Energy Efficiency for Multiple Far-Field Users via Hybrid NOMA-Assisted Near-Field Beamforming

Abstract

This paper investigates the utilization of preconfigured focused beams originally assigned to existing near-field users to simultaneously support multiple additional far-field users and enhance their energy efficiency (EE) within a legacy wireless network. Within this framework, we develop a spatial-domain multiple-access (SDMA)–based hybrid non-orthogonal multiple access (NOMA) strategy, where the preconfigured beams of near-field users are reused as valuable communication resources for far-field transmission. EE-maximization problems for multiple far-field users are formulated under two distinct decoding approaches. To address the inherent non-convexity of the resulting multi-ratio EE formulations, quadratic transformations are employed to convert them into equivalent linear optimization problems, thereby enabling efficient solution procedures. An iterative algorithm is subsequently developed to solve the transformed optimization problem, and its computational complexity is analytically characterized. Simulation results confirm key characteristics of the system and demonstrate the superiority of the proposed SDMA-based hybrid NOMA scheme in improving the joint EE of far-field users. Furthermore, both special and general beam-selection scenarios are analyzed to reveal the factors influencing far-field users beam-selection behaviors and to illustrate the diversity of optimal selection patterns.