  Interconnecting devices in an indoor environment using the illumination
system and white light emitting diodes (LED) requires adaptive networking
techniques that can provide network access for multiple users. Two techniques
based on multilevel signaling and optical orthogonal codes (OOC) are explored
in this paper in order to provide simultaneous multiple access in an indoor
multiuser network. Balanced incomplete block designs (BIBD) are used to
construct multilevel symbols for M-ary signaling. Using these multilevel
symbols we are able to control the optical peak to average power ratio (PAPR)
in the system, and hereby control the dimming level. In the first technique,
the M-ary data of each user is first encoded using the OOC codeword that is
assigned to that user, and then it is fed into a BIBD encoder to generate a
multilevel signal. The second multiple access method uses sub-sets of a BIBD
code to apply multilevel expurgated pulse-position modulation (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 with bandwidth-limited LEDs.
