Enhancing Spectrum Efficiency for Near-Field Communications: Applying Near-Field NOMA

Extremely large-scale multiple-input multiple-output (XL-MIMO) has been considered as a potential key technology for spectrum efficiency enhancement in 6G communications. The XL-MIMO systems introduce spherical- wave based near-field communications and a new multiple access scheme named location division multiple access (LDMA) is adopted. The near-field LDMA communications provides a novel distance dimension for enhancing spectrum efficiency. However, the spectrum efficiency enhancement in near-field LDMA communications is limited, when user are located far away from the base station and the channels become highly correlated. To solve this problem, we apply non-orthogonal multiple access (NOMA) in near-field communications to further enhance spectrum efficiency. Specifically, for two-user near-field NOMA communications, the closed-form solutions for spectrum efficiency in near-field LDMA and near-field NOMA systems and the impact of channel correlation on them are analyzed. Besides, the criteria for applying near-field NOMA is formulated. Moreover, we extend the two-user near-field NOMA to the multi-user scenarios and give a overall framework of applying NOMA in near-field multi-user communications. Finally, simulation results are provided to verify the feasibility and superiority of applying NOMA in near-field communications to enhance spectrum efficiency.