Mitigating Beam-Split Effects in Near-Field Wideband Communications: A Modular Rotatable Array Approach

Abstract

In this paper, we propose a new modular rotatable array (MRA) to mitigate the beam-split effect in near-field multi-user communications, by replacing a large number of true-time-delay (TTD) circuits with a small number of mechanically controlled rotating mechanisms to reduce the associated hardware cost and power consumption. We first consider a typical single-user system and show that by rotating the MRA to align the near-field user with the array boresight direction, the angular-domain beam split can be eliminated, while the residual range-domain beam split still persists. To characterize the beam-split effect, we define a new metric termed the “beam-split-induced effective bandwidth”, within which the analog beamformer designed at the central subcarrier can guarantee the required beamforming gain. We further analyze the impact of imperfect boresight alignment and reveal that the apparent trade-off between angular- and range-domain beam split is only a pseudo trade-off, since the effective bandwidth monotonically decreases with the residual angular mismatch. Then, we derive a closed-form expression for this effective bandwidth, which reveals that the proposed MRA achieves a significantly larger effective bandwidth than fixed-antenna arrays. Next, for the general case with multiple near-field users, we formulate an optimization problem to maximize the spectral efficiency under the power budget and rotation constraints. To solve this non-convex problem, we adopt the block coordinate descent (BCD) framework to decompose it into three subproblems, where the hybrid beamformers, array rotation angle, and subarray-wise rotation vector are optimized by fractional programming (FP), one-dimensional search, and projected gradient ascent (PGA), respectively. Finally, numerical results demonstrate the effectiveness of the proposed MRA in mitigating the wideband beam-split effect, showing a spectral efficiency comparable to that of conventional TTD circuits.