A Gain Matrix Design Method to Ensure Reciprocity in TDD MIMO Two-Hop Relay Systems

In two-hop relay systems, time division duplex (TDD) is usually adopted to support asymmetric traffic and offer channel reciprocity. When a relay is in the amplify-and-forward (AF) mode, the reciprocity is only provided within each hop. The equivalent channel between a base station (BS) and a mobile station (MS) cannot be reciprocal due to the operation at the relay. In some cases, the BS needs the whole downlink channel information to do pre-processing. However, the needed information could just be acquired through feedback in the conventional methods. In this paper, a gain matrix design method is proposed to maintain the whole link reciprocity and avoid large amount of feedback. Simulation results show that the mean square error (MSE) between whole downlink and uplink channel in proposed method is much smaller than that in the conventional gain matrix design schemes. Besides, the proposed method can maintain complete link reciprocity without performance loss.