On the Capacity of RIS-Assisted Intensity-Modulation Optical Channels

Using re-configurable intelligent surfaces (RIS) in optical wireless communication (OWC) systems to solve the signal obstruction and skip-zones dilemmas modifies the transmission channel. It is thus necessary to analyze the capacity of such a system. To this end, the capacity lower- and upper-bounds of RIS-assisted single-input single-output (SISO) OWC systems are discussed in this letter, focusing on the intensity-modulation and direct-detection scheme. This analysis considers two main constraints, namely peak-intensity and average optical power constraints. It also considers two types of structures: the single-layer structure (SLS) and multiple-layer structure (MLS). By exploiting the
$QR$
-decomposition, the analysis is extended to RIS-assisted multiple-input multiple-output (MIMO) OWC systems. As a result, the RIS-based cascaded channel capacities bounds and the achievable rate at a high signal-to-noise ratio are given for free-space RIS-based SISO/MIMO OWC systems. These results show that the channel exhibits a high capacity when the RIS module is closer to the data source, and that the MLS provides a higher achievable rate when compared to the SLS.