Spatially-modulated space-time coding in visible light communications using 2×2 LED array

This paper demonstrates the feasibility of binary spatially-modulated space-time (SM-ST) coding in an outdoor image-sensor-based visible light communication (VLC) system. We showed that at the transmission distance where intensity modulated symbols from two different light emitting diodes (LEDs) are detected at one pixel of the image plane of the image sensor, space time (ST) coding enables to decouple the two symbols and to extend error-free transmission distance. We furthermore aim at increasing transmission rate per symbol duration when the spatial resolution is improved when the receiver approaches the LED array of the transmitter, by superimposing space shift keying (SSK) signal with the diagonal and anti-diagonal set of the 2×2 matrix onto the LED patterns. We built an experimental testbed with LEDs modulated at 1 kbps and a high-speed camera operating at 1,000 fps. Our experimental results validate that SM-ST coding achieves 30 m of the error-free transmission distance for the additional bit based on SSK, without deteriorating the reception quality and range of the space-time coded bits.