Underwater acoustic communication


Underwater acoustic communication 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.
At the beginning of the 20th century some ships communicated by underwater bells as well as using the system for navigation. Submarine signals were at the time competitive with the primitive Maritime radionavigation service. The later Fessenden oscillator allowed communication with submarines.

Types of modulation used for underwater acoustic communications

In general the modulation methods developed for radio communications can be adapted for underwater acoustic communications. 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:
The following is a discussion on the different types of modulation and their utility to UAC.

Frequency shift keying

FSK is the earliest form of modulation used for acoustic modems. UAC prior to modems was by percussion of different objects underwater. This method was also used to measure the speed of sound in water.
FSK usually employs two distinct frequencies to modulate data; for example, Frequency F1 to indicate bit 0 and frequency F2 to indicate bit 1. Hence a binary string can be transmitted by alternating these two frequencies depending on whether it is a 0 or 1. The receiver can be as simple as having analogue matched filters to the two frequencies and a level detector to decide if a 1 or 0 was received. This is a relatively easy form of modulation and therefore used in the earliest acoustic modems. However more sophisticated Demodulator using Digital Signal Processors can be
used in the present day.
The biggest challenge FSK faces in the UAC is multi-path reflections. With multi-path several strong reflections can be present at the receiving hydrophone and the threshold detectors
become confused, thus severely limiting the use of this type of UAC to vertical channels. Adaptive equalization methods have been tried with limited success. Adaptive equalization tries to model
the highly reflective UAC channel and subtract the effects from the received signal. The success has been limited due to the rapidly varying conditions and the difficulty to adapt in time.

Phase shift keying

Phase-shift keying is a digital modulation scheme that conveys data by changing the phase of a reference signal. The signal is impressed into the magnetic field x,y area by varying the sine and cosine inputs at a precise time. It is widely used for wireless LANs, RFID and Bluetooth communication.
Any digital modulation scheme uses a finite number of distinct signals to represent digital data. PSK uses a finite number of phases, each assigned a unique pattern of binary digits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal — such a system is termed coherent.
Alternatively, instead of operating with respect to a constant reference wave, the broadcast can operate with respect to itself. Changes in phase of a single broadcast waveform can be considered the significant items. In this system, the demodulator determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying. DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal. In exchange, it produces more erroneous demodulation.

Orthogonal frequency-division multiplexing

Orthogonal frequency-division multiplexing is a digital multi-carrier modulation scheme. OFDM conveys data on several parallel data channel by incorporating closely spaced orthogonal sub-carrier signals
OFDM is a favorable communication scheme in underwater acoustic communications thanks to its resilience against frequency selective channels with long delay spreads.

Use of vector sensors

Compared to a scalar pressure sensor, such as a hydrophone, which measures the scalar acoustic field component, a
vector sensor measures the vector field components such as acoustic particle velocities. Vector sensors can be
categorized into inertial and gradient sensors.
Vector sensors have been widely researched over the past few decades. Many vector sensor signal processing algorithms have been designed.
Underwater vector sensor applications have been focused on sonar and target detection. They have also been proposed to be used as underwater multi‐channel communication receivers and equalizers. Other researchers have used arrays of scalar sensors as multi‐channel equalizers and receivers.

JANUS

In April 2017, NATO's Centre for Maritime Research and Experimentation announced the approval of JANUS, a standardized protocol to transmit digital information underwater using acoustic sound. Documented in STANAG 4748, it uses 900Hz to 60kHz frequencies at distances of up to. It is available for use with military and civilian, NATO and non-NATO devices; it was named after the Roman god of gateways, openings, etc.
The JANUS specification provides for a flexible plug-in-based payload scheme. This enables multiple different applications such as Emergency location, Underwater AIS, and Chat. An example of a Emergency Position and Status message is.
.
This Emergency Position and Status Message message shows a Portuguese submarine at 38.386547 Latitude -9.055858 Longitude at a depth of 16 Meters. It is moving North at 1.4 meters per second, and has 43 Survivors on board and shows the environmental conditions.