Distributed Synchronization and Beamforming in Uplink Relay Asynchronous OFDMA CoMP Networks

In this paper, we use delay-amplify-and-forward relays in the uplink direction of a coordinated multi-point network based on orthogonal frequency division multiplexing. The key novelty is that we assume delay from users to different base stations (BSs), in general, may exceed cyclic prefix (CP) length as might be the case in large cells and cannot be completely compensated by traditional timing advance strategies. Consequently, interchannel interference and intersymbol interference may arise under such assumptions. We first optimize the adjustable delays added at users, relays and BS nodes in order to reduce the effective delay spread at the receive sites, then we optimize relays´ weights and post-processing matrices at BS nodes to reduce the residual interference. For this purpose, we propose an iterative method that at each step first fixes either the relay weights or the post-processing matrices and then optimizes the signal-to-interference-plus-noise-ratio (SINR) over the other variable. Furthermore, we investigate the optimization problem of minimizing sum mean square error (SMSE) at the receive side under total relay power constraint. Simulation results show significant improvement in terms of SINR and SMSE in comparison with traditional systems in which no relay is deployed and the limited CP length is not addressed properly.