We investigate the downlink outage performance of double-RIS-aided non-orthogonal multiple access (NOMA), where a near-BS and a near-users RISs setup are deployed. To extend the coverage to 360 degrees, we deploy a simultaneously transmitting and reflecting RIS (STAR-RIS) structure to improve communication reliability for indoor and outdoor users. New channel statistics for the end-to-end channel with Nakagami-m considering both the conventional-RIS and the STAR-RIS antenna elements features are derived using the moment-matching (MM) technique. The numerical results reveal that the double-RIS setup can outperform the single-RIS designs when the number of elements of STAR- RIS (RS) and conventional RIS (RC) is suitably adjusted. Moreover, the double-RIS setup outperforms the single-RIS design when the link between the base station and the near-user RIS is in good condition. Finally, the proposed analytical equations are accurate under different channel and system configurations.

翻译：我们研究了双RIS辅助的非正交多址接入（NOMA）下行链路中断性能，其中部署了靠近基站和靠近用户的RIS架构。为将覆盖范围扩展至360度，我们采用同时发射与反射的RIS（STAR-RIS）结构以提升室内外用户的通信可靠性。利用矩匹配（MM）技术，推导了考虑传统RIS与STAR-RIS天线单元特征、基于Nakagami-m衰落的端到端信道新统计特性。数值结果表明，当适当调整STAR-RIS（RS）与传统RIS（RC）的单元数量时，双RIS配置的性能优于单RIS设计。此外，当基站与近用户RIS之间的链路处于良好状态时，双RIS配置同样优于单RIS设计。最后，所提出的解析方程在不同信道与系统配置下均具有准确性。