Can Far-Field Beam Training Be Deployed for Cross-Field Beam Alignment in Terahertz UM-MIMO Communications?

Ultra-massive multiple-input multiple-output (UM-MIMO) is the enabler of Terahertz (THz) communications in next-generation wireless networks. In THz UM-MIMO systems, a new paradigm of cross-field communications spanning from near-field to far-field is emerging, since the near-field range expands with higher frequencies and larger array apertures. Precise beam alignment in cross-field is critical but challenging. Specifically, unlike far-field beams that rely only on the angle domain, the incorporation of dual-domain (angle and distance) training significantly increases overhead. A natural question arises of whether far-field beam training can be deployed for cross-field beam alignment. In this paper, this question is answered, by demonstrating that the far-field training enables sufficient signal-to-noise ratio (SNR) in both far- and near-field scenarios, while exciting all channel dimensions. Based on that, we propose a subarray-coordinated hierarchical (SCH) training with greatly reduced overhead. To further obtain high-precision beam designs, we propose a two-phase angle and distance beam estimator (TPBE). Extensive simulations demonstrate the effectiveness of the proposed methods. Compared to near-field exhaustive search, the SCH possesses 0.2% training overhead. The TPBE achieves 0.16 degrees and 0.01 meters estimation root-mean-squared errors for angle and distance. Furthermore, with the estimated beam directions, a near-optimal SNR with 0.3 dB deviation is attained after beam alignment.

I. Introduction

The Terahertz (THz) bands, ranging from 0.1 THz to 10 THz, could provide an abundance of bandwidth and are promising for enabling wireless links with extreme capacity for six-generation (6G) communications and beyond [2], [3]. At THz frequencies, there exists noticeable signal attenuation, caused by spreading, molecular absorption, and diffusely scattering losses [4]. Fortunately, the sub-millimeter wavelength allows the incorporation of hundreds and even thousands of antenna elements into one antenna array, leading to ultra-massive multiple-input multiple-output (UM-MIMO) systems. These systems could assist in establishing highly directional links that efficiently combat the propagation losses in the THz band. Furthermore, with beam steering, THz UM-MIMO systems can reach Tbps data rates and enable massive connectivity [5].