Underwater Acoustic Communication

- Overview

Underwater Acoustic Communication (UAC) uses sound waves to transmit data between underwater devices because radio waves don't travel well in water, enabling communication for ROVs, sensors, divers, and scientific monitoring. 

It works like sonar, sending acoustic signals (pings) that carry data, though it faces challenges like multipath propagation and low bandwidth, leading to slower data rates than terrestrial systems, but newer tech offers improved speeds. 

1. How UAC works:

• Transmitters & Receivers: Devices called acoustic modems use transducers to convert digital data into acoustic signals (pressure waves) and vice-versa.
• Signal Propagation: Sound travels through water, allowing communication over long distances where other wireless methods fail.
• Data Encoding: Various techniques like Frequency-Shift Keying (FSK) or Orthogonal Frequency-Division Multiplexing (OFDM) encode data onto the sound waves.

2. Key Applications:

• Robotics: Controlling Autonomous Underwater Vehicles (AUVs) and Remotely Operated Vehicles (ROVs).
• Oceanography: Monitoring marine life, tracking pollution, and collecting data for climate studies.
• Offshore Industry: Communication for oil & gas exploration and fish farming.
• Military & Defense: Submarine communication and mine detection.
• Diver Communication: Real-time comms for underwater exploration and rescue.

3. Challenges & Solutions: 

• Multipath: Signals bounce off the surface, seabed, and objects, creating echoes (ISI) that distort data.
• Low Bandwidth: Water attenuates high-frequency sounds, limiting data rates compared to radio.
• Solutions: Advanced modulation, signal processing (like >>array processing), and network design help overcome these issues, with newer systems achieving higher speeds.

Please refer to the following for more information:

• Wikipedia: Underwater Acoustic Communication (UAC)

- 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.

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- 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. 

[More to come ...]