Performance comparisons of relaying schemes in bi-directional demodulate-and-forward channels

Bi-directional relaying (BDR) in which two users wish to exchange independent information in presence of a relay is one of the fundamental wireless network coding applications that attracted intense interests recently. In this paper, maximum likelihood (ML), cooperative maximal ratio combining (CMRC) and soft information relaying (SIR) schemes proposed to combat error propagation in wireless relay systems are applied to the BDR scenario with demodulate-and-forward protocol, which decreases system complexity and cost by removing the decoding process at the relay terminal. Simulation results for Rayleigh fading channels show that detection schemes using ML and CMRC techniques outperform the noncooperative system that involves direct transmissions between user terminals and overcome the diversity gain reduction due to error propagation for different levels of channel state information (CSI). In case of SIR based communication scenario, the diversity gain is contingent on the availability of a coefficient depending on instantaneous CSI.