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, the system being competitive with the primitive Maritime radionavigation service of the time. 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 (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 hopped 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)
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 (DSP) can be used in the present day.
The biggest challenge FSK faces in the UAC is multi-path reflections. With multi-path (particularly in UAC) 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 (PSK) is a digital modulation scheme that conveys data by changing (modulating) the phase of a reference signal (the carrier wave).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 (and referred to as CPSK).
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 (relative to a reference wave) itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). 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 (it is a non-coherent scheme). In exchange, it produces more erroneous demodulation.
Orthogonal frequency-division multiplexing
Orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier modulation scheme. OFDM conveys data on several parallel data channel by incorporating closely spaced orthogonal sub-carrier signals
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.
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.
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 (like modems and fax machines do over telephone lines). Documented in STANAG 4748, it uses 900Hz to 60kHz frequencies at distances of up to 28 kilometres (17 mi). 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.
- I. F. Akyildiz, D. Pompili, and T. Melodia, "Underwater Acoustic Sensor Networks: Research Challenges," Ad Hoc Networks (Elsevier), vol. 3, no. 3, pp. 257-279, March 2005.
- "Submarine Signaling on Steamships". www.gjenvick.com. Retrieved 2016-01-18.
- E. Demirors, G. Sklivanitis, T. Melodia, S. N. Batalama, and D. A. Pados, "Software-defined Underwater Acoustic Networks: Toward a High-rate Real-time Reconfigurable Modem," IEEE Communications Magazine, vol. 53, no. 11, pp. 64 – 71, November 2015.
- S. Zhou and Z.-H. Wang, OFDM for Underwater Acoustic Communications. John Wiley and Sons, Inc., 2014.
- E. Demirors, G. Sklivanitis, G.E. Santagati, T. Melodia, and S. N. Batalama, "Design of A Software-defined Underwater Acoustic Modem with Real-time Physical Layer Adaptation Capabilities," in Proc. of ACM Intl. Conf. on Underwater Networks & Systems (WUWNet), Rome, Italy, November 2014.
- T. B. Gabrielson, “Design problems and limitations in vector sensors,” in Proc. workshop Directional Acoustic Sensors (CD-ROM), New Port, RI, 2001.
- Proc. AIP Conf. Acoustic Particle Velocity Sensors: Design, Performance, and Applications, Mystic, CT, 1995.
- A. Nehorai and E. Paldi, “Acoustic vector-sensor array processing,” IEEE Trans. Signal Process., vol. 42, pp. 2481–2491, 1994.
- K. T. Wong & H. Chi, "Beam Patterns of an Underwater Acoustic Vector Hydrophone Located Away from any Reflecting Boundary," IEEE Journal of Oceanic Engineering, vol. 27, no. 3, pp. 628-637, July 2002.
- A. Abdi and H. Guo, “A new compact multichannel receiver for underwater wireless communication networks,” IEEE Trans. Wireless Commun., vol. 8, pp. 3326‐3329, 2009.
- T. C. Yang, “Temporal resolutions of time-reversal and passive phase conjugation for underwater acoustic communications,” IEEE J. Oceanic Eng., vol. 28, pp. 229–245, 2003.
- M. Stojanovic, J. A. Catipovic, and J. G. Proakis, “Reduced-complexity spatial and temporal processing of underwater acoustic communication signals,” J. Acoust. Soc. Am., vol. 98, pp. 961–972, 1995.
- "A new era of digital underwater communications". NATO. 2017-04-27.
- "JANUS Community Wiki".
- Brown, Eric (2017-08-15). "The Internet of Underwater Things: Open Source JANUS Standard for Undersea Communications". Linux.com. The Linux Foundation.
- Nacini, Francesca (2017-05-04). "JANUS creates a new era for digital underwater communications". Robohub.