Two-Dimensional Adaptive-Bandwidth Bit and Power Loading for MIMO-OFDM VLC Communications

The combination of multiple-input multiple-output (MIMO) and orthogonal frequency-division multiplexing (OFDM) can effectively increase the achievable rate and reliability of visible light communications (VLC). The low-pass characteristic in frequency domain and power diversity in spatial domain leaves the possibility to further enhance the channel capacity of the MIMO-OFDM VLC systems. In this paper, a two-dimensional water-filling (2D-WF) strategy that can fully explore the properties in both frequency and spatial domain, is investigated to maximize the achievable rate. The theoretical water-filling strategy assumes infinite granularity of the information bit, while in practice the number of bits can only be an integer and thus greedy algorithms such as Hughes-Hartogs (HH) algorithm are used for discrete bit and power loading within a fixed frequency band. Toward the practical implementation of the 2D-WF strategy, we further propose adaptive two-dimensional bit-power loading algorithms that optimally utilize the spectrum resources based on the channel state information (CSI). We analyze the energy tightness of the proposed algorithms based on the least incremental energy for the last information bit. Compared to the fixed-bandwidth algorithm, the adaptive-bandwidth bit-power loading algorithm can further enhance the achievable data rate of MIMO-OFDM VLC systems due to its capability to optimally leverage the abundant spectrum resources, which is verified through both simulations and experiments. The proposed algorithms can be directly implemented for high-speed VLC systems.