NOMA-Enhanced Active RIS-Aided MISO ISAC System Under NTN With Hardware Impairment

In this paper, we investigate the performance of a non-orthogonal multiple access (NOMA)-enhanced active reconfigurable intelligent surface (RIS)-assisted integrated sensing and communication (ISAC) system under a non-terrestrial network (NTN) framework. The performance is analyzed in terms of outage probability, ergodic capacity, beampattern gain, and probability of detection (PoD). To this end, the end-to-end equivalent channel distribution under maximum ratio transmission beamforming is derived and modeled as a Gamma distribution. Based on this, a closed-form expression for the outage probability (OP) is obtained in terms of Meijer G-functions. The accuracy of the analytical results is validated through Monte Carlo simulations. To provide further insight, we derive the asymptotic expression of the OP and the corresponding diversity order. Additionally, an approximate expression for the ergodic capacity is derived using the method of moments, along with its upper and lower bounds. To evaluate the impact of the active RIS in mitigating the impact of multiplicative fading through amplification of the incident signal, its performance is compared against a baseline passive RIS scheme, revealing significant performance improvements. In particular, the outage probability is improved by approximately 72.94% and 73.33% for the weak user (D1) and strong user (D2), respectively, when employing an active RIS compared to a passive RIS at a transmit SNR of 22 dBm. The influence of practical impairments, including hardware impairments, imperfect channel state information, and imperfect successive interference cancellation, is also examined. The results show that increasing the number of RIS elements and the number of antennas at the satellite can effectively alleviate the adverse effects of these impairments. On the sensing side, the proposed ISAC system is compared with a conventional radar system in terms of the PoD for aerial targets located at various positions, and it additionally demonstrates beampattern gain. The results are obtained under the assumptions of ideal RIS phase configuration and line-of-sight (LoS) between the satellite and targets, representing key limitations of the analysis.