Exploiting Polarization and Color to Enable MIMO Backscattering with Light

Abstract

Multiple-input multiple-output (MIMO) methods play a pivotal role in increasing the capacity of wireless communication systems, but they have not been analyzed systematically in the nascent area of passive communication with visible light (passive-VLC). The main challenge in passive-VLC is its low data rate. This limitation is caused by the slow switching speed of the most popular modulator used in the state-of-the-art: liquid crystal cells (LCs). Several studies use sophisticated modulation schemes with multiple LCs to increase the data rate. However, these efforts have only led to logarithmic improvements. A transmitter with a single LC can provide 1 kbps, and a transmitter with 64 LCs delivers 8 kbps: resulting in an efficiency of 125 bps per LC cell. Ideally, the capacity should increase linearly with the number of LCs.

We propose a general framework to achieve reliable MIMO communications with passive-VLC. Our approach, which has a theoretical and empirical foundation, has three desirable properties: (i) does not assume orthogonality of the individual channels (overcomes co-channel interference), (ii) can exploit multiple properties of light (polarization and color); and (iii) is agnostic to LC parameters (which some studies rely on). Our results show that a transmitter with 9 LCs increases its capacity almost linearly up to 9 channels, attaining 6.8 kbps (750 bps per LC) using the simplest modulation method in the SoA.