Asymmetric Spatial Modulation for Multiple Access in Near-Field MIMO Systems
This paper investigates asymmetric spatial modulation (ASM) for multi-user multiple-input multiple-output (MIMO) systems operating in the near-field (NF) region. Considering spherical-wave propagation, closed-form expressions for the average bit error probability (BEP) of both users are derived under maximum likelihood (ML) detection. The analytical framework is validated through Monte Carlo simulations using five million transmitted symbols. Numerical results compare the near-field (NF) and far-field (FF) channel models. When users are located in the NF region, for example at a distance of 0.2 m from the array, the NF model achieves lower BER across the entire signal-to-noise ratio range. At an SNR of 20 dB, the BER gap between NF and FF increases from approximately 1.65e-3 for 16 transmit antennas to about $\mathbf{5. 0 1 e}-\mathbf{3}$ for 32 transmit antennas. The proposed ASM scheme also provides comparable BEP performance for the two users while enabling simultaneous transmission within the same MIMO resource block. These results show that near-field spatial characteristics can be effectively exploited for multi-user transmission in large-scale and high-frequency MIMO systems.