Tactical Wireless Communications and Networks: Design Concepts and Challenges

2.4 Signal Fading

Now that you have some fundamental information regarding non-coherent demodulation and know that the received signal has uncertainty in its phase (in addition to the uncertainty created by AWGN), let us visit another uncertainty factor derived from deep amplitude fading of the signal. So far, you have seen how the probability of detecting the signal correctly depends on SNR. This dependency weakens with channel fading. Error control coding techniques developed to remedy the consequences of fading are visited in the next chapter. In this section, we will look at the effect of fading without factoring in error control coding.

The channel gain A(t) is essentially a Rayleigh random process (which means that the signal power varies with time). As a result, in channels where fading is expected, one should shy away from QAM because there are multiple signal energies and the error in estimating the signal amplitude will have greater impact. PSK and FSK (frequency shift keying) are commonly used with fading channels. Also, as the signal amplitude fades, phase shifting can occur as well. This would make non-coherent detection a must for use with channels with deep fading.

Let us try to define the problem, in the sense that we need to know the average error probability calculated over a fading period. In other words, we need to know how much signal detection error we will encounter in the presence of fading. This will allow us to revisit our SNR requirements in order to ...