[Paper Review] Joint Inter-path and Intra-path Multiplexing for Terahertz Widely-spaced Multi-subarray Hybrid Beamforming Systems

Longfei Yan, Yuhang Chen+2 more|arXiv (Cornell University)|Jan 20, 2021

Millimeter-Wave Propagation and Modeling4 citations

TL;DR

This paper proposes a widely-spaced multi-subarray (WSMS) hybrid beamforming architecture for terahertz (THz) communications that jointly exploits inter-path and intra-path multiplexing to achieve a k-fold increase in multiplexing gain over conventional architectures. By optimizing subarray count, spacing, and beamforming matrices via a closed-form solution and a dominant-LoS-relaxation algorithm, the WSMS system achieves 189% higher spectral efficiency and 91% higher energy efficiency than existing FC and AoSA architectures.

ABSTRACT

Terahertz (THz) communications with multi-GHz bandwidth are envisioned as a key technology for 6G systems. Ultra-massive (UM) MIMO with hybrid beamforming architectures are widely investigated to provide a high array gain to overcome the huge propagation loss. However, most of the existing hybrid beamforming architectures can only utilize the multiplexing offered by the multipath components, i.e., inter-path multiplexing, which is very limited due to the spatially sparse THz channel. In this paper, a widely-spaced multi-subarray (WSMS) hybrid beamforming architecture is proposed, which improves the multiplexing gain by exploiting a new type of intra-path multiplexing provided by the spherical-wave propagation among k widely-spaced subarrays, in addition to the inter-path multiplexing. The resulting multiplexing gain of WSMS architecture is k times of the existing architectures. To harness WSMS hybrid beamforming, a novel design problem is formulated by optimizing the number of subarrays, subarray spacing, and hybrid beamforming matrices to maximize the spectral efficiency, which is decomposed into two subproblems. An optimal closed-form solution is derived for the first hybrid beamforming subproblem, while a dominant-line-of-sight-relaxation algorithm is proposed for the second array configuration subproblem. Extensive simulation results demonstrate that the WSMS architecture and proposed algorithms substantially enhance the spectral efficiency and energy efficiency.

Motivation & Objective

• Address the limited spectral efficiency in THz hybrid beamforming due to spatially sparse channels with few resolvable multipaths.
• Overcome the constraint of conventional architectures that only exploit inter-path multiplexing, which is severely limited in sparse THz channels.
• Introduce a new intra-path multiplexing gain by leveraging spherical-wave propagation across widely-spaced subarrays.
• Formulate a joint design problem optimizing subarray count, spacing, and beamforming matrices to maximize spectral efficiency.
• Develop low-complexity algorithms—closed-form solution and dominant-LoS-relaxation algorithm—for practical implementation.

Proposed method

• Propose a widely-spaced multi-subarray (WSMS) hybrid beamforming architecture where subarrays are spaced beyond half-wavelength to enable spherical-wave propagation and intra-path multiplexing.
• Formulate a spectral efficiency maximization problem that jointly optimizes the number of subarrays (k), subarray spacing (ds), and hybrid beamforming matrices.
• Decompose the problem into two subproblems: (1) hybrid beamforming optimization with an optimal closed-form solution derived using channel structure.
• Address array configuration via a dominant-LoS-relaxation (DLR) algorithm that exploits the strong line-of-sight component typical in THz bands.
• Model hardware power consumption using device-specific power values and total power as sum of transmitter and receiver components.
• Use a logarithmic spectral efficiency model and linear power scaling to evaluate energy efficiency across different architectures.

Experimental results

Research questions

• RQ1 Can intra-path multiplexing be effectively exploited in THz hybrid beamforming to overcome the limitations of sparse inter-path multiplexing?
• RQ2 How does the number of widely-spaced subarrays (k) and their spacing (ds) jointly impact spectral efficiency in a THz WSMS system?
• RQ3 What is the optimal beamforming design that maximizes spectral efficiency under the WSMS architecture with spherical-wave propagation?
• RQ4 Can a low-complexity algorithm be developed to jointly optimize array configuration and beamforming for practical deployment?
• RQ5 How does the proposed WSMS architecture compare in spectral and energy efficiency to existing FC and AoSA hybrid beamforming architectures?

Key findings

• The WSMS architecture achieves a k-fold increase in multiplexing gain by jointly exploiting inter-path and intra-path multiplexing, where k is the number of widely-spaced subarrays.
• The proposed closed-form solution for hybrid beamforming achieves optimal spectral efficiency with significantly lower computational complexity than existing methods.
• The dominant-LoS-relaxation (DLR) algorithm for array configuration design provides near-optimal performance while maintaining low complexity.
• Simulation results show 189% higher spectral efficiency for the WSMS system with proposed algorithms compared to conventional FC and AoSA architectures.
• The WSMS system achieves 96% higher spectral efficiency than a baseline WSMS system with only k=2 subarrays, due to optimized k and ds.
• Energy efficiency is improved by 91% compared to counterpart architectures, primarily due to the high spectral efficiency gain and efficient hardware power modeling.