Non-orthogonal Multiple Access for Semantic Communications

Multiple access is one of the primary issues for multi-user semantic communication systems. In this paper, we propose a novel pair of semantic difference (SeD) aware NOMA transceivers for downlink semantic-based image transmission, which mitigates the semantic-level interference among semantic streams. In specific, a SeD-aware superposition coding (SC) technique is proposed to suppress the semantic-level interference by coupling the semantic symbols of higher inter-feature semantic difference, which makes the interfering semantic symbols be identified and filtered out by the corresponding decoding function. The SeD-aware successive interference cancellation (SIC) technique further reduces the semantic-level interference by estimating the transmitted semantic symbols with the joint semantic and channel (JSC) autoencoder. Simulation results show that the proposed transceivers achieve comparable performance with benchmarks of OMA-aided transmission, while outperforming the benchmark of SeD-unaware NOMA transceivers in terms of the quality of reconstructed images and outperforming both benchmarks in terms of semantic transmission efficiency.