NOMA-Assisted OTFS-ISAC for Energy Efficient SAGIN

Space-air-ground integrated networks (SAGIN) are a key technology in 6G, enabling seamless global connectivity through a unified communication framework, while integrated sensing and communication (ISAC) mitigates spectrum congestion by reusing spectrum and transceivers for dual communication and sensing functions. Despite its potential, existing ISAC research has largely focused on terrestrial networks, with limited exploration in SAGIN environments. This paper proposes a novel ISAC framework for SAGIN, incorporating orthogonal time frequency space (OTFS) modulation and non-orthogonal multiple access. OTFS enhances the system's robustness in doubly-dispersive channels and supports precise communication and sensing integration. These channels experience both time dispersion caused by multipath propagation and frequency dispersion caused by Doppler shifts. In this framework, high-altitude platform stations serve as relay nodes, amplifying communication signals and processing echo signals for accurate target parameter estimation. To further enhance system performance, we design a beamforming optimization strategy using successive convex approximation to improve communication energy efficiency. Simulation results demonstrate that the proposed scheme significantly enhances the overall performance of the SAGIN system.