⿻ Underwater Communication

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|UPDATED: 18 July 2025

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The undersea domain is an exceptionally challenging operating environment, and one of the most persistent obstacles is communication. Unlike on land, at sea, or in space, standard electromagnetic transmissions attenuate rapidly when they encounter water. As a result, they cannot cross the air-water interface or propagate effectively through the water column.

There are two primary modes of underwater communication, each with distinct advantages and limitations:

• Acoustic Communication enables long-range transmission but suffers from high latency, low bandwidth, and susceptibility to attenuation, multipath propagation, and ambient noise. Performance is heavily influenced by environmental factors such as salinity, temperature, pH, and pressure (depth), all of which shape how sound travels underwater.

• Optical Communication, which uses blue-green laser light to transmit data, supports very high bandwidth (on the order of megabits or even gigabits per second). However, it requires line-of-sight between transmitter and receiver, and is limited to short ranges—typically under 100 meters— due to rapid attenuation. Optical systems are also sensitive to turbidity, which can degrade signal quality through absorption and scattering.

In addition to acoustic and optical methods, there have been efforts to use electromagnetic (EM) communication underwater.

• The primary focus has been extremely low frequency (ELF) and very low frequency (VLF) ranges.
• While EM signals attenuate quickly in seawater, these low frequencies can penetrate to limited depths, enabling low-data-rate, short-range transmissions suitable for niche applications such as submarine paging or proximity-based networking.

One emerging technique involves generating acoustic vibrations on the surface of the water, which can then be detected and decoded by airborne sensors or satellites.

• Offers a potential solution for cross-domain (subsea-to-air) signaling where traditional methods fail.
• There is also research underway to determine whether acoustic transmissions can be intercepted using this method.
• Though still experimental, this technology represents a creative approach to overcoming the undersea communication barrier.

ACOUSTIC COMMUNICATION (ACOMMS)

ACOMMS involves transmitting information through sound waves in water. Since electromagnetic waves attenuate quickly underwater, sound waves are the primary medium for communication in underwater environments. Acoustic communication is used in a wide range of applications, including defense, scientific research, and environmental monitoring.

▶︎ FUNDAMENTALS:

SOUND PROPAGATION
Sound waves in water are pressure waves that travel through the medium by compressing and rarefying it. These waves are influenced by the unique speed of sound (roughly 1500 m/s) of the particular seawater environment, which varies according to:

• Temperature: Sound travels faster in warmer water.
• Salinity: Higher salinity increases sound speed.
• Pressure (Depth): Greater depths increase sound speed due to higher pressure.

These variations create a Sound Speed Profile (SSP), which affects sound wave propagation and leads to phenomena like refraction and the formation of sound channels. The SOFAR channel (Sound Fixing and Ranging channel), for example, is a naturally occurring horizontal layer in the ocean where sound waves can travel long distances with minimal loss. It forms at depths where the speed of sound is at a minimum, typically around 600 to 1200 meters, depending on latitude. Sound speed in the ocean is influenced by temperature, pressure, and salinity; it decreases with depth due to cooling, then increases again with pressure. This creates a sound speed profile that bends acoustic waves back toward the center of the channel, effectively trapping them and allowing them to refract and reflect within the layer. As a result, even low-power sounds (like a whale song or an underwater explosion) can propagate thousands of kilometers, making the SOFAR channel useful for applications such as long-range passive sonar, oceanographic studies, and submarine communications.

ATTENUATION
Attenuation refers to the gradual loss of acoustic energy as sound travels through water. This loss occurs due to a combination of absorption, where sound energy is converted into heat, and scattering, where sound is redirected by particles, bubbles, or turbulence. Attenuation increases with frequency, meaning high-frequency sounds degrade more quickly than low-frequency ones, which is why long-range communication and detection often rely on lower frequencies. Water temperature, salinity, pressure, and the presence of suspended materials all influence how quickly sound attenuates in a given environment.

Sound attenuation increases with:

• Frequency: High-frequency signals attenuate faster than low-frequency signals.
• Absorption: Water molecules absorb sound energy, which increases with frequency.
• Scattering: Particles, bubbles, and the seafloor scatter sound waves, reducing signal strength.

REFLECTION AND REFRACTION
Reflection and refraction are key behaviors that govern how sound moves through the ocean. Reflection occurs when sound waves bounce off surfaces like the seafloor, sea surface, or submerged objects, often causing echoes or multipath effects. Refraction is the bending of sound waves as they pass through water layers with different sound speeds, which are affected by temperature, salinity, and pressure. This bending can cause sound to curve downward, upward, or become trapped in layers like the SOFAR channel, shaping how sonar and acoustic communications perform across distances.

• Reflection occurs when sound waves bounce off surfaces like the seafloor or the water surface.
• Refraction happens when sound waves bend due to changes in the SSP, affecting their propagation paths.

▶︎ TECHNOLOGIES:

Acoustic Transducers

• Transducers convert electrical signals into sound waves and vice versa.
• Common types include piezoelectric transducers, which rely on materials that generate an electrical charge when mechanically stressed.

Modulation Techniques
Modulation encodes information onto sound waves. Key techniques include:

• Amplitude Shift Keying (ASK) - Varies the amplitude of the carrier signal.
• Frequency Shift Keying (FSK) - Changes the frequency of the carrier signal (commonly used due to its resilience to noise).
• Phase Shift Keying (PSK) - Alters the phase of the signal.
• Orthogonal Frequency Division Multiplexing (OFDM) - Splits the signal into multiple frequencies, enabling higher data rates and robustness.

Error Correction and Coding
Due to noise or distortions in the channel, the receiver may apply error correction and filtering techniques to recover the data accurately, such as Reed-Solomon codes or convolutional coding to minimize data loss.

▶︎ CHALLENGES:

Multipath Propagation
Sound waves can take multiple paths (direct, surface-reflected, and bottom-reflected) due to reflections, refractions, or scattering off various surfaces or objects (like the seafloor, walls, or the water's surface), whic leads to interference and signal distortion. In underwater acoustics, this is particularly common because sound waves can bounce off different layers of water or the seafloor, creating multiple copies of the same signal arriving at the receiver at slightly different times. Multipath is especially challenging in underwater environments, where sound waves can reflect off various underwater structures or objects, causing signal degradation and the need for sophisticated signal processing to correct for it.

Interference
The multiple signal paths can interfere with each other, causing signal distortion** or fading when the paths are out of phase with one another.

Delay Spread
The signal may arrive at the receiver over a period of time, rather than all at once. This can blur the signal and make it harder to decode.

Doppler Effect
Movement of the transmitter, receiver, or medium causes frequency shifts, which must be compensated for in real-time.

Noise
Ambient noise sources include marine life, ship traffic, and natural phenomena, such as waves and wind.

Bandwidth
Available bandwidth depends on the operating frequency and environmental conditions. Lower frequencies (below 10 kHz) allow long-range communication but support lower data rates, while higher frequencies (tens of kHz) provide higher data rates but are limited to shorter distances.

▶︎ APPLICATIONS:

Subsea Networks
Acoustic modems are used for communication between underwater vehicles, sensors, and surface stations in Underwater Wireless Sensor Networks (UWSNs).

Navigation and Control
Remote control and navigation of AUVs or ROVs via acoustic signals from LBL, USBL systems.

▶︎ EMERGING TRENDS:

Adaptive Modulation
Systems that dynamically adjust modulation schemes based on environmental conditions to optimize performance.

Hybrid Communication Systems
Combining acoustic communication with other modalities (e.g., optical or RF) for specific scenarios.

Machine Learning
Using ML algorithms to optimize signal processing, noise reduction, and adaptive error correction.

Energy-Efficient Protocols
Development of low-power communication systems to extend the operational lifespan of underwater devices.

OPTICAL COMMUNICATION (OCOMMS)

Underwater optical communication involves using light waves, typically in the visible spectrum, to transmit information through water. This method is increasingly explored due to its potential for higher data rates compared to acoustic communication. However, it is limited by challenges like light absorption and scattering, making it suitable primarily for short-range, high-bandwidth applications.

Underwater optical communication offers a compelling solution for high-speed data transfer in short-range applications. While it faces challenges like attenuation, scattering, and alignment sensitivity, advancements in laser technology, modulation, and hybrid systems are enhancing its utility in underwater environments. By complementing acoustic communication, optical systems are paving the way for more efficient and versatile underwater networks.

▶︎ FUNDAMENTALS:

LIGHT PROPAGATION
Light waves travel much faster than sound waves in water but are heavily influenced by water's physical properties:

• Absorption: Water absorbs light energy, converting it into heat. Absorption is wavelength-dependent, with red and infrared light absorbed most strongly and blue-green light (400–550 nm) penetrating furthest.
• Scattering: Particles, microorganisms, and air bubbles scatter light, reducing its intensity and coherence.

OPTIMAL WAVELENGTHS
Blue and green light are most suitable for underwater optical communication due to their lower absorption in water:

• Blue Light (~450 nm): Best for clear oceanic waters.
• Green Light (~520 nm): Better for turbid or coastal waters where suspended particles scatter light more effectively.

REFRACTION and TURBULENCE

• Refraction: Light rays bend as they pass through regions with varying water densities, altering their path.
• Turbulence: Temperature gradients and water flow create fluctuations in the refractive index, distorting the light beam.

▶︎ TECHNOLOGIES AND METHODS:

LEDs (Light Emitting Diodes)
Cost-effective, energy-efficient, and capable of operating in turbid water.

Lasers: Provide highly focused, coherent beams, allowing for longer ranges and higher data rates. Common types include:

• Blue-Green Lasers: For medium distances.
• Ultraviolet (UV) Lasers: For non-line-of-sight communication in scattering environments.

Photodiodes
Convert received light signals into electrical signals. Types include:

• Silicon Photodiodes: Effective for the visible spectrum.
• Avalanche Photodiodes (APDs): Provide high sensitivity, crucial for detecting weak light signals.
• CCD/CMOS Sensors: Used in imaging-based systems for optical signal reception.

Modulation Techniques
Optical modulation encodes data onto light waves:

• Intensity Modulation (IM): Varies the brightness of the light source.
• Pulse Position Modulation (PPM): Encodes information in the timing of light pulses.
• Frequency Modulation (FM): Alters the light wave’s frequency.
• Wavelength Division Multiplexing (WDM): Uses multiple wavelengths (colors) of light to transmit parallel data streams, increasing capacity.

▶︎ TRANSMISSION:

Free-Space Optical (FSO)
Wireless communication using lasers or LEDs for untethered systems like AUVs and underwater sensor networks.

Fiber Optics
Often used for tethered systems in ROVs but limits mobility.

▶︎ CHALLENGES:

Light Absorption and Scattering
Severe attenuation limits effective range to tens or hundreds of meters, depending on water clarity.

Alignment Sensitivity
Laser-based systems require precise alignment between the transmitter and receiver, making them vulnerable to misalignment due to platform motion or turbulence.

Background Noise
Ambient light from the sun or artificial sources can interfere with optical signals, especially in shallow waters.

Energy Requirements
Lasers and high-powered LEDs can consume significant energy, limiting operational durations for battery-powered devices.

▶︎ APPLICATIONS:

Short-Range, High-Speed Communication
High-data-rate transfer between AUVs, ROVs, and underwater sensors in a localized area.

Data Harvesting
Collecting data from underwater sensor networks by autonomous platforms using optical links.

Imaging and Video Transmission
Enabling real-time, high-definition video transmission for ROVs in underwater inspections or exploration missions.

Marine Research and Monitoring
Facilitating high-bandwidth communication for real-time data collection from oceanographic instruments.

▶︎ EMERGING TRENDS:

Hybrid Systems
Combining optical communication with acoustic or RF systems for broader coverage and reliability. For instance:

• Acoustic for long-range, low-bandwidth communication.
• Optical for short-range, high-bandwidth data transfer.

Non-Line-of-Sight (NLOS) Communication
UV-based systems that leverage strong scattering properties for indirect light propagation in turbid waters.

Advanced Modulation and Coding
Techniques like MIMO (Multiple Input Multiple Output), adaptive coding, and error correction to improve reliability in dynamic underwater environments.

Energy Harvesting
Integrating energy-efficient optical components with renewable energy sources, such as ocean currents or solar panels, to extend the operational life of communication devices.

Quantum Optical Communication
Exploring the use of quantum properties of light (e.g., entanglement) to enhance underwater communication security.

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ACOMMS:

Acoustic communication continues to pose a great challenge for subsea operations. Current research and development is focusing on secure, Low Probability of Detection (LPD) and Low Probability of Intercept (LPI) communication, and the construction of reliable, scalable undersea communication networks.

LPD/LPI

LPD and LPI are essential for covert undersea operations, ensuring that acoustic signals are difficult for adversaries to detect, locate, or decode. While traditional ACOMMS signals are often strong and structured (and thus easily intercepted), LPD/LPI techniques aim to blend in, evade, or deceive. LPD/LPI is at the heart of secure ACOMMS for modern undersea operations. While traditional spread-spectrum techniques still dominate, biomimetic, chaotic, and AI-driven approaches are pushing the frontier toward truly covert, intelligent, and resilient communication systems.

▶︎ METHODS:

Spread Spectrum Techniques

• Direct Sequence Spread Spectrum (DSSS): Spreads the signal across a wider frequency band using a pseudorandom code.
• Frequency Hopping Spread Spectrum (FHSS): Rapidly changes carrier frequencies in a pseudorandom sequence.
• These reduce detectability by lowering spectral density and resisting jamming or spoofing.

Ultra-Wideband (UWB) Signaling

• Transmits short, low-energy pulses across a wide frequency range.
• Excellent for low SNR environments and resistant to detection.

Chirp Spread Spectrum (CSS)

• Uses linearly modulated chirps with varying frequency over time.
• Robust in multipath and Doppler-heavy environments; harder to intercept without precise synchronization.

Chaotic Modulation

• Inspired by nonlinear, unpredictable natural signals (e.g., dolphin clicks).
• Offers enhanced unpredictability and LPI characteristics, especially when combined with DSSS.

Low Duty Cycle & Time-Hopping

• Transmit only occasionally or irregularly, making it hard for adversaries to lock on or detect patterns.

Directional Transducers and Beamforming

• Physically restricts the transmission to a narrow acoustic cone, reducing signal leakage and detection range.

▶︎ EMERGING POSSIBILITIES:

Chaotic DSSS
Being actively researched, especially in China and some NATO labs, as a way to mimic biological emissions and reduce predictability.

AI-Augmented LPD/LPI
AI is increasingly used to optimize modulation, timing, and power based on the environment to minimize detection risk.

Underwater Steganography
Embedding messages in ambient noise or natural biological sounds.

Environmental Masking
Transmitting during periods of high ambient noise (e.g., shipping, seismic events, biological noise) to hide in the clutter.

Cognitive ACOMMS
Real-time adaptation of signal characteristics based on environmental and threat sensing.

Federated Acoustic Signaling
Cooperative swarms modulating and relaying signals in unpredictable ways.

Quantum-Inspired Security
Exploring uncertainty and entanglement analogs for future stealthy underwater communication paradigms.

▶︎ CHALLENGES:

Tradeoff with Data Rate
Most LPD/LPI techniques reduce data throughput and range.

Synchronization
Spread and chaotic systems often require precise timing and key sharing, which can be hard underwater.

Hardware Constraints
Directional or agile systems may need complex and expensive transducer arrays.

ALGORITHMIC/AI ASSISTANCE

Underwater ACOMMS faces unique challenges: low bandwidth, high latency, multipath distortion, Doppler effects, and ambient noise. AI and advanced algorithms offer tools to mitigate these challenges, enhance performance, and enable adaptive, secure communications. AI and algorithmic processing are transforming ACOMMS from rigid, rule-based systems into adaptive, intelligent, and resilient communication networks. As onboard processing becomes more capable, AI will play an increasingly central role in enabling efficient, secure, and stealthy underwater communications in contested and dynamic environments.

▶︎ METHODS:

Channel Estimation and Equalization

• AI/ML algorithms (e.g., deep learning, LSTM, CNNs) can model and compensate for highly variable underwater channels better than traditional models.
• Enables more reliable signal reconstruction despite distortion from multipath and Doppler shifts.

Adaptive Modulation and Coding

• AI can dynamically select modulation schemes (e.g., QPSK, OFDM variants) based on real-time channel conditions.
• Increases data throughput while minimizing error rates and energy consumption.

Signal Detection and Classification

• Deep learning models trained on acoustic signatures can discriminate between friendly, hostile, and environmental sounds.
• Improves both situational awareness and LPI/LPD by intelligently deciding when and how to transmit.

Noise Reduction and Signal Enhancement

• AI can learn to filter background noise (biological, mechanical, or natural) and enhance weak signals.
• Improves communication quality and range.

Compression and Encoding

• Neural networks and reinforcement learning can help optimize data packaging for transmission over narrowband channels.

Security and Anomaly Detection

• AI can monitor for unusual patterns in communication traffic, enabling intrusion detection, signal spoofing detection, or jammer avoidance.

Forward-Looking Possibilities

• Reinforcement Learning (RL) for mission-aware transmission policies, adjusting when and how to communicate based on energy, threat level, or data urgency.
• Federated Learning among autonomous platforms to build shared models of the acoustic environment without centralized processing.
• Generative AI for synthetic signal generation and spoofing in decoy or deception operations.

▶︎ CHALLENGES:

• Computational Constraints: Onboard processing power and energy are limited.
• Data Scarcity: Training robust models requires high-quality, labeled underwater datasets.
• Generalization: Models trained in one environment may not transfer well to another due to variability in conditions.

BIOMIMICRY FOR SECURITY

Biomimicry in underwater acoustic communications (ACOMMS) involves imitating natural marine communication strategies—particularly those used by marine mammals, fish, or invertebrates—to improve stealth, security, and efficiency. This approach is especially attractive in defense and covert operations, where low probability of detection (LPD) and low probability of intercept (LPI) are critical. Biomimicry offers promising avenues for secure, stealthy, and resilient ACOMMS, especially in contested environments. By mimicking nature’s proven strategies, particularly those of marine organisms evolved for covert and effective communication, engineers can develop novel underwater communication systems that are harder to detect, intercept, or jam.

▶︎ METHODS:

Cetacean Vocalization (e.g., Dolphins, Whales)
Uses a wide range of frequency-modulated signals, clicks, whistles, and chirps. These signals are narrowband, directional, and variable in timing and structure, making them difficult to detect and spoof. Inspired systems may use nonlinear frequency modulation (NLFM) or chaotic signal encoding to mimic these patterns.

Fish Choruses
Some fish species produce irregular, group-based sounds for communication or mating. These bursty, temporally random signals can inspire stochastic transmission patterns to increase covertness.

Invertebrate Sounding (e.g., Shrimp Snaps, Lobster Rumbles)
Background ocean noise is often dominated by biotic sources. Emulating or hiding within this biological noise floor helps achieve acoustic camouflage.

Security Mechanisms via Biomimicry
Chaotic DSSS (Direct Sequence Spread Spectrum): Inspired by the unpredictability of biological signals. Uses chaotic codes that resemble dolphin clicks or whale calls, offering strong LPI/LPD.

Time-Hopping & Randomized Transmission: Mimics irregular call patterns in nature. Prevents predictable signal structures, making interception and jamming difficult.

Adaptive Modulation Based on Environmental Cues: Similar to how marine mammals shift vocalizations in noisy or cluttered environments. ACOMMS systems can adaptively select modulation schemes to optimize security and signal quality.

▶︎ CHALLENGES:

• Environmental Variability: Matching biomimetic signals to complex ocean environments requires sophisticated modeling.
• Processing Overhead: Biomimetic schemes often require advanced signal processing (e.g., machine learning, chaotic maps).
• Bandwidth Limits: Biological signals are typically narrowband; balancing this with data rate needs can be difficult.

DARPA TUNA

The DARPA Tactical Undersea Network Architectures (TUNA) is focused on developing innovative, scalable, and deployable undersea communication networks for temporary use in tactical operations. The program aims to provide reliable undersea communication for military forces in situations where traditional communication infrastructure is compromised, unavailable, or unsuitable.

▶︎ FEATURES:

Temporary Undersea Networks

• These networks are designed to be temporary and flexible, enabling rapid deployment and recovery.
• The networks utilize small, lightweight components that can be deployed by hand, aircraft, or ship.

Components

• Fiber optic relays: To create a scalable undersea network that can transmit high-bandwidth data.
• Autonomous undersea nodes: Capable of managing and routing data through the network.
• Low-power communication links: Designed to extend operational endurance and reduce the size of deployment platforms.

▶︎ Applications:

• Tactical military communications in contested environments.
• Communication restoration after the loss of traditional satellite or terrestrial communication systems.
• Potential use in disaster recovery scenarios.

▶︎ Technologies:

• Focus on integrating buoys, undersea vehicles, and fiber optic systems.
• Emphasis on modular design, enabling easy reconfiguration based on mission requirements.

▶︎ Challenges Addressed::

• Limited range of acoustic communication underwater.
• Difficulty in deploying traditional communication systems rapidly.
• Ensuring security and resilience against electronic or physical disruption.

*THROUGH-ICE ACOMMS

Scientists with U.S. National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL) have investigated wireless through-ice acoustic communication, a potential game-changer for missions targeting subsurface oceans on icy moons like Europa and Enceladus.

▶︎ JPL is seeking to develop an acoustic communications (ACOMMS) system capable of transmitting data wirelessly through kilometers of ice, to support cryobot subsurface missions on oceanic worlds where tethered operations are risky due to ice instability.

▶︎ The [study](https://www.mdpi.com/1424-8220/24/9/2776 details the design, testing, and field validation of a custom acoustic system capable of transmitting data through ice, offering a plausible alternative (or supplement) to tethered solutions.

▶︎ The study shows that acoustic communication through glacial ice is now proven feasible at ranges up to 83 meters, with strong potential for kilometer-scale transmission at lower frequencies. This effort marks a significant step toward untethered, or tether-assisted, communications for robotic systems exploring extraterrestrial oceans.

OCOMMS

LONG-RANGE OCOMMS:

Researchers from Shaqra University (Saudi Arabia) have developed an underwater optical wireless communication (UOWC) system that utilizes novel encoding and efficient data transmission to overcome issues of signal absorption and scattering, while also significantly enhancing data throughput and transmission range. The study presents a major UOWC breakthrough, solving previous challenges related to signal loss, range, data rate, and user capacity, and has broad implications for subsea and seabed operations. The authors' system would enable wide-area, multi-node UOWC networks to share fast, secure, high-bandwidth, reliable transmissions without requiring signal relays or amplification.

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‣ ⿻ S&T Brief: Through-Ice ACOMMS for Ocean Worlds - 17 July 2025
‣ ⿻ S&T Brief: Improved Underwater Optical Wireless COMMS - 20 Feb 2025
‣ ⿻ S&T Brief: Low Probability of Detection (LPD) ACOMMS - 12 February 2025

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