UCA-Based Sub-Connected Hybrid Precoding for MmWave Multi-User Systems

In millimeter-wave communications, full-dimension multiple-input multiple-output has been widely utilized to generate both horizontal and vertical beams by equipping uniform planar array (UPA) at the base station. However, considering the antenna radiation pattern for each sector, UPA offers nonuniform coverage in horizontal domain, where UEs on the boundary of each sector possess lower beamforming gains and hence the loss of spectral efficiency compared to the UEs at the center. In this paper, uniform cylindrical array (UCA) based hybrid precoding is proposed to improve the uniformity of beam coverage in horizontal domain and the fairness of spectral efficiency for UEs at different locations. Firstly, the UCA-based codebook is designed to generate uniform beams in horizontal domain. Next, the sector-free sub-connection transmitter architecture is designed to reduce the hardware complexity. Meanwhile, to serve multi-user transmission, the corresponding cyclic UE scheduling along with grouped digital precoding are introduced to suppress the inter-user interference. Theoretical analyses and simulation results demonstrate that the proposed scheme can improve the fairness of spectral efficiency compared to its conventional UPA counterpart.