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 [1, 2] 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 2 × 2 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 [1, 2] when the relay is close to the sources.