【正文】 relate to SNR as follows. We haveThe parameter is the noise power density in watts/hertz. Hence, the noise in a signal with bandwidth is .Substituting, we have ()Another formulation of interest relates to spectral efficiency. Recall, from Chapter 2,Shannon’s result that the maximum channel capacity, in bits per second, obeys the equationC=B(1+S/N)Where C is the capacity of the channel in bits per second and B is the bandwidth of the channel in Hertz. This can be rewritten as：Using Equation (),and equating with B and R with C, we haveThis is a useful formula that relates the achievable spectral efficiency C/B to.Atmospheric AbsorptionAn additional loss between the transmitting and receiving. antennas is atmospheric absorption Water vapor and oxygen contribute most to attenuation. A peak attenuation occurs in the vicinity of 22 GHz due to water vapor. At frequencies below 15 GHz, the attenuation is less. The presence of oxygen results in an absorption peak in the vicinity of 60 GHz but contributes less at frequencies below 30 GHz. Rain and fog(suspended water droplets) cause of scattering of radio waves that results in attenuation. This can be a major cause of signal loss. Thus, in areas of significant precipitation, either path lengths have to be kept short or lowerfrequency bands should used.MultipathFor wireless facilities where there is a relatively free choice of where antennas are to be located, they can be placed SO that if there are no nearby interfering obstacles, there is a direct lineofsight path from transmitter to receiver. This is generally the case for many satellite facilities and for pointtopoint microwave. In other cases, such as mobile telephony, there are obstacles in abundance. The signal can be reflected by such obstacles so that multiple copies of the signal with varying delays can be received. In fact, in extreme cases, he receiver my capture only reflected signals and not the direct signal. Depending on the differences in the path lengths of the direct and reflected waves, he posite signal can be either larger or smaller than the direct signal. Reinforcement and cancellation of the signal resulting from the signal following multiple paths can be controlled for munication between fixed, well sited antennas, and between satellites and fixed ground stations. One exception is when the path goes across water, where the wind keeps the reflective surface of the water in motion. For mobile telephony and munication to antennas that are not well sited, multipath considerations can be paramount.Microwave line of sight(b)Mobile radioFigure Examples of Multipath InterferenceFigure illustrates in general terms the types of multipath interference typical in terrestrial, fixed microwave and in mobile munications. For fixed microwave, in addition to the direct line of sight, the signal may follow a curved path through the atmosphere due to refraction and the signal may also reflect from the ground. For mobile munications, struct。 Intermodulation noisethat is, there is actually a decrease in loss at higher frequencies.Figure Free Space LossNoiseFor any data transmission event, he received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere between transmission and reception. These unwanted signals are referred to as noise. Noise is the major limiting factor in munications system performance.Noise may be divided into four categories：Multipath.NoiseAttenuation and attenuation distortion wireless local loop Infrared300GHz to400THz1mm to770nmLOSInfrared LANs。 wireless local loopEHF(extremely high frequency)30GHz to300GHz10mm to1mmLOS。 radar。 rainfall attenuation above 10GHz。 microware links。 cellular telephone。 FM broadcast and twoway radio, AM aircraft navigation aidsBandFrequencyFreeSpace WavelengthRangePropagation CharacteristicsTypical UseUHF(ultra high frequency)300MHz to3000MHz100cm to10cmLOS。 scattering because of temperature inversion。 international broadcasting, military munication。 AM broadcastingHF(high frequency)3MHz to30MHz100m to10mSW。 atmospheric noiseMaritime radio。 and night SW。 absorption in daytimeLongrange navigation。 submarine municationLF(low frequency)30kHz to300kHz10km to1kmGW。 low attenuation day and night。 Power line frequencies。自動(dòng)化與電氣工程英文資料與中文翻譯畢業(yè)論文 ANTENNASAn antenna can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy. For transmission of a signal, radiofrequency electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water). For reception of a signal, electromagnetic energy impinging on the antenna is converted into radiofrequency electrical energy and fed into the receiver.Radiation PatternsAn antenna will radiate power in all directions but, typically, does not perform equally well in all directions. A mon way to characterize the performance of an antenna is the radiation pattern, which is a graphical representation of the radiation properties of an antenna as a function of space coordinates. The simplest pattern is produced by an idealized antenna known as the isotropic antenna. An isotropic antenna is a point in space that radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center. However, radiation patterns are almost always depicted as a twodimensional cross section of the threedimensional pattern. The pattern for the isotropic antenna is shown in distance from the antenna to each point on the radiation pattern i