POLI: Long-Range Visible Light Communications Using Polarized Light Intensity Modulation

Abstract

Recent studies have demonstrated the potential of light-to-camera communications for realizing applications such as localization, augmented reality and vehicle-to-vehicle communications. However, the fundamental requirement of visible light communications, flicker-free illumination, becomes a key limitation hindering existing technologies from serving a camera at larger distances. To break this limitation, this paper presents POLI, a light-to-camera communication system that exploits a novel POlarized Light Intensity modulation scheme to provide reliable communications for a wide range of distances. The key idea of POLI is to hide the information with the polarization direction of the light, to which human eyes are insensitive. Taking advantage of this, POLI can change the intensity of the polarized light as slowly as possible, at a rate determined by the range the system would support, but does not generate flickers. By using an optical component to "transform polarization directions to colors", POLI allows a camera to leverage its received RGB values as the three spatial dimensions to recover the information carried in different data streams. POLI further incorporates a number of designs to tackle the non-linearity effect of a camera, which is especially critical for an intensity-based modulation scheme. We implemented a prototype using the USRP N200 combined with off-the-shelf optical components. The experimental results show that POLI delivers to its camera receiver a throughput proportional to the dynamic channel conditions. The achievable throughput can be up to 71 bytes per second at short distances, while the service range can be up to 40 meters.