Two-User Relay Transmission with Shorter Delay

In this paper, a cooperative system with two users and one relay is considered. An amplify-and-forward relaying protocol is applied in a half duplex mode. To make the transmission delay as short as possible, the relay forwards for both users simultaneously. We consider transmission schemes with symbol rates 2/3 to demonstrate the tradeoff among performance, delay, and symbol rate. Based on the analysis for the pairwise error probability (PEP), an optimal precoder is proposed to achieve the full diversity and best coding gain. Simulations show that the proposed precoder can provide about one dB improvement over some conventional space-time block codes at bit error rate 10^-4.