Exposure Synchronization in Optical Camera Communications for Time Division Multiplexing

Abstract

Optical camera communication (OCC) is a type of visible light communication that employs a camera as the receiver. Various flicker-free OCC methods using a general-purpose low-frame rate camera have been proposed. In particular, the undersampling method uses a short exposure camera and a light-emitting diode that blinks faster than the camera's frame rate. Despite the short exposure time, the transmitter has to send a symbol for a frame period or longer because the relationship between the symbol timing of the transmitter and exposure timing of the receiver is indeterminate. We propose resolving this indeterminacy by synchronizing the exposure timing to the transmitter. In the proposed method, the transmitter sends multiple data channels in a frame period by time division multiplexing, and the receiver selects a channel. Then, a single symbol of multilevel phase shift keying is extracted from a single pixel value in a frame. Experiments confirmed that the exposure timing could be synchronized even with a degraded signal–noise ratio and that the receiver could select the desired channel. The proposed method achieved a bit error rate of $10^{-6}$ at a signal–noise ratio of 46.2 dB under various modulation schemes.

Keywords

• Coherent demodulation
• non-flickering communications
• optical camera communication (OCC)
• synchronization
• visible light communications (VLC)