LD-NOMA: Multiple Access for Mixed Near-Field and Far-Field Communications
Different from existing works that mostly considered multiple access in either the far-field or near-field, we consider in this paper a new and practical scenario, and propose the concept of location division-assisted non-orthogonal multiple access (LD-NOMA) in mixed-field (near-field and far-field) communications to suppress multi-user interference and further enhance system spectrum efficiency. Specifically, we formulate an optimization problem to maximize the spectrum efficiency for all users under a total transmit power constraint by optimizing user clustering, hybrid precoding design, and power allocation. To gain useful insights, we first consider a two-user mixed-field NOMA system, deriving a closed-form solution for spectrum efficiency and analyzing the impact of channel correlation on it, while establishing the criteria for applying mixed-field LD-NOMA. Interestingly, we find that the coverage area of NOMA clusters is significantly expanded compared to that in previous near-field NOMA systems, due to the energy-spread effect in mixed-field scenarios. Furthermore, we extend the two-user mixed-field LD-NOMA to a multi-user scenario, develop a location/correlation-based user clustering algorithm, and present an overall framework for the deployment of LD-NOMA in multi-user mixed-field communications. Finally, simulation results validate the effectiveness of the proposed LD-NOMA in mixed-field communications.