Quotient coding for fading channels

Abstract

Multiplicative Rayleigh fading is a frequent problem in wireless communications. If the channel is relatively benign and fading is not severe, one may obtain higher bit rates for an equivalent bandwidth by using M-ary QAM modulation (MQAM). A variation, used to combat channel fading while still retaining MQAM, is differential MQAM (i.e., DQAM). The term differential refers to the phase which is coded exactly as in DPSK, however, the amplitude is still subject to distortion by the fading channel. In this paper, we propose a technique called quotient coding, which is designed to remove channel effects from the symbol amplitude as well as its phase. In particular, we shall apply it to MQAM resulting in modulation which we term QQAM. In contrast to DQAM, QQAM is just as effective at suppressing the effects of channel fading with respect to the entire symbol as DPSK is for the phase alone. In fact, the scaling of the amplitude at the receiver is entirely irrelevant to QQAM.