Performance of 16-QAM demodulation in the rapid multipath environment

Land mobile communication systems must overcome the rapid changes in received signal amplitude and phase that result from multipath effects. The effects of multipath are most significant in applications where coherent demodulation is required, namely high-order QAM. In this paper, the effects of noise and multipath distortion on direct 16-QAM at microwave frequencies is described with the techniques being applicable to 64-QAM, 256-QAM and other high-order modulation types. Multipath performance is characterized herein using dynamic characteristics, for example, the change rates of power, amplitude, and phase. The distributions of these characteristics are predicted using analytical methods and then verified by direct measurement of shadowed multipath channels on a ground mobile collection platform. It is shown that the multipath effects may be overcome with the use of adaptive demodulation techniques to compensate for amplitude and phase distortion, and with the use of diversity.