  In multi-user optical wireless communication (OWC) systems interference
between users and cells can significantly affect the quality of OWC links.
Thus, in this paper, a mixed-integer linear programming (MILP) model is
developed to establish the optimum resource allocation in wavelength division
multiple access (WDMA) optical wireless systems. Consideration is given to the
optimum allocation of wavelengths and access points (APs) to each user to
support multiple users in an environment where Micro, Pico and Atto Cells
co-exist for downlink communication. The high directionality of light rays in
small cells, such as Pico and Atto cells, can offer a very high signal to noise
and interference ratio (SINR) at high data rates. Consideration is given in
this work to visible light communication links which utilise four wavelengths
per access point (red, green, yellow and blue) for Pico and Atto cells systems,
while the Micro cell system uses an infrared (IR) transmitter. Two 10-user
scenarios are considered in this work. All users in both scenarios achieve a
high optical channel bandwidth beyond 7.8 GHz. In addition, all users in the
two scenarios achieve high SINR beyond the threshold (15.6 dB) needed for 10-9
on off keying (OOK) bit error rate at a data rate of 7.1 Gbps.
