A New RLL Code for Visible Light Communication System

Visible light communications (VLC) based on light-emitting diodes (LED) are receiving growing interest in academia and in industry due to its many desirable advantages: high data rate over unregulated broad spectrum, cost-effectiveness (data transmission achieved with little additional cost to illumination), security (no eavesdropping or jamming from outside intruders), and little electro-magnetic (EM) pollution. A vast variety of applications are being seriously explored for VLC, including vehicle-to-vehicle communication using headlights and smart transport systems, indoor position detection and tracking, image sensor communication, and indoor video streaming. A big technical challenge for VLC is the need to suppress severe inter-symbol interference (ISI) and to avoid flicker during the data transmission. For this, researchers are seriously evaluating various communication technologies including orthogonal frequency division multiplexing (OFDM) mechanism and forward error correction (FEC) and run-length limit (RLL) coding strategies. This is also the primary purpose of this study.Our biggest contribution presented here is the construction of a new and innovative coding framework that provides a convenient way to construct highly efficient nonlinear trellis-based codes. The resultant code serves the dual purpose of FEC and RLL control and can be efficiently decoded using a single soft-in soft-out (SISO) decoding algorithm.Our first development is a new trellis-based coding structure that is nonlinear, capable of RLL control and FEC capability, optimally decodable using linear-complexity trellis-based decoding algorithms, and demonstrating high flexibility in code lengths and cod rate. Even more exciting is that this framework allows us to control the reclusiveness of the nonlinear trellis codes. Since recursive codes are the necessary and unique building blocks for capacity-approaching (linear) turbo codes, we adopt the same parallel concatenation structure with random interleaving. With this, we further extend our nonlinear trellis coding framework to the even more powerful nonlinear turbo coding framework. Soft-iterative decoding algorithm based on BCJR algorithms is exploited, and extensive simulations are conducted. Evaluation of the proposed coding system on OFDM VLC channels demonstrate significant BER performance over the existing schemes. It should also be noted that in addition to powerful error correction, this code simultaneously provides the much needed RLL control without any rate degradation! The last feature is particularly desirable as conventional RLL codes are known to noticeably undermine the data throughput.