Evaluation of Link Quality and Constellation Distance for RIS-Based Spatial Modulation System

In recent years, reconfigurable intelligent surface (RIS) technology is stimulating the usage on the field of wireless communication due to its low-cost hardware and powerful functionality. RIS is composed of many unit cells (reflective cells), and each one can be designed to control the phase of input electromagnetic waves. This enables the adjustment of reflected wireless signals towards the desired direction. By leveraging this feature, RIS can significantly enhance link power and capacity by concentrating reflected waves on the receiver. In addition, by controlling the phases of all unit cells, it can increases the total number of different channel patterns or channels, which can be used as the constellations of spatial modulation (SM) and improve the capacity of wireless system. The main purpose of RIS based SM is to find the appropriate set of cell phases to maximize the minimum distance of the constellation points which is an NP-complete problem. In this paper, we will propose a genetic algorithm (GA) based method to find the appropriate set of cell phases. In addition, we divide all cells into two groups for increasing link quality and generating the constellation points of SM respectively. This paper also evaluates the trade-off between link quality and GA-based constellation distance when the ratio of two groups are changed. The evaluated results show that it exists some ‘good’ ratios to make the system has appropriate constellation distance with good link quality.