Downlink Hybrid Non-Orthogonal Multiple Access Energy Minimization Design for Ultra-Massive MIMO Near-Field Communications

Abstract

This paper proposes a hybrid non-orthogonal multiple access (NOMA) framework for tacking the energy efficiency optimization problem of serving additional users with pre-configured beams in the near-field downlink of ultra-massive multiple-input multiple-output (MIMO) systems. This proposed approach formulates a resource allocation model aimed at minimizing the total transmit power of the base station. For the conventional orthogonal multiple access (OMA) scheme, the time division multiple access (TDMA) protocol is employed, and the optimal power is solved using KKT conditions. For the Hybrid NOMA scheme, zero-force (ZF) beamforming and beam orthogonality are leveraged to simplify the problem. Simulation results for both random and deterministic user scenarios demonstrate that Hybrid NOMA significantly reduces system energy consumption compared to OMA, and the performance gain is further augmented as the antenna count grows. The superiority of Hybrid NOMA is more pronounced under high data rate requirements, and either increasing the transmit power or lowering the target rate threshold contributes to a reduction in total energy consumption.