Wideband Beam Tracking Based on Beam Zooming for THz Massive MIMO

Terahertz (THz) multiple-input multiple-output (MIMO) is becoming a promising technology for future 6G network, where using beam tracking scheme to track mobile users is essential. However, existing beam tracking schemes designed for narrowband systems with the traditional hybrid precoding structure suffer from severe performance loss caused by the wideband beam split effect. To solve this problem, we propose a beam zooming based beam tracking scheme by considering the recently proposed delay-phase precoding structure. At first, we prove the beam zooming mechanism to flexibly control the angular coverage of frequency-dependent beams over the whole bandwidth, i.e., the degree of the wideband beam split effect, which is achieved by the elaborate design of time delays in the delay-phase precoding structure. Based on this mechanism, we then propose to track multiple physical directions in each time slot by generating multiple beams. The angular coverage of these beams are flexibly zoomed to match the angular variation range of user physical direction. After several time slots, the base station can obtain the new physical direction by finding out the beam with the largest received power. The proposed scheme can track multiple physical directions simultaneously with reduced training overhead, which is verified by simulation results.