Novel Codebook Search Method for FDD Massive MU-MIMO Systems

Limited feedback communication systems rely on codebook schemes for informing the base station (BS) with the favorable downlink precoding weights. The shortcoming of those methods is that the codebook size increases as the number of antennas at the BS scales up and/or there is rich scattering. The exhaustive search method proposed in most of the current literature should be replaced with new codebook indexing techniques that can reduce the search complexity without jeopardizing the sum-rate performance. This paper presents a novel codebook search scheme which makes use of the spatial correlation structure in massive multi-user multi-input multi-output (MU-MIMO) systems to minimize the set complex operations needed to select the best codeword. Moreover, we derive the closed form analytical expression for the probability of indexing the whole codebook under the presented search scheme. Simulation results show that with the presented method we can achieve a considerable percentage of the sum-rate performance realized by the more complex brute force search method yet with less number of indexed codewords.