Delay-Phase Precoding for Wideband THz Massive MIMO

Abstract

Benefiting from tens of GHz bandwidth, terahertz (THz) communication is\nconsidered to be a promising technology to provide ultra-high speed data rates\nfor future 6G wireless systems. To compensate for the serious propagation\nattenuation of THz signals, massive multiple-input multiple-output (MIMO) with\nhybrid precoding can be utilized to generate directional beams with high array\ngains. However, the standard hybrid precoding architecture based on\nfrequency-independent phase-shifters cannot cope with the beam split effect in\nTHz massive MIMO systems, where the directional beams will split into different\nphysical directions at different subcarrier frequencies. The beam split effect\nwill result in a serious array gain loss across the entire bandwidth, which has\nnot been well investigated in THz massive MIMO systems. In this paper, we first\nreveal and quantify the seriousness of the beam split effect in THz massive\nMIMO systems by analyzing the array gain loss it causes. Then, we propose a new\nprecoding architecture called delay-phase precoding (DPP) to mitigate this\neffect. Specifically, the proposed DPP introduces a time delay network as a new\nprecoding layer between radio-frequency chains and phase-shifters in the\nstandard hybrid precoding architecture. In this way, conventional\nphase-controlled analog beamforming can be converted into delay-phase\ncontrolled analog beamforming. Unlike frequency-independent phase shifts, the\ntime delay network introduced in the DPP can realize frequency-dependent phase\nshifts, which can be designed to generate frequency-dependent beams towards the\ntarget physical direction across the entire THz bandwidth. Due to the joint\ncontrol of delay and phase, the proposed DPP can significantly relieve the\narray gain loss caused by the beam split effect. Furthermore, we propose a\nhardware structure by using true-time-delayers to realize the concept of DPP.\n