The demand of underwater communication grows rapidly to control unmanned underwater vehicles (or remotely controlled vehicles (ROV)) performing critical missions such as storing liquid CO2 in the seabed, mining methane hydrate, monitoring earthquake, and operating military missions. In particular, the wireless underwater communication is becoming more important because
i) there can be massive underwater sensors that cannot be handled via wired network,
ii) cables can be easily damaged due to fisher dragnet, and
iii) buoys can be damaged due to ships or climate.
The major differences compared to the ground communication are three-fold:
i) the ROV and ground station in general communicate with each other via a multi-hop relay communication due to the long distance, as shown in the left figure above,
ii) we need to use sonar radio for communication, which propagates very slowly and has highly limited bandwidth (as shown in the right figure above, the gateway node, a relay, usually floats on the sea, communicating with the ground station via cellular RF and with underwater ROVs or sensors via sonar radio),
iii) limited power is allowed at ROVs or sensors.
To overcome these problems, an advanced relay communication should be carefully designed for high-data-rate and low-latency wireless underwater communication.