Exploiting non-orthogonal multiple access in integrated sensing and communications

This chapter explores the employment of non‐orthogonal multiple access (NOMA) for developing the novel concept of integrated sensing and communications (ISAC), i.e., harmoniously accommodating the sensing and communication functionalities from a multiple access perspective. It first reveals that the developing trend of ISAC is from orthogonality to non‐orthogonality. Then, the fundamental downlink and uplink ISAC models are introduced, and the design challenges caused by the inter‐functionality interference are identified. For each ISAC model, novel NOMA designs are discussed, namely NOMA‐empowered downlink ISAC and semi‐NOMA‐based uplink ISAC. As a further advance, a case study of NOMA‐empowered downlink ISAC is provided. An ISAC base station serves multiple communication users employing NOMA, while the superimposed NOMA communication signal is simultaneously exploited for target sensing. An efficient double‐layer penalty‐based algorithm is proposed for the ISAC beamforming design, with the aim of maximizing the weighted sum of the communication throughput and the effective sensing power. Numerical results show that the proposed NOMA‐empowered downlink ISAC outperforms the conventional ISAC in the underloaded regime experiencing highly correlated channels and in the overloaded regime.