯曲程度与地球曲率相同，因而比直线广播传输的远些 • Refraction – bending of microwaves by the atmosphere • Velocity of electromagnetic wave is a function of the density of the medium • When wave changes medium, speed changes • Wave bends at the boundary between mediums

18. 光波地平线和无线电地平线

19. Line-of-Sight Equations • Optical line of sight • Effective, or radio, line of sight • d = distance between antenna and horizon (km) • h = antenna height (m) • K = adjustment factor to account for refraction, rule of thumb (经验法) K = 4/3

20. Line-of-Sight Equations • Maximum distance between two antennas for LOS propagation: • h1 = height of antenna one • h2 = height of antenna two

21. 天线高度计算 • 例5.2:假定天线高度100m, 另一天线在地平线上，则两天线直线传播的最大距离是 • 假设接收天线10 m，试问为获得同样传播距离，发送天线高度几何？ • h1=7.842/1.33=46.2 m

22. LOS Wireless Transmission Impairments (传输损伤) • Attenuation and attenuation distortion • Free space loss • Noise • Atmospheric absorption • Multipath • Refraction • Thermal noise

23. Attenuation • Strength of signal falls off with distance over transmission medium • Attenuation factors for unguided media: • Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal • Signal must maintain a level sufficiently higher than noise to be received without error • Attenuation is greater at higher frequencies, causing distortion • 跨频带的衰减均等化

24. Free Space Loss • Free space loss, ideal isotropic antenna • Pt = signal power at transmitting antenna • Pr = signal power at receiving antenna •  = carrier wavelength • d = propagation distance between antennas • c = speed of light (3 10 8 m/s) where d, , and c are in the same units (e.g., meters)

25. Free Space Loss • Free space loss equation can be recast:

27. Free Space Loss • Free space loss accounting for gain of other antennas • Gt = gain of transmitting antenna • Gr = gain of receiving antenna • At = effective area of transmitting antenna • Ar = effective area of receiving antenna

28. Free Space Loss • Free space loss accounting for gain of other antennas can be recast as 表5.2: 天线增益与等效面积

29. Categories of Noise • Thermal Noise • Intermodulation noise (互调噪声) • Crosstalk (串扰) • Impulse Noise

30. Thermal Noise • Thermal noise due to agitation of electrons （电子的热搅动） • Present in all electronic devices and transmission media • Cannot be eliminated • Function of temperature • Particularly significant for satellite communication • 因为卫星地面站收到的卫星信号较弱

31. Thermal Noise • Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is: • N0 = noise power density in watts per 1 Hz of bandwidth • k = Boltzmann's constant = 1.3803  10-23 J/K • T = temperature, in kelvins (absolute temperature)

32. Thermal Noise • Noise is assumed to be independent of frequency • Thermal noise present in a bandwidth of B Hertz (in watts): or, in decibel-watts

33. Noise Terminology • Intermodulation noise – occurs if signals with different frequencies share the same medium • Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies • Crosstalk – unwanted coupling between signal paths （不希望的信号路径之间的偶合） • Impulse noise – irregular pulses or noise spikes • Short duration and of relatively high amplitude • Caused by external electromagnetic disturbances, or faults and flaws in the communications system （外部电磁干扰或通信系统的故障或缺陷） 脉冲噪音是数字数据误码的一个主要来源 如：0.01秒的脉冲在56kbps的传输中，会破坏560bits数据

34. Expression Eb/N0 • Ratio of signal energy per bit to noise power density per Hertz R: 数据比特率 ; S信号功率 • The bit error rate for digital data is a (decreasing) function of Eb/N0 • Given a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selected • As bit rate R increases, transmitted signal power must increase to maintain required Eb/N0

35. Other Impairments • Atmospheric absorption – water vapor （水蒸气） and oxygen contribute to attenuation • Multipath – obstacles reflect signals so that multiple copies with varying delays are received • Refraction – bending of radio waves as they propagate through the atmosphere

36. Multipath Propagation 反射(R), 散射(S), 衍射(D)

37. Multipath Propagation • Reflection (反射) - occurs when signal encounters a surface that is large relative to the wavelength of the signal • Diffraction(衍射) - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave • Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

38. The Effects of Multipath Propagation • Multiple copies of a signal may arrive at different phases • If phases add destructively, the signal level relative to noise declines, making detection more difficult • Intersymbol interference (ISI) • One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

39. Types of Fading • Fast fading • Slow fading • Flat fading (平面衰落) • Selective fading • Rayleigh fading (瑞利衰落) • Rician fading (瑞森衰落)

40. Error Compensation Mechanisms(差错补偿机制) • Forward error correction • Adaptive equalization • Diversity techniques

41. Forward Error Correction • Transmitter adds error-correcting code to data block • Code is a function of the data bits • Receiver calculates error-correcting code from incoming data bits • If calculated code matches incoming code, no error occurred • If error-correcting codes don’t match, receiver attempts to determine bits in error and correct

42. Adaptive Equalization（自适应均衡） • Can be applied to transmissions that carry analog or digital information • Analog voice or video • Digital data, digitized voice or video • Used to combat intersymbol interference • Involves gathering dispersed symbol energy back into its original time interval • Techniques • Lumped analog circuits （集总模拟电路） • Sophisticated digital signal processing algorithms

43. Diversity Techniques（分集技术） • Diversity is based on the fact that individual channels experience independent fading events • Space diversity – techniques involving physical transmission path • Frequency diversity – techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers • 如：扩频 • Time diversity – techniques aimed at spreading the data out over time • 交织(interleaving) • 代价是时延。。交织度越大，接收端重构原序列的时延越大

44. 作业 • 命名并简要定义四类噪声 • 抛物反射天线的优点是什么? • 假设发送器产生50W的功率 (1) 以dBm和dBW为单位表示传输功率 (2) 如果上述发送器的功率输入到一个增益为1 、载波频率为900MHz的发送天线上,则在距离100m远的自由空间处, 接收功率是多少dBm? （假定接收天线增益也为1） (3) (2)中,如果距离为10km,距离如何? (4) (3)中,假设接收天线有2的增益,结果又如何? • 确定信号必须能够达到80km远的用户处的电视台天线的高度

45. 下周六停一次课！