Resource Allocation in Multi-User MIMO-OFDMA VLC Systems Over Low-Pass LED Channels

With consideration of the inherent low-pass characteristics of LEDs, this letter proposes a resource allocation strategy for multi-user multiple-input multiple-output visible light communication (MU MIMO-VLC) based on orthogonal frequency division multiple access (OFDMA). We derive theoretical expressions for the achievable rates of MU MIMO-OFDMA VLC systems and propose a three-dimension waterfilling strategy (3D-WF) that fully exploits the freedom in the spectral and spatial dimensions across all the users to maximize the rates. Both theoretical and simulation results demonstrate an approximately 10% increment in achievable rate compared to the uniform (UF) strategy. In particular, we observe a rate gap between multi-user and single-user scenarios, as the power allocation in the multi-user case exhibits significant discontinuities due to the low-pass nature of the VLC channel. Furthermore, our findings reveal that at low input power, diversity gain results in a greater rate improvement than multiplexing gain, whereas the opposite is true at high input power.