Selective DF relaying in multi-relay networks with different modulation levels

Despite the rich literature on cooperative networks, employment of different modulation levels by the source and relay terminals has not been investigated thoroughly from the physical layer perspective. In this paper, we investigate the bit error rate (BER) performance of selective relaying in a multi-relay decode-and-forward cooperative network where the source and the relays transmit using different modulation levels. Specifically, we derive a closed form expression for the end-to-end (uncoded) BER. To draw further insights on the BER performance, we also provide a simpler approximate BER expression that is accurate in the high signal-to-noise ratio regime. Finally, simulation results are presented to verify the analytical results. The derived BER expressions can be utilized in various other scenarios in which the destination selects the best signal (in terms of minimizing BER) among a set of signals which use different modulation levels. The set of signals to choose from may have already been received through orthogonal channels (selection combining), or this signal set may correspond to a set of “candidate” transmissions. The latter scenario is often referred to as selective transmission; applications of this scenario include selective relaying (the setting in this paper), fast base-station selection, and coordinated multipoint transmission and reception (CoMP).