Several industrial indoor positioning systems utilize LEDs as beacons and cameras as sensors: The LED beacons transmit their identity, using various means of visible light communication (VLC) techniques. To avoid flickering effects, the transmission frequency is usually much higher than the sampling frequency of ordinary cameras, thus undersampling occurs. In this paper, a potential problem of undersampled protocols is highlighted: If the transmitter and receiver are not synchronized, the frequency slip between the transmitter and receiver clocks will periodically cause a burst of potential decoding errors. If the frequency slip is small (i.e., good-quality clocks are used in both the transmitter and the receiver), the time between bursts of errors is longer but at the same time the length of the bursts are also longer. An error analysis is provided as a function of protocol parameters and various error sources. Based on the results the robust-undersampled phase-shift on-off keying (UPSOOK) protocol is introduced, which guarantees the correct operation even in the presence of clock inaccuracies, as well as other error sources such as sensor noise, jitter, camera saturation, without the utilization of any error correcting codes. The properties of the proposed robust-UPSOOK protocol are demonstrated using simulations and measurements.
Keywords: UPSOOK; adaptive; autonomous vehicles; camera communication; indoor positioning; responsiveness; robustness; visible light communication (VLC).