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Underwater wireless communication is a rapidly growing field, especially with the recent emergence of technologies such as autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs). To support the high-bandwidth applications using these technologies, underwater optics has attracted significant attention, alongside its complementary technology – underwater acoustics. In this paper, we propose a hybrid opto-acoustic underwater wireless communication model that reduces network power consumption and supports high-data rate underwater applications by selecting appropriate communication links in response to varying traffic loads and dynamic weather conditions. Underwater optics offers high data rates and consumes less power. However, due to the severe absorption of light in the medium, the communication range is short in underwater optics. Conversely, acoustics suffers from low data rate and high power consumption, but provides longer communication ranges. Since most underwater equipment relies on battery power, energy-efficient communication is critical for reliable underwater communications. In this work, we derive analytical models for both underwater acoustics and optics, and calculate the required transmit power for reliable communications in various underwater communication environments. We then formulate an optimization problem that minimizes the network power consumption for carrying data from underwater nodes to surface sinks under varying traffic loads and weather conditions. The proposed optimization model can be solved offline periodically, hence the additional computational complexity to find the optimum solution for larger networks is not a limiting factor for practical applications. Our results indicate that the proposed technique yields up to 35% power savings compared to existing opto-acoustic solutions.

The visible light communication (VLC) technique using a rolling-shutter sensor-based commercial camera has an advantage in that the LED and camera mounted on an underwater device (e.g., drone, sensor) can be effectively used as a transmitter (Tx) and a receiver (Rx), respectively, under water. On the other hand, a suitable modulation technique has not been well considered to realize a VLC system using a rolling-shutter sensor in an underwater channel that includes high ambient light conditions. In this paper, we design VLC systems using phase-shift keying (PSK), non-return-to-zero on- off keying (NRZ-OOK), and orthogonal frequency division multiplexing (OFDM), and evaluate them through simulations and experiments. The results suggested that the VLC system using PSK or OFDM is more resilient than that using NRZ-OOK in an underwater channel with low-frequency ambient light, and that a VLC system using PSK or NRZ-OOK is more tolerant than that using OFDM in environments with high ambient light, resulting in pixel saturation. In conclusion, the present study has demonstrated that PSK is the most suitable modulation technique for a VLC system using a rolling-shutter sensor in an underwater channel that includes high ambient light conditions.