Synchronization issues in visible light communication

In the preceding chapters, many facets of VLC have been outlined demonstrating the promising features of this emerging illumination and communication technology. It is anticipated that VLC will become a familiar facility in the office, on the road (V2V communications) and even in toys (Disney Research). The broad range of VLC applications is driving an exponential rise in the required data rate, while the semiconductor industry is keeping pace in developing light sources that could provide the necessary high data rate. The high data rate VLC paradigm opens up new opportunities for modulation methods that are being developed to meet the particular requirements of simultaneous illumination and communication. Demodulation of the signal and, hence, extraction of the transmitted information require stringent synchronization. In this chapter, we present four VLC modulation methods: on off keying (OOK), pulse position modulation (PPM), inverse pulse position modulation (IPPM), and variable pulse position modulation (VPPM), and develop expressions that describe the bit error rates (BER) for each method. We provide examples for the effect of clock time shift and jitter on the system BER performance for the inverse pulse position modulation (IPPM) method. Introduction The many different applications of VLC systems, such as V2V [1–5], short range communication for toys [6–8] and high data rate indoor implementations [7–13] have been elaborated upon in the preceding chapters. While the underlying VLC principle is simple – an electrical signal from the transmitter is converted to a modulated optical signal by the light source and carried by the illuminating light to the receiver, different modulation methods that encapsulate the information within the illumination light are possible. The common modulation methods can be divided into three families [11, 13, 14], based on a) illumination color e.g. color shift keying (CSK), b) many subcarriers e.g. discrete multitone (DMT) or orthogonal frequency division multiplexing (OFDM), and c) intensity measured in the time domain, including on off keying (OOK), pulse position modulation (PPM), inverse PPM (IPPM) and variable pulse position modulation (VPPM).