science

Underwater acoustic communication

Underwater acoustic communication is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Underwater acoustic communication rather than just read about it. In short: Underwater acoustic communication is a technique of sending and receiving messages in water. There are several ways of employing such communication but the most common is by using hydrophones.

Key takeaways

Underwater acoustic communication belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Underwater acoustic communication to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Underwater acoustic communication from memory before moving on to harder problems.

Reference excerpt

Underwater acoustic communication is a technique of sending and receiving messages in 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. 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-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) Continuous Phase Modulation (CPM) 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. 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.

Orthogonal frequency-division multiplexing Orthogonal frequency-division multiplexing (OFDM) is a digital multi-carrier modulation scheme. OFDM conveys data on several parallel data channels 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.

Continuous phase modulation Continuous phase modulation (CPM) is a modulation technique, which is a continuous phase shift, where the phase of the carrier signal varies over time and avoids abrupt changes between successive symbols. This smooth phase trajectory reduces spectral side lobes. Reducing spectral side lobes increases the spectral efficiency of CPM and enables it to transmit data within a narrower bandwidth. Notable variants of CPM include minimum shift keying (MSK) and Gaussian minimum shift keying (GMSK), which uses a Gaussian filter to smooth out phase shifts. Since the underwater environment is highly scattered, it can cause multipath propagation and signal degradation. The CPM's continuous phase feature mitigates these effects and maintains signal integrity. Besides its high spectral efficiency helps make optimal use of limited bandwidth underwater.

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.

Applications

Underwater telephone The underwater telephone, also known as UQC, AN/WQC-2, or Gertrude, was used by the U.S. Navy in 1945 after in Kiel, Germany, in 1935 different realizations at sea were demonstrated. The terms UQC and AN/WQC-2 follow the nomenclature of the Joint Electronics Type Designation System. The type designation "UQC" stands for General Utility (multi use), Sonar and Underwater Sound and Communications (Receiving/Transmitting, two way). The "W" in WQC stands for Water Surface and Underwater combined. The underwater telephone is used on all crewed submersibles and many Naval surface ships in operation. Voice or an audio tone (morse code) communicated through the UQC are heterodyned to a high pitch for acoustic transmission through water.

… excerpt ends here. Continue reading the full article.

Worked examples

Example 1 — a first encounter with Underwater acoustic communication

Start with the simplest possible case. Write down what Underwater acoustic communication claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Underwater acoustic communication before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Underwater acoustic communication ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Underwater acoustic communication

In research: Underwater acoustic communication appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Underwater acoustic communication in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Underwater acoustic communication is common in secondary-school and first-year university syllabi. It links to neighbouring topics Acoustics, Telecommunications techniques, so understanding it makes those chapters shorter.
In everyday life: Look for Underwater acoustic communication outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Underwater acoustic communication in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Underwater acoustic communication means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Underwater acoustic communication out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Underwater acoustic communication in simple terms?

Underwater acoustic communication is a technique of sending and receiving messages in water. There are several ways of employing such communication but the most common is by using hydrophones.

Why does Underwater acoustic communication matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Underwater acoustic communication?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Underwater acoustic communication.