Phase Shift Optimization for RIS Element-Based Index Modulation.

This study investigates a phase shift optimization of reconfigurable intelligent surface (RIS) element-based index modulation (IM). The RIS-based IM employs the spatial modulation (SM) technique, which utilizes active indices of RIS elements as additional bits. Therefore, it can enhance bits per channel usage (BPCU) for each user with lower modulation. However, due to the user’s mobility and RIS hardware limitation, it is challenging to determine the phase shifter of RIS elements. This study formulates a non-convex problem to decrease symbol error rate (SER) by maximizing the minimum Euclidian distance. The successive convex approximation algorithm is adopted in this study to obtain an approximate optimal solution for RIS phase shifter. The computer simulation result shows that the RIS-based IM could enhance BPCU. In addition, it has a lower SER than SM-based transmission without RIS.