Joint Precoding and Combining Design for Hybrid Beamforming Systems With Subconnected Structure

Hybrid analog and digital structure with reduced radio frequency (RF) chains is a key enabler to reach a compromise between system complexity and performance in future fifth-generation high-frequency communications. A fully connected architecture that each antenna connects to all RF chains has been proven to achieve similar performance compared to a traditional full-digital system where the number of RF chains is equal to the number of antennas. However, such a structure is difficult for practical implementation owing to the problem of circuit layout for high-frequency antenna array structures. Therefore, a subconnected alternative where each RF chain only connects to a disjoint subset of antennas becomes more attractive. In this paper, we study the system with the subconnected subarray architecture employed by both the transmitter and the receiver, and aim to maximize the sum-rate by taking joint precoding and combining into consideration. We propose two schemes, named joint-SIC and MU-SIC, for single-user and multi-user communications, respectively. Moreover, we provide an upper bound of the sum-rate achieved by such a subconnected subarray structure. The simulation results show the fast convergence and effectiveness of the proposed schemes.