Subarray-Based Coordinated Beamforming Training for mmWave and Sub-THz Communications

Millimeter-wave (mmWave) and sub-Terahertz (THz) communications are compelling as an enabler for next-generation wireless networks. In this paper, we study mmWave and sub-THz systems with array-of-subarray architecture. To accommodate the ultrabroad bandwidth in the mmWave and sub-THz bands, time-delay phase shifters are introduced in system design. Our goal is to investigate beamforming training with hybrid processing to extract the dominant channel information, which would fully exploit channel characteristics while respecting the nature of circuit hardware. In particular, codebooks based on time-delay phase shifters are defined and structured. Then, two multi-resolution time-delay codebooks are designed through subarray coordination. One is built on adaptation of physical beam directions, and the other relies on dynamic approximation of beam patterns. Also, a low-complexity system implementation with modifications on the time-delay codebooks is studied. Furthermore, based on the proposed codebooks, a hierarchical beamforming training strategy with reduced overhead is developed to enable simultaneous training for multiple users. Simulation results show that the proposed multi-resolution time-delay codebooks could provide sufficient beam gains and are robust over large bandwidth. Also, the effectiveness of the hierarchical beamforming training is verified.

I. Introduction

Recent years have witnessed a phenomenal development of wireless services along with a change of data consumption in a larger, faster, and denser manner. In the meantime, an irreconcilable contradiction arises between quality-of-service (QoS) demands and spectrum supplies. Against this backdrop, next-generation wireless systems are pushing new higher frequency bands for larger bandwidth delivery. In particular, communications over the millimeter-wave (mmWave) band, from 30 GHz to 300 GHz, and the sub-Terahertz (THz) band, from 0.1 THz to 1 THz, emerge as key enabling technologies [1]–[7].