Impact of Pointing Errors on the Performance of IRS-Assisted Multi-User THz-NOMA

This paper considers an intelligent reflecting surface (IRS)-assisted multi-user non-orthogonal multiple access (NOMA) system operating in the terahertz band. It aims to assess the impacts of pointing errors on the system performance under two practical IRS phase shifting schemes: partitioned IRS with discrete phase shifts under quantization errors, and IRS with random phase shifts. By taking into account the joint effects of pointing errors and phase shift quantization errors, accurate analytical expressions for the ergodic rate and outage probability are derived, which are validated via numerical and simulation results. The results demonstrate that the pointing errors have a more pronounced effect on the system performance than random phase shifts and phase shift quantization errors. Furthermore, the results reveal that although the random phase shifting scheme achieves a lower performance in contrast to the partitioned IRS, it remains a promising candidate for multi-user NOMA. This is attributed to its ability to circumvent the costly channel estimation and computational complexity that are proportional to the numbers of IRS reflecting elements and users.