Security, Technology

summary

Underwater wireless communication (UWC) equipment provides the ability to transmit information and exchange data in underwater environments. It is an important type of equipment that supports applications such as marine scientific research, underwater networking monitoring, underwater collaborative operations, and marine safety maintenance. Starting from the four main types of UWC equipment, namely underwater acoustic communication, underwater optical communication, underwater electromagnetic wave communication, and underwater magnetic induction communication, this article deeply analyzes the technical difficulties faced by each of them, comprehensively sorts out the current development status of related equipment at home and abroad, and then summarizes the future development trend of UWC equipment. Focusing on the development of my country’s UWC industry, this paper identifies the development difficulties in terms of overall gaps, underlying common problems, and top-level systems, and puts forward development suggestions such as tackling basic mechanisms and common problems, focusing on breaking through the core direction of the industry, clarifying the top-level system architecture of equipment, and improving guarantee measures and support policies. The relevant content can provide reference and inspiration for grasping the development trend of UWC equipment and planning the development and application of UWC equipment.

The ocean attracts human beings to explore it because of its vast area and rich resources. Countries have started to compete and cooperate in the development of marine resources, maintenance of marine security, and protection of territorial rights. After the strategy of building a strong marine nation was proposed ^[ 1 ] , China has further increased its attention to the strategic space of the ocean. Among them, conducting environmental observation and monitoring of target marine areas and acquiring and transmitting large-scale marine environmental data are key links in achieving the goals of marine access, protection, and development.

Underwater wireless communication (UWC) equipment is a key component of marine environment observation and monitoring systems and underwater sensor networks. At present, the more mature UWC equipment mainly includes underwater acoustic communication equipment  , underwater optical communication equipment , and underwater electromagnetic wave communication equipment  ; the emerging underwater magnetic induction communication has also been studied in practical applications . In the civilian field, UWC equipment has played an important role in marine biological observation, marine environmental pollution monitoring, offshore oil and natural gas resource exploration, marine natural disaster monitoring and early warning, and marine environmental change research  . In the military field, UWC equipment can assist in completing various tactical operations, such as underwater target information feedback, port and target sea area monitoring, coastal and territorial waters security, and underwater carrier platform cluster coordination.

For most types of UWC equipment, my country is still in the development stage of “late start, slow development, and few applications”, which is not conducive to maintaining marine rights and interests under the background of gradually intensifying competition for marine rights and interests, and the development of related equipment technology is urgent. To this end, this article starts from the perspective of UWC equipment technical difficulties and solutions, sorts out the current development status at home and abroad, summarizes future development trends, analyzes the gaps in domestic equipment and the core bottlenecks of industry development, and then puts forward corresponding development suggestions to provide reference for the development layout of advanced marine equipment and the improvement of marine communication equipment capabilities.

The underwater environment has characteristics such as poor permeability and high pressure ^[ 7 ] , which makes underwater data difficult to perceive and transmit, increasing the challenges of ocean exploration and investigation. Sound waves, light waves, and electromagnetic waves can all be used as potential waveforms for UWC and can be used for information transmission in underwater environments. A large number of UWC technology research and equipment development have been carried out around this ( see Figure 1 ).

1.Underwater acoustic communication technology

Underwater acoustic communication (UAC) is the only reliable means of wireless long-distance information transmission covering hundreds to thousands of kilometers underwater. The underwater acoustic channel is the wireless transmission environment that the acoustic signal experiences from the transmitter to the receiver. It has limited communication bandwidth, large frequency-related attenuation, strong colored environmental noise, high multipath delay spread, fast channel time-varying speed, and severe Doppler effect. It is considered one of the most complex wireless transmission channels. The underwater acoustic channel directly causes energy attenuation and signal distortion of the UAC signal, affecting the communication quality of UAC, which is the main problem restricting the development of UAC technology.

1 Attenuation and colored noise
In terms of energy attenuation, one of the important characteristics of the underwater acoustic channel is that the absorption energy loss during the propagation process depends on the frequency of the acoustic signal, and the absorption coefficient increases rapidly with the increase of frequency. The noise contained in the acoustic channel is mainly composed of marine environmental noise and specific regional noise: the sound source of the former is very complex, including wind and wave noise, turbulence noise, ship noise, thermal noise, etc.; the latter is closely related to the regional location, such as the ice breaking noise in the Arctic Ocean and the approximate impact noise emitted by the claws of shrimps and crabs farmed in shallow waters. The superposition of different noises causes the ocean noise to present obvious non-white power spectrum characteristics. Attenuation increases with the increase of frequency, and ocean noise decreases with the increase of frequency, which makes the signal-to-noise ratio in the communication band change significantly.

Attenuation and noise reduce the signal-to-noise ratio of the received signal, which may cause demodulation errors. Introducing redundant bits in the transmitted information and using channel error correction coding such as convolutional codes, low-density parity check codes, and polarization codes are effective solutions in UAC. Using a receiving array for signal acquisition and processing at the receiving end can also improve the received signal-to-noise ratio.

2 Severely limited bandwidth
Unlike the wide frequency band resources of radio propagation in the air, underwater acoustic transmission is severely restricted by energy absorption and attenuation. For example, the ideal available signal bandwidth for a transmission distance of 10 km is only tens of kilohertz, and the available signal bandwidth for a transmission distance of 100 km is only 1 kHz. It can be seen that such limited communication bandwidth seriously restricts the underwater communication rate.

In order to maximize the communication rate within a limited bandwidth, the development process of UAC technology is: transition from analog communication technology to digital communication technology, from incoherent communication technology to coherent communication technology, from single-carrier communication to multi-carrier communication, and from single-transmit and single-receive to multiple-transmit and multiple-receive. The application of multiple-transmit and multiple-receive technology, simultaneous full-duplex technology, non-orthogonal multiple access technology, etc. can also increase the communication rate and improve the frequency band utilization within a limited bandwidth.

3 Multipath Delay Spread

The multipath effect in the marine environment is mostly caused by the superposition of two phenomena. For example, sound waves are reflected by the sea surface and the seabed in the marine waveguide environment, and bend during the propagation process; the essential reason for the bending of the sound line is the change in the speed of sound in the ocean. In shallow water, the temperature and pressure are relatively stable, and the sound speed changes little (relatively constant); as the propagation distance increases, the sound waves are constantly reflected by the sea surface and the seabed to form multipath ^[ , resulting in delay expansion and increased inter-code interference. In deep water, in addition to the reflection of the sea surface and the seabed, the sound speed changes with depth, and the sound line is also constantly “reflected” in the waveguide environment within the channel axis, thus showing a strong multipath effect. The time domain impulse response function of the acoustic channel is affected by reflection, which determines the number, intensity and delay of the propagation path.

In order to deal with the inter-symbol interference introduced by multipath delay spread, the academic community has conducted a lot of research on UAC technology. Inter-symbol interference can be eliminated through channel estimation combined with zero-forcing equalization, minimum mean square error equalization and other methods. Single-carrier and multi-carrier systems can be combined with more advanced Turbo equalization technology to eliminate inter-symbol interference and additional noise effects. Multi-carrier signals such as orthogonal frequency division multiplexing (OFDM) can introduce protection intervals such as cyclic prefixes and zero prefixes to eliminate inter-symbol interference.

4 Fast channel time-varying speed

The underwater acoustic channel has a strong time-varying property, which is caused by slow large-scale changes caused by factors such as seasonal changes and daily tides, and fast small-scale changes caused by factors such as sea waves and bubbles. Differentiating various scale changes according to the duration of the transmitted signal helps improve the communication quality. Slow large-scale changes mainly affect the average power of the signal, and fast small-scale changes affect the instantaneous level of the signal by changing the instantaneous impulse response of the channel. Modeling and analysis of large-scale changes supports adaptive power control of the signal to improve the signal-to-noise ratio of the signal, and modeling and analysis of small-scale changes supports adaptive signal processing in aspects such as channel estimation and equalization.

In the case of slow large-scale changes, since time-varying mainly affects the power of the signal, adaptive power control technology can produce good results in power saving and performance improvement. When facing the influence of fast-changing small-scale time-varying, adaptive modulation and demodulation technology is a good solution. However, any scale change requires both the transmitter and receiver to have feedback capabilities and form a feedback link, so that both ends have the ability to perceive the underwater environment; the performance improvement obtained from feedback technology, whether it is adaptive modulation or command transmission, depends on the quality of the channel state information fed back to the transmitter.

5 The Doppler effect is serious

The speed of sound underwater is about 1500 m/s, and underwater acoustic signals have a large Doppler shift when facing a moving platform. The signal bandwidth and center frequency in UAC are close in magnitude, so UAC generally belongs to broadband communication; when facing a large Doppler scale factor, each communication frequency point will suffer from uneven and non-uniform Doppler shift. When a multi-carrier UAC system faces non-uniform large-scale Doppler shift, it will produce serious signal distortion, thereby deteriorating the performance of the communication system.

The Doppler shift in UAC is characterized by large scale and non-uniform characteristics, and it is impossible to use the consistent Doppler shift compensation method similar to that in narrowband radio communication. Only carrier phase tracking and carrier frequency compensation can be used. In UAC, it is generally necessary to estimate the large-scale Doppler factor first, and then use frequency domain interpolation, time domain resampling and other methods to offset the underwater acoustic Doppler effect. Orthogonal time-frequency air modulation and other new multi-carrier waveforms with Doppler robustness can also be used to replace traditional OFDM waveforms for underwater information transmission.

The demands of various communication scenarios have given rise to more targeted UAC technologies. In response to the needs of UAC confrontation, technologies such as UAC signal detection and interference, and UAC interference suppression under interference background are usually applied on both the offensive and defensive ends. In response to the needs of underwater acoustic covert communication, technologies such as bionic communication imitating marine life such as whales or dolphins, and camouflage communication based on ship radiation noise are more commonly used.

2.Underwater optical communication technology

UAC bandwidth is severely limited, and the communication rate is difficult to increase even when the distance between the UAC machines at both ends is close. Underwater wireless optical communication (UWOC) technology with higher bandwidth potential has become a research focus. However, given the complexity of the marine water environment, there are also high technical challenges in establishing a reliable UWOC link.

Water absorbs light waves, and most of the light waves in the spectrum have a large energy attenuation in water, so the propagation distance cannot be compared with the kilometer-level UAC technology. However, the study of the propagation characteristics of light waves in seawater found that the blue-green band in the spectrum is an optical window with relatively weak underwater attenuation, which provides a theoretical basis for the short-distance and high-speed transmission of light waves underwater. The UWOC machine using a blue-green high-power laser transmitter can propagate hundreds of meters underwater under experimental conditions. At present, the transmission rate and underwater transmission distance of the UWOC system are mainly improved by developing high-performance transmitter equipment and integrating new technologies to increase the system bandwidth. In the laser communication system, the external light source is injected by optical injection locking and photoelectric feedback technology, which can significantly increase the modulation bandwidth of the communication system. For light-emitting diode (LED) equipment, new materials such as indium gallium nitride and the transformation of a single large LED into a multi-pixel LED array are preferred to improve the communication system bandwidth and communication rate.

UWOC has high requirements for hydrological conditions such as water turbidity, ocean turbulence, and suspended bubbles. Ocean turbulence is usually caused by changes in seawater temperature, salinity, pressure, and suspended bubbles in the water, and can last for a long time. Underwater wireless laser communication systems have strict requirements for beam positioning, capture, and tracking, and the presence of ocean turbulence and suspended bubbles will cause beam fluctuations and further beam misalignment, making it particularly difficult to maintain beam tracking capabilities. Ocean turbulence can also cause random changes (scintillation) in optical signals, resulting in random changes in the propagation direction of photons in the water medium, and slight changes in the direction of the beam will also produce severe signal attenuation. Analyzing and modeling the statistical characteristics of underwater turbulence and its impact on light propagation can help alleviate the performance degradation caused by turbulence. The scintillation effect decreases significantly with the increase in the wavelength of the light wave, and the use of larger wavelengths can enhance the communication capability to cope with underwater turbulence. Using wider beams can also improve the performance of underwater optical communication links, such as beam expansion and spatial diversity in multiple-transmitter and multiple-receiver systems.

Commonly used photodetectors have only a small effective detection area and require precise alignment, otherwise the wireless optical communication link cannot be established. This means that most wireless optical communication systems can only communicate within the line-of-sight range. The rapid changes in the seawater environment, underwater turbulence, turbidity, underwater obstacles and other factors make it difficult to avoid link misalignment in line-of-sight UWOC systems. Using light beams with strong scattering characteristics for water surface reflection or scattering transmission, and using synchronization and channel estimation algorithms related to dedicated optical systems to build non-line-of-sight UWOC systems is an effective way to increase the transmitter coverage area and alleviate link mismatch.

Optical communication media are visible, so the concealment of UWOC is relatively poor. High-power light source transmitters will cause light pollution, which has an adverse effect on the daily activities of marine life and also poses a potential threat to the marine ecological environment.

 3.Underwater electromagnetic wave communication technology

Although underwater optical communication has a high communication rate, in cross-media communication scenarios, light waves are not easy to pass through the air-water interface, and repeaters are usually required for signal forwarding. Compared with acoustic/optical communication systems, underwater electromagnetic wave communication has advantages: electromagnetic waves can be directly emitted from the transmitting base station and communicate with underwater targets, smoothly passing through the air-water interface, significantly expanding the scope of application, and facilitating the establishment of a comprehensive information network system across medium space; electromagnetic waves have higher robustness in the face of water turbulence, turbidity, etc. When deploying underwater electromagnetic wave communication systems, it is necessary to focus on optimizing design parameters such as communication rate, antenna design, and transmission power intensity.

Similar to the underwater transmission of light waves, the transmission attenuation of electromagnetic waves in seawater is also large, and it also shows obvious frequency correlation. For example, the common 2.4 GHz wireless Bluetooth module can only propagate tens of centimeters underwater. The underwater environment has unique physical characteristics. Factors such as salt concentration, pressure, temperature, wind and waves cause the electromagnetic waves in seawater to attenuate more seriously (and the attenuation degree increases sharply with the increase of electromagnetic wave frequency), so the propagation distance of electromagnetic waves underwater is limited. Although ultra-low frequency electromagnetic waves (30~300 Hz) can be transmitted in seawater for more than 100 m, large-scale transmitting antenna base stations and large-sized receiving antennas are required, which has no practical value for smaller underwater platforms. In order to improve the applicability of electromagnetic wave communication, improving the design of magnetic antennas is the most likely solution, and electric dipole antennas can also be used to transmit transverse electromagnetic waves. In addition to the attenuation factor, the RF signal is adversely affected by environmental noise, and it is necessary to integrate the functional modules such as channel estimation and noise suppression.。

4.Underwater magnetic induction communication technology

As an emerging UWC method, underwater magnetic induction communication (UMIC) has received widespread attention in the past decade. In 2001, the essential difference between magnetic induction theory and electromagnetic wave theory was clarified, and the foundation for the theoretical construction of magnetic induction communication was established. The advantage of magnetic induction communication is that the channel experienced during underwater propagation has the characteristics of weak multipath, weak Doppler interference, and cross-medium transmission. The radiation resistance of the coil is much smaller than the radiation resistance of the electric dipole. Only a very small amount of energy is radiated to the far field through the magnetic induction channel and forms multipath, which is weak multipath; the propagation speed is close to the speed of light, and there is almost no Doppler interference. The temperature, turbidity, and salinity of seawater affect the underwater transmission of sound, light, and electromagnetic waves, but the magnetic permeability of seawater is almost the same as that of air, so the channel response of magnetic induction waves is more stable and predictable, which also makes magnetic induction communication have good cross-medium application prospects. The transmission and reception of magnetic induction communication are both completed through small-sized Faraday coils, so magnetic induction technology can achieve miniaturization of equipment and improve communication concealment; but similar to the underwater transmission of light and electromagnetic waves, it can only achieve UWC at a distance of tens of meters.

During the UMIC process, the frequent changes in the coil direction lead to uncontrollable received signal-to-noise ratio, so the reliability of UMIC demodulation performance is poor. The focus of related research is to design antennas that are insensitive to the coil direction, and gradually develop from traditional unidirectional magnetic induction antennas to multi-directional magnetic induction antennas, such as three-way magnetic induction antennas, metamaterial-enhanced magnetic induction antennas, and spherical coil array closed loop antennas. After optimizing the underwater antenna design and ensuring the transmission quality and reliability as much as possible, the “long-distance and fast transmission” of magnetic induction communication underwater has become the focus of attention. For long-distance, large-scale interconnected underwater applications represented by remote monitoring of underwater platforms and buoy systems, the transmission distance of UMIC actual application is a key indicator. In order to solve the shortcoming of UMIC transmission distance, relay units can be deployed between the transmitter and the receiver to build a multi-hop magnetic induction transmission network. Depending on whether the relay requires additional power and processing units, magnetic induction relay transmission can be divided into two categories: passive multi-coil magnetic induction waveguide transmission and active active relay transmission. UMIC has limited inherent bandwidth and serious eddy current energy loss, and the corresponding data transmission rate is low. Generally, multi-band extended resonators and spatial domain multi-transmitting and receiving antenna arrays are used to increase the communication rate. From a technical perspective, existing methods can be roughly divided into two categories: multi-band magnetic induction communication that extends the communication bandwidth and multi-input multi-output magnetic induction communication.

5.Future development trend of underwater wireless communication equipment

（1） High-speed and robust underwater communication system based on acoustic, optical, electrical and magnetic multi-mode complementarity

Improving the reliability and transmission speed of underwater communication systems and enhancing the surface/underwater maneuvering and operation capabilities of various types of carrier platforms through multi-mode complementation of acoustic, optical, electrical, and magnetic is an important development direction for underwater communication systems in the future. Integrate hydroacoustic, optical, electromagnetic and other communication means, conduct in-depth research on the coupling mechanism of cross-medium magnetic induction communication, short-range visible light communication, long-range UAC and other communication modes, and form complementary advantages in communication delay, rate, distance, and power consumption. According to the needs of actual communication scenarios, flexibly select various communication modes to enable underwater communication systems to have cross-medium communication capabilities and realize efficient information interconnection between underwater platforms and shore bases.

（2） Integrated equipment architecture for underwater acoustic “detection, communication and navigation” functions

Most of the current underwater acoustic detection, UAC, and acoustic positioning and navigation equipment are designed and applied independently. The volume occupancy, power consumption, and frequency band resource allocation of the related equipment are strictly constrained, which is not conducive to their deployment and use on small underwater platforms. However, from a functional perspective, the working principles, system architecture, signal processing, and working frequency bands of underwater acoustic detection, UAC, and acoustic positioning and navigation systems are similar, creating feasibility for the design of integrated equipment with “detection, communication, and navigation” functions. With the development and growth of marine information networks, various types of underwater platforms are showing an application trend of collaborative operations; integrating detection, communication, and navigation and positioning technologies and carrying out integrated equipment architecture design are important development directions for realizing underwater platform resource sharing, improving operational efficiency, enhancing concealment performance, and reducing platform size and power consumption.

（3） Intelligent multi-mode integrated and low-power communication network for underwater Internet of Things

The Internet of Everything is the development theme of the digital age. Deploying the Internet of Things to the underwater environment has become an important development trend of the future underwater communication network, and it is also a key link in the formation of the “air, space, sea and land” integrated information Internet of Things. The main features of the underwater Internet of Things that are different from the traditional underwater communication network are miniaturization, low power consumption, organic coupling of multi-modal communication systems, and intelligent services. In addition, it faces multiple challenges in the harsh underwater environment, so it has become one of the key directions for the technological breakthrough of underwater wireless communication equipment in the future. The traditional acoustic single-mode communication network has inherent defects such as high transmission delay, low rate, and limited application scenarios. It is necessary to develop an intelligent multi-mode integrated underwater communication network. ① In response to the changing channel environment and complex communication scenarios, it can be selected and adjusted intelligently, and a cross-media communication chain from the seabed to the air can be constructed by using multi-mode fusion to provide a physical layer support foundation for stable and efficient network services. ② The intelligence of network nodes can enhance the adaptive ability of underwater communication networks to complex environments. Algorithms such as deep reinforcement learning support the training system to find the optimal strategy in the process of interacting with the working environment, so as to make adaptive adjustments according to the time-varying environment and optimize deployment decisions. ③ The low power consumption level of each node and the network as a whole is crucial for the long-term and wide-range coverage services of underwater communication networks. Sharing the sensor arrays at both ends of the transmission and reception, adopting common component solutions and combining low-complexity algorithms can achieve the purpose of reducing node power consumption. ④ Research low-power network routing protocols that adapt to underwater communication environments, optimize propagation path planning, and support low-latency, low-energy information collection and transmission.

（4）All-mode spectrum integrated collaborative precision confrontation network

In order to meet the needs of underwater communication confrontation in the future, underwater communication confrontation equipment should have the capabilities of full coverage of communication system, high degree of coordinated integration, and precise attack and defense confrontation. As the complexity of battlefield communication environment increases, confrontation equipment with a single communication system no longer meets application needs, and communication equipment with acoustic, optical, electrical, and magnetic fusion characteristics has become a development trend. Future communication confrontation equipment also needs to integrate various communication modes and cover the confrontation capabilities of the entire frequency domain to achieve indiscriminate interference and defense. In the face of rapidly changing battlefield confrontation situations, improving the comprehensive application efficiency of underwater communication confrontation equipment has become a key direction; building an integrated collaborative information network to support shortening decision-making time, improve the timeliness of command transmission, form an integrated, networked, and intelligent equipment system, and enhance the joint command and control capabilities of the confrontation system. In the scenario where the number of equipment and the scale of the confrontation network are expanding simultaneously, further strengthening the accuracy of underwater communication confrontation equipment in situational awareness, enemy identification, etc. will help improve the underwater asymmetric information balance capability.