Performance analysis of a new transmission scheme for multi-relay channels

Cooperative diversity provides reliable communications between nodes in a network through relay nodes. In this paper, we introduce a new transmission protocol for relay fading channels. We examine the performance of the proposed protocol using both the amplify-and-forward (AF) and decode-and-forward (DF) modes. Our results prove that using this protocol, one can achieve full spatial diversity at full rate. We also show that our protocol with M relays is equivalent to a delay diversity scheme with M + 1 transmit antennas. At the receiver side, a maximum likelihood sequence detector is used to recover the transmitted symbols. Comparing our protocol with existing ones, we noted large performance degradations in all protocols when the relay is operating in the DF mode where detection errors exist. This is different from the AF mode, where diversity is always maintained and only a SNR loss is incurred (relative to the ideal case of error-free relay transmission). This, in turn, suggests that even with the large cost/complexity involved in the DF mode, the ensuing performance may be poor compared to the AF mode. Motivated by this fact, we obtain a bit-error rate upper bound for a multi-relay configuration where all relay nodes operate in the AF mode. At high signal-to-noise ratio (SNRs), this error bound is shown to be tight when compared to simulation results.