Performance Evaluation of Power Allocation Schemes for Non-Orthogonal Multiple Access in MIMO Visible Light Communication Links

Visible light communication (VLC) is an enabling technology which provides ubiquitous indoor wireless access but the capacity of VLC systems is severely limited by the narrow modulation bandwidth of the light emitting diode (LED) transmitters. Non-orthogonal multiple access (NOMA) is envisioned to address this challenge by circumventing the need for spectrum allocation (as in orthogonal multiple access schemes) which limits the number of users. We formulate an analytical model to evaluate the total system capacity achievable by using NOMA in a multiple-input-multiple-output (MIMO) VLC system. We analyze the total capacity of a NOMA based $2 \times 2$ MIMO-VLC system employing two different power allocation schemes, namely, gain ratio power allocation (GRPA) and normalized gain power allocation (NGDPA), for power domain superposition coding at the transmitter. We evaluate and compare the performance of these two schemes for various system parameters like system coverage, user location and number of users. The results and analyses presented herein provide critical insights into the modelling of NOMA based MIMO-VLC systems.