On the optimality of multiple photodiode receivers using precoding schemes for visible light communications

The use of multiple light sources for visible light communications (VLC) naturally configures a multi-user multiple-input multiple-output (MU-MIMO) channel. Several transmit precoding (TPC) schemes based on Channel State Information (CSI) have been proposed for increasing the achievable rates in multi-user scenarios regarding the orthogonal resource allocation schemes. However, the particular characteristics of the light transmission leads to more likely correlated channel responses among the users. In this work, we propose the use of photodetectors equipped with multiple photodiodes able to provide distinct linearly independent channel responses by varying the architecture, i.e., orientation, filtering or shaping connected to a single signal processing chain. This approach allow to generate a photodiode selection scheme that obtains an uncorrelated set of channel responses improving considerably the performance of the TPC schemes in VLC systems. Moreover, since only one of the photodiodes of the photodetector is selected in each symbol time, the proposed approach involves a energy consumption reduction regarding traditional multi-photodiode architectures.