Application of Expurgated PPM to Indoor Visible Light Communications—Part II: Access Networks

Providing network access for multiple users in a visible light communication (VLC) system that utilizes white light emitting diodes (LED) as sources requires new networking techniques adapted to the lighting features. In this paper we introduce two multiple access techniques using expurgated PPM (EPPM) that can be implemented using LEDs and support lighting features such as dimming. Multilevel symbols are used to provide M-ary signaling for multiple users using multilevel EPPM. In the first technique, the M-ary data of each user is first encoded using an optical orthogonal code assigned to the user, and the result is fed into a EPPM encoder to generate a multilevel signal. The second multiple access method uses subsets of the EPPM constellation to apply MEPPM to the data of each user. While the first approach has a larger Hamming distance between the symbols of each user, the latter can provide higher bit-rates for users in VLC systems using bandwidth-limited LEDs. Both techniques are able to support up to 15 simultaneous users transmitting 200 Mb/s with a bit error rate of 10^-3 under normal lighting conditions.