Two stage beamforming and combining scheme for FDD massive MIMO systems with multi-antenna users

This paper proposes a two-stage beamforming and combining scheme in frequency division duplex (FDD) massive multiple-input multiple-output (MIMO) systems with multi-antenna users. Specifically, the proposed scheme is a two-stage scheme, where the pre-beamforming matrix and pre-combining matrix are designed using the channel covariance matrix (CCM) in the first stage. The problem of the pre-beamforming matrix and pre-combining matrix design are formulated as an 0–1 quadratic integer programming problem. To solve this problem, it is further transformed into an 0–1 mixed linear integer programming problem. In the second stage, the sparse code multiple access and multi-user precoding and combining are adopted to mitigate the inter-user interference. Different from the previous transmission scheme using CCM, the proposed scheme consider the multi-antenna user system, and uses the CCM to design both the pre-beamforming and pre-combining matrices to sparsify the effective channel matrix, such that the overhead of pilot and feedback can be reduced. Moreover, the sparse code multiple access can help to better reduce the inter-user interference. Simulation results validate the good performance of the proposed scheme.