雙工無線語音數據傳輸系統外文翻譯(編輯修改稿)

　

【文章內容簡介】 ere 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 雙工無線語音數據傳輸系統的設計 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 demodulation in order to extract the message from the sinusoidal carrier. Since the signal demodulation is performed in the presence of additive noise and possibly other signal distortion, the demodulated message signal is generally degraded to some extent by the presence of these distortions in the received signal. As we shall see, the fidelity of the additive noise, the type and strength of any other additive interference, and the type of any nonadditive interference. Besides performing the primary function of signal demodulation, the receiver also performs a number of peripheral functions, including signal filtering and noise suppression. Digital Communication System An electrical munication system in rather broad terms based on the implicit assumption that message signal is a continuous timevarying waveform. We refer to such continuoustime signal waveforms as analog sources. Analog signal can be transmitted directly via modulation over the munication channel and demodulated accordingly at the receiver. We call such a i munication system an analog munication system. Alternatively, an analog source output may be converted into a digital form and the message can be transmitted via digital modulation as a digital signal at the receiver. There are some potential advantage to transmitting an analog signal by means of digital modulation. The most important reason is that signal fidelity is better controlled though digital transmission than analog transmission. In particular, digital transmission allows us to regenerate the digital signal in longdistance transmission, thus eliminating effects of noise at each regeneration point. In contrast, the noise added in analog transmission is amplified a