On the Performance of Sub-THz/THz NOMA Systems Considering Practical Impairments

The integration of non-orthogonal multiple access (NOMA) with sub-Terahertz and Terahertz (sub-THz/THz) wireless communications can be a key enabler for future 6G networks, offering enhanced spectral efficiency and massive connectivity in highly directional and bandwidth-rich frequency bands. In this work, we investigate the performance of a power-domain downlink terrestrial-to-UAV NOMA system under realistic propagation conditions and hardware impairments. We develop a comprehensive analytical framework to evaluate the outage probability (OP) and average bit error probability (ABEP) of the considered system operating over multi-cluster fluctuating two-ray (MFTR) fading channels, which provide a flexible and physically motivated model for small-scale fading in sub-THz and THz propagation environments. The proposed model jointly accounts for frequency-dependent path loss with molecular absorption, pointing error-induced beam misalignment, and imperfect successive interference cancellation (SIC), while supporting practical square M-QAM modulation. An exact analytically tractable expression is derived for OP and a high-signal-to-noise ratio (SNR) approximation is obtained for ABEP, along with asymptotic results in the high-SNR regime that provide insight into the system's coding gain and diversity behavior. The analytical results reveal two distinct operating regimes. Under strong-interference conditions, persistent multiuser interference leads to irreducible ABEP floors, whereas in mild-interference scenarios, the ABEP decays with the average SNR according to a power-law behavior, with performance limited by the dominant physical impairment, either multipath fading or beam misalignment. Monte Carlo simulations validate the analysis and highlight key design trade-offs involving power allocation, SIC imperfection, modulation order, beamwidth, carrier frequency, and channel richness.