Nonlinearity Mitigation based on Modulus Pruned Look-up Table for Multi-bit Delta-sigma 32-CAP Modulation in Underwater Visible Light Communication System

Application of light-emitting diodes (LEDs) based visible light communication (VLC) in underwater wireless communication has become a hot topic. However, on the one hand, the performance of underwater VLC (UVLC) system is limited due to power attenuation in water medium. On the other hand, increasing the driving power will induce nonlinearity from the imperfect optoelectronic components. Particularly at high signal amplitude, nonlinearity will become the dominant bottleneck. In this paper, we focus on nonlinearity mitigation method which preprocesses signal at transmitter-side. Multi-bit delta-sigma modulation combined carrier-less amplitude and phase modulation (DSM-CAP) is proposed to obtain quantized signal with lower peak-to-average power ratio (PAPR). Furthermore, as assuming the quantization noise being additive white noise is not appropriate in nonlinearity, modulus pruned weighted look-up table (MP-WLUT) is employed to modify the signal before DSM quantization to mitigate pattern dependent nonlinear distortion. Experimental results indicate that the DSM-CAP has better nonlinearity mitigating performance than normal CAP. At the data rate of about 1.69Gbp/s, utilizing MP-WLUT pre-distorted DSM-CAP an improvement of 1.44 dB Q-factor is realized compared with normal CAP without WLUT, and the size of MP-WLUT can be reduced to 62.5% of the full-size WLUT.