Performance Analysis of Active RIS-Assisted Downlink NOMA With Transmit Antenna Selection

Active reconfigurable intelligent surfaces (RISs) overcome the severe multiplicative fading effect observed in conventional passive RIS-assisted systems by amplifying the reflected signal. This paper investigates the performance of an active RIS-assisted downlink non-orthogonal multiple access (NOMA) network with transceiver hardware impairments and imperfect successive interference cancellation (ipSIC). By employing the transmit antenna selection technique at the base station, closed-form expressions of the outage probability (OP) and ergodic capacity are derived in terms of the Meijer-G function. Regarding the former, we formulate the asymptotic OP and asymptotic ergodic capacity and obtain the diversity order of the system. Additionally, we explore higher-order modulation schemes such as rectangular quadrature amplitude modulation (QAM), cross QAM, and hexagonal QAM, which are integral for achieving increased data rates while maintaining power and bandwidth efficiency. For these modulation schemes, we derive closed-form expressions of the average symbol error rate (ASER) and asymptotic ASER using the Gaussian Chebeyshev quadrature approximation. The performance of the considered NOMA network is compared with an orthogonal multiple access technique. In addition, the system performance is compared to that with a passive RIS. We show that the diversity order of the considered network scales with the number of transmit antennas, the number of REs, and fading mitigation parameters, and the hexagonal QAM outperforms several other QAM schemes. Finally, simulation results affirm the accuracy of the derived closed-form expressions.