Cooperation through ARQ

We consider the cooperative multiple access channel with single antenna sources and a multiple antenna destination. In order to fully exploit the degrees of freedom in this channel, multiple sources must transmit independent streams of information simultaneously. Under the standard coherent model, we argue that no half-duplex cooperation protocol can simultaneously achieve the maximum diversity and multiplexing gains of this channel. We avoid this limitation by developing a framework for cooperation through an automatic retransmission request (ARQ) mechanism. In the proposed cooperation protocol, users are only allowed to cooperate after receiving the first round of ACK/NACK signals. Our characterization of the achievable diversity-multiplexing tradeoff with the proposed protocol reveals that it attains both full-diversity and full-rate. Furthermore, our analysis reveals the asymptotic optimality of the proposed protocol, as the number of ARQ rounds grows. To establish the gain offered by the proposed protocol, we compare its tradeoff curves with that of the multiple access ARQ channel.