Improved dual frequency multiple access relay scheme for spatially correlated sources

We consider the non-cooperative multiple access relay channel, where N users transmit their information to a common receiver with the assistance of a shared relay. For N = 2 independent sources and dual-frequency operation (i.e., two separate frequency channels are available for transmission), it has been recently shown that additional diversity gain can be attained by simply switching the transmit frequency bands between users at the relay site, thus reducing the scenario to a 2times2 MIMO channel with achievable diversity order equal to 2. In this paper we propose to extend this previous work to the case where the sources are spatially correlated. Under a detect-and-forward (DF) relay approach, and using a maximum likelihood (ML) detector, we show that the performance of the aforementioned dual-frequency scheme can be enhanced by preprocessing the detected symbols and properly designing the constellation mapping at the relay site. In addition, we derive closed-form expressions for the symbol error probability of our proposed scheme. Simulation results show that our system outperforms the scheme in when the relay is close to the sources.