This article is selected from the journal of the Chinese Academy of Engineering, China Engineering Science, No. 2, 2024
Authors: Liu Xinyu, Zhou Heng, Ge Xiyun, Jiao Huifeng
Source:Research on the development of underwater wireless communication equipment[J].Strategic Study of Chinese Academy of Engineering,2024,26(2):38-49.
Editor's note
Carrying out environmental observation and monitoring in target marine areas, and obtaining and transmitting large-scale marine environmental data are the key links to achieve the goals and tasks of marine access, protection and development. Underwater wireless communication (UWC) equipment can provide the ability of information transmission and data exchange in the underwater environment, and is an important type of equipment to support marine scientific research, underwater network monitoring, underwater cooperative operation, marine safety maintenance and other applications.
The second issue of the journal of the Chinese Academy of Engineering, China Engineering Science, in 2024 published an article "Research on the Development of Underwater Wireless Communication Equipment" by the research team of researcher Jiao Huifeng of the China Ship Scientific Research Center. This paper introduces the four main types of UWC equipment such as underwater acoustic communication, underwater optical communication, underwater electromagnetic wave communication and underwater magnetic induction communication, deeply analyzes the technical difficulties faced by them, comprehensively sorts out the development status of related equipment at home and abroad, and then condenses the future development trend of UWC equipment. Focusing on the development of the UWC industry in mainland China, this paper identifies the development dilemmas of the overall gap, underlying common problems, and top-level systems, and puts forward development suggestions such as tackling the basic mechanism and common problems, focusing on breaking through the core direction of the industry, clarifying the top-level system architecture of equipment, and improving safeguard measures and support policies. The relevant content can provide reference and inspiration for grasping the development trend of UWC equipment and laying out the development and application of UWC equipment.
I. Preface
Because of its vast area and abundant resources, the ocean has attracted mankind to continue to explore, and countries have launched competition and cooperation in the development of marine resources, the maintenance of maritime security, and the protection of rights and interests in territorial waters. After the strategy of building a maritime power was proposed, the mainland has further increased its attention to the strategic maritime space. Among them, environmental observation and monitoring in the target marine area, and the acquisition and transmission of large-scale marine environmental data are the key links to achieve the goals and tasks of marine access, protection and development.
Underwater wireless communication (UWC) equipment is a key component of marine environmental 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 for practical applications. In the civilian field, UWC equipment has played an important role in marine biological observation, marine environmental pollution monitoring, offshore oil and gas resource exploration, marine natural disaster monitoring and early warning, and marine environmental change research. In the military field, UWC equipment can assist in various tactical operations, such as underwater target information return, port and target sea area monitoring, coastal and territorial sea security, underwater launch platform cluster coordination, etc.
For most UWC equipment types, the mainland is still in the development stage of "late start, slow development, and few applications", which is not conducive to safeguarding maritime rights and interests in the context of the gradual intensification of competition for maritime rights and interests, and the development of related equipment technology needs urgently. To this end, from the perspective of technical difficulties and solutions of UWC equipment, this paper sorts out the development status at home and abroad, summarizes the future development trend, analyzes the gap between domestic equipment and the core bottleneck of industry development, and then puts forward corresponding development suggestions, so as to provide reference for the development layout of advanced marine equipment and the improvement of marine communication equipment capabilities.
2. Difficulties in underwater wireless communication technology
The underwater environment has the characteristics of poor permeability and high pressure, which makes it difficult to perceive and transmit underwater data, which increases the challenge of marine exploration and investigation. Acoustic waves, light waves, and electromagnetic waves can all be used as potential waveforms for UWC for information transmission in the underwater environment. A large number of UWC technical research and equipment development have been carried out around this (see Figure 1)
Figure 1 UWC scenario
(1) Underwater acoustic communication technology
Underwater acoustic communication (UAC) is the only reliable means to achieve wireless long-distance transmission of information in the range of hundreds to thousands of kilometers under water. Underwater acoustic channel is the wireless transmission environment experienced by the acoustic signal from the transmitter to the receiver, which is regarded as one of the most complex wireless transmission channels, due to the limited communication bandwidth, large frequency-dependent attenuation, strong noise in the colored environment, high multipath delay expansion, fast channel time-varying speed, and serious Doppler effect. The underwater acoustic channel directly leads to the energy attenuation and signal distortion of the UAC signal, which affects the communication quality of the 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 underwater acoustic channel is that the absorbed energy loss during propagation depends on the frequency of the acoustic signal, and the absorption coefficient increases rapidly with the increase of frequency. The noise contained in the sound channel is mainly composed of marine environmental noise and specific area noise: the former has a very complex sound source composition, including wind and wave noise, turbulent noise, ship noise, thermal noise, etc.; The latter is closely related to regional locations, such as the noise of ice breakage in Arctic seas, and the approximate impact noise emitted by the pincers of shrimp and crabs farmed in shallow waters. Different noises are superimposed, resulting in obvious non-white power spectral characteristics of marine noise. The attenuation increases with the increase of frequency, and the ocean noise decreases with the increase of frequency, which makes the signal-to-noise ratio in the communication frequency band change significantly.
Attenuation and noise reduce the signal-to-noise ratio of the received signal, and demodulation errors may occur. It is an effective solution in UAC to introduce redundant bits into the transmission information and use channel error correction coding such as convolutional code, low-density parity check code, and polarization code. The use of a receive array for signal acquisition and processing at the receiving end can also improve the received signal-to-noise ratio.
2. Bandwidth is severely limited
Different from the vast frequency band resources of radio propagation in the air, underwater acoustic transmission is severely restricted by energy absorption attenuation, such as the ideal usable 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 a limited communication bandwidth seriously restricts the underwater communication rate.
In order to maximize the communication rate within the limited bandwidth, the development process of UAC technology is as follows: the 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 single-receiver to multi-transmit and multi-receiver. The application of multi-transmit and multi-receiver technology, simultaneous co-frequency full-duplex technology, and non-orthogonal multiple access technology can also increase the communication rate and improve the frequency band utilization in the limited bandwidth.
3. Multipath delay extension
The multipath effect in the marine environment is mostly caused by the superposition of two phenomena. For example, sound waves are reflected from the sea surface and seabed in the ocean waveguide environment, and bend during propagation. The essential cause of the curvature of the voice is the change in the speed of sound in the ocean. In shallow water, the temperature and pressure are relatively stable, and the sound velocity changes little (relatively constant); With the increase of propagation distance, the sound waves are continuously reflected through the sea surface and seabed to form multiple paths, resulting in the expansion of time delay and the increase of inter-symbol interference. In deep water, in addition to the reflection of the sea surface and the seabed, the sound velocity varies according to the depth, and the sound line is also constantly "reflected" in the waveguide environment in the channel axis, so it shows a strong multipath effect. The time-domain shock response function of the acoustic channel is affected by reflection, which determines the number, intensity, and delay of propagation paths.
In order to deal with the inter-symbol interference introduced by multipath delay extension, a lot of research has been carried out on UAC technology in academia. The inter-symbol interference can be eliminated by channel estimation combined with zero-forcing equilibrium and minimum mean square error equalization. Both single-carrier and multi-carrier systems can be combined with more advanced Turbo equalization techniques to eliminate inter-symbol interference and additional noise effects. For multi-carrier signals such as orthogonal frequency-division multiplexing (OFDM), protection intervals such as cyclic prefixes and zero prefixes can be introduced to eliminate inter-symbol interference.
4. The channel has a fast time-varying speed
The time variation of underwater acoustic channels is strong, and its causes include slow large-scale changes caused by seasonal changes and daily tides, and rapid small-scale changes caused by sea surface waves and bubbles. Differentiating between changes in various scales based on the duration of the transmitted signal helps to improve the quality of communication. Slow large-scale changes mainly affect the average power of the signal, and rapid small-scale changes affect the instantaneous level of the signal by changing the instantaneous shock response of the channel. The modeling and analysis of large-scale changes supports the adaptive power control of the signal to improve the signal-to-noise ratio of the signal, and the modeling and analysis of small-scale changes supports the adaptive signal processing in channel estimation and equalization.
In the slow large-scale transformation, since the time-varying mainly affects the power of the signal, the adaptive power control technology can produce good results in power saving and performance improvement. In the face of the rapidly changing small-scale time-varying influence, adaptive modulation and demodulation technology is a good solution. However, any scale change requires feedback capabilities at both ends of the transmitter and receiver and the formation of a feedback link, so that the transmitter and receiver ends have the ability to perceive the underwater environment. The performance improvements obtained from feedback techniques, whether adaptive modulation or instruction transmission, depend on the quality of the channel state information fed back to the transmitter.
5. The Doppler effect is severe
The underwater sound velocity is about 1500 m/s, and the underwater acoustic signal has a large Doppler frequency shift when facing the moving platform. The magnitude of signal bandwidth and center frequency in UAC is similar, so UAC generally belongs to broadband communication. In the face of a large Doppler scale factor, each communication frequency will suffer from an uneven non-uniform Doppler shift. When a multi-carrier UAC system faces a non-uniform large-scale Doppler shift, it will produce serious signal distortion, which will deteriorate the performance of the communication system.
The Doppler frequency shift in UAC is large-scale and non-uniform, and the consistent Doppler frequency shift compensation method similar to that in narrowband radio communication cannot be adopted, and 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. It can also use new multi-carrier waveforms with Doppler robustness, such as orthogonal time-frequency air conditioning system, to replace traditional OFDM and other waveforms for underwater information transmission.
The requirements of various communication scenarios have given rise to more targeted UAC technologies. To meet the requirements of UAC confrontation, UAC signal detection and jamming, UAC jamming suppression in the context of interference are usually applied at both offensive and defensive ends. To meet the needs of underwater acoustic covert communication, bionic communication imitating marine organisms such as whales or dolphins, and camouflage communication based on ship radiation noise are often used.
(2) Underwater optical communication technology
The bandwidth of the UAC is severely limited, and it is difficult to increase the communication rate even when the UAC devices at both ends are close to each other. Underwater wireless optical communication (UWOC) technology with higher bandwidth potential has become the focus of research. However, given the complexity of the marine water environment, there are also high technical challenges in establishing reliable UWOC links.
Water has an absorption effect on light waves, and the energy attenuation of most of the light waves in the spectrum in water is large, so it cannot be compared with kilometer-level UAC technology in terms of propagation distance. However, the study of the propagation characteristics of light waves in seawater shows 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, which uses a blue-green high-power laser emitter, can travel up to several hundred meters underwater under experimental conditions. At present, the transmission rate and underwater transmission distance of the UWOC system are mainly improved through the development of high-performance transmitter equipment and the integration of new technologies to increase the system bandwidth. In the laser communication system, the external light source is injected by the optical injection locking and photoelectric feedback technology, which can significantly increase the modulation bandwidth of the communication system. For light-emitting diode (LED) devices, new materials such as indium gallium nitride and designs such as transforming a single large LED into a multi-pixel LED array are preferred to increase the bandwidth and communication rate of the communication system.
UWOC has high requirements for hydrological conditions such as water turbidity, ocean turbulence, and suspended bubbles. Ocean turbulence is usually caused by changes in the temperature, salinity, and pressure of seawater, as well as suspended bubbles in the water column, 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 air bubbles will lead to beam fluctuations and further beam misalignment, so it is particularly difficult to maintain beam tracking capabilities. Ocean turbulence can also cause random changes (scintillation) in the light signal, resulting in random changes in the direction of photons propagating in the water medium, and small changes in the direction of the beam can also produce severe signal attenuation. Analyzing and modeling the statistical characteristics of underwater turbulence and its impact on light propagation can help mitigate performance degradation caused by turbulence. The scintillation effect decreases significantly as the wavelength of the light wave increases, and the use of larger wavelengths can enhance the ability to communicate with underwater turbulence. The use of wider beams can also improve the performance of underwater optical communication links, such as beam spreading, spatial diversity in multiple-transmit, multiple-receive systems.
Commonly used photodetectors only have a small effective detection area, which needs to be accurately aligned, otherwise the link of wireless optical communication cannot be established, which leads to most wireless optical communication systems can only communicate within the line-of-sight range. The rapid change of seawater environment, underwater turbulence, turbidity, underwater obstacles and other factors make it difficult to avoid the link misalignment of the line-of-sight UWOC system. It is an effective way to improve the coverage area of the transmitter and alleviate the link mismatch by using the beam with strong scattering characteristics for water surface reflection or scattering transmission, and using the synchronization and channel estimation algorithms related to the special optical system to construct a non-line-of-sight UWOC system.
Optical communication media has visibility, so UWOC is relatively poorly concealed. High-power light emitters can cause light pollution, which has an adverse impact on the daily activities of marine organisms and 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 the cross-media communication scenario, light waves are not easy to pass through the air-water interface, and repeaters are often 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 pass through the air-water interface, significantly expand the scope of application, and facilitate the establishment of a cross-media space integrated information network system; Electromagnetic waves have higher robustness to water turbulence and turbidity. When deploying underwater electromagnetic wave communication systems, it is necessary to focus on optimizing design parameters such as communication rate, antenna design, and transmit 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 a significant frequency correlation. For example, a common 2.4 GHz wireless Bluetooth module can only travel a few tens of centimeters underwater. The underwater environment has unique physical characteristics, and the attenuation of electromagnetic waves in seawater is serious due to factors such as salt concentration, pressure, temperature, wind and waves (and the attenuation degree increases sharply with the increase of electromagnetic wave frequency), so the propagation distance of electromagnetic waves under water is limited. Although ultra-low frequency electromagnetic waves (30~300 Hz) can transmit more than 100 m in seawater, they require large-scale transmitting antenna base stations and large-size receiving antennas, which are not of practical value for small underwater platforms. In order to improve the applicability of electromagnetic wave communication, it is most likely to improve the design of magnetic antennas, and electric dipole antennas can also be used to transmit transverse electromagnetic waves. In addition to the attenuation factor, RF signals are adversely affected by environmental noise, and functional modules such as channel estimation and noise suppression need to be integrated.
(4) Underwater magnetic induction communication technology
Underwater Magnetic Induction Communication (UMIC), as an emerging UWC method, has gained extensive attention in the past decade. In 2001, the essential difference between magnetic induction theory and electromagnetic wave theory was clarified, and the foundation of theoretical construction in the field of magnetic induction communication was established. The advantage of magnetic induction communication is that the channel experienced in the underwater propagation process has the characteristics of weak multipath, weak Doppler interference, and cross-media transmission. The radiation resistance of the coil is much smaller than that of the electric dipole, and only a very small amount of energy is radiated to the far field through the magnetic induction channel and forms a multipath, which is a weak multipath; The speed of propagation is close to the speed of light, and Doppler interference is almost non-existent. 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 a good prospect for cross-media applications. The transmission and reception of magnetic induction communication are completed by a small Faraday coil, so magnetic induction technology can realize the miniaturization of equipment and improve the concealment of communication. However, similar to the underwater transmission of light and electromagnetic waves, UWC can only be achieved at a distance of tens of meters.
In 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 not sensitive to the direction of the coil, 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 design of underwater antennas and ensuring the transmission quality and reliability as much as possible, magnetic induction communication has become the focus of attention in "transmitting far and fast" underwater. For the long-distance and large-scale interconnection of underwater applications represented by the remote monitoring of underwater platforms and submersible systems, the transmission distance of UMIC practical applications is the key indicator. In order to solve the shortage of UMIC transmission distance, a relay unit can be deployed between the transmitter and receiver to build a multi-hopping magnetic induction transmission network. According to whether the relay requires additional power supply and processing unit, magnetic induction relay transmission can be divided into two categories: passive multi-coil magnetic waveguide transmission and active active relay transmission. The inherent bandwidth of UMIC is limited, the eddy current energy loss is serious, and the corresponding data transmission rate is low. Multi-band extended resonators and multi-transceiver antenna arrays in the space domain are usually used to improve the communication rate. From a technical point of view, the existing methods can be roughly divided into two categories: multi-band magnetic induction communication and multi-input multiple-output magnetic induction communication that extend the communication bandwidth.
3. The development status of underwater wireless communication equipment at home and abroad
(1) Water Voice Communication Equipment1. Overseas Water Voice Communication Equipment
Foreign UAC equipment has gone through the development process from UAC technology research to principle prototype research and development, and then to pedigree equipment manufacturing. The ATM pedigree underwater acoustic communication machine developed by Teledyne Marine in the United States adopts phase shift keying, multi-frequency keying (MFSK), frequency hopping and other communication modulation methods to achieve from 80 bps (>6 km) to 15.4 kbps (>2 km). The SoundLink UWM series underwater acoustic communication machine developed by LinkQuest in the United States has a horizontal communication distance of more than 10 km, and can achieve short-range UAC with a very low bit error rate (maximum rate of 38.4 kbps). The S2C-R series, S2C-M series and S2C-T series underwater acoustic communication machines developed by Evologics in Germany cover the medium and long-range communication distance through the medium and high frequency communication frequency bands, and the farthest horizontal communication distance is more than 10 km. The Modem 6 series underwater acoustic communication machine developed by Sonardyne in the United Kingdom can cover a range of less than 5 km with an effective communication rate of 9 kbps. It is equipped with various underwater environment perception sensors, which can be monitored for a long time and a wide range, and the underwater working time is up to 4 years.
2. Domestic underwater acoustic communication equipment
The mainland was relatively late in carrying out UAC technology research, marked by the analog communication sonar developed in the 70s of the 20th century and the digital UAC technology principle research completed in the 80s of the 20th century, which opened the development process of domestic UAC equipment. At present, the construction of a maritime power has been promoted to a new height, and scientific research institutes, universities and enterprises in the field have actively carried out UAC technology research and UAC equipment development, such as the 715th Research Institute of China State Shipbuilding Corporation, the Institute of Acoustics of the Chinese Academy of Sciences, Harbin Engineering University, Northwestern Polytechnical University, Zhejiang University, Xiamen University, Shenzhen Smart Ocean Technology Co., Ltd., Suzhou Santai Marine Instrument R&D Co., Ltd., Beijing United Shengxin Marine Technology Co., Ltd., etc. At present, the mainland has relatively complete independent research and development capabilities for UAC equipment, and has achieved a smooth transition from UAC technical theory to scientific research prototype and then to test prototype, and the developed UAC equipment has passed various lake and sea test verifications.
In order to meet the communication needs of the Jiaolong manned submersible, a hydroacoustic communication machine with data, text, voice and image transmission functions was developed, and the UAC transmission of 10-3 bit error rate and 10-4 bit error rate at 10 kbps communication rate was realized in the 5000-meter and 7000-meter sea trials, respectively. In order to meet the deep-sea vertical communication requirements of the "Struggler" manned submersible, a full-sea deep-sea acoustic communication system was developed, which realized the wireless transmission of data and commands, including images, at an inclination distance of 12.8 km. The research team of Harbin Engineering University has experience in the development of equipment for UAC systems such as MFSK, spread spectrum, single carrier, and multi-carrier, and has developed a highly reliable ultra-long-range spread spectrum underwater acoustic communication machine with a horizontal transmission distance of 100 km and a bit error rate of 10-4, which is suitable for real-time transmission of marine submersible target information and remote control command transmission of underwater unmanned underwater vehicles. The underwater acoustic high-speed communication system developed by the "Wukong" is equipped with the "Wukong" full-sea deep-water unmanned aerial vehicle, and has achieved a communication index of 2 kbps ×15 km in the Mariana Trench, reaching the international advanced level. The research team of Northwestern Polytechnical University mainly uses single-carrier and multi-carrier communication systems to complete theoretical research in near/medium/long-range UAC scenarios: in terms of long-range robust UAC, the Danjiangkou long-range UAC test has been completed, and the bit-free communication rates of 3 kbps and 4.5 kbps have been achieved at a distance of 10.8 km; The short-range high-speed UAC has passed the lake test and sea test verification.
There is still a lot of room for the pedigree development of UAC equipment in the mainland. Shenzhen Smart Ocean Technology Co., Ltd. has developed a pedigree commercial UAC equipment that includes multiple communication frequency bands, covers shallow/deep waters, and performs near/medium-range underwater acoustic transmission, with the functions of communication and navigation integration and networking expansion, and the theoretical bit error rate is 10-4 under good hydrological conditions.
(2) Underwater optical communication equipment
1. Foreign underwater optical communication equipment
The UWOC research was launched earlier abroad, and the process from technical theory research to principle prototype and then to pedigree equipment manufacturing was completed. In 2008, the laser emitter was used to achieve underwater transmission at a distance of 2 m and the transmission rate reached 1 Gbps by using near-infrared light in the 1064 nm band in the laboratory environment, which verified the feasibility of underwater light transmission. Using laser as the medium of UWOC (the emission power of the laser emitter is generally larger), it can realize high-rate and long-distance wireless transmission of underwater visible light; However, there are inherent shortcomings in laser communication, such as coherent flicker, and the two ends of the transmission and reception need to be accurately aligned and unobstructed in the communication process, which makes the practicability poor. UWOC equipment based on blue-green LED light source mostly uses incoherent light source, which does not require strict precise alignment, and the LED light source takes into account both lighting and communication functions, which significantly improves the feasibility and convenience of UWOC. Therefore, the development of UWOC equipment for LED light sources is the current mainstream choice.
The BlueComm 100 UWOC machine developed by Sonardyne in the United Kingdom uses 450 nm blue light as the light source, which is suitable for various underwater lighting conditions, and can provide a transmission rate of 5 Mbps within 15 m. The BlueComm 200 series UWOC machine, which can realize real-time underwater video transmission, uses 450 nm blue light and 400~800 nm band white light as light sources, and can achieve a transmission rate of more than 2.5 Mbps in the range of 150 m under the condition of high underwater ambient light interference. The BlueComm 200 UV underwater optical communication machine uses ultraviolet light as the light source, which has stronger anti-interference ability to natural ambient light, and is suitable for underwater platforms operating in strong light environments close to the water surface. The LUMA X series UWOC machine developed by Hydromea in Switzerland includes the LUMA X Blu-ray communication machine suitable for use in deep water environment, and the LUMA X-UV ultraviolet communication machine suitable for use in near water environment, with a short communication distance (up to only 50 m), but the maximum communication rate reaches 10 Mbps.
2. Domestic underwater optical communication equipment
In 2017, a research team from Zhejiang University used a genuine frequency division multiplexing technology with high spectrum efficiency to achieve aggregate data transmission based on red, green and blue trichromatic light at a rate of 9.51 Gb/s in a 10 m long underwater channel, and the bit error rate met the forward error correction standard. The research team at Fudan University has built a UWOC system based on green laser diodes, which uses non-return-to-zero on-off key control (NRZ-OOK) modulation to achieve data transmission at a distance of 34.5 m and a rate of 2.7 Gbps. These technological advances show that underwater laser communication can indeed achieve extremely high communication rates, although the transmission distance needs to be improved.
In terms of UWOC principle prototype and commercial equipment development, Xi'an Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences and Wuhan Liubo Optoelectronic Technology Co., Ltd. represent the leading level in China. The underwater blue-green light communication engineering prototype developed by the Xi'an Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences has been used for high-speed data transmission between underwater submersibles and high-speed data recovery of submarine observation networks. In 2020, the 11 000 m full-sea high-speed wireless blue-green optical communication engineering prototype supported the world's first 10,000-meter submarine live broadcast of the "Struggler" manned submersible. The 30 series, 50 series and 75 series underwater optical communication machine products developed by Wuhan Liubo Optoelectronic Technology Co., Ltd. have the longest communication distance of 50 m, 80 m and 100 m respectively.
(3) Underwater electromagnetic wave communication equipment
1. Foreign underwater electromagnetic wave communication equipment
During the First World War, France used electromagnetic wave communication for submarine communication tests, which was the early research and application of underwater electromagnetic wave communication technology. At present, most of the electromagnetic wave communication frequency bands used by submarines are ultra-low frequency and very low frequency bands. The penetration depth of ultra-low frequency in seawater is more than 100 meters, which is convenient for submarines to receive information in deep waters, so ultra-low frequency communication is of great value. The ultra-low frequency communication of the United States and Russia adopts the center frequency of 76 Hz and 82 Hz respectively, and can realize the command and communication of submarines over 80 m underwater without relay through electromagnetic waves. However, ultra-low frequency communication has obvious disadvantages: the communication rate is only 0.01 bps, which cannot meet the transmission needs of complex instructions, and is usually only used as a "ringing" function to notify the submarine to carry out the receiving operation; Ultra-low frequency signals usually require the use of large-size transceiver antennas, and the length of the ground-based antenna of the related system is tens of kilometers, the length of the towed antenna is more than 1 km, and the transmission power is in the megawatt range, which significantly increases the difficulty of practical application. VLF is also a common frequency band for submarine underwater communication, and the penetration ability of seawater is usually greater than 20 m, and submarines can communicate by towing antennas at periscope depth or in a submersible state. However, due to the extremely limited available bandwidth, VLF can only transmit low-speed telegraph and command information, and cannot meet the needs of high-speed underwater transmission of information. Even though there are drawbacks in electromagnetic wave communication, foreign countries are still continuing to develop miniaturized underwater electromagnetic wave communication equipment because it can conveniently carry out long-distance trans-media transmission without a ground repeater.
In 2016, the United States launched a series of R&D projects on underwater communication technology to promote the development of underwater radio communication, underwater optical communication, and underwater network communication technology. For example, the U.S. Defense Advanced Research Projects Agency (DARPA) has proposed a miniature lightweight radio transmitter project based on mechanical antennas to develop miniature radio equipment for underwater information interconnection between unmanned underwater vehicles, submarines, surface boats, and frogmen. Electromagnetic waves are generated by mechanical vibrations of special materials with strong electric fields and strong magnetic fields, and the size of wireless transmitters can be significantly reduced and equipment miniaturized.
2. Domestic underwater electromagnetic wave communication equipment
There are few institutions engaged in the research and development of underwater electromagnetic wave communication equipment in China, and the existing related research focuses on feasibility theoretical analysis and simulation, and there is still a certain gap from equipment research and development to practical application. The research team of Naval Aviation University and Naval University of Engineering completed the simulation analysis of the propagation characteristics of electromagnetic waves of different frequencies under water, the design of underwater ring antennas, and the radiation characteristics of antennas. The research team of the National University of Defense Technology has conducted comprehensive research on underwater electromagnetic wave communication technology and underwater electromagnetic wave communication system based on deep-sea direct wave model. The research team of Northwestern Polytechnical University used the current method to establish a model of underwater electromagnetic wave high-speed communication system and passed the tank test, and achieved high-speed communication with a maximum rate of 1 Mbps at a distance of 0.5 m, 0.8 m and 1 m, respectively.
(4) Underwater magnetic induction communication equipment
1. Foreign underwater magnetic induction communication equipment
UMIC technology is an emerging underwater communication method, and foreign countries are undergoing a development process from theoretical research to principle prototype development. In the early days, UMIC mainly verified the feasibility of magnetic field to transmit information underwater by experimental means. In 2001, the UMIC system for shallow water communication achieved a communication rate of 100~300 bps in the 250~400 m transmission range of air and water mixed across media. In 2010, a British research team developed a voice communication system for divers based on magnetic induction communication (with a center frequency of 12 kHz) to achieve cross-media transmission (distance of 30 m) between divers on surface, air, and underwater. UMIC has the characteristics of being inaudible and invisible, and there are no countermeasures and reconnaissance means at present, so the military prospects are good and have been highly valued by developed countries. University research teams in the United States have realized underwater magnetic induction high-speed communication under laboratory environmental conditions; Using the magnetic inductance coil, general-purpose software defined radio and MSP432 microcontroller, a magnetic communication machine model suitable for unmanned remotely operated vehicles was developed, and the binary phase-shift keying communication was carried out with the surface remotely controlled ship.
2. Domestic underwater magnetic induction communication equipment
The domestic UMIC research is also in the stage of theoretical and experimental verification, and universities are the main research force. The research team of the Naval University of Engineering has completed the research on UMIC array antenna design, antenna characteristics and magnetic field simulation. The research team of China University of Mining and Technology conducted simulation and scheme design for the underwater safety monitoring scenario, such as path loss and physical system construction of UMIC. The research team of Harbin Engineering University designed the communication circuit of the transceiver terminal based on the general software defined radio peripheral module, and built a UMIC system, which realized the 10 kbps error-free communication transmission under the condition of mixed media transmission and the transceiver coil of 20 m.
Fourth, the future development trend of underwater wireless communication equipment
(1) A high-speed and robust underwater communication system based on acoustic, optical, electrical, and magnetic multi-mode complementarity
It is an important development direction of underwater communication systems in the future to improve the reliability and transmission speed of underwater communication systems and enhance the surface and underwater maneuvering navigation and operation capabilities of various types of carrier platforms through the complementarity of sound, optical, electrical and magnetic. Integrating underwater acoustic, optical, electromagnetic and other communication means, the coupling mechanism of cross-medium magnetic induction communication, short-range visible light communication, long-range UAC and other communication modes is deeply studied, and complementary advantages are formed in terms of communication delay, rate, distance and power consumption. According to the needs of actual communication scenarios, various communication modes can be flexibly selected, so that the underwater communication system has cross-media communication capabilities, and the information between the underwater platform and the shore base can be efficiently interconnected.
(2) The integrated equipment architecture of underwater acoustic "exploration and guidance" function
At present, most of the underwater acoustic detection, UAC, and acoustic positioning and navigation equipment are independently designed and applied, and the volume occupation, power consumption, and frequency band resource allocation of related equipment are strictly constrained, which is not conducive to the deployment and use on small underwater platforms. However, from a functional point of view, the working principle, system architecture, signal processing, and working frequency bands of underwater acoustic detection, UAC, and acoustic positioning and navigation systems are similar, which creates a feasibility for the design of "exploration and guidance" functional integration equipment. With the development and growth of marine information networks, various types of underwater platforms show an application trend of cooperative operation. Integrating detection, communication, navigation and positioning technologies and carrying out integrated equipment architecture design are important development directions to realize the resource sharing of underwater platforms, improve operational efficiency, enhance concealment performance, and reduce platform volume 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 era, and the deployment of 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 integration of the "air, space and sea" information Internet of Things. The main features of underwater Internet of Things that distinguish it from traditional underwater communication networks are miniaturization, low power consumption, organic coupling of multi-modal communication system, intelligent services, and multiple challenges in the harsh underwater environment, so it has become one of the key directions for technological breakthroughs in 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, and it is necessary to develop an intelligent multi-mode integrated underwater communication network. (1) In view of the changing channel environment and complex communication scenarios, it can intelligently select and adjust, and use multi-mode fusion to build a cross-media communication chain from the seabed to the air, providing a physical layer support foundation for stable and efficient network services. (2) The intelligence of network nodes can improve the adaptive ability of the underwater communication network to complex environments, and 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 very important for the long-term and large-scale coverage service of the underwater communication network, and the purpose of reducing the power consumption of the node can be achieved by sharing the sensor base array at both ends of the transmission and receiving ends, adopting a common component scheme and combining with a low-complexity algorithm. (4) Research on low-power network routing protocols that adapt to the underwater communication environment, optimize propagation path planning, and support low-latency and low-energy information collection and transmission.
(4) All-mode spectrum integration and collaborative precision countermeasure 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 coordination and integration, and precision of offensive and defensive confrontation. As the complexity of the battlefield communications environment increases, the countermeasure equipment of a single communication system no longer meets the needs of application, and the communication equipment with the characteristics of acoustic, optical, electrical, and magnetic fusion has become a development trend. In the future, communication countermeasures equipment also needs to integrate various communication modes and cover the full frequency domain of countermeasure capabilities to achieve indiscriminate interference and defense. In the face of the rapidly changing battlefield confrontation situation, improving the comprehensive application efficiency of underwater communication countermeasure equipment has become a key direction. Build an integrated collaborative information network, support shortening decision-making time, improve the timeliness of instruction transmission, form an integrated, networked, and intelligent equipment system, and enhance the ability of the confrontation system to make joint accusations. In the scenario of simultaneous expansion of the number of equipment and the scale of the confrontation network, further strengthening the accuracy of underwater communication and countermeasure equipment in terms of situational awareness and identification of friend or foe will help improve the ability of underwater asymmetric information counterbalance.
5. The development dilemma faced by the mainland's underwater wireless communication equipment
(1) Overall gaps
On the whole, due to its late start, the mainland UAC equipment field lags behind developed countries by about 5 years in terms of productization and maturity. Domestic institutions also lag behind the international advanced level in terms of propagation distance, bit error rate performance, equipment reliability, and product genealogy. It is necessary to deepen the research on UAC algorithms, network protocols, experiments and applications, strengthen the hardware development of transducers and digital systems, and promote the development of UAC genealogy products.
In the field of underwater optical communication equipment, there is a gap between domestic equipment and foreign products in terms of reliability, miniaturization, integration, manufacturing technology, etc., and a UWOC equipment system dedicated to small underwater mobile platforms has not yet been formed, and most of them stay in the stage of test verification and prototype design.
In the field of underwater electromagnetic wave equipment, it is an emerging development direction for foreign countries to develop practical miniature underwater electromagnetic wave communication machines, extend wireless communication and data transmission to the seabed, underground and other environments that do not yet have wireless communication capabilities, realize cross-media communication and enhance application capabilities. In contrast, no similar research has been launched in China, and there is an urgent need to lay out research and development projects of underwater electromagnetic wave communication technology to support the innovation and development of related technology applications.
In the field of magnetic induction communication, at present, both at home and abroad are in the stage of theoretical research and experimental verification, and no practical underwater communication equipment has been formed. In view of the inherent advantages of magnetic induction communication in terms of cross-medium, concealment, and communication rate, it is necessary to strengthen the research and development of magnetic induction communication equipment and accelerate the underwater practical application of magnetic induction communication equipment.
(2) Underlying common problems
Traditional marine equipment powers have achieved modularization, digitization, standardization, genealogy, and intelligent development in core sensor systems and processor chips, and related products occupy a dominant position in the international market. In the mainland, the core sensors and processor chips required for the "exploration and guidance" equipment are mostly imported, and they are facing potential monopoly, blockade and embargo risks. Although most conventional sensors are basically localized, there are few practical applications, resulting in the domestic market still dominated by imported products, and the high-end sensors required for deep-sea scenarios are particularly prominent.
The accuracy, stability, and adaptability of domestic sensors are still insufficient. In terms of UWOC equipment, the maturity of domestic high-performance optical devices is not as good as that of imported products, and it cannot meet the actual needs of underwater high-speed wireless communication. It is necessary to focus on the development of high-speed and high-power indium gallium nitride LED devices, so as to solve the bottleneck problem of "not far and fast" in the UWOC system. Only by breaking through the core technology of such new materials and devices and accelerating the process of productization and industrialization can we truly get rid of the status quo of import dependence and effectively improve the development level of domestic deep-sea communication equipment.
For processes such as pressure resistance and sealing under deep sea conditions, the underlying theoretical analysis ability required for systematic design is still lacking, and the breadth and depth of relevant numerical simulations are also insufficient. Only after solidly improving our basic research capabilities can we lay a solid foundation for tackling key technological problems and engineering research and development.
(3) Top-level system
At present, the development focus of the field of deep-sea communication equipment in the mainland is still based on high-speed and long-distance data transmission, equipment stability and reliability, data security and privacy protection, equipment energy supply and maintenance, etc., while the relevant top-level system is not clear enough, and the direction of industry development is not strong. Although some deep-sea communication equipment has completed prototype development and related tests, there is still a certain gap in the application of the platform compared with the international advanced level. It is necessary to sort out the technical foundation, formulate equipment planning, and determine key technical directions and medium and long-term development goals through system traction.
6. Suggestions for the development of underwater wireless communication equipment in the mainland
(1) Tackling key problems in basic mechanisms and common problems
Carry out research on the propagation mechanism of various communication media and establish corresponding mathematical and physical models, so as to provide a solid theoretical foundation for the high-quality development of UWC equipment. In view of the weak links in the development of high-end sensors, we will deploy technical research projects in marine sensor materials and processes, and strive to solve the common problems of marine sensors restricting the development of UWC equipment. Based on independent innovation, we will support the basic and original research of UWC equipment, carry out research on the development and application of new materials, the integration of new principles and new methods, and new processes, improve the research level of sensor structure design, material development, and common generalization, and overcome the "bottleneck" problem that restricts the development of UWC equipment.
(2) Focus on breaking through the core direction of the industry
The R&D and application of UWC equipment has the particularity of large investment, long cycle and low demand, and the activity and participation of enterprises are low due to the immaturity of the relevant application market. It is suggested to adopt a diversified financing model, set up an underwater communication equipment R&D and venture capital fund, aim at the key development direction of UWC equipment, concentrate industry forces to carry out key core technology research, and improve the UWC equipment industry chain. It will focus on enhancing the capabilities of UWC equipment intelligent environment perception, surface-underwater cross-domain communication between various types of underwater information equipment, large-scale interactive communication networking of underwater multi-modal information, multi-dimensional information fusion integrating "exploration, communication and guidance", and intelligent unmanned network confrontation of underwater attack and defense.
(3) Clarify the top-level system architecture of equipment
To meet the needs of deep-sea communication, carry out top-level design in the field of deep-sea UWC equipment, and form a medium- and long-term development plan for deep-sea UWC equipment and technology. Driven by the needs of platform design, we will accurately carry out UWC equipment development and technical research, and then promote the improvement of platform capabilities with the development of relevant equipment and technology, and build a sustainable development mechanism. The corresponding development strategies are: to develop independent UWC equipment, to carry out various types of platform applications, to form an independent functional system; Carry out the intelligent and networked function upgrade of the UWC equipment of the platform, and build a deep-sea information integrated system including deep-sea vigilance, observation, communication, navigation and positioning functions; Form a three-dimensional and multi-dimensional deep-sea information system to expand the application scope of deep-sea equipment on the mainland.
(4) Improve safeguard measures and support policies
It is recommended to set up a public test platform for UWC equipment to provide consistent test guarantee conditions for universities, scientific research institutions and enterprises in the industry. Coordinate the development of the UWC equipment sharing management platform, solve the actual application needs of users and improve the utilization rate of equipment, widely obtain feedback from front-line users, and form a virtuous cycle development model of UWC equipment. According to the development plan of UWC equipment system, industry standards and specifications are formulated in a collaborative way of "production, education, research and application" to improve the standardization level of UWC equipment manufacturing and application. Issued long-term and stable incentive policies to support the development of UWC equipment manufacturing industry, deepen domestic application and high-level "going out" at the same time, and expand the UWC equipment industry in mainland China.
Note: The presentation of the content of this article has been slightly adjusted, if necessary, you can view the original article.
Note: This paper reflects the progress of research results and does not represent the views of Chinese Journal of Engineering Science.