Optical True Time Delay-Based Hybrid Beamforming for Limited-Feedback Millimeter-Wave Massive MIMO Systems

Optical true time delay-based (OTTD-based) beamforming can widen the instant bandwidth of phased antenna array systems dramatically. Herein, the low-cost implementation of OTTD-based, sub-connected hybrid beamforming (SC-HBF) is proposed for limited-feedback millimeter-wave massive MIMO systems. In the proposed OTTD-based SC-HBF, weight control modules (WCMs) of analog beamforming are decoupled from all subarrays and centrally deployed. The number of WCMs is less than that of subarrays, and each WCM can be reused. Meanwhile, we develop an optical codebook-based (OC-based) hybrid precoding algorithm, where the use of sub-optimal beam modes is considered. Based on the proposed OC-based hybrid precoding, we analyze the performance of the OTTD-based SC-HBF. The obtained spectral efficiency shows that the OTTD-based SC-HBF achieves near-optimal performance, although the number of WCMs is less than that of subarrays.