In this paper, we experimentally demonstrate a 450-nm laser underwater optical wireless transmission system, in which a comprehensive investigation is made to show the significant performance improvement of the discrete Fourier transform spread discrete multi-tone (DFT-S DMT) as compared to the conventional quadrature amplitude modulation discrete multi-tone (QAM-DMT) modulation. DFT-S DMT outperforms QAM-DMT in terms of bit error rate (BER) performance due to low peak-to-average-power ratio (PAPR) and capability of counteracting high frequency fading in a band-limited underwater optical wireless communication (UOWC) system. In order to avoid signal-to-noise-ratio (SNR) degradation at fringe subcarriers, several zeros are padded at the edge of each block before DFT-S operation. The experimental results show the superiority of DFT-S DMT compared with QAM-DMT in different water environments, including turbidities, bubbles, and water flow. Data rates of ∼16.16 Gbps and ∼13.96 Gbps at a BER of 3.8 × 10−3 are achieved by 16-QAM DFT-S DMT and 16-QAM-DMT over a 5-m water channel, respectively, which indicates that capacity enhancement of ∼2.2 Gbps is obtained by the DFT-S DMT. Meanwhile, over 3-dB receiver sensitivity improvement can always be achieved by the DFT-S DMT at the tested underwater transmission distances. Combined with adaptive bit-power loading, 20.04 Gbps over a 5-m “clear ocean” channel transmission with a single laser diode (LD) is demonstrated. For a 35-m water link, the distance-data rate product reaches 498.4 Gbps*m. To the best of our knowledge, both the data rate and distance-data rate product are the largest among all the results reported for a single visible LD. Aimed at high-speed deep ocean applications, the studies are promising for future UOWC research.

中文翻译：

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不同水环境下UOWC系统中DFT-S DMT和QAM-DMT的综合性能比较

在本文中，我们通过实验演示了一个 450 nm 激光水下光学无线传输系统，其中进行了全面的研究，以表明与离散傅立叶变换扩展离散多音 (DFT-S DMT) 相比，离散傅立叶变换扩展离散多音 (DFT-S DMT) 的显着性能改进传统的正交幅度调制离散多音（QAM-DMT）调制。DFT-S DMT 在误码率 (BER) 性能方面优于 QAM-DMT，因为它具有低峰均功率比 (PAPR) 和抵消频带受限水下光无线通信 (UOWC) 中高频衰落的能力） 系统。为了避免边缘子载波的信噪比 (SNR) 下降，在 DFT-S 操作之前，在每个块的边缘填充了几个零。实验结果表明，DFT-S DMT在不同水环境，包括浊度、气泡、水流等方面均优于QAM-DMT。16-QAM DFT-S DMT 和 16-QAM-DMT 在 3.8 × 10−3 的 BER 下分别实现了 16.16 Gbps 和 13.96 Gbps 的数据速率，这表明容量增强通过 DFT-S DMT 获得约 2.2 Gbps 的速度。同时，在测试的水下传输距离上，DFT-S DMT 始终可以实现超过 3-dB 的接收器灵敏度改进。结合自适应位功率加载，演示了使用单个激光二极管 (LD) 在 5 米“清澈海洋”信道传输上的 20.04 Gbps。对于 35 米的水上链路，距离数据速率乘积达到 498.4 Gbps*m。据我们所知，在针对单个可见光 LD 报告的所有结果中，数据速率和距离数据速率乘积都是最大的。这些研究针对高速深海应用，对未来的 UOWC 研究很有希望。