Underwater Acoustic Communication

- Overview

Underwater acoustic communication (UAC) is a technique of sending and receiving messages below water. There are several ways of employing such communication but the most common is by using hydrophones. Underwater communication is difficult due to factors such as multi-path propagation, time variations of the channel, small available bandwidth and strong signal attenuation, especially over long ranges. Compared to terrestrial communication, underwater communication has low data rates because it uses acoustic waves instead of electromagnetic waves.

Driven by the huge demand to explore oceans, underwater wireless communications have been rapidly developed in the past few decades. Due to the complex physical characteristics of water, acoustic wave is the only media available for underwater wireless communication at any distance. As a result, underwater acoustic communication (UAC) is the major research field in underwater wireless communication.

- Types of Modulation Used for UAC

In general the modulation methods developed for radio communications can be adapted for underwater acoustic communications (UAC). However some of the modulation schemes are more suited to the unique underwater acoustic communication channel than others. Some of the modulation methods used for UAC are as follows:  

• Frequency-shift keying (FSK)
• Phase-shift keying (PSK)
• Frequency-hopping spread spectrum (FHSS)
• Direct-sequence spread spectrum (DSSS)
• Frequency and pulse-position modulation (FPPM and PPM)
• Multiple frequency-shift keying (MFSK)
• Orthogonal frequency-division multiplexing (OFDM)

- Underwater Sound Communication

Special underwater communication systems have been developed to allow scuba divers to talk to each other underwater. A transducer is attached to the diver’s face mask, which converts his or her voice into an ultrasound signal. A fellow diver has an ultrasound receiver, which accepts the signal and converts it back to a sound that the diver can hear, allowing for communication. The same system can be used for communication between the diver and a surface ship. Acoustic communication systems allow divers to talk to each other underwater. 

Can similar systems be used for communication between submerged submarines or between a submerged submarine and a surface ship? Radios will not work, because radio signals cannot propagate any significant distance through water. Acoustic systems called underwater telephones have been developed for this purpose. U.S. Navy submarines use a specialized telephone system to communicate underwater. It works much like an AM radio, except that it transmits and receives sound waves instead of transmitting and receiving radio waves. Similar to land-based systems, underwater telephone systems use microphones and audio amplifiers. 

These systems are designed for voice communications. What if one wants to send e-mail or surf the Internet? More advanced systems designed to transmit and receive digital data have been developed.

- Underwater Acoustic Data Transmission 

Submarines do not have telephone or cable connections, and radio signals do not propagate underwater, so a submarine relies on sound to send and receive digital data. Underwater telephone systems use sound to communicate underwater. These systems produce very low quality signals, similar to poor telephone connections that cellular phones may experience. Telephone modems allow computers to transmit and receive information over telephone lines, however, they have small data transmission rates, and do not work very well with poor connections.

Special acoustic modems that can successfully transmit digital data underwater have been developed. These modems convert digital data into underwater sound signals that can be transmitted between two submerged submarines or between a submerged submarine and a surface ship. These digital signals can represent words and pictures, allowing submarines to send and receive communications. Underwater acoustic modems are relatively slow compared to telephone or cable modems on land. Nonetheless, this technology is very important because it provides an accurate and efficient means to send and receive data underwater. Acoustic links are used to control underwater instruments and acquire the data remotely. 

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