【正文】 ulse noise is generally only a minor annoyance tor analog data. For example, voice transmission may be corrupted by short clicks and crack les with no loss of intelligibility. However, impulse noise IS the primary source of error in digital data transmission. For example, a sharp spike of energy of duration would not destroy any voice data but would wash out about 560 bits of data being transmitted at 56 kbps.The Expression Chapter 2 introduced the signaltonoise ratio(SNR).There is a parameter related to SNR that is more convenient for determining digital data rates and error rates and that is the standard quality measure for digital munication system performance. The parameter is the ratio of signal energy. per bit to noise power density per Hertz, .Consider a signal, digital or analog, that contains binary digital data transmitted at a certain bit rate R. Recalling that 1 watt=1 J/s, the energy per bit in a sign al is given by, where S is the signal power and is the time required to send one bit. The data rate R is just R=1/.Thus ()or, in decibel notation, The ratio is important because the bit error rate (BER) for digital data is a (decreasing) function of this ratio. Figure illustrates the typical shape of a plot of BER versus . Such pots are monly found in the literature and several examples appear in this text. For any particular curve, as the signal strength relative to the noise increases (increasing),the BER performance at the receiver decreases. This makes intuitive sense. However, there is not a single unique curve that expresses the dependence of BER the performance of a transmission/reception system, in terms of BER versus,also depends on the way in which the data is encoded onto the signal. Thus, Figure show two curves, one of which gives better performance than the other. A curve below and to the left of another. curve defines. superior performance. Chapter 6 explores the relationship of signal encoding to performance. A more detailed discussion of is found in [SKLA01]Given a value of needed to achieve a desired error rate, the parameters in Equation () may be selected. Note that as the—bit rate R increases, the transmitted signal power, relative to noise, must increase to maintain the required Let us try to grasp this result. intuitively by considering again Figure signal here is digital, but the reasoning would be the same for an analog signal. In several instances the noise is sufficient to alter the value of a bit. If the data rate were doubled, the bits would be more tightly packed together and. the same passage of noise might destroy two bits. Thus, or constant signal and noise strength, an increase in data rate increases the error rate.The advantage of pared to SNR is that the latter quantity depends on the bandwidth.Figure General Shape of BER Versus CurvesWe can 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, struc