An Overlapped Subarray Structure in Hybrid Millimeter-Wave Multi-User MIMO System

Hybrid beamforming architecture is a practical implementation in millimeter-wave (mmWave) communication systems with large-scale antenna arrays for future fifth-generation (5G) cellular networks, which offers a compromise between hardware complexity and system performance. Fully-connected and sub-connected are two popular connected structures in the hybrid beamforming architecture. However, the hardware complexity and the required number of phase shifters in the fully-connected structure is rather high when the number of RF chains increases. Sub-connected structure can effectively decrease the required number of phase shifters but at the expense of beamforming gain loss of each RF chain. In this paper, we consider an overlapped subarray structure between fully-connected and sub-connected structures and determine how many antenna elements should be connected for each RF chain. For the mmWave downlink multi-user multiple-input multiple-out (MIMO) communication, we design a two-stage hybrid beamforming algorithm based on the overlapped subarray structure. Simulation results indicate that there exists a best cost-effective overlapped subarray spacing associated with the required number of phase shifters for the sparse mmWave channel, which provides a guideline for the design of hybrid beamforming architecture.