A dual-resonance magnetic coupling stealth communication unit

Abstract

The application relates to a dual-resonance magnetic coupling invisible communication unit. The unit comprises a three-axis orthogonal annular coil group, a Litz wire wound coil, a dual-resonance matching network and a single-carrier QPSK frequency domain equalization modem. The three-axis orthogonal annular coil group synthesizes a directional magnetic dipole moment by independently controlling the three-axis current amplitude and phase, realizes omnidirectional space coverage and directional beam forming; the multi-strand twisted structure of the Litz wire suppresses the high-frequency skin effect, and the equivalent alternating current resistance is 20% to 30% of that of a solid copper wire; the dual-resonance matching network makes the system bandwidth more than 5 times that of a single-resonance design, and the data rate reaches 30 Mbps; in combination with AES-128 hardware encryption and hierarchical adaptive rate control, the application provides a front-end solution for deep-sea magnetic induction communication.

Description

Technical Field

[0001] This invention relates to the field of deep-sea covert communication technology, specifically to a magnetically coupled covert communication unit employing a triaxial orthogonal loop coil group, Litz wire winding, and a dual-resonant matching network, suitable for providing magnetic induction communication support for communication beacons, underwater unmanned vehicles, and diver handheld terminals in a deep-sea environment up to 1000 meters deep. Background Technology

[0002] Underwater wireless communication is a key technological bottleneck restricting the development of deep-sea equipment. Existing mainstream solutions all have shortcomings: underwater acoustic communication utilizes sound waves to propagate in water, but passive sonar can detect and locate sound sources from several kilometers away. Even with low probability of intercept (PRC) technologies such as spread spectrum or frequency hopping, the sound signal cannot be inherently hidden in the background noise of the ocean. Blue-green light communication has good directionality, but high-power light sources can be detected by optical sensors at long distances underwater, and the light signal will leak into the air through scattering on the water surface, making it easy to be detected by satellites or drones. Optical channels require line-of-sight alignment, and their performance drops sharply in turbid water.

[0003] Magnetic induction communication utilizes the near-field attenuation characteristic of magnetic fields in seawater, where the field strength decreases with the cube of the distance, and does not generate sound wave radiation, thus possessing natural concealment. However, existing magnetic induction communication systems have the following drawbacks: when using a single-coil or non-orthogonal multi-coil structure, the antenna directivity limitation prevents the transmitted energy from effectively covering the omnidirectional space, requiring precise alignment for communication between nodes; in single-resonant operating mode, the system bandwidth is limited, with data rates typically below 1Mbps, failing to meet the high-speed requirements such as deep-sea high-definition video backhaul; the coil design is not optimized for deep-sea low-frequency magnetic communication, and the skin effect of solid copper wire at high frequencies leads to increased AC resistance and low energy transmission efficiency; the periodicity and cyclic stationary characteristics of direct sequence spread spectrum signals are easily identified by eavesdroppers through high-order spectral analysis.

[0004] This invention provides a stealth communication unit optimized for deep-sea low-frequency magnetic coupling communication. It employs a triaxial orthogonal loop coil, a dual-resonant matching network, and Litz wire winding technology to achieve a balance between Mbps-level data rates and low detection probabilities. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a dual-resonant magnetic coupling stealth communication unit that can solve the problems of the existing technology, such as the inability to effectively cover the omnidirectional space when using a single coil or non-orthogonal multi-coil structure to transmit energy, low data transmission rate, low energy transmission efficiency, and poor natural stealth.

[0006] To solve the above-mentioned technical problems, the technical solution of the present invention is as follows: a dual-resonant magnetic coupling stealth communication unit, characterized in that: it includes a triaxial orthogonal ring coil group, wherein the triaxial orthogonal ring coil group is composed of three ring coils with the same geometric parameters and mutually perpendicular to each other, and each coil is wound on an engineering plastic skeleton; A dual-resonance matching network, which consists of an adjustable capacitor array connected in parallel with each coil, compensates for the resonant point offset in real time under varying temperature and voltage conditions, thereby achieving resonant tuning at the transmitting and receiving ends. The modulation and demodulation unit adopts single-carrier QPSK modulation and frequency domain equalization, with a data rate of 1Mbps-30Mbps adaptive and automatically adjusting the rate based on the received signal strength indication feedback.

[0007] Furthermore, the toroidal coil is wound with Litz wire, which is composed of 60-100 strands of copper wire with a diameter of 0.04mm-0.10mm, and each strand of copper wire is insulated from the others.

[0008] Furthermore, the number of turns of the toroidal coil is 35-40, and the coil diameter is 100mm-200mm.

[0009] Furthermore, the adjustable capacitor array consists of 12 parallel branches, including 10 fixed capacitor branches and 2 fine-tuning capacitor branches, which are switched by a microcontroller-controlled switching network. The total tuning capacitance range of the adjustable capacitor array is 5pF to 2000pF.

[0010] Furthermore, the three coils of the triaxial orthogonal loop coil group are each controlled by an independent drive circuit. By adjusting the amplitude and phase of the current in each loop coil, a synthetic magnetic dipole moment in a specific direction is synthesized in space to achieve directional beamforming.

[0011] Furthermore, the modulation and demodulation unit also includes an adaptive rate controller, which switches between QPSK modulation and BPSK modulation in stages according to the received signal strength indication and signal-to-noise ratio fed back from the receiver, and adjusts the coding rate accordingly: QPSK modulation with a code rate of 3 / 4 is used when the signal-to-noise ratio is higher than 20dB; QPSK modulation with a code rate of 1 / 2 is used when the signal-to-noise ratio is 15dB to 20dB; QPSK modulation with a code rate of 1 / 2 is used when the signal-to-noise ratio is 10dB to 15dB; BPSK modulation with a code rate of 1 / 2 is used when the signal-to-noise ratio is 8dB to 10dB; and BPSK modulation with a code rate of 1 / 4 is used when the signal-to-noise ratio is lower than 8dB.

[0012] A deep-sea communication beacon, characterized in that it includes a dual-resonant magnetically coupled stealth communication unit, wherein the triaxial orthogonal ring coil group is assembled inside the beacon housing and connected to a PSIC power generation module and a signal camouflage module via a watertight connector.

[0013] The advantages of this invention are: this invention integrates a triaxial orthogonal loop coil structure, Litz wire winding technology, dual resonant matching network and spread spectrum camouflage module into a unified magnetic communication front end, achieving a balance between Mbps-level data rate and low detection probability; The triaxial orthogonal loop coil group can synthesize magnetic dipole moments in any direction in space by independently controlling the amplitude and phase of the triaxial current, achieving omnidirectional spatial coverage and solving the problem of mechanical alignment required by traditional single coils. The dual-resonant matching network increases the system bandwidth by more than 5 times compared to the single-resonant design, with a data rate of 30Mbps, supporting deep-sea high-definition video backhaul and large data transmission of sensor arrays. The multi-strand twisted structure of Litz wire suppresses the high-frequency skin effect, and the equivalent AC resistance is 20% to 30% of that of solid copper wire, achieving high-efficiency transmission. The cubic attenuation of the magnetic field in seawater is faster than the cylindrical attenuation of sound waves. Combined with dual-resonant matching and adaptive power control, the intensity of the radiated magnetic field is controlled below the ambient magnetic noise within a preset safe distance. The dual-resonant technology adds parallel compensation capacitors to the transmitting and receiving ends, so that the transmitting and receiving ends work in a resonant state at the same time. The parallel adjustable capacitor array compensates for the resonant point offset in real time, maintaining optimal performance under the conditions of temperature and pressure changes in the deep sea. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the triaxial orthogonal ring coil group structure of the present invention; Figure 2 This is a schematic diagram of the cross-sectional structure of the Litz wire, solid copper wire, and the stranding of multiple fine copper wires according to the present invention. Figure 3 This is a block diagram of the dual-resonant matching network principle of the present invention; Figure 4 This is a flowchart of the single-carrier QPSK SC-FDE baseband signal processing of the present invention; Figure 5 This is a schematic diagram of the triaxial current amplitude-phase control synthesis directional magnetic dipole moment vector of the present invention; Figure 6 This is a comparison curve of the transmitted signal power spectrum and the Wenz deep-sea magnetic noise spectrum of the present invention; Figure 7 This is a schematic diagram of the adjustable capacitor array circuit of the present invention. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. The following embodiments will enable those skilled in the art to more fully understand this invention, but do not limit the invention to the scope of the described embodiments.

[0016] This invention discloses a dual-resonant magnetically coupled stealth communication unit suitable for providing magnetic induction communication for communication beacons, underwater unmanned vehicles, and diver handheld terminals in a deep-sea environment up to 1000 meters deep. It offers a data rate of 1 Mbps to 30 Mbps and features a low probability of intercept. This unit is a sub-module of the parent invention patent, "An Integrated End-to-End Low Physical Characteristics Communication System Based on Deep-Sea Hydrostatic Pressure."

[0017] Example 1: A dual-resonant magnetically coupled stealth communication unit includes a triaxial orthogonal ring coil group, such as Figure 1 As shown, the triaxial orthogonal ring coil group consists of three ring coils with identical geometric parameters. The axes of the three ring coils are perpendicular to each other in pairs. Each coil is wound on an engineering plastic skeleton, which is supported by a skeleton made of PEEK polyether ether ketone material by injection molding or machining. The skeleton wall thickness is 3mm. The number of turns of the ring coil is 35-40 turns, and the coil diameter is 100mm-200mm. In this embodiment, each coil is wound with 37 turns, and the diameter of each coil is 150mm.

[0018] The toroidal coil is wound with Litz wire, such as Figure 2 As shown, the Litz wire is made of 60-100 strands of copper wire with a diameter of 0.04mm-0.10mm twisted together. In this embodiment, the winding material is Litz wire with a specification of 66×0.07mm, that is, 66 strands of copper wire with a diameter of 0.07mm twisted together, with a total equivalent diameter of 0.57mm. The basis for selecting this specification is that, within the target operating frequency band of 1MHz-45MHz, the skin depth of copper decreases with increasing frequency, being 66μm at 1MHz, 21μm at 10MHz, and 9.8μm at 45MHz. With a diameter of 0.07mm per strand, the wire has a skin depth of less than 1MHz to 10MHz, allowing full utilization of the conductor cross-sectional area. Although the skin depth is greater than 9.8μm at 45MHz, the multi-strand stranded structure of the Litz wire disperses the current to each independent copper wire, suppressing the skin effect and proximity effect. All strands can fully utilize their conductivity. The equivalent AC resistance of the Litz wire is 20% to 30% of that of solid copper wire. Driven by a power amplifier, the coil carrier frequency reaches the optimal resonance state near 10MHz, with a current amplitude of 0.5A to 1A.

[0019] The comparison between Litz wire and solid copper wire is shown in the table below: .

[0020] Three loop coils are fixed to the internal frame of the beacon housing using a mechanically orthogonal assembly method, with assembly errors controlled within ±1°. Mechanical limiting devices are installed on the orthogonal axes to ensure that the relative positions of the three axes do not shift during deep-sea transportation and deployment. After assembly, all loop coils are vacuum-sealed with epoxy resin. The epoxy resin used is EP-6010HT type, a hydrolysis-resistant product, to fill the tiny gaps between the coils and the frame, preventing seawater from seeping in under the high pressure of the deep sea.

[0021] Each strand of the Litz wire is insulated from the others and is evenly twisted at a pitch of 3-5 strands per centimeter to suppress high-frequency proximity effects. During the winding process, a constant tension winding machine is used to maintain uniform tension, which is set at 0.8N-1.2N. Each turn is isolated by a 0.025mm thick polyimide film, and insulation is also provided between layers. The coil inductance is measured using an LCR meter, and the deviation of the triaxial inductance value is less than 5%. If the deviation exceeds 5%, the number of turns is adjusted or the coil is rewound.

[0022] The advantages of Litz wire in underwater magnetic induction communication have been experimentally verified. A study published in Discover Applied Sciences in 2026 analyzed the magnetic flux density distribution and loss region through finite element simulation and established an empirical loss model for a saltwater conductive environment. At high frequencies, the current tends to flow to the surface of a solid conductor, significantly increasing the resistance. The multi-strand stranded structure of Litz wire can suppress the skin effect and reduce AC resistance.

[0023] The dual-resonant matching network consists of an array of adjustable capacitors connected in parallel with each coil. The adjustable capacitor array compensates for the resonant point offset in real time under varying temperature and voltage conditions, thereby achieving resonant tuning at the transmitting and receiving ends.

[0024] The physical basis of dual-resonant magnetic quasi-static coupling (DQ-MQS) is that Faraday's law of electromagnetic induction causes a changing current to generate a magnetic field, and the changing magnetic field induces an electromotive force in the receiving coil.

[0025] In traditional single-resonant systems, the transmitting coil and matching capacitor form an LC resonant circuit, but the receiving end lacks resonant matching, limiting the system bandwidth to the inherent Q value of the coil. Dual-resonant technology adds parallel compensation capacitors at both the transmitting and receiving ends, allowing both ends to operate in a resonant state simultaneously. The parallel resonance significantly increases the coil impedance, allowing energy to be efficiently coupled into the magnetic field. Dual-resonance increases the system bandwidth by more than five times compared to a single-resonant design, boosting data rates to the Mbps level.

[0026] In this embodiment, each loop coil is connected in parallel with a set of adjustable capacitor arrays to achieve resonant tuning at the transceiver end, with the goal of making the resonant frequency of the LC resonant circuit consistent with the carrier frequency of 10MHz; for example Figure 3 and Figure 7As shown, the capacitor array consists of 12 parallel branches: 10 fixed capacitor branches (1pF, 2pF, 5pF, 10pF, 20pF, 50pF, 100pF, 200pF, 500pF, 1000pF) and 2 trimmer capacitor branches (5pF-20pF, 10pF-50pF). Switching is performed by an analog switch network controlled by a microcontroller, with a switching on-resistance of less than 4Ω and a switching time of less than 50ns. The total tuning capacitance of the capacitor array ranges from 5pF to 2000pF, covering the resonance matching requirements when the coil inductance varies from 0.5μH to 2.0μH. The bandwidth is increased by ≥5 times, and the resonant frequency is 10MHz. The adjustable capacitor array compensates for the resonant point shift in real time under varying temperature and voltage conditions, maintaining optimal coupling.

[0027] The matching algorithm is as follows: (1) In the initial stage, the transmitter switches different capacitance values ​​in sequence and monitors the reflected power through the built-in VSWR detection circuit to find the capacitor combination with the minimum reflected power; (2) During the communication process, the calibration process shall be restarted every 1 minute or when the RSSI reported by the receiver drops by more than 3dB. (3) During rapid resynchronization, only scan the range of ±20% of the optimal capacitance value in the previous step to reduce scanning time and energy consumption.

[0028] The modulation and demodulation unit employs single-carrier modulation and frequency domain equalization. Compared to OFDM, single-carrier frequency domain equalization has a lower peak-to-average power ratio, which can improve power amplifier efficiency under energy-constrained conditions in the deep sea. The data rate is adaptive from 1Mbps to 30Mbps, automatically adjusting the rate based on the received signal strength indication feedback.

[0029] The transmitted data frame format is as follows: synchronization header (512 known symbols, used for time-frequency synchronization), channel estimation sequence (256 symbols), data payload (variable length), and CRC checksum (32 bits). The total length of the data frame ranges from 1ms to 10ms, depending on the data rate and payload size. Figure 4 The diagram shows the processing flow of a single-carrier QPSK SC-FDE baseband signal.

[0030] like Figure 6 The figure shows a comparison curve between the transmitted signal power spectrum and the Wenz deep-sea magnetic noise spectrum. As can be seen from the comparison curve, the transmitted signal power spectrum, after shaping, basically overlaps with the Wenz deep-sea magnetic noise spectrum, achieving a balance between Mbps-level data rates and low detection probabilities.

[0031] Example 2: The data rate of the modulation and demodulation unit is adaptively controlled by an adaptive rate controller. Based on the received signal strength indication and signal-to-noise ratio fed back from the receiver, it switches between QPSK modulation and BPSK modulation in stages and adjusts the coding rate accordingly.

[0032] The rate adaptive algorithm is as follows: (1) The receiver calculates the signal-to-noise ratio (SNRest) of the received signal in real time, calculates the received signal strength index (RSSI) based on the average value of ADC sampling, and calculates the received bit error rate (BERest).

[0033] (2) The receiver encodes the SNRest and recommended data rate in the ACK frame and sends it to the transmitter through the reverse channel.

[0034] (3) The transmitter adjusts the transmission data rate and transmission power based on the feedback.

[0035] The rate rating table is as follows: 1 SNR ≥ 20dB 30 Mbps QPSK 3 / 4 2 15dB ≤ SNR <20dB 20 Mbps QPSK 1 / 2 3 10dB ≤ SNR <15dB 10 Mbps QPSK 1 / 2 4 8dB ≤ SNR <10dB 5 Mbps BPSK 1 / 2 5 SNR <8dB 1 Mbps BPSK 1 / 4 (repeated encoding) The encryption engine uses an AES-128 hardware acceleration engine, which implements 128-bit AES algorithm encryption and decryption operations with a dedicated hardware module.

[0036] Example 3: The three coils of the triaxial orthogonal loop coil group are each controlled by an independent drive circuit, such as Figure 5 As shown, in order to achieve point-to-point secure communication and reduce the probability of signal interception from non-target directions, this embodiment adjusts the amplitude and phase of the current in each axis loop coil to synthesize a synthetic magnetic dipole moment in a specific direction in space, forming a directional beam pointing towards the target node, replacing the omnidirectional broadcast mode, and reducing the probability of signal interception from non-target directions.

[0037] Let the azimuth angle of the target direction in the spherical coordinate system be... (Relative to the Z-axis), the pitch angle is (Relative to the X-axis). Ideally, the projection components of the resultant magnetic moment on each axis are proportional as follows: .

[0038] In practical applications, the above ratios need to be corrected to address the mutual inductance effect between coils when the three axes are orthogonal. The correction coefficients are obtained through pre-calibration experiments or electromagnetic simulations. The phase of the three-axis current is independently adjusted by the PWM DAC. After synthesis, the main lobe gain of the magnetic field in the target direction is increased by 8dB to 12dB compared to the omnidirectional mode, while the sidelobe attenuation in non-target directions reaches more than 20dB.

[0039] Example 4: Omnidirectional Broadcast and Directional Handover: In multi-node networking scenarios, the system determines whether broadcasting is needed through an intelligent handover controller. Global broadcast time slots are reserved every 30 seconds and last for 2ms; data packets are extremely short beacon frames (containing only synchronization and node ID). During directional communication, after the master node determines the target node through pre-negotiation, the synthesized magnetic dipole moment vector points directly at the target.

[0040] Example 5: A deep-sea communication beacon includes a dual-resonant magnetically coupled stealth communication unit, with a triaxial orthogonal ring coil assembly assembled inside the beacon housing, and connected to a PSIC power generation module and a signal camouflage module via a watertight connector.

[0041] The foregoing has shown and described the basic principles and main features of the present invention, as well as its advantages. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A dual-resonant magnetically coupled stealth communication unit, characterized in that: It includes a triaxial orthogonal ring coil group, which consists of three ring coils with the same geometric parameters and which are mutually perpendicular to each other. Each coil is wound on an engineering plastic skeleton. A dual-resonance matching network, which consists of an adjustable capacitor array connected in parallel with each coil, compensates for the resonant point offset in real time under varying temperature and voltage conditions, thereby achieving resonant tuning at the transmitting and receiving ends. The modulation and demodulation unit adopts single-carrier QPSK modulation and frequency domain equalization, with a data rate of 1Mbps-30Mbps adaptive and automatically adjusting the rate based on the received signal strength indication feedback.

2. The dual-resonant magnetically coupled stealth communication unit according to claim 1, characterized in that: The toroidal coil is wound with Litz wire, which is made of 60-100 strands of copper wire with a diameter of 0.04mm-0.10mm, and each strand of copper wire is insulated from the others.

3. The dual-resonant magnetically coupled stealth communication unit according to claim 1, characterized in that: The number of turns of the toroidal coil is 35-40, and the coil diameter is 100mm-200mm.

4. The dual-resonant magnetically coupled stealth communication unit according to claim 1, characterized in that: The adjustable capacitor array consists of 12 parallel branches, including 10 fixed capacitor branches and 2 fine-tuning capacitor branches, which are switched by a microcontroller-controlled switching network. The total tuning capacitance range of the adjustable capacitor array is 5pF to 2000pF.

5. The dual-resonant magnetically coupled stealth communication unit according to claim 1, characterized in that: The three coils of the triaxial orthogonal loop coil group are controlled by independent drive circuits. By adjusting the amplitude and phase of the current of each loop coil, a synthetic magnetic dipole moment in a specific direction is synthesized in space to achieve directional beamforming.

6. The dual-resonant magnetically coupled stealth communication unit according to claim 1, characterized in that: The modulation and demodulation unit further includes an adaptive rate controller, which switches between QPSK modulation and BPSK modulation in stages according to the received signal strength indication and signal-to-noise ratio fed back from the receiver, and adjusts the coding rate accordingly: QPSK modulation with a code rate of 3 / 4 is used when the signal-to-noise ratio is higher than 20dB; QPSK modulation with a code rate of 1 / 2 is used when the signal-to-noise ratio is 15dB to 20dB; QPSK modulation with a code rate of 1 / 2 is used when the signal-to-noise ratio is 10dB to 15dB; BPSK modulation with a code rate of 1 / 2 is used when the signal-to-noise ratio is 8dB to 10dB; and BPSK modulation with a code rate of 1 / 4 is used when the signal-to-noise ratio is lower than 8dB.

7. A deep-sea communication beacon, characterized in that: The invention comprises a dual-resonant magnetically coupled stealth communication unit as described in any one of claims 1 to 6, wherein the triaxial orthogonal ring coil assembly is assembled inside the beacon housing and connected to the PSIC power generation module and the signal camouflage module via a watertight connector.

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