  The abundant optical spectrum is a promising part of the electromagnetic
spectrum for 6G communication systems. The visible light spectrum which is a
part of the optical spectrum, can be used to provide communication and
illumination simultaneously. Visible light communication (VLC) systems have
been widely researched, however, little work has focused on the area of
multiple access. This chapter studies wavelength division multiple access
(WDMA) techniques in VLC systems to support multiple users. In addition, the
optimization of resource allocation is considered in this chapter by developing
a mixed-integer linear programming (MILP) model that can be used to maximize
the signal to noise and interference ratio (SINR) while supporting multiple
users. The optimized resource allocation results in the best assignment of
access points (APs) and wavelengths to users. Different indoor environments
such as office, data center and aircraft cabins are evaluated in this chapter.
A laser diode (LD) with four wavelengths (red, green, yellow and blue) is used
to provide high bandwidth for communication and white light for illumination.
Also, an angle diversity receiver (ADR) is utilized to receive signals and
reduce noise and interference by exploiting the spatial domain.
