1、Antennas and Propagation (天线与传播),Chapter 5,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 a
2、nd reception 假定：两个方向上使用同一频率,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
3、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 antennas equivalent to radiation pattern,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
4、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）,半波偶极,波垂直天线,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
5、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.,Antenna Gain,Relationship between antenna gain and effective areaG = antenna gain Ae = effective area f = carrier frequency c = speed of light ( 3 108 m/s) = carri
6、er wavelength,Propagation Modes,Ground-wave propagation Sky-wave propagation Line-of-sight propagation,Ground Wave Propagation,Ground Wave Propagation,Follows contour of the earth (沿地表传播) Can Propagate considerable distances Frequencies up to 2 MHz1km Example AM radio (调幅无线电),Sky Wave Propagation,Sk
7、y 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 earths 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),Line-of
8、-Sight Propagation,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
9、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,光波地平线和无线电地平线,Line-of-Sight Equations,Optical line of sightEffective, or rad
10、io, line of sightd = 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,Line-of-Sight Equations,Maximum distance between two antennas for LOS propagation:h1 = height of antenna one h2 = height of antenna two,天
11、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,LOS Wireless Transmission Impairments (传输损伤),Attenuation and attenuation distortion Free space loss Noise Atmospheric absorption Multipath Refraction Thermal noise,Attenuation,Strength of signal falls
12、 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
13、 at higher frequencies, causing distortion 跨频带的衰减均等化,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
14、 c are in the same units (e.g., meters),Free Space Loss,Free space loss equation can be recast:,Free Space Loss,Free space loss accounting for gain of other antennasGt = gain of transmitting antenna Gr = gain of receiving antenna At = effective area of transmitting antenna Ar = effective area of rec
15、eiving antenna,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,Categories of Noise,Thermal Noise Intermodulation noise (互调噪声) Crosstalk (串扰) Impulse Noise,Thermal Noise,Thermal noise due to agitation of electrons （电子的热搅动） Present in all electron
16、ic 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,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 ban
17、dwidth k = Boltzmanns constant = 1.3803 10-23 J/K T = temperature, in kelvins (absolute temperature),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,Noise Terminology,Intermodulation noise occurs if signals
18、 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 relativ
19、ely 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,Expression Eb/N0,Ratio of signal energy per bit to noise power density per HertzR: 数据比特率 ; S 信号功率 The bit error
20、 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,Other Impairments,Atmospheric absorption water vap
21、or （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,Multipath Propagation,反射(R), 散射(S), 衍射(D),Multipath Propagation,Reflection (反射) - occu
22、rs 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 o
23、f the signal or less,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 arri
24、ve at the same time as the primary pulse for a subsequent bit,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),Error Compensation Mechanisms (差错补偿机制),Forward error correction Adaptive equalization Diversity techniques,Forward Erro
25、r 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 dont match, receiver attempts to determine bit
26、s in error and correct,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 in
27、terval Techniques Lumped analog circuits （集总模拟电路） Sophisticated digital signal processing algorithms,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 diver
28、sity 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,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,下周六停一次课！,
