Hybrid Precoding with True-Time-Delay for Terahertz Near-Field Ultra-Massive Antenna Arrays

Abstract

To address the severe path loss, wideband beam squint effect, and near-field spherical wave propagation chal-lenges in Terahertz (THz) ultra-massive multiple-input multipleoutput (UM-MIMO) systems for 6G communications in this paper, we propose a true-time-delay (TTD)-assisted near-field wideband hybrid precoding scheme. To maximize the achievable system sum rate while tackling the prohibitive computational complexity of the highly non-convex problem, an efficient joint alternating optimization (AO) framework is developed. Specifically, the precoder design is decoupled into three tractable subproblems: the digital baseband (BB) precoder design is solved via a closed-form least-squares solution, the phase-shifting (PS) matrix design is optimized using manifold optimization (MO) to address the constant modulus constraint, and the discrete TTD parameter design is solved via a randomized coordinate descent (RCD) method based on an element-wise phase matching. Simulation results demonstrate that the proposed scheme effectively mitigates the beam squint effect, exhibiting fast convergence speed and superior sum-rate performance, closely approximating the fully-digital precoder compared to existing baseline architectures.