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Brief research report article, research on underwater wireless dynamic optical communication system based on ppm modulation.

• 1 National and Local Joint Engineering Research Center of Space Optoelectronics Technology, Changchun University of Science and Technology, Changchun, China
• 2 College of Optoelectronic Engineering, Changchun University of Science and Technology, Changchun, China

In order to achieve underwater wireless dynamic optical communication, a laser communication system is proposed based on Pulse Position Modulation (PPM). In order to achieve underwater laser communication accurately, the mathematical model of underwater laser communication was constructed with small angle analysis. The pulse position modulation demodulation algorithm is designed, and the workflow of modulation and demodulation is given in the transmit module and the receive module. In the experiment, Lumileds-470 nm light source was selected for data communication for testing at a communication rate of 15 Mbp/s. In the servo control process, the square wave signal used for stepping motor drive had a stable amplitude output and a stable time width. It can well simulate the testing process of underwater dynamic scanning. In the experiment, laser light spots were obtained under different attenuation states, and the characteristics of the light spot distribution were analyzed. The numerical reconstruction of the light spot energy was completed in MATLAB. Three types of light attenuators, 1.0%, 0.1%, and 0.01%, were used to simulate different light attenuations underwater. The test results show that the system error rate is better than 10 −6 when attenuation chip is 1.0%. When attenuation chip is 0.1%, the error rate of the system is reduced to 10 −4 . When attenuation chip is 0.01%, a valid signal cannot be obtained by the system. The feasibility of the system is verified.

1 Introduction

Underwater wireless communication technology is widely used in resource exploration [ 1 ], abnormal sea state monitoring [ 2 ], sensor monitoring [ 3 ], underwater imaging [ 4 ], underwater robots [ 5 ], and airspace integration [ 6 ]. It is important advantages for blue or green laser, such as good penetration ability, strong anti-interference, large communication depth, and high transmission rate in seawater. It has become a research hotspot.

The characteristics of underwater laser communication include: a) high transmission rate. Due to the use of high-frequency information transmission, the transmission rate can reach Mbit/s, far due to traditional acoustic equipment. b) Large information capacity. The blue light of laser communication is about 620 THz, which can support the construction of large capacity link systems. It has the underwater transmission requirements of image signals and multi-channel video signals, and its high bit rate is generally around the GHz level. c) Strong anti-interference. The laser transmission process is not affected by electromagnetic interference. It is not significantly affected by seawater temperature, concentration, etc. d) High safety and density. Light wave has strong directionality and small beam divergence angle. If the communication signal is truncated, the receiving end will lose the signal, which is easy to be detected in a timely manner, so its security and confidentiality are very good. e) The system structure is compact. Underwater laser communication systems have a compact structure and low power, making them more concealable.

Duntley [ 7 ] found that lasers with a wavelength range of 470–525 nm can effectively reduce the light wave absorption of seawater. Shimura S [ 8 ] had experimentally verified that the visible light band has a low loss window, and its propagation loss was only 1% of that of other light waves. This discovery will have greatly advanced for the underwater laser communication research. Tivey et al. [ 9 ] developed an underwater wireless communication system. It still had the ability to transmit data underwater. The system achieves effective communication of signals within a range of 5 m. Huang A P et al. [ 10 ] designed an optical communication system with acoustic systems. This system used low-power receivers and it could achieve underwater data transmission of 10–20 Mbit/s within 100 m. Campagnaro F et al. [ 11 ] used a 405 nm wavelength laser to conduct communication tests in 4.8 m long clear water, when the communication rate is 1.45 Gbit/s, the bit error rate is only 9.1 × 10 −4 . Fu et al. [ 12 ] studied the impact of communication aperture on the bit error rate of underwater wireless optical communication systems under medium intensity turbulence. Han et al. [ 13 ] used electro-optical crystal modulation to generate communication signals, and completed the extinction characteristics of underwater communication systems on modulated signals by Monte Carlo method. Based on the analysis of transmission link span and laser divergence angle, Vali et al. [ 14 ] analyzed and discussed the effects of system aperture and field of view on underwater communication under turbulent conditions.

In summary, this paper studies an underwater laser communication system based on PPM (Pulse Position Modulation [ 15 ]) modulation. The system uses a servo mechanism to align the optical path at both ends of the laser communication, then uses PPM modulation technology to complete data communication, and simulates underwater communication by using an attenuator.

2 Underwater laser communication system design

In this paper, a blue laser communication experimental system based on PPM modulation and demodulation is constructed. The overall structure of the system is shown in Figure 1 .

FIGURE 1 . Receiving and transmitting module of underwater laser communication system.

The entire laser communication system consists of a transmitter and a receiver. The transmitter is mainly composed of a data transmission module, a PPM modulator, and a laser. The data transmission module is used to input the data to be transmitted and transmit it to the PPM modulator through USB interface. The PPM modulator converts the modulated data into an output signal and drives the laser to emit light through the laser interface. A laser beam emits a signal in the form of a carrier wave. The laser adopts a tunable semiconductor blue laser with a central wavelength of 470 nm and a maximum communication rate of 15 Mbps. In the experiment, in order to simulate the optical attenuation effect from 10 to 20 m underwater, variable optical attenuators were added to the communication path, achieving 30%, 50%, and 80% optical attenuation, respectively. In actual experiments, the distance between the two laser communication modules after using the attenuator will be set to be 2–4 m.

The receiving module is mainly composed of a receiving optical path, a PPM demodulator, and a data receiving terminal. After the certain attenuation, the laser pulse containing modulation information is transmitted to the receiving optical path. After photoelectric conversion is completed by the APD (Avalanche Photodetector [ 16 ]) detector, it is sent to the PPM demodulation module. The demodulated data can be displayed on the LCD panel of the PPM demodulator. Data can also be transferred to a computer through a USB port to complete operations such as storing and recording.

3 Small angle analysis mathematical model

Small angle analysis is a typical method for studying underwater laser propagation [ 17 ]. During communication, the laser transmitter and receiver can achieve good position alignment through a servo mechanism, so their transmission divergence angle is small. The mathematical model is constructed from the particle nature of light, and the scattering process between photons and suspended particles is described using a small angle approximation method, which is more consistent with the actual test situation.

The laser beam is transmitted in water along the z -axis, and the light source coordinates are set to the origin (0, 0, 0). During transmission, seawater molecules and impurity molecules can cause laser light to scatter, assuming this scattering angle is θ i .

The movement displacement L per unit time is

x, y, z represents the numerical value on the corresponding coordinate axis, and t represents time. According to the scattering theory, assume that the single scattering rate is w, the unilateral scattering angle is θ , and the optical thickness is τ . There are

Then r is the photon scattering projection angle.

Due to the small value of r , the above formula can be simplified and approximated as follows:

According to the principle of optical communication [ 18 ], it is possible to calculate the time delay of the small angle analysis method as follows:

Where c is the speed of light and n is the refractive index of the water transmitting medium [ 19 ]. Therefore, the optical power at the receiving end of the system can be expressed as

Then E L is the energy value of the received light. In communication systems, the width of laser pulses affects time domain broadening. After the laser pulse passes through the water, since only the pulse broadening is considered, the shock response of the initial time domain waveform p (t) can be assumed to be P c (t). After the transmission distance z of the initial pulse p (t), the broadened waveform can be expressed as

I(t) represents the energy value that varies with time t. From this energy value, the energy distribution of the communication laser spot at the receiving end can be calculated, thereby completing the inversion of communication efficiency.

4 PPM demodulation algorithm

The PPM communication pulse sequence is composed of several frames, each frame being divided into protection segments and information segments. When a pulse is included in the K time slots of the communication period, the pulse width is set to ΔT. One frame period is equivalent to one optical pulse period, which is also the repetition frequency of the laser.

The protection segment is related to the characteristics of the laser and is the minimum time interval for the laser to reach the threshold condition again. Use J light pulse lengths JΔT represents. K determines the number of bits k of modulated information, which can be expressed as:

In PPM modulation, the transmitted information corresponds to the position of the laser pulse one by one. The analysis of the pulse position can calculate the transmitted data value, which belongs to a phase modulation.

PPM modulation transmits information using a laser pulse from K time slots in the information segment. In a system, a frame period is the period of a light pulse, i.e., (K + J)ΔT. When the energy of a single laser pulse is high, the average power over the entire cycle is very small, reducing the need for average power in signal transmission. In terms of transmission rate, PPM modulation achieves log2M bit data transmission, and its speed can be expressed as

To further improve speed, you can increase the information time slot or reduce the protection time slot. However, this increase and decrease is not arbitrary, and is limited by the characteristics of the laser.

4.1 Transmission module

The laser emission module with a microprocessor as its core achieves timing output through a high-speed timer. Controllable output at a certain port is achieved by controlling the timer amount. Specific process: First, the timer operates in a high-speed timing state, and counts with the minimum resolution time slot. Then, when the algorithm controls the time corresponding to the modulated information data, a pulse is generated, and the rising edge of the pulse is its modulation information. Next, a similar rising edge is generated through the PPM conversion module to control this narrow pulse and complete the PPM modulation output. Finally, the output signal can be connected to the modulation interface of the laser to control the laser to emit a modulated optical pulse sequence.

4.2 Receiving module

The PPM demodulator, as the core device of the reception section, converts the received pulses containing modulation information into slot positions and demodulates the information. Firstly, it is implemented using the comparison and capture module of high-speed timers. As long as the received signal meets the trigger conditions after processing, it can accurately capture the pulse jump edge and achieve accurate timing. Secondly, a high-speed clock is used for timing decisions, and the positions of each pulse are recorded in the form of time counts. Finally, the modulation information is obtained through the corresponding demodulation algorithm.

5 Experiments

5.1 experimental system.

The main structure of the system includes a transmitting light module and a receiving light module. In the receiving module, the computer controls the modulation module to complete the uploading and modulation of data. In the receiving module, the computer controls the demodulation module to complete data download and demodulation. Lumileds-470 nm type light source is selected as the emission laser. At a communication rate of 15 Mbps, a single LED has a luminous power of about 200 mW and a divergence angle of 120°. The output power of the entire LED array is above 1 W, which can meet the system requirements. In the receiving module, the APD detector selected by Hamamatsu Company is C12702-12. The processing part uses ALINX’s AN108 A/D module and FPGA development board to work together. The system is shown in Figure 2 .

FIGURE 2 . Laser communication experimental system.

Before the experimental test, first use a test target to calibrate the position of the receiving module and the transmitting module, and then add attenuation pads to the communication link to complete the simulation of underwater communication attenuation. The emitting laser spot is collected by the receiving end after passing through the medium. The original spot signal is restored through demodulation processing. Processing system is used to calculate the energy loss of the light spot, thereby analyzing the communication efficiency of the system.

5.2 Servo control

Servo control modules are installed in the transmitting and receiving parts respectively, which are used to adjust the laser and detector by step scanning, thereby achieving effective alignment of communication angles. The servo control module is shown in Figure 3A , and its control response signal is shown in Figure 3B .

FIGURE 3 . Wavelength offset values at different positions. (A) Servo control module. (B) Servo control signal

The system drives the stepper motor through a servo control module to achieve adjustable control of the position and angle of the optical module, achieving alignment of the irradiation position and posture during communication. By Figure 3B , the maximum signal value is 0.75 V, and the minimum signal value is 0.14 V in Ch1 channel. This is a control command issued by the control module that completes stepping control through a square wave level signal. After adjustment by the servo module, the corresponding command signal is displayed on the Ch2 channel. The maximum value of the signal in Ch2 channel is 2.41 V, and the minimum value is 1.23 V. After amplifying the step signal, the amplitude has significantly increased, and the time interval has not changed. This control module can effectively control the scanning system through a stepper motor. In the servo system, a stepper motor is used to complete the scanning of the communication module, simulating the characteristics of underwater dynamic communication.

5.3 Receiving spot energy test

In underwater laser communication, the spot energy of the communication laser is an important indicator to judge the communication quality. The attenuation and turbulent disturbance of water on the laser spot can significantly affect the energy distribution characteristics of the laser spot. Therefore, it is necessary to measure the energy of laser spots with energy attenuation and turbulence disturbances. In the experiment, 50% attenuation plates were used to simulate the attenuation effect of seawater medium on laser light. The energy of the actual test laser spot obtained by the APD detector is shown in Figure 4A . The energy distribution is reconstructed using MATLAB software, as shown in Figure 4B .

FIGURE 4 . Laser spot acquisition and energy distribution reconstruction. (A) Grayscale image from APD. (B) Energy Reconstruction by MATLAB

As shown in Figure 4A , over a communication distance of 10 m, when 1.0% attenuation chip was used, the laser spot still undergoes significant deformation. When 0.1% attenuation chip was used, the energy peak decreases significantly and the signal-to-noise ratio decreases. When 0.01% attenuation chip was used, the laser communication receiver cannot obtain a valid signal. The deformation of the laser spot tends to be elliptical, and the analysis shows that its deviation angle is caused by alignment deviation between the receiving module and the transmitting module. This situation will exist in actual laser communication. On this basis, the energy distribution of the light spot is simulated and reconstructed, and the reconstruction results are shown in Figure 4B . The energy map shows that the energy distribution of the light spot is elliptical, consistent with the actual test results. The intensity of energy meets the energy requirements of laser communication, which can achieve effective information transmission.

As the pass rate of the attenuator decreases, the input power decreases. The laser spot energy becomes weaker, and the packet loss of communication data gradually increases. Under the condition of using 1.0% attenuator, the test results show that the error rate of the system is better than 10 −6 . Under the condition of using 0.1% attenuator, the test results show that the error rate of the system is better than 10 −4 . It can be seen that when the energy attenuation exceeds an order of magnitude, the bit error rate significantly increases. With a 0.01% attenuator, the effective signal is submerged in noise.

6 Conclusion

An underwater laser communication system was proposed based on PPM modulation. The system is mainly composed of a transmitting module and a receiving module. The system applies PPM modulation and demodulation technology to the transmitting module and the receiving module. The control of step scanning in the servo module was tested and analyzed in the experiment, for verifying the stability of clock matching. In the spot energy analysis test, the laser spot intensity at different attenuation states was obtained. The system communication error rate under different laser spot energy intensities was tested and studied. The feasibility of this system has been verified, and it has certain application prospects in the field of underwater laser communication.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

Author contributions

BD completed the paper. ST completed the system theoretical analysis. PZ completed the simulation calculation. JW completed the experimental test.

This work was supported in part by Natural Science Foundation of Jilin Province (YDZJ202301ZYTS394); Science and Technology Research Project of Jilin Provincial Department of Education (JJKH20230814KJ).

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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Keywords: laser communication, underwater communication, servo control, dynamic testing, PPM

Citation: Dong B, Tong S, Zhang P and Wang J (2023) Research on underwater wireless dynamic optical communication system based on PPM modulation. Front. Phys. 11:1195052. doi: 10.3389/fphy.2023.1195052

Received: 28 March 2023; Accepted: 19 April 2023; Published: 05 May 2023.

Reviewed by:

Copyright © 2023 Dong, Tong, Zhang and Wang. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Shoufeng Tong, [email protected] ; Peng Zhang, [email protected]

This article is part of the Research Topic

Advances in High-Power Lasers for Interdisciplinary Applications

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• Published: 09 June 2022

Underwater wireless communication via TENG-generated Maxwell’s displacement current

• Hongfa Zhao   ORCID: orcid.org/0000-0003-1929-3184 1 , 2   na1 ,
• Minyi Xu   ORCID: orcid.org/0000-0002-3772-8340 1   na1 ,
• Mingrui Shu   ORCID: orcid.org/0000-0001-9139-5092 1   na1 ,
• Jie An   ORCID: orcid.org/0000-0002-0028-0079 3 ,
• Wenbo Ding   ORCID: orcid.org/0000-0002-0597-4512 2 ,
• Xiangyu Liu   ORCID: orcid.org/0000-0003-0386-9089 1 ,
• Siyuan Wang   ORCID: orcid.org/0000-0001-7174-9754 1 ,
• Cong Zhao   ORCID: orcid.org/0000-0003-3647-7567 1 ,
• Hongyong Yu   ORCID: orcid.org/0000-0002-7195-7139 1 ,
• Hao Wang   ORCID: orcid.org/0000-0002-9238-9791 1 ,
• Chuan Wang 1 ,
• Xianping Fu   ORCID: orcid.org/0000-0001-9888-9327 1 ,
• Xinxiang Pan   ORCID: orcid.org/0000-0003-0460-0620 1 ,
• Guangming Xie   ORCID: orcid.org/0000-0001-6504-0087 1 , 4 , 5 &
• Zhong Lin Wang   ORCID: orcid.org/0000-0002-5530-0380 3 , 6  

Nature Communications volume  13 , Article number:  3325 ( 2022 ) Cite this article

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• Applied physics
• Electrical and electronic engineering
• Sensors and biosensors

Underwater communication is a critical and challenging issue, on account of the complex underwater environment. This study introduces an underwater wireless communication approach via Maxwell’s displacement current generated by a triboelectric nanogenerator. Underwater electric field can be generated through a wire connected to a triboelectric nanogenerator, while current signal can be inducted in an underwater receiver certain distance away. The received current signals are basically immune to disturbances from salinity, turbidity and submerged obstacles. Even after passing through a 100 m long spiral water pipe, the electric signals are not distorted in waveform. By modulating and demodulating the current signals generated by a sound driven triboelectric nanogenerator, texts and images can be transmitted in a water tank at 16 bits/s. An underwater lighting system is operated by the triboelectric nanogenerator-based voice-activated controller wirelessly. This triboelectric nanogenerator-based approach can form the basis for an alternative wireless communication in complex underwater environments.

Introduction

In the booming ocean exploration, underwater equipment and technology is attracting more and more attention 1 , 2 , 3 , 4 , 5 . Particularly, obtaining underwater wireless communication has always been a critical challenge. The current underwater communication is achieved through different physical fields, such as acoustic field, optical field, and electromagnetic field 6 , 7 , 8 .

Acoustic communication is most widely used underwater communication as sound wave is not absorbed by water so easily like electromagnetic wave and optical wave. However, acoustic communication has always been accompanied by considerable transmission delays while the transmission is subject to influences from temperature, pressure, and salinity, which leads to multipath effects and Doppler frequency shift. What’s more, echo and reverberation from obstacles could make acoustic communications inaccessible in certain environments (such as confined space, narrow pipes, tunnels, and caves) 9 , 10 . Underwater optical communication can realize large-capacity data transmission, but it is subject to absorption, scattering, beam divergence, and ambient light interruptions 11 , 12 . Compared to the acoustic and optical waves, the electromagnetic waves are not affected by acoustic noise or turbulence. Underwater displacement current communication usually has high transmission rate and low delay 10 . While high-frequency electromagnetic waves will be largely absorbed by water 13 , 14 , low-frequency electromagnetic waves can transmit through an antenna of several kilometers. In sum, complex and sometimes confined underwater space turns out to be a considerable challenge to traditional underwater communication technologies.

Under those conditions, an alternative communication that can work well in underwater space is definitely needed. An inspiration comes to our mind from the Maxwell’s equations, foundation of modern wireless electromagnetic communication. The displacement current, corresponding to ∂ D /∂ t in the Maxwell’s equations, is what unified electricity and magnetism theoratically 15 . Of the two terms in displacement current, the first term ∂ E /∂ t induces electromagnetic waves widely used in information technology, especially in wireless communications. The second term ∂ P /∂ t in the displacement current is induced by the polarization of media 16 , 17 . Previous studies by Prof. Z. L. Wang reveal that the second term ∂ P /∂ t in the Maxwell’s displacement current can be directly related to the output electric current of the nanogenerator 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 . A few studies have been performed on the energy transmission or communications (in air), based on the Maxwell’s displacement current generated by the triboelectric nanogenerator (TENG). Recently, Zi et al. (2021) used TENGs to generate a rapidly alternating electric field so that wireless communication in air can be realized by the displacement current ∂ P /∂ t 26 . Compared with air, water is of a larger dielectric constant, which is more conducive to the propagation of the polarization electric field. Therefore, based on the second term of the displacement current, i.e., the polarization electric field, underwater communication in complex waters is feasible.

In this study, an underwater wireless communication via Maxwell’s displacement current, ∂ P /∂ t , is proposed. The TENG that converts sound to electricity is connected to a transmitting electrode to generate the time-varying polarization electric field underwater. The corresponding time-varying current is measured with a receiving electrode connected to an electrometer. The study reveals that the current signals generated by the TENG yield good anti-interference ability to underwater disturbances. Through a 100 m long salt water pipe, the peak value of the current signal decreases by 66% from the original signal, while the waveforms of the electric signals are not distorted. With the on-off keying method, texts and images can be successfully transmitted in a water tank. No errors appeared in the continuous transmission for about 20,000 digital signals, and an underwater lighting system has been voice-controlled wirelessly via the TENG. What’s more, the current signals output by a sandwich-like TENG can be transmitted in a 50 m × 30 m × 5 m basin with the signals displayed on screen in real time. Therefore, it is believed that the presented work could become an effective communication approach in underwater environments.

Working principle of the underwater electric field communication

A conceptual diagram of the application of the studied underwater communication is shown in Fig.  1 . The TENG converts sound (i.e. sonic waves in air) to electric signals in water (Fig.  1a ), and the electric signals carrying the voice information can be transmitted in water and received by the diver. In this way, an underwater wireless communication is established via Maxwell’s displacement current generated by the TENG. Figure  1b is the flow chart of the underwater communication. It needs to be noted that this method is different from the electric field communication generated from a pair of electric dipoles (see Supplementary Note  1 ).

(signals generated by TENG is directly transmitted without amplification by an external power source). a The application and ( b ) the flow chart of the underwater wireless communication. c Schematic diagram of the capacitance model, \({{{\varepsilon }}}_{{{r}}}\) is relative permittivity, E is the original electric field, P is the polarization electric field, E′ is the combined electric field of ( E ) and ( P ), and all about Q are amount of charge.

The working principle of the underwater communication can be understood, approximately, with a capacitance model (Fig.  1c ). The propagation of underwater electric field is analyzed from the perspective of displacement current. The transmitting and receiving electrodes form the positive and negative electrodes of the capacitor, while the water solution is the dielectric. With the presence of electric field E , the dielectric can be polarized where a polarization electric field P can be generated. It should be noted that the polarization electric field P is originated from the negative polarization charge to the positive polarization charge. \({{{{{\bf{E}}}}}}^{\prime}\) is the combined electric field of E and P , If the relative permittivity is defined as \({\varepsilon }_{r}={{{{{\bf{E}}}}}}{{{{{\boldsymbol{/}}}}}}{{{{{\bf{E}}}}}}^{\prime}\) , the relationship between the polarization charge and the charge Q on the transmitting electrode is \({Q}^{\prime} =(1-1/{\varepsilon }_{r})Q\) . Due to the attenuation of the electric field during through propagation medium, the received charge \(Q^{\prime\prime}\) at the receiving electrode is < \(Q^{\prime}\) .

The underwater communication can be demonstrated theoretically with the Maxwell’s equation. Remind that the Gauss’s law of the Maxwell’s equations is

where ρ is the distribution of free charges in space, and D is the electric displacement vector, which can be expressed as

where permittivity in vacuum is \({{{\varepsilon }}}_{{{0}}}\) . The Maxwell’s displacement current density can be defined as

From Eq. ( 3 ), the first term \({{{\varepsilon }}}_{{{0}}}\partial {{{{{\bf{E}}}}}}/\partial {t}\) gives rise to electromagnetic wave. Studies of Prof. Zhonglin Wang reveal that the second term (∂ P /∂t) in the Maxwell’s displacement current can be directly related to the output electric current of the TENG 15 .

It is worth mentioning that the internal circuit in the TENG is dominated by the displacement current, and the observed current in the external circuit is the capacitive conduction current (see Supplementary Note  2 ). The research on the underwater electric field propagation is inspired by the built-in electric field of the TENG. Comparing the TENG-based underwater electric field with electromagnetic waves, the propagation of electromagnetic waves does not require a medium, and the propagation effect is best in vacuum. At this time, ∂ E /∂t reaches the maximum value, and ∂ P /∂t is 0. The propagation of the polarization electric field requires a medium (see Supplementary Note  3 ). As ∂ E /∂t gets significantly reduced in water, the propagation effect of ∂ P /∂t gets improved.

To examine the performance of the underwater communication, an acoustic-driven TENG is applied to convert sound in air to electrical signals in water. The output performance of the acoustic-driven TENG has been investigated systematically in our previous study 27 . As shown in Fig.  2a , the TENG consists of a Helmholtz resonant cavity, an aluminum film with evenly distributed acoustic holes, and a fluorinated ethylene propylene (FEP) film with a conductive ink-printed electrode (details about the TENG is shown in Supplementary Notes  4 and 5 ). The transmitting electrode in water is connected to the aluminum electrode of the TENG, while the other electrode of the TENG is grounded so that the TENG operates in the single-electrode mode. In reaching an electrostatic equilibrium state, higher electrical output can be obtained by acquiring ground charges 28 . It is worth noting that one piece of conduct materials, such as metal and salt water, can serve as a charge reservoir for the TENG.

a Schematic diagram of the experimental process. I D represents displacement current in all figures. b Schematic diagram of the working principle. E is the underwater electric field, and v is the speed of the TENG for contact and separation. c The short-circuit current signals (measured by connecting an electrometer to the aluminum electrode). d The short-circuit current signal obtained by connecting the electrometer to the receiving electrode.

Figure  2b shows the working principle of the underwater communication, which is based on the interface polarization from the Maxwell-Wagner effect. The electrical output is generated from the variation of the built-in electric field in the TENG, which is directly related to the second term (∂ P /∂t) in the Maxwell’s displacement current 15 . A transmitting electrode is connected to one electrode of the TENG, thus an electric field \({{{{{{\bf{E}}}}}}}_{{{{{{\bf{0}}}}}}}\) is induced in water as the TENG works (see Fig.  2b ). Corresponding to the variation of electric field \({{{{{{\bf{E}}}}}}}_{{{{{{\bf{0}}}}}}}\) , the positive and negative ions in the water move reciprocally, generating a polarization electric field \({{{{{{\bf{P}}}}}}}_{{{{{{\bf{0}}}}}}}\) . The current in the receiving electrode induced by the polarization electric field can be measured with an electrometer. The electric field \({{{{{{\bf{E}}}}}}}_{{{{{{\bf{0}}}}}}}\) generated by the TENG is related to the charge density \({{{{{{\boldsymbol{\rho }}}}}}}_{{{{{{\boldsymbol{0}}}}}}}\) in the transmitting electrode in the following form:

\({{{{{{\bf{P}}}}}}}_{{{{{{\bf{0}}}}}}}\) is the polarization electric field generated from the electric field \({{{{{{\bf{E}}}}}}}_{{{{{{\bf{0}}}}}}}\) , which is

Therefore, the second term \({{{{{{\bf{J}}}}}}}_{{{{{{\bf{p}}}}}}}\) in the Maxwell’s displacement current (generated by the polarization electric field) is

For the acoustic driven HR-TENG, when the FEP film contacts with the aluminum film, the electron clouds on the surfaces of the two films overlap, and some of the electrons from the aluminum film enter the deeper potential well of the FEP film. Due to the much higher electronegativity of FEP than aluminum, the free electrons on the surface of the aluminum film transfer to the lowest unoccupied molecular orbital of the FEP interface. So the aluminum film becomes positively charged (Supplementary Fig.  1a ). Since the transmitting electrode is connected to the aluminum film, positive charges are also distributed on the surface of the transmitting electrode. Negatively charged ions in the water are attracted by the transmitting electrode, while positively charged ions are repelled to the surroundings. When positive ions contact with the receiving electrode, electrons in the receiving circuit flow to the receiving electrode, so the electrometer detects a positive current. Due to the change in the acoustic pressure difference, the FEP film is separated from the aluminum electrode. At the moment, electrons flow from the ground to the conductive ink electrode to balance the electric field between the FEP film and the conductive ink electrode. Due to the negative charge distributed on the surface of the FEP film, the free electrons on the aluminum film are repelled, so the electrons flow from the aluminum film to the transmitting electrode. Opposite to before, positive charged ions in the water are attracted by the transmitting electrode, while negative charged ions are repelled to the surroundings (Supplementary Fig.  1b ). When negative ions contact with the receiving electrode, electrons in the receiving electrode flow to the receiving circuit, so the electrometer detects a negative current.

Figure  2c, d compares the electric signals in air with those in ordinary water. Under acoustic waves (80 Hz, 80 dB), the corresponding periodic output short-circuit current signals yield the peak value of 14.9 μA (Fig.  2b ), which is directly measured with the electrometer connected to the aluminum electrode. When the (electrometer-connected) receiving electrode is two meters away from the submerged transmitting electrode, the peak value of the current decreases slightly to 14.5 μA while the waveform of electric signals remain constant (Fig.  2c ). The peak value of open-circuit voltage output of the TENG decreases from 28.5 V in air to 13 V in water (see Supplementary Fig.  2 ). When the water tank is grounded by a wire, the output current decreases significantly, but the waveform of electric signals stay consistent with the original signal (Supplementary Fig.  3 ). This can be explained by the tendency that charges from ground would balance the electrical potential field in water. Furthermore, the current signal could still be measured even when the transmitting electrode is insulated from water by a Kapton tape (Supplementary Fig.  4 ), and the electric field generated by the TENG can propagate across both gas and liquid media (Supplementary Fig.  5 ). These prove that the transmission of the signals depends on the electric field radiated by the TENG rather than the direct exchange of electrons between water and electrode plates. Both theoretical analysis and experiments have shown that for the whole system, the propagation of underwater electric field has demonstrated the characteristic of displacement current (see Supplementary Note  6 ). Previous study 29 proved that when the electric field propagates in a medium, conduction current dominates when \({{{{{\boldsymbol{\sigma }}}}}}{{{{{\boldsymbol{/}}}}}}{{{{{\boldsymbol{\omega }}}}}}\; > \;{{\varepsilon }}\) and displacement current dominates when \({{\varepsilon }}\, > \,{{{{{\boldsymbol{\sigma }}}}}}{{{{{\boldsymbol{/}}}}}}{{{{{\boldsymbol{\omega }}}}}}\) (σ is conductivity, ω is angular frequency, and ε is permittivity). The Rayleigh distance of the TENG generated electric field can be calculated by \({{{{{\boldsymbol{R}}}}}}=2{{{{{{\boldsymbol{D}}}}}}}^{{{{{{\boldsymbol{2}}}}}}}{{{{{\boldsymbol{/}}}}}}{{{{{\boldsymbol{\lambda }}}}}}\) , where R is Rayleigh distance, D is the maximum size of the transmitting electrode, and λ is the wave length. As a result, R is so small that it can be neglected. According to these theories, for the TENG-based underwater electric field communication, \({{\varepsilon }}\, > \,{{{{{\boldsymbol{\sigma }}}}}}{{{{{\boldsymbol{/}}}}}}{{{{{\boldsymbol{\omega }}}}}}\) and the transmitting distance is larger than the Rayleigh distance. So the displacement current domains the underwater electric field while conduction current only appears in a very short distance.

It is worth mentioning that underwater communication can also be realized by various types of TENGs, such as the TENGs that harvest wave energy and vibration energy (see Supplementary Fig.  6 , and the first and second items in Supplementary Table  1 ). The development of the electric field communication depends on the development of TENG technology. The techniques for designing TENGs with high frequency and good output performance (see Supplementary Table  1 ) provides good potential for the application of the TENG in underwater communication 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 .

Transmission performance of the underwater electric field

The characteristics of the underwater electric field under different parameters such as water volume, electrode position/size, salinity, water turbidity, and underwater obstacles, have been studied. Figure  3 shows the attenuation of the underwater electric field. In a 3 m × 2 m × 0.4 m water tank (Fig.  3a ), the receiving electrode is placed certain distance away from the transmitting electrode. As the distance increases from 1 m to 3 m, the current signals remain almost unchanged (Fig.  3b ). This is also verified by the simulation of underwater polarization electric field shown in Supplementary Fig.  7 . Furthermore, the complete signals output by the TENG and received signals underwater, and their Fourier transforms are shown in Supplementary Figs.  8 – 9 , showing that the frequency-domain features of the signals remain unchanged underwater. As more water is added into the tank, the peak value of the current decreases. Actually, the peak value decreases by 30% from the original signals as the water volume reaches 6 m 3 (Fig.  3c ). This can be understood, as the electric field energy W in water is

According to Eq. ( 7 ), the energy density of the electric field decreases with the water volume V while using a TENG with constant power output. Therefore, the peak value of the current decreases with larger water volume.

a A photo of the experiment. b Comparison between the original TENG short-circuit current and received current signals in water. c Comparison between the original short-circuit current of the TENG and received signals under different water volume. d Schematic diagram of the electrode. e Comparison of the current signals received in water with different transmitting and receiving electrodes. Tr, Re, and Pl represents transmitting electrode, receiving electrode, and electrode plate respectively. f Comparison of the peak values of the received current signals with different receiving electrodes. Simulation diagram of the distribution of polarization electric field ( g ) without and ( h ) with the receiving electrode. Color represents polarization intensity. i Variation of polarization electric field and charges on the receiving electrode with the distance between the two electrodes.

As shown in Fig.  3d–f , the current can be enhanced by using a receiving electrode plate of larger area. With a 10 cm × 5 cm electrode plate, the peak value of the current signal increases by 18%, compared to that with a thin electric wire. This means more ions in water contacting with the electrode, which is verified by the underwater polarization electric field simulation shown in Supplementary Fig.  10 . In addition, the peak value of the current is subject to the angle between the transmitting and receiving electrode plates, but the effect caused by the angle is very small in the water tank (see Supplementary Figs.  11 – 12 ), which is quite different from the case in a bipolar electric field 38 .

By comparing the distribution of the polarization electric fields without and with the receiving electrode (Fig.  3g, h ), it is found that the receiving electrode can change the distribution of the polarization electric field. This can be explained by the fact that the electrode is equivalent to a terminal with a potential of zero voltage. The variation of the polarization electric field and the variation of the terminal charges in the receiving electrode are shown in Fig.  3i . A 2D simulation is performed and the polarization electric field distribution corresponding to Fig.  3i is shown in Supplementary Figs.  13 and 14 . The attenuation of the polarization electric field and terminal charges at the receiving electrode with distance can be fitted respectively

k 1 , k 2 , k 3 and a are parameters in the fitted curves of the output power of the TENG (Supplementary Figs.  15 and 16 ). According to the simulation results, the exponent of x is close to −1 for two dimension simulations (Supplementary Figs.  15 and 16 ), and close to −2 for three dimension simulations (Supplementary Figs.  17 and 18 ), which is consistence with the Gauss’s law.

The dependence of the underwater electric field on disturbances in water is shown in Fig.  4 . It is found that the peak value of the current signal in salty water (5 g L −1 , adjusted by adding salt to water) increases by 40% on top of that in pure water (Fig.  4a ). This indicates that the ions in salt water can enhance the polarization electric field. However, water salinity increase beyond 15 g L −1 will not further promote the current signals. Similarly, when acid or alkali is added to the pure water to change the pH, the ion concentration in the water will change, indicating that as the pH of water deviates from 7, the received current signals will increase (Supplementary Fig.  19 ). This may be attributed to the improved relative permittivity of the aqueous solution. Figure  4b, c reveal that the waveform of the received signals identical to original ones, regardless of obstacles or turbidity in the water tank. In this sense, the polarization electric field has shown robustness to obstacles and water turbidity.

a Effect of water salinity on the peak value of the current. Error bars indicate standard deviations, with all values ≤0.53. b Influence of an obstacle on the current signals. c Comparison between the received current signals in clean water and those in turbid water. d Schematic diagram of the drilling platform with oil pipeline. e Variation of the peak value of short-circuit current output transmitted in a water pipe of 100 meters. Tr and Re represent transmitting and receiving electrodes respectively. v is the liquid flowing speed. Error bars indicate standard deviations, with all values ≤0.46. f Comparison between the short-circuit current signals transmitted in a straight and those in a curved water pipe.

As achieving reliable communication across the pipe is very important for the pipeline robot system 39 , 40 , the performance of the polarization electric field in liquid pipes is investigated (Fig.  4d–f ). Figure  4d is a schematic diagram of the drilling platform with the oil pipeline. In a pipe filled with salt water, the peak value of the current is also found to decreases with the distance between the transmitting electrode and receiving electrode. In fact, the value decreases by 66% when the distance in-between is 100 m (Fig.  4e ), which is consistent with the simulation result of the polarization electric field in the pipe (as shown in Supplementary Fig.  20 ).

In fact, the collision between ions and water molecules may influence the performance of the electric field. In addition, it is interesting to find that the received signals in a spiral pipe are the same with those in a straight pipe (Fig.  4f ). From Supplementary Fig.  21 , it is found that independent of the flow status, the current signals can also be obtained in the mixture of oil and water, which means the polarization electric field communication can be applied in complex pipelines.

It is worth noting that the electric field communication is also insensitive to water temperature and ambient lightness (Supplementary Fig.  22 ). Further comparisons between acoustic, optical, and electromagnetic waves methods are shown in Supplementary Table  2 . What’s more, by studying the effect of the ground on the electric field, it is theoretically proved that this system may work in open water area as shown in Supplementary Note  7 .

Modulation and demodulation of the underwater electric field communication

The modulation and demodulation process of current signals for data transmission in water is shown in Fig.  5 . The current signals converted from sound waves by the TENG can be modulated to digital signals containing the information of texts or images in water via the electric field communication (Fig.  5a ). The signal modulation method is based on the on-off keying (OOK), in which longer signals with time intervals of 50 ms is set as “1”, and shorter signals with time intervals of 25 ms is set as “0”. A 25 ms interval is inserted between each digital signal to separate adjacent digital signals. After transmission in water, the modulated digital signals can be received by the electrometer (Fig.  5b ). The fundamental frequency of the signals generated by the TENG is 80 Hz, and the frequency of the modulated digital signals is 16 Hz. Alternatively, the digital signals can be modulated with other frequencies or other methods (see Supplementary Fig.  23a, b ). Higher frequency yields a fast information transmission rate, while lower frequency yields a strong anti-interference ability.

a Schematic diagram of the modulation and demodulation process. b The modulated digital signals transmitted in water. c The demodulated current signals to a word after transmitting in water. d Part of current signals transmitted for an image.

By demodulating the received signal (with the MATLAB codes), the signals of “0” and “1” can be identified accurately (Fig.  5b and Supplementary Fig.  23c ). The current signals can be modulated into text by the standard encoding. The received signals can be accurately demodulated into the original text (Fig.  5c ). Supplementary Movie  1 shows that the real-time current signals generated by the TENG is modulated, transmitted, and demodulated, and the text obtained after demodulation is displayed on a computer screen. This electric field communication can also be used for image transmission. Figure  5d shows part of the received current signals, and the complete signals are shown in Supplementary Fig.  23d . A 2.7-KB image is transmitted within 1353 s at 16 bits/s (owing to the low fundamental frequency of the TENG). There is no error signal in the continuous transmission of ~20,000 digital signals (100,000 working cycles of the TENG). In addition, by applying an external alternating current on the dielectric material’s electrode, the electric signals with higher frequencies (up to kilohertz) can be modulated and transmitted in water, demonstrating that this approach can be used for high frequencies communication (see Supplementary Note  8 ).

It should be noted that the current signals output by the TENG and received signals underwater at a range of 60–200 Hz are compared. It turns out that the signals received underwater are always consistent in waveforms with the signals output by the TENG (see Supplementary Fig.  24 ). What’s more, the power spectrum is obtained by performing Fourier transform to the modulated digital signals and noise. The power spectrum shows that energy is evenly distributed in the frequency range from 40 kHz to 85 kHz (see Supplementary Fig.  25 ), proving that the bandwidth of the system with water channel is greater than 85 kHz.

Realization of wireless control using the underwater electric field communication

To further study the ability of the underwater wireless communication, a demo voice control of an underwater lighting system is performed (Fig.  6 ). A microphone-style TENG that converts voice to electrical signals is to control the underwater lighting system wirelessly (Fig.  6a ). The signals containing the voice information (e.g. “red” and “green”) are transmitted in water and received by the electrometer (Fig.  6b ). By performing short-time Fourier transform to the signals, the words “red” and “green” can be distinguished with a neural network algorithm (see Fig.  6c and Supplementary Fig.  26 ). Subsequently, the words are converted into digital signals to control the lights. This approach can be applied in the real-time voice control of underwater lights (Fig.  6d and Supplementary Movie  2 ). It is worth mentioning that the entire underwater communication realized by the TENG is self-powered.

a The schematic diagram of underwater light wirelessly controlled by voice. b The received signals of “red” and “green”. c The short-time Fourier transform of “red” and “green”. Colors represent amplitudes. d The photo of the voice control experiment setup. e The experiment of the button-type TENG controlling an independent system. f The photo of the touch control experiment. g The schematic diagram and ( h ) The photo of the experiment in a 50 m × 30 m × 5 m basin. D is the distance between two electrodes. i The received current signals underwater.

At the same time, the signals receiving and controlling device in the water can be designed independently. By touching a (contact-separation mode) button-type TENG, people can use the electric to control the independent working system in water (see Fig.  6e, f and Supplementary Movie  3 ). The independent working system consists of a weak current acquisition board, a single-chip microcomputer, batteries, a relay, and an underwater working light. The pulse signals generated by the TENG are collected by the weak current acquisition board, and the analog signals are converted into digital signals sent to the microcontroller. The single-chip microcomputer processed the digital signals and controlled the underwater working light.

In another demo experiment, a sandwich-like TENG (S-TENG) with an output current of 60 μA is deployed in a 50 m × 30 m × 5 m basin (with all boundaries connected to the ground, see Fig.  6g, h . When the S-TENG is shaken, the current signals outputted by the S-TENG can be transmitted in water and received by the receiving electrode 5 m away from the transmitting electrode. The signals are detected by a current acquisition board (Supplementary Note  9 ), which sends the signals to a computer through WiFi and then the waveforms are displayed on the screen (Fig.  6i and Supplementary Movie  4 ).

In summary, an underwater communication via Maxwell’s displacement current is proposed and investigated. In the Maxwell’s displacement current, the first term ∂ E /∂ t gives rise to electromagnetic waves. However, in underwater environments, the high-frequency electromagnetic waves can be easily absorbed, and the low-frequency electromagnetic waves can only be transmitted through an antenna of several kilometers. In this study, the second term (∂ P /∂ t ) in the Maxwell’s displacement current is utilized for underwater communication. An acoustic-driven TENG connected to a transmitting electrode is applied to generate alternating electric field in water, so that the sound in air can be converted into underwater electrical signals, which can be measured with a receiving electrode connected to an electrometer. Through a salt water pipe of 100 m length, the peak value of the current signal decreases by 66% compared to the original signal, while the electric signals are not distorted in waveform during transmission.

Based on the on-off keying method, texts and images have been successfully transmitted by modulated current signals in a water tank at 16 bits/s. Throughout the continuous transmission of about 20,000 digital signals, no error appears. By successfully converting voices into current signals, the TENG is capable of controlling an underwater lighting system wirelessly. What’s more, the current signals output by a sandwich-like TENG can be transmitted in a 50 m × 30 m × 5 m basin with the signals displayed on screen in real time. Compared to traditional sonic, optical, and electromagnetic communications, the underwater communication via Maxwell’s displacement current appears to be less vulnerable to disturbances, which exhibits considerable potential for applications in complex underwater environments.

Fabrication of the TENGs

The HR-TENG in the experiments consists of a Helmholtz resonant cavity, an aluminum film with acoustic holes, and an FEP film with a conductive ink-printed electrode. The resonant cavity has a dimension of 73 mm × 73 mm × 40 mm. Two tubes with an inner diameter of 5.0 mm and a length of 32 mm are fixed on the resonant cavity. The aluminum film with 440 uniformly distributed acoustic holes acts as the electropositive triboelectric layer. The length, width, and thickness of the film are 45 mm, 45 mm, and 0.1 mm, respectively and the diameter of the holes is 0.5 mm. The FEP film is used as the electronegative triboelectric layer on observation of its strong electronegativity and good flexibility. It has a thickness of 12.5 μm and a working area of 45 mm × 45 mm. Given that the FEP material is insulated, a conductive ink electrode with a micron thickness is attached to the other side of the FEP film to transfer electrons. A screen printing device (Tou) is used to print the conductive ink (CH-8(MOD2)) on the FEP film (WitLan). The shell is printed by a 3D printer with PLA material.

The TENG to recognize voice is similar to the HR-TENG, except that it has no dule-tube structure but has a 45 mm × 45 mm opening on one side of the resonance cavity. The contact separation distance between the FEP and aluminum film is ~0.2 mm. The membrane structure of the button-type TENG is the same as HR-TENG without a cavity.

The acrylic plate of a single layer S-TENG is of 5 mm thickness and 10 cm diameter. Two aluminum electrodes with a thickness of 50 µm and an area of 6 cm × 4 cm are parallel attached onto two sides of the acrylic plate. PTFE balls with 10.5 mm diameter are filled between two acrylic plates and they are produced by 3 M company. Each S-TENG unit consists of 10 layers stacked S-TENG in parallel connection and acrylic block shell. The acrylic block shell has 10 cm diameter and 20 cm height. There are four AC output copper ends in an S-TENG unit, one pair at the top and the other pair at the bottom. The buoy consists of 5 S-TENG units as the power module and an acrylic shell as the frame structure. The S- TENG units integrated inside are in parallel connection to make the AC electrical output in-phase and the they are fixed through packing tape.

Experimental process and measuring equipment

The output signals are measured with a Keithley 6514 electrometer. The HR-TENG is mounted on an optical plate with a loudspeaker (JBL), driven by sinusoidal waves from a function generator (YE1311). One electrode of the TENG is grounded and the other electrode is immersed in water. The wires used for electrodes has a 0.3 mm copper core, and the electrode plates are copper films with the size of 100 × 50 × 0.06 mm. The water pipe used in the experiment is an ordinary PVC rubber water pipe with an inner diameter of 13 mm. A 12 V DC motor is used to control the flow of liquid in the pipe. The signal modulator consists of a microcontroller development board (STM32F7) and a relay. The MATLAB interface in LABVIEW is used to demodulate and display the real-time signals measured with the electrometer. The signals generated by the voice-driven TENG need to be filtered at 50 Hz and its harmonics after being received.

The transmitting electrode in water and the aluminum electrode of the TENG is connected by an ordinary copper wire. The HR-TENG is applied to convert sound in air to electrical signals in water. In this way, acoustic waves with specific frequencies can be got by controlling the signal generator and a loudspeaker. Furthermore, under the excitation of the acoustic waves, electrical signals with specific frequencies can be generated by the HR-TENG to examine the performance of the underwater communication. Button type TENG is a basic and commonly used TENG with the simplest structure. The application of the button type TENG prove that underwater communication can be realized by general TENGs, demonstrating the application potential of this approach.

Numerical simulations

In order to verify the accuracy of the derived theory and experimental results, COMSOL Multiphysics software has been used for numerical simulations. The AD/DC modules, electrostatic interfaces, and transient state analysis are used in simulations. The distribution of the polarization electric field and terminal charge of the receiving electrode has been simulated. As the element size influences the calculation results, the ultra-fineness meshing option has been adopted in the simulation. For 3-D simulations, the size of the model is limited to 15 m. The 2-D simulation is used to analyze the attenuation of the electric field through longer distance. The error of the 3D simulation depends on the mesh size and model scale.

Data availability

The data supporting this study are available within the article and the  Supporting Information .  Source data are provided with this paper.

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Acknowledgements

The authors are grateful for the joint support from the National Key R & D Project from Minister of Science and Technology (2021YFA1201600, Z.L.W.), the National Natural Science Foundation of China (Grant Nos. 51879022, 51979045, M.Y.X.), the Fundamental Research Funds for the Central Universities, China (Grant No. 3132019330, M.Y.X.), and Tsinghua-Foshan Innovation Special Fund (TFISF, Grant No. 2020THFS0109, W.B.D.).

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These authors contributed equally: Hongfa Zhao, Minyi Xu, Mingrui Shu.

Authors and Affiliations

Marine Engineering College, Dalian Maritime University, 116026, Dalian, China

Hongfa Zhao, Minyi Xu, Mingrui Shu, Xiangyu Liu, Siyuan Wang, Cong Zhao, Hongyong Yu, Hao Wang, Chuan Wang, Xianping Fu, Xinxiang Pan & Guangming Xie

Tsinghua-Berkeley Shenzhen Institute, Tsinghua Shenzhen International Graduate School, Tsinghua University, 518055, Shenzhen, China

Hongfa Zhao & Wenbo Ding

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, 100085, Beijing, China

Jie An & Zhong Lin Wang

Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou, 511458, P. R. China

Guangming Xie

College of Engineering, Peking University, Beijing, 100871, P.R. China

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA, 30332-0245, USA

• Zhong Lin Wang

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Contributions

M.X., Z.L.W., and G.X. supervised and guided the project; H.Z., M.X., M.S., and H.W. conceived the idea and designed the experiment. H.Z., J.A., C.Z., S.W., and H.Y. fabricated the devices and performed the experiments; H.Z., X.L., and C.W. did the theoretical calculation; H.Z, W.D., X.F., and X.P. discussed the experiment and results; H.W., M.X., and H.Z. wrote the manuscript. All authors discussed and reviewed the manuscript.

Corresponding authors

Correspondence to Minyi Xu , Guangming Xie or Zhong Lin Wang .

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Zhao, H., Xu, M., Shu, M. et al. Underwater wireless communication via TENG-generated Maxwell’s displacement current. Nat Commun 13 , 3325 (2022). https://doi.org/10.1038/s41467-022-31042-8

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Published : 09 June 2022

DOI : https://doi.org/10.1038/s41467-022-31042-8

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Underwater wireless communication is intended to share information effectively among connected sensors, underwater vehicles, computing devices, surface stations, robotic equipment, etc. It helps to monitor real-world underwater assets, micro-organisms, human activities, sudden disasters, etc. 

This article provides you both the current and future scope of underwater wireless communication!!!

Further, underwater wireless communication is also technically strong in resource sharing, system integration, and protocol optimization which are baseline factors for effective underwater communication . Below, we have given you the important requirements of underwater wireless communication. 

Need of Underwater Wireless Communication

• For avoiding huge cost of wired network deployment
• For conducting experiments over extensive-distance
• For performing temporary trials over communication

Now, we can see about the system architecture of the underwater wireless communication in 3 major terminologies as an acoustic link, local sink, and virtual sink . These terms are used throughout the underwater wireless communication research field. So, scholars need to know the fundamentals of underwater wireless communication systems . By the by, we are proud to say that our resource team has the strong groundwork in the basics of underwater wireless communication to develop future technological advancements. 

Architecture of Underwater Wireless Communication

• Network link among 2 nodes for the purpose of communication
• Also, it is called as acoustic communication channel
• Act as interface to external world
• Instigate the processes of sensor network for management and restoration
• Integrate all available local sink nodes via multi-hops
• This connected sinks collectively represent a virtual network

In addition to underwater wireless communication architecture , we have given the core functionalities of underwater wireless communication since it is also part of underwater wireless communication fundamentals. In any underwater wireless communication application and service, you can find the functions as the essentials. The researches may vary by means of how efficiently you perform the following functions in underwater wireless communication systems . Our experts will suggest all enhanced solutions based on your wireless project needs.

Important Functions of Underwater Wireless Communication

• Provide assurance of continuous network links even in environmental attacks or node failure
• Function with all network related process for effective management
• For instance: Achieve high network connectivity, security, channel quality, etc.
• Detect the locality of the deployed network devices
• Monitoring the resource handling, data transmission, resource allocation and other network operations
• Identify the network related issues and solve them in an simplified way
• Control the resource wastage and increase the resource conservation
• For instance: memory, power, battery lifespan, etc.
• Performance assessment, traffic control, security, dynamic traffic monitoring, etc.,

So far, we have discussed the needs, architecture, and primary functions of underwater wireless communication . We hope, you have fine-tuned your fundamental knowledge of underwater wireless communication from the above sections. Now, we can see the research issues faced by UWC while practically implementing the theoretical concepts. In order to gather up-to-date research shortcomings, our technical professionals have to dive into in-depth reviews on underwater wireless communication journal papers and magazines . From that research ocean, we have collected pearls of creative research ideas based on top-demand research issues to craft wireless security research topics. Here, we have given a few of them for your awareness.

Research Challenges of Underwater Wireless Communication

• Trust and Integrity in Data Transmission 
• Fault tolerance, Flexibility and Security
• Improvisation of Multiple Access Technique in underwater wireless communication
• Secure PHY and MAC layers in Acoustic Communication
• Traffic Congestion Monitoring and Management
• Advance Time Synchronization Method for Data Transmission
• Adaptive Routing Technique for Multi-hop Communication
• Efficient Node Localization in Distributed Environment

Next, we can see the influential elements of Critical Factors that Influence Underwater Wireless Communication for affecting system performance. More than underground wireless communication, underwater wireless communication systems have several performances influencing factors. All these factors have the intention to degrade the functionalities/efficiencies of communication . And, some of them are discussed below for your reference.

Critical Factors that Influence Underwater Wireless Communication

• Large Latency
• Incorporative Transmission
• High Re-Connectivity
• Non-directional Antenna
• Network Environment Noise
• Current Speed / Network Flow Variation
• Instability of Temperature
• No Optimization of Cross Layer
• Short Network Lifespan
• High Retransmission
• Inefficient Clustering
• Signal Falsification
• Inter-symbol Interference
• Mismatched Time in Multipath Signal Delivery
• Weak Multipath Signal Strength (Rayleigh Fading)
• Lack of Robustness
• High Energy Consumption
• Horizontal Coverage
• Unrealistic Sensing
• Underwater Noise (organisms, marine movement, etc.)
• Large Salt Content of Water
• Enlarged Pressure

Latest Technologies of Underwater Wireless Communication

Our research team is not only great at handpicking research problems but also proficient in framing optimal problem-solving solutions . Since we are well-experienced in working with complex mathematical problems and numerical analysis. As a result, we are skillful in designing our own protocols / pseudo-code / algorithms. 

Further, we are also currently dealing with hybrid technologies to ensure your research work with the Future scope of underwater wireless communication . Below, we have itemized few globally preferred techniques of underwater wireless communication projects . Additionally, we also support you in Underwater Mobile Ad-hoc networks (UMANet), Underwater Wireless Sensor Networks (UWSNs), Autonomous Underwater Vehicles (AUV) Design , etc.

• Support large-scale network via numerous hydrophones arrays
• Enable to broadcast multimedia data (image, audio, video, signal)
• For instance: Live video streaming, audio-visual conferencing
• Guaranteed to accomplish high network capacity, throughput and energy efficiency
• Popular for large network capacity feature
• Close to optical wireless signals characteristics
• So, includes properties of high efficiency and bandwidth
• Support data rates about <10 Gbps for hybrid communication
• Convert the unused energy into useful electric power
• Improve energy harvesting by wireless low-power communication
• Node batteries are getting charge by itself from remote location via electromagnetic emission
• Efficient performance in short distance rather than long-distance application
• In general IoT, it connects the sensors without wire for environmental sensing and data sharing from remote location
• Internet of Underwater Things (IoUTs) performs the same process in underwater environment
• Through this, one can monitor marine organisms’ life, realize underwater territories, underwater environment changes, etc.
• Sense the underwater data transmit to the nearer AUVs or surface base stations for human access
• Provide favorable multi-access technique with other rewards of communication
• Concurrently link with multiple users in low latency
• Equal Transmission Times (ETT) power distribution
• Allocation techniques in Underwater Acoustic Networks Architecture (UWASNs)

Integration of 5G in Underwater Communication

As a matter of fact, the future scope of underwater wireless communication in a 5G network mainly focuses on two significant parameters as tremendously high data rate and low delay. Further, it incorporates two advanced technologies as GFDM and FBMC, especially for underwater applications . Here, the GFDM technique is derived from the FBMC technique in terms of frequency and time parameters.

• Generalized Frequency Division Multiplexing (GFDM)
• Filter bank multicarrier (FBMC)

Besides, here we also included the limitation of integrating the 5G network in underwater communication. Although it has some limitations, it vastly growing in the direction of future technologies . Our team also performed countless research on the following areas and designed more useful optimal solutions, 

Core Research Challenges in Underwater Wireless Communication

• Large Industrial Needs
• Mobility Management
• Improvement of Uplink performance
• mmWave Coverage

Future Directions of 5G in Underwater Wireless Communication

In the above, we have seen the future scope of Underwater Wireless Communication (i.e., 5G technology). Here, we are going to see about the future directions of 5G on holding hands with Underwater Wireless Communication. Therefore, you can get an idea about the beyond 5G and 6G technologies in underwater environments . Let’s see the future research direction of 5G in Underwater Wireless Communication

• To accomplish network parameters of ultra-low connectivity delay, ultra-speed, ultra-large capacity, etc.
• To provide very cheap and low energy communications
• To enlarge communication areas in space, sea and sky
• To introduce new spectrum with terahertz (THz) frequencies
• To improve abilities for large-scale network sensing
• To achieve extreme trust and integrity assured data transmission

Overall, we have debated on all the future research perspectives of underwater wireless communication. All these are expected to meet the requirements of QoS and QoE in a widespread environment . Then, we have discussed the responsibilities of the 5G network in Underwater Wireless Communication with its future directions. 

In specific, 5G supports all RF, electromagnetic waves, optic signals, and acoustic signals for advanced UWC systems. To the great extent, the 6G networks are combined with Space-Air- Ground-Sea Integrated Networks (SAGSIN) to attain universal coverage . Last but not least, we have enumerated the next generation expectation of underwater sensor networks. 

Future Scope of Underwater Sensor Networks

• Learning of Underwater Environment Variations
• Performance Assessment in UWC systems
• Designing Adaptive Protocols for UWSNs
• Efficient Mobile Node Localization in Underwater
• Impact of Delay in Synchronization for Critical Systems
• Improving Underwater Network Scalability and Measurement
• Enhanced Mechanism for Access control and Authorization in UWSNs
• Prediction of Low-power Sensor Mobility
• Replenishing and Depth Control in Mechanical Systems
• Dynamic Varying Network Structure Management
• Reduction of Electronic System Transmission Power

To the end, we are pleased to inform you that we support you in both present and upcoming technologies of Underwater Wireless Communication . Hence, we are ready to develop any kinds of applications in your desired areas to fulfill your requirements. So, make your research dream into true by holding our expert’s guidance. Reach our Expert Panel Team to know more about future scope of underwater wireless communication projects .