Channel estimation for OFDM-based underwater wireless optical communication systems

Underwater wireless optical communications (UWOC) could transmit data using blue or green light beams with high data rate and safety in a relatively short range. OFDM based UWOC systems are able to further increase data rate, however, highly dependent on the accuracy of channel estimation. In this paper, we consider the channel estimation problem for OFDM based UWOC systems. We firstly apply Monte Carlo simulation to obtain the channel impulse response (CIR) of UWOC links under different conditions to facilitate the design of the subsequent OFDM systems. Secondly, we evaluate the pilot-based least squares (LS), and two types discrete Fourier transform (DFT) channel estimation methods and compare their performance. Numerical results have suggested that the temporal pulse spread strongly degrades the performance of the channel estimation. These two DFT methods especially DFT channel estimation with noise threshold method achieved the best performance among these prior works. While for the signal-to-noise ratio (SNR) less than 10 dB, the performance of DFT with noise threshold method is still poor. To solve this problem, we propose a new channel estimation approach of DFT with adaptive noise threshold (DFT-ANT) which adaptively adjusts the noise threshold based on SNR, and analyze its complexity and normalized mean square error (NMSE) performance in underwater environment. Numerical results have validated the proposed approach which outperforms existing channel estimation methods especial DFT with noise threshold method in terms of accuracy for various water types.