Hybrid Near/Far-Field Frequency-Dependent Beamforming via Joint Phase-Time Arrays

TL;DR

The paper tackles frequency-dependent, wideband beamforming for simultaneous near- and far-field users using a Joint Phase-Time Array (JPTA) with a single RF chain. It formulates a system utility maximization problem and develops a 3-step alternating optimization (AO) algorithm along with a novel unsupervised Graph Attention Network (GAT) design to jointly optimize subband allocation and analog beamformers. Numerical results show that JPTA outperforms traditional phased arrays and achieves a favorable balance between spectral efficiency and energy efficiency, with the logarithmic utility improving fairness among users. The DL approach matches AO performance with far lower online complexity, highlighting its practicality for real-time deployment in 6G-scale, NF/FF wideband systems.

Abstract

Joint phase-time arrays (JPTA) emerge as a cost-effective and energy-efficient architecture for frequency-dependent beamforming in wideband communications by utilizing both true-time delay units and phase shifters. This paper exploits the potential of JPTA to simultaneously serve multiple users in both near- and far-field regions with a single radio frequency chain. The goal is to jointly optimize JPTA-based beamforming and subband allocation to maximize overall system performance. To this end, we formulate a system utility maximization problem, including sum-rate maximization and proportional fairness as special cases. We develop a 3-step alternating optimization (AO) algorithm and an efficient deep learning (DL) method for this problem. The DL approach includes a 2-layer convolutional neural network, a 3-layer graph attention network (GAT), and a normalization module for resource and beamforming optimization. The GAT efficiently captures the interactions between resource allocation and analog beamformers. Simulation results confirm that JPTA outperforms conventional phased arrays (PA) in enhancing user rate and strikes a good balance between PA and fully-digital approach in energy efficiency. Employing a logarithmic utility function for user rates ensures greater fairness than maximizing sum-rates. Furthermore, the DL network achieves comparable performance to the AO approach, while having orders of magnitude lower computational complexity.

Figures (8)

• Figure 1: The JPTA with a single RF chain, $N_{\rm{T}}$ TTDs and $N$ PSs in a hybrid near-far field OFDM communication system.
• Figure 2: Structure of the proposed network architecture, comprising a 2-layer CNN for feature extraction, a 3-layer graph attention module and a normalization module for subband allocation and beamforming optimization.
• Figure 3: Average array gain of FD beamforming with assigned subbands in a two-user scenario.
• Figure 4: Array gain for 2-user scenario at the distance of 1 meters achieved by different approaches.
• Figure 5: CDF of user rate under different optimization goals and approaches.