How to Enhance Spectrum Efficiency for Near-Field Communications: From LDMA to NOMA?

Extremely large-scale multiple-input multiple-output (XL-MIMO) has been viewed as a potential key technology for spectrum efficiency enhancement in 6G communications. With the spherical-wave channel model in XL-MIMO systems, the direct-link channels tend to be orthogonal as the number of antennas scales up. In this context, extra distance dimension can be utilized for multiple access and the location division multiple access (LDMA) was proposed for spectrum efficiency enhancement. However, the spectrum efficiency enhancement in near-field LDMA communications is limited when users are situated far away from the base station and the channels become highly correlated. To tackle 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, we analyse its closed-form solution for spectrum efficiency and the impact of channel correlation on its spectrum efficiency. Besides, we compare near-field NOMA with LDMA which employs zero-forcing (ZF) precoders. Furthermore, the criteria for applying near-field NOMA is formulated and the impact of near-field beamfocusing position on near-field NOMA performance is analyzed. Moreover, we extend the two-user near-field NOMA communications to the multi-user scenarios and give a overall framework of applying NOMA in near-field multi-user communications. Finally, simulation results verify the effectiveness of applying NOMA in near-field communications to enhance spectrum efficiency.