Two-Dimensional Power Allocation for Optical MIMO-OFDM Systems Over Low-Pass Channels

Multiple-input multiple-output (MIMO) combined with orthogonal frequency-division multiplexing (OFDM) technique can dramatically increase the achievable rate of visible light communication (VLC) systems. The channel in a VLC system with light-emitting diode (LED) luminaires is generally low-pass, which highly limits the achievable rate of practical VLC systems. This aspect has largely been ignored in the analysis for MIMO-OFDM VLC systems. As wide parts of the bandwidth at higher frequencies are severely attenuated, the choice of the power loading on every frequency bin has a large impact on the achieved bit rate. Thus, in this paper, we propose a two-dimensional water-filling (2D-WF) power allocation algorithm that operates both in frequency and space domains to efficiently improve the rate achieved by MIMO-OFDM VLC systems over low-pass VLC channels. The achievable rates and optimal bandwidths of a MIMO-OFDM VLC system are derived analytically using the proposed 2D-WF power allocation algorithm and five conventional power allocation strategies, including uniform (UF) power allocation, pre-emphasis (PE) power allocation, beamforming (BF) power allocation, and two kinds of one-dimensional water-filling (1D-WF) power allocation. Our simulation results show that the achievable rate of a MIMO-OFDM VLC system in a typical indoor environment can be significantly degraded by the low-pass effect. The proposed 2D-WF power allocation outperforms all other schemes in terms of achievable bit rate.