【正文】 M) is yet a third method for impressing the information signal on a sinusoidal carrier. In general, carrier modulation such as AM, FM, and PM is performed at the transmitter, as indicated above, to convert the information signal to a form that matches the characteristics of the channel. Thus, though the process of modulation, the choice of the type of modulated in frequency to match the allocation of the channel. The choice of the type of modulation is based on several factors, such as the amount of bandwidth over the channel, the type of noise and the interference that the signal encounters in transmission. In any case, the modulation process makes it possible to acmodate the transmission of multiple messages from many users over the same physical channel. In addition to modulation, other functions that are usually performed at the transmitter are filtering of the informationbearing signal , amplification of the modulated signal, and in case of wireless transmission, radiation of the signal by means of a transmitting antenna. The channel. The munications channel is the physical medium that is used to send the signal from the transmitter to the receiver. In wireless transmission, the channel is usually the atmosphere (free space). On the other hand, telephone channels usually employ a variety of physical media, including wirelines, optical fiber cables, and wireless (microwave radio). Whatever the physical medium for signal transmission, the essential feature is that the transmitted signal is corrupted in a random manner by a variety of possible mechanisms. The most mon from of signal degradation es in the form of additive noise ,which is generated at the front end of the receiver, where signal amplification is performed. This noise is often called thermal noise. In wire less transmission, additional additive disturbances are manmade noise, and atmospheric noise picked up by a receiving antenna. Automovile ignition noise is an example of manmade noise, and electrical lightning discharges from thunderstorms is an example of atmospheric noise. Interference from other users of the channel is another form of additive noise that often arises in both wireless and wire line munication systems . In some radio munication channels, such as the ionospheric channel that is used for long range ,shortwave radio transmission, another form of signal degradation is multipath propagation. Such signal distortion is characterized as a nonadditive signal disturbance which manifests itself as time variations in the signal amplitude, usually called fading . Both additive and nonadditive signal distortions are usually characterized as random phenomena and described in statistical terms. The effect of these signal distortions must be taken into account on the design of the munication system. In the design of a munication system, the system, the system designer works with mathematical models that statistically characterize he signal distortion encountered on physical channels. Often, the statistical description that is used in mathematical model is a result of actual empirical measurements obtained from experiments involving signal transmission over such channels .In such cases , there is 雙工無線語音數(shù)據(jù)傳輸系統(tǒng)的設計 a physical justification for the mathematical model used in the design of munication systems. On the other hand, in some munication system designs ,the statistical characteristics of the channel may vary significantly with time. In such cases, the system design may designer may design a munication system that is robust to the variety of signal distortions. This can be acplished by having the system adapt some of its parameters to the channel distortion encountered. The receiver. The function of the receiver is to recover the message signal contained in the received signal. If the message signal is transmitted by carrier modulation, the receiver performs carrier demodulat