Gram-Schmidt based Precoding Algorithms for Multi-User Relay Communications and Optimized Power Allocation

In this paper we propose new simple precoding solutions to be used at the relay station in the context of a multi-relay wireless network where the different mobile stations belong to the same network, i.e. we don't consider the case of competing users from different networks as in some recent works based on game theory. Our proposed algorithm is based on the Gram-Schmidt Orthonormalization since our goal is to mitigate the multiuser interference at each mobile station. The strength of this method is that it only requires the knowledge of all channel impulse responses from a given relay to all the mobile stations. In other words, to compute its precoding vectors, each relay does not need to know the channel impulse responses of the other relays. Using our algorithm, we find that, when compared to the centralized reference method where each mobile station benefit from the same diversity gain, some mobile stations will improve their diversity gain whilst the other ones will suffer from a loss. This constitutes a simple solution to supply different quality of service (QoS) in the case of a multi-services network. We propose then an optimized power allocation strategy at each relay and we give complete accurate performance derivations for the different studied contexts.