Analysis and Compensation of Spatial Correlation in Data Transmission Using RIS

Reconfigurable intelligent surfaces (RIS) are currently drawing a lot of attention in the research community as a key technology for future wireless networks. In addition to boosting the signal-to-noise ratio, they can also be used to transmit data in the same way spatial modulation (SM) transmits data by mapping it to the activated antenna indices in MIMO systems. The problem of this transmission technique which we refer to as RIS-SM is that spatial correlation between elements of the RIS array strongly degrades bit error rate performance. In this paper, we analyze this degradation and introduce two techniques to compensate for spatial correlation. The first employs tile-specific phase shifts in the RIS elements and the second employs dynamic phase shifts that are specific to the RIS patterns activated by the information bits to be transmitted. The analysis and the simulation results show that the proposed techniques provide substantial performance improvements and make RIS-SM transmission reliable even in the presence of very strong spatial correlation.