Delay-Phase Precoding for THz Massive MIMO with Beam Split

Abstract

Benefiting from tens of GHz bandwidth, Terahertz (THz) communications has been considered as one of the promising technologies for the future 6G wireless communications. To compensate the serious attenuation in THz band and avoid huge power consumption, massive multiple input multiple output (MIMO) with hybrid precoding is widely considered. However, the traditional phase-shifter (PS) based hybrid precoding architecture cannot cope with the effect of beam split in THz communications, which means that the path components of THz channel split into different spatial directions at different subcarrier frequencies, leading serious array gain loss. In this paper, we first point out the seriousness of beam split effect in THz massive MIMO by analyzing the array gain loss caused by the beam split effect. To compensate this array gain loss, we propose a new hybrid precoding architecture called delay-phase precoding (DPP). In the proposed DPP, a time delay (TD) network is introduced between radio- frequency chains and the traditional PS network, which converts phase-controlled analog precoding into delay-phase controlled analog precoding. When carrying out precoding, the time delays in the TD network are dedicatedly designed to generate frequency-dependent beams which are aligned with the spatial directions over the whole bandwidth. Thanks to the joint control of delay and phase, the proposed DPP can significantly alleviate the beam split effect. Simulation results reveal that the proposed DPP can generate beams with the near- optimal array gain over the whole bandwidth, and achieve the near-optimal achievable rate performance.