Underwater acoustic communications

Underwater wireless communication is an enabling technology for various applications, ranging from basic sciences such as oceanography and marine biology to offshore industries such as fish farming or oil and gas drilling, search and rescue operations, and military and exploratory missions. It is also critical for climate monitoring, pollution control and military operations. Radio frequencies do not propagate well through water except over short distances, making acoustic waves the preferred choice for many of these applications. However, acoustic waves are confined to low frequencies, limiting the communication bandwidth. Additionally, sound travels underwater at a relatively low speed and propagates over multiple paths. Delay spreading over tens of milliseconds results in frequency-selective distortion, whereas motion induces significant Doppler effects. The worst properties of radio channels — poor link quality of a mobile terrestrial channel and long delay of a satellite channel — are combined in an underwater acoustic channel, which is often said to be the most challenging communication medium in use today. In this Review, we discuss existing efforts in modelling underwater acoustic propagation channels, processing communication signals and establishing networks. We then summarize some of the future research directions in underwater acoustic communications.