Analysis of differentially modulated cooperative communications over asymmetric fading channels

Differential modulation has largely re-attracted the attention of academia and industry due to its advantages relating to simple implementation and no need for knowledge of channel state information. The present work analyzes the average bit error rate performance of dual-hop cooperative systems over generalized multipath fading conditions. The considered system is differentially modulated and is assumed to operate based on the amplify-and-forward relaying protocol. Therefore, the main advantage of the considered set up is that it does not require any channel state information neither at the relay nor at the destination nodes. Novel closed-form expressions are derived for the end-to-end error rate under asymmetric generalized multipath fading conditions, which are encountered in realistic wireless communication scenarios. These expressions are subsequently employed in quantifying the effect of generalized fading conditions on the achieved bit error rate performance. It is shown that the impact of multipath fading and shadowing effects is detrimental at both high and low signal-to-noise ratio regimes as the corresponding deviations are often close to an order of magnitude. The incurred difference is also significantly different than the conventional Rayleigh fading conditions, which verifies that accurate channel characterization is of paramount importance in the effective design of conventional and emerging wireless technologies. In addition, it indicates that differential modulation can be a suitable modulation scheme for relay systems, under certain conditions, since it can provide adequate performance at a reduced implementation complexity.