A Demodulation Method Using a Gaussian Mixture Model For Unsynchronous Optical Camera Communication With on-off Keying

We consider optical camera communication (OCC) between a camera receiver with optical lenses and distributed transmitters. This article investigates the features of an OCC system when the periods of reception and transmission are slightly different from each other. We describe a received light signal model for the OCC system with on-off keying and regard the received signals generated from a probability distribution of a Gaussian mixture model. We obtain the parameters of the probability distributions by applying a variational Bayesian inference method and utilize them for channel estimation. In addition, we define cost functions and minimize them to demodulate the transmitted bit sequences. The demodulation procedure uses a maximum-likelihood sequence detection method, which can be implemented by the Viterbi algorithm and estimates a synchronization parameter by minimizing the cost functions. Our new demodulation method requires neither synchronization devices nor training sequences for estimating the parameters. Moreover, the receiver does not need the precise transmission period, which is difficult to know in advance in practical situations because of the frequency tolerance of the clock generator in the transmitter. To validate our developed method, we conducted numerical simulations and compared the results with those from an oracle estimator that knows the parameters other than the bit sequence in advance. We also experimented in a real setup situation, and the results show the efficiency of our developed method.