Demo: Lightweight Visible Light Communication forIndoor Positioning

Abstract

Visible light positioning (VLP) is an emerging positioning technique that utilizes indoor light sources to broadcast archer locations through visible light communication (VLC). Benefited by the densely deployed light lamps, VLP holds the promise for more accurate positioning accuracy than RF based approaches, and thus enables interesting applications such as retail navigation and shelf-level advertising in supermarkets and shopping malls. However, we observe that in existing VLP systems, receiving devices are heavily burdened in handling light flickering for VLC. In order to prevent human from being disturbed, the VLC transmitter lamps have to transmit pulses in a high rate (over 1kHz). Since the pulse rate far exceeds the cameras' sampling capability (30fps), receiver design either requires customized light sensor with cumbersome calibration on receiving signal strength [2] or processing high resolution images to leverage rolling shutter effect of the CMOS camera to decode message [1]. The heavy burden on receiving devices motivated us to try getting rid of light flickers. Since human eyes normally can not perceive changes in polarization, the idea is to modulate polarization instead of intensity for communication. In this demo, we demonstrate the VLC system in our VLP design [3] which realizes this idea. It enables lightweight VLC that is even affordable in wearables (Google Glass), without incurring hardware modification or computation off-loading. Moreover, it also makes other types of illuminating light beyond LED light (even sun light) possible to construct communication, therefore eliminates the barrier for deploying future VLC-based applications. As detailed in the full paper [3], our design was inspired by the liquid crystal display (LCD), from which we borrowed polarizer and liquid crystal. Liquid crystal has the property to change the polarization of bypassing polarized light according to the applied voltage. We use polarizer to polarize illuminating light and leverage liquid crystal to modulate bits through changing the light's polarization. The receiver can thus detect changes in polarization with a polarizing film, through which different polarizations result in different intensities. Since the SNR of such communication channel may significantly vary with the mobility of receiving device, we propose to add a dispersion filter to the transmitter. The dispersion filter can cast different frequency of lights into different polarization directions so that SNR in different receiving directions are evened. With the above techniques, VLC transmitter generates signal in low baud rate and mobile devices can perform VLC through camera directly. The communication is long in distance and lightweight enough for resource-constrained devices since the information is carried in intensity changes rather than certain image patterns in rolling shutter approaches. In this demo, we will illustrate the prototype implementation of the VLC communication system. We use incandescent lamps as illuminating light sources. The transmitter consists of a polarizing film, a liquid crystal layer, a dispersor and a control board. The receiver is demonstrated in Android smartphone with polarizing film in front of the camera. The following demo scenarios are planed to show our design (the demo space higher than 2.4m is preferred): Polarization based VLC modulation can solve the flickering problem. The modulated light provides normal illumination. When viewing the modulated light through a polarizing film, people can feel strong intensity changes. Dispersor and binary color shift keying (BCSK). Transmitter without dispersor results in bad SNR in certain receiving angles. For normal transmitters, color changes can always be observed in different angles. End-to-end VLC communication system. Smartphones with a polarizing film in front of the camera can receive and show transmitted messages from VLC transmitters. The VLC method enables accurate indoor positioning. After successfully receiving location beacons from VLC transmitters, smartphone performs location estimation.