M-point Combinatory for Parallel Transmission Image Sensor-based Visible Light Communications

An M-point combinatory (MC) modulation is proposed for parallel transmission (PT) image sensor (IS)-based visible light communication (VLC). In the conventional PT system, $M_{0}$ discrete signals which incorporate $N=(M_{0}-1)/2$ spatial frequencies are transmitted in parallel with $M_{0}$ transmitting regions of liquid crystal display (LCD). When each of spatial frequencies is modulated with P-ary phase shift keying (P- PSK), the number of bits transmitted in one frame is given by $k=N\log_{2}P$ bits. $k$ increases when the number of spatial frequencies $N$ increases with increasing $M_{0}$, but at same time, the peak-to-average power ratio (PAPR) becomes high because the signal of the PT system is essentially orthogonal frequency modulation. Therefore, the number of spatial frequencies $N$ without any error remains at a very low number. In this paper, we propose an MC modulation, which is combined with the PT system to increase the number of bits per frame that the system carries without any error. In our proposed MC PT system, $M$ out of the $M_{0}$ transmitting regions are used, where $M\leq M_{0}$. Since there are $\left(\begin{array}{c}M_{0} \\ M\end{array}\right)$ ways in choosing $M$ out of the $M_{0}$ ones, we map them to $\ell=\left\lfloor\log _{2}\left(\begin{array}{c}M_{0} \\ M\end{array}\right)\right\rfloor$ bits. According to $\ell$ bits, $M$ transmitting regions are chosen for a frame, and then the $M$ chosen ones are used to transmit $M$ discrete signals of the PT system. Our experimental results show that our proposed MC PT system with 8-PSK achieves 697 bits/frame with keeping no error being observed, while the conventional PT system achieves only 180 bits/frame.