Abstract
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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> 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.
Highlights
The ongoing expansion of human activities in ocean, such as oceanographic research, offshore oil exploration, underwater oil pipelines monitoring, tactical surveillance, etc., have triggered the demand for high-speed underwater communications [1], [2]
The performance of the underwater optical wireless communication (UOWC) systems based on Discrete Fourier transform spread (DFT-S) discrete multi-tone (DMT) and quadrature amplitude modulation discrete multi-tone (QAM-DMT) is experimentally investigated in static tap water
bit error rate (BER) performance for the cases of DFT-S DMT and QAM-DMT become worse as the data rate increases under different transmission distances. 7% hard-decision forward error correction (HD-FEC) limit of 3.8 × 10−3 is drawn by the dashed line as reference
Summary
The ongoing expansion of human activities in ocean, such as oceanographic research, offshore oil exploration, underwater oil pipelines monitoring, tactical surveillance, etc., have triggered the demand for high-speed underwater communications [1], [2]. The emerging promising technology could be applied to high-speed data harvesting from seabed observatories, tetherless autonomous underwater vehicles (AUVs) or remotely operated vehicles (ROVs) control, live streaming from remote subsea cameras during operations and the like [5]. Submarine can communicate with vessels or buoys on the sea surface more securely by virtue of optical links. Buoys on the sea surface and terrestrial base stations could set up communication links to satellites or aircrafts via RF approach. The aquatic environment poses serious challenges to UOWC, such as absorption, scattering, turbulence, etc. These effects translate into large attenuation or fluctuations (scintillations) in the optical signal intensity at the receiver, which results in system performance degradation [6], [7]
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