Multi-level Optical On-Off Line Code for Flat-like Power Spectral Density: Outdoor Visible Light Communications

This paper suggests a multi-level optical line code using an On-Off technique that enhances the performance of optical communication systems with flat-like power spectral density (PSD) such as outdoor visible light communications. The proposed technique based on a controlled multilevel coding and a multistage decoding of M = 2^l modulation signals, consists of designing the code using a balance distance rule that maximizes the minimum Euclidean distance and minimizes the number of nearest neighbours, with the aim to achieve a better system capacity and error protection. Through a controlled correlative level technique by an optical orthogonal code (OOC) time sequence, each obtained symbol is assigned an elementary signal within the same symbol duration using the conventional Miller coding. The constraints of an OOC sequence add a coding complexity that is dealt with using a synchronized error technique, where the error is recovered through an adapted time diversity decoding technique during the multi-stage decoding (MSD). The resulting M asymmetric signals are differentiated by l time transition levels and M = 2^l 1-dimension space distances to produce Mary On-Off Keying (OOK) signals. These signals are transmitted in accordance with the correlation properties of the OOC sequence, hence OOK OOC. An analytical investigation conducted on an 8-OOK OOC, compared with an amplitude shift keying (8-ASK) of the classical Ungerboeck (UG) and the unequal error protection (UEP) methods, has shown an improved level of signal-to-noise ratio (SNR), and consequently a better possible data rate.