【正文】 d calculator.對于通信（調制/檢測）類型、信道衰落模型、分集類型的每一種組合，得到系統(tǒng)的平均誤碼率（BER）和符號錯誤率（SER），并以容易求值的形式表示。5. Modulation is the systematic variation of some attribute of a carrier waveform such as the amplitude, phase, or frequency in accordance with a function of the message signal. It is used in munication systems for matching signal characteristics to channel characteristics, for reducing noise and interference, for simultaneously transmitting several signals over a single channel, and for overing some equipment limitations. A considerable portion of this book is devoted to the study of how modulation schemes are deigned to achieve the above tasks. The success of a munication system depends to a large extent on the modulation.調制是載波波形的某些屬性，例如幅度、相位和頻率，根據消息信號的函數(shù)有規(guī)則的變化。本書相當大部分內容是研究如何設計調制方案以實現(xiàn)上述任務。Unit 5 Exercises (1) Translate the following passage into Chinese.OFDM consists of a large number of subcarriers equally spaced in a frequency band, each may be digitally modulated by a same scheme such as PSK, QAM, etc., or by different schemes. A serially transmitted sequence is divided into a number of sections, each having N symbols, and the N symbols in each section are used to modulate N carriers for simultaneous transmission. Therefore OFDM is essentially a parallel modulation system. When the number of subcarriers is sufficiently large, the system can resist multipath interference. This is because that, in the time domain, a symbol duration longer than the multipath delay can be chosen, while in the frequency domain, each symbol only occupies a small portion of the channel’s frequency band. Thus, the effect of multipath fading spreads over many symbols, resulting in slight distortion to many symbols rather than plete destroy of a few symbols. In this way, correct demodulation is not affected so that the signal can be accurately recovered at the receiver. In an OFDM system, the principle of choosing the subcarrier interval is to make the subcarriers mutually orthogonal within the entire symbol period. Thus, even if spectral overlap exists between the subcarriers, the symbols can still be recovered without loss. In order to realize maximum spectral efficiency, the interval between subcarriers is usually chosen to equal the reciprocal of the symbol duration T. Therefore the subcarrier frequencies in the base band are fn = n/T, (n = 0, 1, ... , N1). Denoting the nth modulating symbol as X(n), the OFDM waveform within a symbol duration can be expressed as: (1)Sampling this waveform at t = T/N yields (2)It is observed from the above expression that x(k) and X(n) form a discrete Fourier transform pair, therefore the baseband OFDM waveform can be generated from the discrete Fourier transform of N modulating symbols. When N=2m where m is an integer, the fast algorithm of IDFT is easy to implement.OFDM由大量在頻率上等間隔的子載波構成。將串行傳輸?shù)姆栃蛄蟹殖稍S多段，每一段有N個符號，每段的N個符號被用來分別調制N個載波同時發(fā)送。這是因為在時域上符號周期能選取得比多徑延遲時間長，而在頻域上每個符號只占整個信道帶寬的一小部分，因此多徑衰落的影響被分散到許多符號上，其結果是許多符號都發(fā)生輕度的畸變，而不是使某些符號被完全破壞。在OFDM系統(tǒng)中，選擇子載波間隔的原則是使子載波在整個符號周期內相互正交，這樣。為了實現(xiàn)最大的頻譜效率，子載波之間的間隔通常選取為符號周期的倒數(shù)。當N=2m，其中m為整數(shù)，IDFT的快速算法很容易實現(xiàn)。這種傳送要求信號隨時間發(fā)生的變化不能在接收端預測。3. Since the very beginning, munications systems have consisted of three major parts, namely, the equipment, the medium, and the protocol. It is still the case today. However, the close link of telemunications with the puter technology has brought about tremendous changes in munications, from the concepts, to the contents and the methods.從一開始通信系統(tǒng)就由設備、媒介和協(xié)議三個要素組成。然而電信與計算機技術的緊密聯(lián)系使得通信從概念到內容和手段都發(fā)生了重大的變化。5. The channel provides the electrical connection between the information source and the user. The channel can have many different forms such as a microwave radio link over free space, a pair of wires, or an optical fiber.信道提供信息源和用戶之間的電路聯(lián)接。6. The degradation of the transmitted signal is a result of signal distortion due to imperfect response of the channel and due to undesirable electrical signals (noise) and interference. Noise and signal distortion are two basic problems of electrical munication.傳輸信號的退化是信號失真的結果，而這種失真又是由信道的非理想響應及不想要的電信號（噪聲）和干擾引起的。Unit 8 Exercises (1) Translate the following sentences into Chinese.1. Early in the twentieth century, it was found that light could cause atoms to emit electrons and that, when light released an electron from an atom, the energy possessed by the electron very greatly exceeded that which the atom could, according to the electromagnetic wave theory, have received.在二十世紀早期，人們發(fā)現(xiàn)光能夠使原子放出電子，而且當光從原子中釋放一個電子時，電子所包含的能量大大超過由電磁波理論得到的原子所接收的能量。3. If one were to transmit such information without impressing it on an optical carrier but instead on a radio frequency (RF) carrier a bit higher than the maximum rate, the transmission wavelength of the RF carrier would be centimeters or larger.如果傳輸這樣的信息不是將它加載到光纖上，而是加載在略高于最大速率的射頻載波上，則此射頻載波就會是厘米波或是波長更長一些的波。5. Although all the advantages of coherent optical munication systems have yet to be brought to fruition, another property of optical radiation has made today’s optical munication systems not desirable for applications.雖然相干光通信系統(tǒng)的所有優(yōu)點還有待于落實在具體成果中，光輻射的另一性質卻使目前的光通信系統(tǒng)不利于應用。7. As mentioned previously, these applications have tended to open up more slowly than originally predicted, as cost was really not much of a consideration in telemunications, where equipment costs are swamped by other considerations.如前所述，（線路）成本并非電信系統(tǒng)中真正重要的考慮因素，通信設備的成本主要受到其他因素的制約，因此這些應用比預料的出現(xiàn)得慢。UNIT 10 principles which underlie almost all digital audio applications and devices, be they digital synthesis, sampling, digital recording or CD or iPod playback, are based on the basic concepts which follow in this chapter. New forms of playback, file formats, pression and storage of data are all changing on a seemingly daily basis, but the underlying mechanisms for converting realworld sound into digital values, manipulating those data and finally converting them back into realworld sound has not varied much since Max Mathews developed MUSIC I