Modulation Precoding for MISO Visible Light Communications

Indoor Visible Light Communication is recognized as a viable and smart solution to provide both illumination and wireless connectivity by conveniently modulating optical signals generated by Light Emitting Diodes. When dealing with multiple independent sources, the communication reliability is tied to the capability of separating signals. The accomplishment of such task becomes very challenging especially in Multiple-Input Single-Output scenarios where the effectiveness of spatial equalization is limited by the presence of a unique photodetector equipping the receiver, and channels spatial correlation may severely impact on the communication performance. In this regard, we propose a modulation precoding scheme that allows the minimum distance between received symbols to be maximized under explicit lighting constraints, thus simultaneously addressing communication and illumination issues as demanded by VLC systems. Following this approach, a twofold result is achieved. By operating adaptive power allocation on LEDs, we realize a spatial pre-filtering that counterbalances the co-channel interference caused by channels spatial correlation. Furthermore, constellation optimization is performed so that the symbol error probability is kept below $10^{-3}$ even in strong interference scenarios.