90-m/660-Mbps Underwater Wireless Optical Communication Enabled by Interleaved Single-Carrier FDM Scheme Combined With Sparse Weight-Initiated DNN Equalizer
Zihao Du, Wenmin Ge, Chengye Cai, Haipeng Wang, Guangbin Song, Jianmin Xiong, Yanlong Li, Zejun Zhang, Jing Xu
Abstract
In this paper, we first utilize the interleaved single-carrier frequency division multiplexing (I-SC-FDM) scheme with a novel sparse weight-initiated deep neural network (SWI-DNN) equalizer for underwater optical communication (UWOC) systems. In addition to the superior characteristics of I-SC-FDM over the original OFDM and localized SC-FDM (L-SC-FDM), such as lower peak-to-average power ratio (PAPR) and lower computational complexity, the implementation of a special sparse weight-initiated (SWI) structure can significantly reduce the necessary training epochs up to 10.3%, which enables the proposed SWI-DNN equalizer to outperform the traditional random weight-initiated DNN equalizer. Besides, further pruning operation can dramatically enhance the final sparsity of the SWI-DNN equalizer to 96.88% at the expense of an inappreciable performance penalty, thereby effectively saving occupied computing resources. Following this, a data rate of up to 660 Mbps over a 90-m underwater transmission can be achieved in a standard 50-m swimming pool, with a relatively low received optical power of −31.12 dBm (at the BER of 3.8 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> ). Such a data rate is 17.9% higher than that obtained with a common hybrid time-frequency domain (TFD) equalizer. Moreover, the computational complexity of the SWI-DNN equalizer after pruning operation is only 11.83% of that of the hybrid TFD equalizer and can still support a 625-Mbps transmission. This is the first time to employ I-SC-FDM combined with an SWI-DNN equalizer for long-distance high-speed UWOC transmission, and it can be highly beneficial to the cost-sensitive and power-sensitive systems in future deployment.