Application of Phase Shift in Coherent Multi-Relay MIMO Communications

In this paper we propose a new low feedback and low complexity scheme that adjusts the phases at the relays, thus allowing for the coherent combining of signals from multiple relays in a cooperative multi-input-multi-output (MIMO) system using space division multiple access (SDMA). It is assumed that the channel state information (CSI) is not fed back to the source and the relays and the case of imperfect CSI at the destination is also taken into consideration. In determining the parameters, conventional convex optimization schemes cannot be applied due to the non-convex nature of the function. A low complexity method is used to optimize the system that utilizes bisection search. Numerical calculations and BER simulations demonstrate a significant performance gain over conventional AF.