Underwater Communication Modalities

• Underwater communication modalities are a suite of techniques enabling data transfer in marine environments, balancing range, data rate, and robustness via acoustic, optical, electromagnetic, and bio-inspired methods.
• Recent innovations demonstrate multiplexing strategies, including acoustic orbital angular momentum and hybrid integration, to effectively enhance channel capacity and overcome environmental constraints.
• Future research focuses on cross-modal integration, semantic compression, and adaptive protocols, aiming to optimize performance amid variable underwater conditions and limited energy budgets.

Underwater communication modalities are the suite of physical and protocol-layer techniques enabling data exchange in marine environments, where propagation constraints fundamentally diverge from terrestrial or aerial contexts. The principal modalities are acoustic, optical, electromagnetic (including both radio-frequency and low-frequency quasi-static electric/magnetic fields), and various hybrid/multimodal integrations. Each realizes a trade-off among achievable range, data throughput, environmental robustness, latency, and hardware complexity, with distinct mechanisms rooted in the underlying physics of wave propagation and signal interaction with the aquatic medium. Recent advances include multiplexing along previously underutilized physical degrees of freedom, cross-layer hybrid networks, and semantic communication schemes optimized for ultra-constrained underwater channels.

1. Physical Principles and Channel Characteristics

Underwater communication must contend with high absorption and scattering rates for electromagnetic and optical waves, contrasted to relatively low attenuation for mechanical (acoustic) waves. Acoustic propagation exploits the compressional sound channel at typical velocities of order 1500 m/s, with path loss described by combined spreading and frequency-dependent absorption mechanisms:

$TL(d, f) = 20 \log_{10}(d) + \alpha(f)d$

with absorption $\alpha(f)$ increasing steeply with frequency (empirical Thorp’s formula). Acoustic channels, while supporting O(10 km) ranges, exhibit severe multipath, time-varying Doppler, and ambient noise from natural and anthropogenic sources (Bommisetty et al., 2021, Ramesh et al., 18 Jan 2026).

Electromagnetic (RF) waves experience exponential decay from Ohmic losses: skin depths at 50 MHz are $\sim$cm in seawater, with special configurations (e.g., Zenneck-type surface electromagnetic waves at water/air interface) enabling “surface wave” propagation over several meters with path loss orders of magnitude smaller than bulk waves (Smolyaninov et al., 2018).

Optical transmission leverages blue-green windows (450-550 nm) with e-folding attenuation lengths of $O(30\ \mathrm{m})$ in pure water, exponentially reduced in turbid environments. Scattering, alignment, and ambient light contribute to severe channel variability (Kernbach et al., 2011, Ramesh et al., 18 Jan 2026). Quantum and OAM-enhanced optical communication further utilize polarization and spatial mode structure but are highly sensitive to turbulence and path loss (Hufnagel et al., 2020).

Magnetic Induction (MI) modalities exploit the quasi-static, near-field of time-harmonic magnetic fields via compact coil antennas. Propagation is less sensitive to salinity and turbidity, with moderate ranges (up to tens of meters), and sub-$\mu$s group delays, making MI a synchrony-enabling complement to acoustic links (Li et al., 2019, Zhang, 2024).

Bio-inspired electric field (electrocommunication) uses low-frequency dipole signaling for very short-range analog/digital exchange, with range decay scaling as $1/r^3$ in the quasi-static regime (Wang et al., 2020, Kernbach et al., 2011).

2. Modality-Specific Approaches and Recent Multiplexing Innovations

Acoustic Modalities: Traditional scalar-pressure communication is limited to a single information channel. Recent advances unlock a “polarization-like” multiplexing via particle velocity vector components $(v_x,v_y,v_z)$, enabling simultaneous, orthogonal channels in guided wave structures (e.g., ocean ducts, cylindrical pipes). Laboratory implementation at $2.5\ \mathrm{kHz}$ carrier yields three independent 125 bit/s channels over a 1.1 m air guide, with aggregate BER $<10^{-2}$, effectively tripling channel capacity without extra bandwidth or added hardware (Liu et al., 5 Aug 2025).

Acoustic OAM Multiplexing: Acoustic orbital angular momentum modes of varying topological charge $\ell$ form a theoretically infinite set of orthogonal eigenfunctions. Beamforming and demultiplexing permit parallel transmission (e.g., 8 channels, each at 2.5 kbaud, achieving $8\ \mathrm{bit/s/Hz}$ spectral efficiency), as experimentally demonstrated for real-time image transmission (Zhang et al., 2019). Dynamic Modal Decomposition (DMD) reduces array hardware requirements, offering improved BER and robustness to array misalignment compared to classical orthogonal demodulation (Li et al., 2024).

Electromagnetic/Magnetic Induction and Surface Waves: Portable 50 MHz monopole antennas matched via deionized water enclosures launch surface electromagnetic waves at the air/seawater interface, with experimental links up to 9 m and theoretical attenuation length $L_p\sim60\ \mathrm{m}$—far exceeding bulk RF skin depth ($\sim$4 cm). Bulk propagation is feasible only near the surface; range and performance are strongly modulated by surface conditions and salinity (Smolyaninov et al., 2018).

Inductive Coupling: Power-carrier based inductive coupling leverages mooring cables as underwater single-turn transformers. With high-Q MnZn ferrite rings and 4-turn coils, resonant operation at 1.67 MHz achieves $115.2\ \mathrm{kbps}$ at $<2.2\,\mathrm{mW}$ static power over 700 m, with measured BER $<10^{-5}$. Differential PSK encoding is favored for robustness without carrier recovery needs (Zhang, 2024).

Electrocommunication: Dipole-driven E-field signaling, modulated via binary FSK and demodulated by compact deep learning-based classifiers, achieves robust 5 kbps links up to 10 m at sub-0.1 W power. Quasi-static attenuation severely limits range but enables robust swarm localization and low-interference short-range networking (Wang et al., 2020).

3. Hybrid, Multimodal, and Semantic Communication

Hybrid architectures maximize aggregate throughput and robustness by integrating acoustic, optical, MI, and EM links. A canonical scenario uses MI for intra-swarm high-precision synchronization, then exploiting distributed acoustic MIMO beamforming for long-range links (up to 2–3× improvement in effective communication time, 1–2 orders of magnitude in throughput vs. acoustic alone) (Li et al., 2019). Table summary of typical modality attributes (Ramesh et al., 18 Jan 2026):

Modality	Range	Data Rate	Robustness	Typical Use
Acoustic	100 m–20 km	10 bps–100 kbps	High, env. noise	Long-range coordination
Optical	1–100 m	1 Mbps–1 Gbps	Low, turbidity	Short-range, high rate
RF/MI	<2 m	100 kbps–10 Mbps	Low, salinity dep.	Near-surface, docking
Hybrid	Adaptive	Adaptive	Adaptive	Multi-modal swarm links

Semantic communication (SC) leverages visual LLMs at the transmitter for semantic-level image compression and prioritization. A recent framework extracts key semantic regions (e.g., as bounding boxes), encodes these regions and global context at different resolutions, and transmits with up to $99.2\%$ payload reduction, enabling robust, query-driven underwater image exchange even under harsh acoustic channel constraints. Reconstruction at the receiver via diffusion models and ControlNets preserves semantic content with state-of-the-art CLIP, SSIM, and FID metrics for $SNR \geq 0$ dB (Chen et al., 2024).

4. Protocols, Network Design, and Practical Performance

CDMA enables multiuser acoustic networking, leveraging spreading codes for robustness to multipath and multiple-access interference. A typical system achieves BER $\sim10^{-4}$ at 12 dB $E_b/N_0$ with $L=4$ resolvable paths and $SF=32$. Rake receivers and distributed power control algorithms mitigate the impact of channel variability and node heterogeneity. Three-tier architectures (seabed, mobile relays/AUVs, surface gateways) are prevalent (Bommisetty et al., 2021).

OFDM-based systems (e.g., AquaApp) leveraging software-only implementations on commodity mobile devices attain $100$ bps–$1.8$ kbps at up to $30$ m range, and beaconing to $100$ m, via adaptive subcarrier selection, channel estimation, and differential coding (Chen et al., 2022). For ultra-low energy operation, passive wake-up receivers harvesting acoustic energy from the communication signal achieve valid UUID address decoding at $0.2$ kbps and $5$ m with zero idle power draw—enabling batteryless, event-driven activation of underwater sensor nodes (Schulthess et al., 2024).

Implicit data compression protocols exploiting environmental and kinematic model predictability can reduce effective acoustic channel occupancy by up to $8\times$ and mitigate latency for dynamic vehicle interaction tasks (Rahmati et al., 2019).

5. Environmental Factors and Modality Selection

Environmental parameters—turbidity, temperature, salinity, pressure, and ocean motion—govern path loss, scattering, multipath, Doppler, and device alignment constraints. For acoustic channels, ambient noise (biological/anthropogenic), fluctuating sound speed profiles, and surface/bottom reflections impose stringent multipath and delay spread, affecting both single and multiuser (MIMO/CDMA/OAM) performance (Ramesh et al., 18 Jan 2026, Li et al., 2024, Liu et al., 5 Aug 2025). Optical modalities are extremely sensitive to water clarity, particulate concentration, and alignment, limiting practical range and dictating network topology in swarms (Kernbach et al., 2011).

Hybrid or adaptive-multimodal systems mitigate these constraints by dynamically selecting or multiplexing physical layers based on in-situ link quality, driven by real-time SNR, latency, and energy metrics (Diamant et al., 2016, Li et al., 2019, Ramesh et al., 18 Jan 2026). RF and MI are favored for near-surface, through-metal, or docking scenarios where acoustic or optical links are infeasible.

6. Research Trends and Future Directions

Key research challenges and trends include:

• Realization of robust velocity and OAM multiplexing in field-deployable, large-scale underwater networks (Liu et al., 5 Aug 2025, Li et al., 2024).
• Integration of cross-modal hybrid hardware, with optimized routing and cross-layer design (e.g., the OMR protocol dynamically balances traffic between acoustic and optical/RF links in field trials, reducing delay and improving throughput) (Diamant et al., 2016).
• Development of semantic and model-based compression schemes tightly coupled to application-layer mission requirements, reducing payload and matching the strict latency/energy constraints of underwater environments (Chen et al., 2024, Rahmati et al., 2019).
• Minimized-power and passive systems for long-duration IoT deployments, exploiting zero-idle architectures and wake-up signaling harvested from ambient communication energy (Schulthess et al., 2024).
• Simulation-to-field transfer with standardized channel, mobility, and benchmark datasets for fair and reproducible comparisons of modality performance (Ramesh et al., 18 Jan 2026).
• Emergence of bio-inspired and sub-modal signaling, pairing continuous (analog gradient) cues for spatial navigation with discrete messaging, as in light-field coordinated swarms (Kernbach et al., 2011).
• Adaptive, in situ learning protocols governing both physical and MAC layers, for sustained operation under rapidly varying oceanographic and operational conditions.

Deployment scenarios demand consideration of the sharply constrained trade-space between range, bandwidth, energy, and environmental dynamics. Thus, next-generation underwater networks will likely integrate velocity/OAM acoustic multiplexing, MI-assisted synchrony, low-power optical/RF transfer, and semantic-layer compression, with cross-layer coordination driven by real-time environmental and mission context.