Reconfigurable Intelligent Surface Enabled Spatial Modulation for Visible Light Communications

An important issue in visible light communications (VLC) is how to boost up the system throughput even when the line-of-sight (LOS) link is blocked by obstacles. In this paper, we propose reconfigurable intelligent surface (RIS) based spatial modulation (SM) designs for VLC, which enable RIS to modulate its own message on the reflected signals. Compared with conventional RIS-assisted VLC systems, RIS enabled SM VLC can further increase the overall system throughput. To enable RIS based SM for VLC, we first study two straight-forward methods by exploiting the reconfigurability of RIS whose elements are divided into groups, namely RIS enabled SM (RIS-SM) and RIS enabled generalized SM (RIS-GSM), which activate one and multiple groups, respectively. Furthermore, we find that the bit error rates (BER) performance of RIS-SM and RIS-GSM are highly affected by channel correlation. That is, there is a trade-off between the channel correlation coefficients and received signal power. To solve this problem in RIS-SM and RIS-GSM where only part of the RIS elements are active, we further propose RIS enabled receive SM (RIS-RSM), where all the RIS elements can be exploited to reflect signals. In the three proposed designs, the reflected light signals received at the destination contain two types of information streams: one is spatially modulated at the RIS node and the other is from the light source, and the two signal streams are decoded by the receiver in a joint manner using maximum-likelihood detections (MLD). We then derive the corresponding upper bounds according to BER metric for the three schemes, and simulation results verify the tightness of our analytical upper bounds. Interestingly, we find that blockage on the RIS link may have different effects for RIS-SM, RIS-GSM and RIS-RSM. Without blockage, it is shown that RIS-RSM always outperforms RIS-SM and RIS-GSM, and the SNR gain can be 30 dB when BER = 1e-6, showing the advantage of RIS-RSM for future VLC applications.