【正文】 Impulse noiseThermal noise is due to thermal agitation of electrons. It is present in all electronic devices and transmission media and is a function of temperature. Thermal noise is uniformly distributed across the frequency spectrum and hence is often referred to as white noise. Thermal noise cannot be eliminated and therefore places an upper bound on munications system performance. Because of the weakness of the signal received by satellite earth stations, thermal noise is particularly significant for satellite munication.The amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor isWhere=noise power density in watts per 1 Hz of bandwidthk=Boltzmann’s constant=J/KT=temperature, in kelvins (absolute temperature)The noise is assumed to be independent of frequency. Thus the thermal noise in watts present in a bandwidth of B Hertz can be expressed asN=kTBor, in decibelwatts,N=101gk+10lgT+10lgB =(dBW)+101gT+10lgBWhen signals at different frequencies share the same transmission medium, the result may be intermodulation noise. Intermodulation noise produces signals at a frequency that is the sum or difference of the two original frequencies or multiples of those frequencies. For example, the mixing of signals at frequencies and might produce energy at the derived signal could interfere with an intended signal at the frequency.Intermodulation noise is produced when there is some nonlinearity in the transmitter, receiver, or intervening transmission system. Normally, these ponents behave as linear systems；that is, the output IS equal to the input times a constant. In a nonlinear system, the output is a more plex function of the input. Such nonlinearity can be caused by ponent malfunction, the use of excessive signal strength, or just the nature of the amplifiers used. It is under these circumstances that the sum and difference frequency terms occur.Crosstalk has been experienced by anyone who, while using the telephone, has been able to hear another conversation；it is an unwanted coupling between signal paths. It can occur by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying multiple signals. Crosstalk can also occur when unwanted signals are picked up by microwave antennas；although highly directional antennas are used, microwave energy does spread during propagation. Typically, crosstalk is of the same order of magnitude as, or 1ess than, thermal noise. However, in the unlicensed ISM bands, crosstalk often dominates. All of the types of noise discussed so far have reasonably predictable and relatively constant magnitudes. Thus it is possible to engineer a transmission system to cope with them. Impulse noise, however, is noncontinuous, consisting of irregular pulses or noise spikes of short duration and of relatively high amplitude. It is generated from a variety of causes, including external electromagnetic disturbances, such as lightning, and faults and flaws in the munications system. Impulse 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