Code Design for Visible Light Communications Under Illumination Constraints

Abstract

Visible light communication (VLC) uses the same LEDs which are an efficient source of illumination to transmit information concurrently using optical direct-detection. As a result of modulating the LED to convey information, there may be a perceived change in the light perception which besides being annoying, may produce physiological consequences under prolonged exposure. The aim of this research is to propose code design methodologies for controlling the effects of light intensity flickering, brightness control, and color shifts due to the modulation, encoding information bits in organized optical symbol sequences, and improving the coding gain by the use of the Viterbi algorithm. In order to mitigate the effect of intensity flickering presented in On-Off Keying modulation, five codes are designed with two proposed algorithms using finite-state machines (FSMs) for constraining the runs of zeros or ones. The codes are compared with the codes proposed in the IEEE 802.15.7 standard on VLC (Manchester code, 4B6B code, and the 8B10B code) in terms of flicker mitigation using the perceived flicker index (PFI) (a mathematical measure of flicker introduced in this study) and error-rate performance. The designed codes show asymptotic coding gains between 1:25 and 6 dB with a low sacrifice in PFI. To avoid color shifts in color-shift keying (CSK) modulation, four codes were designed from optimally CSK constellations and two classes of codes where one class is based on FSMs and the other on trellis-coded modulation (TCM) according to the desired color perception constraint. The designed codes show asymptotic coding gains between 1:5 to 3:5 dB with respect to uncoded transmission. For brightness control, variable-weight multipulse pulse-position modulation (VW-MPPM) is introduced as an alternative for increasing the spectral efficiency by the selection of multipulse pulse-position modulation symbols of diverse weight to attain the desired dimming level. Combining VM-MPPM symbols with Huffman codes and TCM, two designed codes are compared with MPPM codes for dimming level of 0:67 and 0:40 showing an asymptotic coding gain of 0:94 and 1:29 dB, respectively. Finally, we show the trade-offs between coding gain improvement and their effects on light perception.