Abstract
We experimentally demonstrated high-speed diffuse line-of-sight optical wireless communication across a wavy water-air-interface. The testbed channel was evaluated, in terms of data rate, coverage and robustness to the dynamic wave movement, based on the performance of different modulation schemes, including non-return-to-zero on-off keying (NRZ-OOK) and quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM). Under the emulated calm water condition, 8-QAM-OFDM offers a data rate of 111.4 Mbit/s at the aligned position, while only 55 Mbit/s is achieved using NRZ-OOK. On the other hand, effective communication can still be maintained at a high data rate of 11 Mbit/s when the photodetector is off aligned laterally by 5 cm based on NRZ-OOK modulation, leading to a coverage of ~79 cm2. By utilizing OFDM modulation scheme, a data rate of 30 Mbit/s can be achieved up to 2.5-cm misalignment, leading to a coverage of ~20 cm2. Furthermore, in the presence of strong waves (15-mm wave height, causing a scintillation index of 0.667), 4-QAM-OFDM modulation showed a better resilience to channel instability than NRZ-OOK modulation. Our studies pave the way for the eventual realization of communication across a challenging water-air interface without the need for an interface relay, which is much sought-after for implementing a robust and large-coverage underwater-to-terrestrial internet-of-things.
Highlights
Owing to growing underwater activities [1], underwater wireless communication is of great interest to the military, industry, and scientific communities
To investigate the optimum modulation scheme for across-medium communication in the presence of waves, we studied the performance of 4-quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM)
By analyzing the obtained results, we summarized the highest data rate achieved for calm water, weak waves, and strong waves for non-return-to-zero on-off keying (NRZ-OOK), 4-QAMOFDM, and 8-QAM-OFDM
Summary
Owing to growing underwater activities [1], underwater wireless communication is of great interest to the military, industry, and scientific communities. In addition to focusing on communications in the sea, it is important to construct communication links between underwater platforms [e.g., autonomous underwater vehicles (AUVs) and sensors] and terrestrial platforms [e.g., unmanned aerial vehicles (UAVs) and base stations] for data transmission. The U.S Navy conducted Project ELF in 1989 using an ELF as a carrier for communication with submerged submarines This system used a huge antenna (23 km long) to send very high-power signals (2.6 MW) to submarines at depths of several hundred meters whether in open water or under an ice pack
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