Enhanced Signal Sets for Hybrid Reflection Modulation

Hybrid reflection modulation (HRM) is a promising modulation technique which employs a combination of active and passive reflecting elements in reconfigurable intelligent surfaces (RIS). In addition to applying phase tuning, the active elements apply an amplification to its incident signal, which can alleviate the double-fading effect inherent in RIS systems. The choice of the elements to be made active conveys information bits through indexing. Additional bits can be conveyed through symbols from a modulation alphabet. In this letter, we propose methods to design improved HRM signal sets that achieve better bit error performance. The idea is to schedule the activation of tiles of elements in an ordered way that picks tiles with highest effective response and ignores those with lowest effective response. Specifically, we propose a signal set design procedure that efficiently packs non-equidistant signal points on the positive real line when all active elements use the same amplification factor. We further improve this signal set by obtaining the optimum amplification factors that maximize the minimum Euclidean distance of the signal set. Simulation results show improved performance of the proposed HRM signal sets compared to the that of basic HRM signal set introduced in the recent literature.