A Low-Jitter Fully-Digital M-CAP Receiver for Visible Light Communications

Abstract

[EN] In recent years, there has been extensive research in carrier-free phase and amplitude (CAP) modulation for optical communications, especially in the field of visible light communications (VLC). While the majority of the published works have been focused on characterizing its performance in optical channels, both theoretically and experimentally, only a limited number of publications have presented practical solutions to solve signal synchronization and demodulation, probably because experiments are usually conducted with externally synchronized equipment. The interest in CAP modulation comes from its lower complexity compared to quadrature amplitude modulation (QAM), but this advantage comes at the expense of a worse performance with time jitter. This paper presents a low-complexity, low-jitter, fully-digital CAP receiver with synchronization that improves CAP demodulation performance. Its enhanced behavior with time jitter is achieved by preprocessing the input of the timing error detector (TED) without changing the rest of the stages of standard CAP processing. That is, the receiver preserves the main advantage of CAP: it does not require frequency synchronization as neither the transmitter nor the receiver uses oscillators for passband up and down conversion. The proposed receiver operates with 4 samples per symbol and comprises only three stages: a matched filter, a time synchronizer, and a blind equalizer.