NOMA-Based Spatial Modulation

Spatial modulation (SM) has emerged as a low-complexity and energy-efficient multiple-input multiple-output transmission technique, where the information bits are not only transmitted by amplitude phase modulation but also conveyed by the index of activated transmit antenna (TA). By deploying SM in downlink multi-user (DL-MU) scenarios, conventional orthogonal multiple access-based SM (OMA-SM) allocates exclusive time-frequency resources to users, but suffers from low spectral efficiency. TA grouping-based SM (TAG-SM) divides TAs into sub-groups to serve different users independently, but suffers from severe inter-user interference. By introducing non-OMA (NOMA) into SM for DL-MU transmission, NOMA-based SM (NOMA-SM) is proposed to mitigate inter-user interference, while maintaining high spectral efficiency. Specifically, by applying successive interference cancellation at user side, the inter-user interference could be effectively eliminated with the sacrifice of increased computational complexity. Afterward, based on a symbol error rate analysis, a low-complexity power allocation scheme is provided to achieve high spectral efficiency through power domain multiplexing. When considering near-far effect from user distribution, user pairing issue is also discussed. Numerical simulation compares NOMA-SM with OMA-SM and TAG-SM, and verifies the effectiveness of the proposed low-complexity power allocation and user pairing methodologies.