Indexed Multiple Access with Reconfigurable Intelligent Surfaces: The Reflection Tuning Potential

Indexed modulation (IM) is an evolving technique that has become popular due to its ability of parallel data communication over distinct combinations of transmission entities. In this article, we first provide a comprehensive survey of IM-enabled multiple access (MA) techniques, emphasizing the shortcomings of existing non-indexed MA schemes. Theoretical comparisons are presented to show how the notion of indexing alleviates the limitations of non-indexed solutions. We also discuss the benefits of utilizing a reconfigurable intelligent surface (RIS) when deployed as an indexing entity. In particular, we propose an RIS-indexed multiple access (RIMA) transmission scheme that utilizes dynamic phase tuning to embed multi-user information over a single carrier. The performance of the proposed RIMA is assessed using simulation results that confirm its performance gains. The article further includes a list of relevant open technical issues and research directions.