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扩频通信相关文章

简介
扩展频谱是凭借将已调信号再次调整的方式，产生新的波形，这种波形对其他信号的干扰微乎其微，并且与它们处于同一频段。因此，调频某个AM或FM广播频道的接收者可能并不会注意到，在同一个频段还出现了另外一个扩展频谱。同样的，接收扩展频谱的那方也不会注意到同时出现的AM或者FM信号。所以我们可以说，干扰信号对扩展频谱信号来说相当于透明，而扩展频谱信号对干扰信号来说也是透明的。

要想得到前文中提到的“透明度”，扩展频谱技术是指使用幅度调频或者宽带调频来调整一个已调波形，以期得到一个非常宽频的信号。例如，常规的AM信号使用10KHz带宽。考虑到扩展频谱信号在和AM信号的同一个载波频率运作，而且能量P也和它相等，但是它的带宽是1MHz。这样，在AM信号10KHz的带宽中，二次信号的能量是 Ps x (104/106) = Ps/100。因为AM信号的能量是Ps,那么干扰扩展频谱信号的噪音就要是20dB,低于AM信号。

An Essay About Spread Spectrum Communications Essay
Published: 23, March 2015

简介——INTRODUCTION
Spread spectrum is a technique whereby an already modulated signal is modulated a second time in such a way as to produce a waveform, which interferes in a barley noticeable way with any other signal operating in a same frequency band. Thus, a receiver tuned to receive a specific AM or FM broadcast would probably not notice a presence of a spread spectrum signal operating over the same frequency band. Similarly, the receiver of the spread spectrum would not notice the presence of the AM or FM signal. Thus, we say that interfering signals are transparent to spread spectrum signals and spread spectrum signals are transparent to interfering signals.

To provide the “transparency” described above the spread spectrum technique is to modulate an already modulated waveform, either using amplitude modulation or wideband frequency modulation, so as to produce a very wideband signal. For example, an ordinary AM signal utilizes a bandwidth of 10KHz. Consider that a spread spectrum signal is operating at the same carrier frequency as the AM signal and has the same power P, as the AM signal but a bandwidth of 1 MHz. Then, in the 10 KHz bandwidth of the AM signal, the power of the second signal is Ps x (104/106) = Ps/100. Since the AM signal has a power of Ps, the interfering spread spectrum signal provides noise, which is 20 dB below the AM signal. The widest application at this time is its use in military communications systems where spread spectrum serves two functions. The first is that it allows a transmitter to transmit a message to a receiver without the message being detected by a receiver for which it is not intended i.e. the transmitter is transparent to an unfriendly receiver. To achieve this transparency the spread spectrum modulation decreases the transmitted power spectral density so that it lies well below the thermal noise level of any unfriendly receiver. The second major application of spread spectrum is found, when, as a matter of fact, it turns out not to be possible to conceal the transmission. Police radars can employ spread spectrum to avoid detection by radar detector employed by drivers. In such a case the operator of an unfriendly receiver might attempt to begin transmitting an interfacing signal to block communication between transmitter and receiver. Here again spread spectrum act to reduce the effective power of the interference so that communication can process with minimal interference.

In the commercial communication field, spread spectrum has many applications, a major application being the transmission of a spread spectrum signal on the same carrier frequency as an already existing microwave signal. By communicating in the manner additional signal can be transmitted over the same band thereby increasing number of user. In additio