The Application of Distributed RIS to Massive Access MISO Systems: NOMA or OMA?

The application of distributed reconfigurable intelligent surface (RIS) to massive access multiple-input single-output (MISO) is significant to extend the communication coverage. In this paper, a novel framework is proposed in distributed RIS-aided massive access MISO systems with supporting non-orthogonal multiple access (NOMA) and orthogonal multiple access (OMA) transmissions simultaneously, where a joint active and passive beamforming scheme is designed to fully eliminate the inter-cluster interference and improve the channel gains of prioritized users, respectively. Based on the proposed framework, firstly, we derive two exact channel statistics to characterize the equivalent channel gains of prioritized and non-prioritized users, respectively. Then, by taking into account the influence of imperfect channel state information (CSI) and successive interference cancellation (SIC), the approximate expressions of outage probability and ergodic rate for all users of one cluster under MISO-NOMA and MISO-OMA transmissions are analyzed to obtain their corresponding system throughput. Moreover, by utilizing the above results, we also determine the diversity order and high slope of these users to attain more viewpoints. Finally, simulation results prove our analyses and reveal that: 1) enhancing the estimated accuracy of CSI and the ability of SIC process can remarkably enhance the system performance; 2) the performance of priority users will be significantly improved with the increase of the number of reflecting elements and Rician factor; 3) heterogeneous quality of service requirements and deployment behaviors of users are beneficial for NOMA, while homogenous settings are competitive for OMA.