  Optical wireless communication (OWC) systems are a promising communication
technology that can provide high data rates into the tens of Tb/s and can
support multiple users at the same time. This paper investigates the optimum
allocation of resources in wavelength division multiple access (WDMA) OWC
systems to support multiple users. A mixed-integer linear programming (MILP)
model is developed to optimise the resource allocation. Two types of receivers
are examined, an angle diversity receiver (ADR) and an imaging receiver (ImR).
The ImR can support high data rates up to 14 Gbps for each user with a higher
signal to interference and noise ratio (SINR). The ImR receiver provides a
better result compared to the ADR in term of channel bandwidth, SINR and data
rate. Given the highly directional nature of light, the space dimension can be
exploited to enable the co-existence of multiple, spatially separated, links
and thus aggregate data rates into the Tb/s. We have considered a visible light
communication (VLC) setting with four wavelengths per access point (red, green,
yellow and blue). In the infrared spectrum, commercial sources exist that can
support up to 100 wavelengths, significantly increasing the system aggregate
capacity. Other orthogonal domains can be exploited to lead to higher
capacities in these future systems in 6G and beyond
