This paper focuses on the design of transmission methods and reflection optimization for a wireless system assisted by a single or multiple reconfigurable intelligent surfaces (RISs). The existing techniques are either too complex to implement in practical systems or too inefficient to achieve high performance. To overcome the shortcomings of the existing schemes, we propose a simple but efficient approach based on \textit{opportunistic reflection} and \textit{non-orthogonal transmission}. The key idea is opportunistically selecting the best user that can reap the maximal gain from the optimally reflected signals via RIS. That is to say, only the channel state information of the best user is used for RIS reflection optimization, which can in turn lower complexity substantially. In addition, the second user is selected to superpose its signal on that of the primary user, where the benefits of non-orthogonal transmission, i.e., high system capacity and improved user fairness, are obtained. Additionally, a simplified variant exploiting random phase shifts is proposed to avoid the high overhead of RIS channel estimation.

翻译：本文聚焦于单/多个可重构智能表面（RIS）辅助无线系统的传输方法设计与反射优化。现有技术要么因过于复杂而难以在实际系统中部署，要么因效率不足而无法实现高性能。为克服现有方案的局限性，我们提出一种基于"机会反射"与"非正交传输"的简洁高效方法。其核心思想是机会性地选择能够从RIS最优反射信号中获得最大增益的最佳用户——换言之，仅利用该最佳用户的信道状态信息进行RIS反射优化，从而显著降低系统复杂度。随后，通过将次优用户信号叠加至主用户信号上，实现非正交传输的效益，即提升系统容量与用户公平性。此外，为规避RIS信道估计的过高开销，我们进一步提出采用随机相位移位的简化变体方案。