LCD-Based Optical Filtering Suitable for Non-Imaging Channel Decorrelation in VLC Applications

Abstract

A novel approach for optical channel decorrelation in visible light communication (VLC) multiple-input multiple-output (MIMO) systems is presented. Frequently, illumination fixtures employing an array of white light emitting diodes (LEDs) are suggested for simultaneous illumination and data transmission. As white LEDs share the same optical spectrum and due to diffuse light propagation, a separation of the data streams at the receiver side is difficult, even if multiple photodetectors are applied, but necessary. As opposed to software-based solutions, our approach to the interference problem works in the hardware domain: a liquid crystal display (LCD) is placed in front of the photodetectors. This LCD suppresses the interference caused by modulated light sources and by ambient light. The LCD can be configured to form dynamic receiver apertures in the MIMO setup under investigation. In order to assess the main functionality of the proposed concept, a mixed hardware/software approach is performed. In an experimental testbed, we chose a black-and-white LCD with high contrast, i.e., with high transmittance ratio. We carried out measurements of the transmittance factors in “on” and “off” states, and created a numerical model of the selected LCD. Based on this model and on ray-tracing simulations of the environment, an analytical upper bound on the bit error rate is evaluated for a given VLC indoor scenario. Spatial modulation benefits with an improvement of up to 27 dB from the proposed kind of optical filtering.