Magnetic induction through-the-earth communication method based on OFDM

Abstract

The invention discloses a magnetic induction through-the-earth communication method based on OFDM (Orthogonal Frequency Division Multiplexing), which is applied to mine rescue and comprises the following steps: in a signal transmitting end, converting an analog voice signal into a digital signal, encoding an information source, mapping into a multi-system code element, carrying out serial-to-parallel conversion, modulating, synthesizing an OFDM symbol, carrying out parallel-to-serial conversion and carrying out first filtering processing; transmitting the signal to a magnetic induction through-the-earth communication channel by using a transmitting antenna; in a signal receiving end, acquiring a signal by using a receiving antenna; and after second filtering processing, serial-to-parallel conversion, separation to original OFDM subcarriers, demodulation, parallel-to-serial conversion, conversion back to binary data and information source decoding processing, a recovered digital signal is converted to an analog form, and original voice information is output. According to the method, a multipath effect generated by electromagnetic waves in a multilayer geological environment is avoided through magnetic induction through-the-earth communication, secondary modulation is performed by using an OFDM technology, the communication bandwidth utilization rate is effectively improved, and remote mine rescue communication with stable signals is realized.

Description

Technical Field

[0001] The present invention relates to the field of magnetic induction through-the-earth communication, and more particularly to an OFDM-based magnetic induction through-the-earth communication method. Background Art

[0002] In the mine emergency rescue scenario, when a disaster occurs, conventional communication means are easily damaged, resulting in communication interruption, which not only seriously affects the rescue efficiency, but also threatens the life safety of trapped personnel. Therefore, through-the-ground communication technology, as a technical means that can penetrate the strata for wireless communication, is particularly important.

[0003] Traditional ground-penetrating communication methods, such as low-frequency electromagnetic waves and elastic wave communications, face many challenges in their application in mine environments. First, although very low frequency electromagnetic waves have strong penetration capabilities, they require transmitting antennas that are several kilometers or even longer, which is almost impossible to achieve in a mine environment with limited space. Secondly, when electromagnetic waves propagate in multi-layer geological structures, reflection, refraction and scattering phenomena will inevitably occur. The multipath effect formed by the superposition of these phenomena seriously interferes with the communication signal, resulting in a significant reduction in communication effect and stability. Even if lower frequency electromagnetic waves are used to reduce path loss, the large size of the required antenna is still a difficult problem to overcome.

[0004] In comparison, magnetic induction communication has unique advantages in the field of through-the-ground communication due to its characteristics of no multipath effect and stable channel state. However, magnetic induction communication is not without its disadvantages. Its communication bandwidth is relatively narrow and the path loss is also large, especially in long-distance communication scenarios, these problems are particularly prominent. The low received signal strength and low signal-to-noise ratio make the application of high-order modulation methods difficult, and cannot meet the requirements of real-time voice communication for information transmission rate. Although the data transmission method of low-base code elements can alleviate this problem to a certain extent, the use of low-base code elements requires sufficient communication bandwidth for through-the-ground communication, which is inconsistent with the current situation of relatively narrow communication bandwidth of magnetic induction communication.

[0005] In addition, existing magnetic induction ground-penetrating communication solutions also have deficiencies in receiver design. Some solutions require that the receiving antenna must be placed in a specific posture to achieve better communication effects. This limitation not only affects the flexible layout of the receiver, but also reduces the adaptability of the communication system under complex geological conditions. At the same time, the effective coverage range of the receiving antenna is limited, and it is difficult to ensure that sufficient signal strength can be received at all possible locations, thus affecting the reliability and practicality of the overall communication.

[0006] In summary, the traditional ground-penetrating communication method has problems such as large antenna size, serious multipath effect, narrow magnetic induction communication bandwidth, and large path loss. The existing magnetic induction ground-penetrating communication solution also has deficiencies in receiver design. These problems together limit the effective application of ground-penetrating communication technology in mine emergency rescue scenarios.

[0007] Therefore, how to design a magnetic induction through-the-ground communication method that can adapt to the complex environment underground and can transmit real-time voice signals over long distances through the ground under narrow communication bandwidth and large path loss is an urgent problem that technical personnel in this field need to solve. Summary of the invention

[0008] In view of this, the present invention provides a magnetic induction through-the-ground communication method based on OFDM. By combining the stability of magnetic induction through-the-ground communication and the efficient bandwidth utilization of OFDM technology, the problems of signal attenuation and multipath effect faced by traditional through-the-ground communication methods in long-distance communication are overcome, and long-distance real-time through-the-ground voice calls can be ensured between rescue personnel and trapped persons.

[0009] In order to achieve the above object, the present invention adopts the following technical solution:

[0010] A magnetic induction ground-penetrating communication method based on OFDM, applied to mine rescue, comprising:

[0011] At the signal transmitter,

[0012] Convert analog voice signals into digital signals;

[0013] Perform source encoding on digital signals and output compressed binary data;

[0014] Mapping the encoded binary data into multi-binary code elements;

[0015] Performing serial-to-parallel conversion on the multi-binary code elements to generate a first parallel data stream;

[0016] Modulating the first parallel data stream and loading it onto an OFDM subcarrier to generate an OFDM subcarrier signal;

[0017] The OFDM subcarrier signals are synthesized into OFDM symbols through IFFT;

[0018] Performing parallel-to-serial conversion on the OFDM symbols to generate a first serial data stream;

[0019] Performing a first filtering process on the first serial data stream;

[0020] Using a transmitting antenna to transmit the signal after the first filtering process to a magnetic induction through-the-earth communication channel;

[0021] At the signal receiving end,

[0022] Using a receiving antenna to obtain a signal in a magnetic induction through-the-earth communication channel;

[0023] performing a second filtering process on the signal;

[0024] Performing serial-to-parallel conversion on the signal after the second filtering process to generate a second parallel data stream;

[0025] Separating the second serial data stream into original OFDM subcarriers by FFT;

[0026] Demodulate the signals separated into the original OFDM subcarriers to recover the parallel data streams;

[0027] Performing parallel-to-serial conversion on the parallel data stream to generate a second serial data stream;

[0028] converting the second serial data stream back into binary data;

[0029] Perform source decoding on the converted binary data to restore the digital signal;

[0030] Convert the recovered digital signal to analog form and output the original voice information.

[0031] Furthermore, the transmitting antenna adopts a single-axis multi-turn air coil made of copper wire, with a radius of 1 meter, 1000 turns, and a copper wire diameter of 0.4 mm.

[0032] Furthermore, the receiving antenna uses a three-axis orthogonal multi-turn air coil made of copper wire, with a radius of 10 cm, 200 turns, and a copper wire diameter of 0.14 mm.

[0033] Furthermore, in the receiving antenna, based on the magnitude of the magnetic flux on each receiving coil, the signal in the receiving coil with the largest magnetic flux is selected for output.

[0034] Furthermore, parameter design is performed on the signal transmitter and the signal receiver, including:

[0035] Construct a magnetic induction through-the-ground communication model between the transmitting antenna and the receiving antenna, as well as a receiving antenna attitude model;

[0036] Determine the equivalent circuits of the signal transmitter and the signal receiver;

[0037] The signal transmitting end equivalent circuit includes: the transmitting antenna internal resistance R t , transmitting antenna inductance L t , transmitting circuit current i t , EMF of the transmitting circuit u t ;in, I0 represents the current i tThe effective value of , e represents the base of the natural logarithm, j represents the imaginary unit, π represents the circumference of a circle, f represents the frequency of the alternating magnetic field generated by the transmitting antenna coil, and t represents the time variable;

[0038] The signal receiving end equivalent circuit includes: the receiving antenna internal resistance R r , receiving antenna inductance L r , receiving loop current i r 、Receiving circuit electromotive force u e , compensation capacitor C L , load resistance R L and load electromotive force u r ;

[0039] Based on the magnetic induction through-the-ground communication model and the receiving antenna attitude model, as well as the equivalent circuits of the signal transmitter and the signal receiver, the average power P of the signal transmitter is determined. t and load resistance R L The received power P r ;in,

[0040]

[0041] Based on the average power P of the signal transmitting end t and load resistance R L The received power P r , determine the equivalent path loss L(f) of magnetic induction through-the-ground communication; where,

[0042]

[0043] Based on the equivalent path loss L(f), a curve of path loss versus transmission current frequency is constructed to determine the available communication bandwidth;

[0044] The number of OFDM subcarriers and the cyclic prefix length are determined based on the coherence time ΔT and coherence bandwidth ΔB of the magnetic induction through-the-earth communication channel.

[0045] Furthermore, the number of OFDM subcarriers is 8, and the corresponding multi-ary codewords are octal codewords.

[0046] Furthermore, the cyclic prefix length is 0, and when generating OFDM symbols, the IFFT result without adding the cyclic prefix is ​​directly output.

[0047] It can be seen from the above technical solution that compared with the prior art, the technical solution of the present invention has the following advantages:

[0048] Beneficial effects:

[0049] 1. By using magnetic induction to communicate through the ground, the multipath effect of electromagnetic waves in a multi-layer geological environment is avoided, ensuring the stability of the communication channel. At the same time, the OFDM technology is used for secondary modulation to improve the communication bandwidth utilization rate, so that even in long-distance communication, a high signal quality can be maintained, thereby increasing the real-time voice call distance in mine rescue.

[0050] 2. The transmitting antenna uses a single-axis multi-turn air coil, and the receiving antenna uses a three-axis orthogonal multi-turn air coil, which not only reduces the size of the antenna, making it more suitable for deployment in a narrow mine environment, but also improves communication efficiency through reasonable coil parameter design (such as radius, number of turns, copper wire diameter, etc.). In addition, the receiving effect is further optimized through magnetic flux detection and selective merging output technology.

[0051] 3. The entire communication process uses mature technologies such as LDPC coding and QPSK modulation to ensure the reliability and stability of communication. At the same time, through the design of modules such as A / D conversion, source coding, binary to multi-binary mapping, it can process analog voice signals and efficiently transmit them to the receiving end, and then restore them to the original voice information, providing strong communication support for mine rescue.

[0052] 4. In the specific parameter design process, by building the communication model and attitude model, determining the equivalent circuit, calculating the path loss, and determining the available communication bandwidth, it can flexibly adjust the parameters such as the number of OFDM subcarriers, communication symbol system, and subcarrier modulation mode according to the specific mine environment and communication requirements to ensure the optimal communication performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0054] Figure 1 A flow chart of a magnetic induction through-the-earth communication method based on OFDM provided in an embodiment of the present invention;

[0055] Figure 2 A schematic diagram of a parameter design process for a signal transmitting end and a signal receiving end provided in an embodiment of the present invention;

[0056] Figure 3 A schematic diagram of a magnetic induction through-the-earth communication model between a transmitting antenna and a receiving antenna provided in an embodiment of the present invention;

[0057] Figure 4A schematic diagram of a receiving antenna attitude model provided in an embodiment of the present invention;

[0058] Figure 5 A schematic diagram of an equivalent circuit of a signal transmitting end provided by an embodiment of the present invention;

[0059] Figure 6 A schematic diagram of an equivalent circuit of a signal receiving end provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0060] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0061] Embodiment 1;

[0062] like Figure 1 As shown, this embodiment provides an OFDM-based magnetic induction ground communication method, which is applied to mine rescue, including:

[0063] At the signal transmitter,

[0064] Convert analog voice signals into digital signals;

[0065] Perform source encoding on digital signals and output compressed binary data;

[0066] Mapping the encoded binary data into multi-binary code elements;

[0067] Performing serial-to-parallel conversion on the multi-binary code elements to generate a first parallel data stream;

[0068] Modulating the first parallel data stream and loading it onto an OFDM subcarrier to generate an OFDM subcarrier signal;

[0069] The OFDM subcarrier signals are synthesized into OFDM symbols through IFFT;

[0070] Performing parallel-to-serial conversion on the OFDM symbols to generate a first serial data stream;

[0071] Performing a first filtering process on the first serial data stream;

[0072] Using a transmitting antenna to transmit the signal after the first filtering process to a magnetic induction through-the-earth communication channel;

[0073] At the signal receiving end,

[0074] Using a receiving antenna to obtain a signal in a magnetic induction through-the-earth communication channel;

[0075] performing a second filtering process on the signal;

[0076] Performing serial-to-parallel conversion on the signal after the second filtering process to generate a second parallel data stream;

[0077] Separating the second serial data stream into original OFDM subcarriers by FFT;

[0078] Demodulate the signals separated into the original OFDM subcarriers to recover the parallel data streams;

[0079] Performing parallel-to-serial conversion on the parallel data stream to generate a second serial data stream;

[0080] converting the second serial data stream back into binary data;

[0081] Perform source decoding on the converted binary data to restore the digital signal;

[0082] Convert the recovered digital signal to analog form and output the original voice information.

[0083] In the specific application scenario of mine rescue, the signal transmitter is usually deployed on the ground of the mine or in a safe area near the rescue entrance so that rescuers can easily operate and monitor the communication process. Rescuers transmit key data such as rescue instructions and voice information to the trapped people through the signal transmitter. The signal receiver is deployed inside the mine, as close as possible to the location of the trapped people, to ensure that the trapped people can receive clear voice information and rescue instructions.

[0084] The method converts the analog voice signal into a digital signal at the signal transmitting end, and then transmits it through a special transmitting antenna after source coding, mapping into multi-binary code elements, serial-to-parallel conversion, modulation, OFDM symbol synthesis, parallel-to-serial conversion, and first filtering. The receiving end captures the signal through the receiving antenna, and finally restores the original voice information after filtering, FFT separation, demodulation, parallel-to-serial conversion, reverse mapping, source decoding, and D / A conversion.

[0085] It takes advantage of magnetic induction communication to avoid multipath effects and combines it with OFDM technology to improve bandwidth utilization and communication distance, ensuring the stability and reliability of mine rescue communications under complex geological conditions.

[0086] The following is a further detailed description of each step and related features in the above method;

[0087] In this embodiment, the purpose of the source coding in the signal transmitting end is to compress the collected original voice signal and reduce the information transmission rate required for transmitting the real-time voice signal as much as possible. Specifically, the compression processing process of the voice signal in the GSM standard can be referred to, and the voice signal with an information rate of 13Kbps can be regarded as a typical voice signal, and the original voice information can be compressed from 64Kbps to 13Kbps, thereby effectively reducing the amount of data and improving the transmission efficiency.

[0088] In this embodiment, the transmitting antenna uses a single-axis multi-turn air coil made of copper wire, with a radius of 1 meter, 1000 turns, and a copper wire diameter of 0.4 mm.

[0089] In this embodiment, the receiving antenna uses a three-axis orthogonal multi-turn air coil made of copper wire, with a radius of 10 cm, 200 turns, and a copper wire diameter of 0.14 mm.

[0090] And in the receiving antenna, based on the magnitude of the magnetic flux on each receiving coil, the signal in the receiving coil with the largest magnetic flux is selected for output.

[0091] The design of the transmitting antenna and the receiving antenna not only reduces the antenna size to adapt to the narrow mine environment, but also improves the communication efficiency through reasonable parameter selection. The receiving sensitivity is enhanced, and the signal reception effect is further optimized through magnetic flux detection and selective merging output technology, ensuring that the strongest magnetic induction signal can be obtained even under complex geological conditions.

[0092] like Figure 2 As shown, parameter design is performed on the signal transmitter and the signal receiver, including:

[0093] 1) Construct a magnetic induction through-the-ground communication model between the transmitting antenna and the receiving antenna, as well as a receiving antenna attitude model;

[0094] like Figure 3 As shown, constructing a magnetic induction through-the-ground communication model between a transmitting antenna and a receiving antenna includes:

[0095] With the center position O of the transmitting antenna coil as the origin, an XYZ coordinate system is constructed; where Q is the center position of the receiving antenna coil, and the coordinate parameters of point Q are expressed as Where r is the straight-line distance between position O and position Q, θ is the angle between the line connecting position O and position Q and the Z axis, is the angle between the projection of the line connecting position O and position Q on the XOY plane and the X-axis; n1, n2, n3 represent different orthogonal multi-turn air coils.

[0096] When the current on the transmitting antenna coil is I, the magnetic induction intensity component B along the X, Y, and Z axes of the rectangular coordinate system at point Q can be obtained. X , BY , B Z The modulus values ​​are:

[0097]

[0098]

[0099]

[0100] Among them, α t represents the radius of the transmitting antenna coil, n t represents the number of turns of the transmitting antenna coil, μ represents the magnetic permeability of the earth medium, and G represents the attenuation coefficient caused by the eddy current effect;

[0101] The attenuation coefficient G caused by the eddy current effect is expressed as:

[0102]

[0103] Among them, f represents the frequency of the alternating magnetic field generated by the transmitting antenna coil, σ represents the conductivity of the earth medium, and π represents pi.

[0104] like Figure 4 As shown, constructing the receiving antenna attitude model includes:

[0105] With the center position Q of the receiving antenna coil as the origin, an XYZ coordinate system is constructed. The normal vector of each coil plane in the coordinate system X'Y'Z' is expressed as:

[0106]

[0107] Among them, i is 1, 2 and 3, corresponding to coils n1, n2, n3, e X 、e Y 、e Z Unit vector in rectangular coordinate system, θ i is the normal vector e i The angle with the positive direction of the Z axis; is the normal vector e i The angle between the projection on the XOY plane and the positive direction of the X-axis.

[0108] Furthermore, the signal in the receiving coil with the largest magnetic flux is selected for output. At this time, the maximum magnetic flux Φ that the receiving antenna coil can receive is max for:

[0109]

[0110] Among them, α r represents the radius of the receiving antenna coil, n r Indicates the number of turns of the transmitting antenna coil, J maxIt represents the spatial attitude factor of the coil that can receive the maximum magnetic flux among the three receiving antenna coils;

[0111]

[0112] 2) Determine the equivalent circuits of the signal transmitter and the signal receiver;

[0113] like Figure 5 As shown, the equivalent circuit of the signal transmitter includes: the internal resistance of the transmitting antenna R t , transmitting antenna inductance L t , transmitting circuit current i t , EMF of the transmitting circuit u t ;in, I0 represents the current i t The effective value of , e represents the base of the natural logarithm, j represents the imaginary unit, π represents the circumference of a circle, f represents the frequency of the alternating magnetic field generated by the transmitting antenna coil, and t represents the time variable;

[0114] like Figure 6 As shown, the equivalent circuit of the signal receiving end includes: the internal resistance of the receiving antenna R r , receiving antenna inductance L r , receiving loop current i r 、Receiving circuit electromotive force u e , compensation capacitor C L , load resistance R L and load electromotive force u r ;

[0115] When magnetic induction through-the-ground communication is performed, there is an alternating current i in the transmitting circuit. t , alternating current i t It will stimulate an alternating magnetic field. When the alternating magnetic field passes through the stratum medium and passes through the plane of the receiving antenna coil, it will generate a receiving loop electromotive force u in the receiving antenna coil. e , thus generating a receiving loop current i in the receiving antenna coil loop r , receiving loop current i r Flowing through the load resistor R L Generate load electromotive force u r , completing magnetic induction through-the-ground communication.

[0116] 3) Based on the magnetic induction through-the-ground communication model and the receiving antenna attitude model, as well as the equivalent circuits of the signal transmitter and the signal receiver, determine the average power P of the signal transmitter t and load resistance R L The received power P r ;in,

[0117] Receiving circuit electromotive force u e for:

[0118]

[0119] Receiving loop current i r for:

[0120]

[0121] The average power P of the signal transmitting end t for:

[0122] P t =I0 2 R t

[0123] Load resistance R L The received power P r :

[0124] P r =i r 2 R L

[0125] 4) Based on the average power P of the signal transmitter t and load resistance R L The received power P r , determine the equivalent path loss L(f) of magnetic induction through-the-ground communication; where,

[0126]

[0127] 5) Based on the equivalent path loss L(f), a curve showing the change of path loss with the frequency of the transmitting current is constructed to determine the available communication bandwidth; specifically, by substituting the relevant design parameters in this embodiment into the expression of L(f), a curve showing the change of path loss with the frequency of the transmitting current can be obtained, and the frequency range covered by a 3 dB increase in the trough position of the curve is taken as the available bandwidth, and the available communication bandwidth is about 6.4 kHz.

[0128] 6) Determine the number of OFDM subcarriers and the cyclic prefix length based on the coherence time ΔT and coherence bandwidth ΔB of the magnetic induction through-the-ground communication channel.

[0129] Because the magnetic field is closed and has no dispersion, and the magnetic permeability of materials such as soil and rocks in the stratum is close to the magnetic permeability of vacuum, the magnetic field signal will not be reflected, refracted or scattered during the propagation process. Therefore, there is no multipath effect when using magnetic field signals for through-the-ground communication, and the maximum delay spread of the channel is τ max Therefore, in this embodiment, when OFDM is used, it is chosen not to add a cyclic prefix, thereby avoiding the power and information rate loss caused by adding a guard interval.

[0130] During underground emergency rescue, the position of the transceiver for magnetic induction through-the-ground communication will basically not change, there is no relative motion between the transceivers, and the maximum Doppler frequency shift f m To obtain a relatively simplified communication process, the magnetic induction through-the-ground communication method in this scheme selects as few OFDM subcarriers as possible while meeting the required communication rate.

[0131] Furthermore, the number of OFDM subcarriers is 8, and the corresponding multi-ary codeword is an octal codeword. And the cyclic prefix length is 0, when generating OFDM symbols, the IFFT result without adding the cyclic prefix is ​​directly output.

[0132] After setting the relevant parameters, the specific communication process includes:

[0133] At the signal transmitter, the analog voice signal is converted into digital form, that is, a binary data stream.

[0134] Then, these binary data streams are converted into octal code elements in order to achieve a higher information transmission rate within a limited bandwidth, minimize the requirements for signal-to-noise ratio, and make long-distance real-time voice communication possible.

[0135] Next, the octal code elements are distributed to multiple parallel OFDM subcarriers and modulated by 8QAM (octal quadrature amplitude modulation). Each subcarrier carries a portion of information, and the difference in carrier frequency between adjacent subcarriers is set to 0.542kHz. Real-time voice information with a rate of 13kbit / s can be dispersed to 8 OFDM subcarriers and transmitted in parallel at an information rate of 1.625kb / s and a code element rate of 541.7B.

[0136] The modulated subcarrier signals are synthesized into a composite OFDM symbol through inverse fast Fourier transform (IFFT), and then transmitted by the transmitting coil in the form of a magnetic field signal after further parallel-to-serial conversion and first filtering through the transmitting filter. The roll-off factor of the transmitting filter is 0.2 to ensure smooth transition of the signal and reduce out-of-band interference. After this processing, the total bandwidth occupied by the signal is 6.2kHz.

[0137] At the signal receiving end, the received magnetic field signal is captured by the receiving coil, and after the second filtering process is performed by the receiving filter, the composite OFDM symbol is decomposed into the original 8 subcarrier signals using the fast Fourier transform (FFT), and then 8QAM demodulation is performed separately to restore the octal codewords. Finally, through a series of conversion steps, including converting the octal codewords back to binary data streams, source decoding, and digital-to-analog conversion, the original voice information is finally restored.

[0138] This method combines the advantages of magnetic induction communication and OFDM technology, overcomes the narrow bandwidth problem of traditional magnetic induction communication by optimizing the modulation scheme, and realizes more efficient long-distance real-time voice communication. It is particularly suitable for emergency rescue work in complex environments such as mines, and can provide more stable and reliable communication guarantee.

[0139] In this specification, each embodiment is described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the embodiments can be referred to each other. For the system disclosed in the embodiment, since it corresponds to the method disclosed in the embodiment, the description is relatively simple, and the relevant parts can be referred to the method part.

[0140] The above description of the disclosed embodiments enables those skilled in the art to implement or use the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A magnetic induction ground communication method based on OFDM, applied to mine rescue, characterized in that: include: At the signal transmitter, Convert analog voice signals into digital signals; Perform source encoding on digital signals and output compressed binary data; Mapping the encoded binary data into multi-binary code elements; Performing serial-to-parallel conversion on the multi-binary code elements to generate a first parallel data stream; Modulating the first parallel data stream and loading it onto an OFDM subcarrier to generate an OFDM subcarrier signal; The OFDM subcarrier signals are synthesized into OFDM symbols through IFFT; Performing parallel-to-serial conversion on the OFDM symbols to generate a first serial data stream; Performing a first filtering process on the first serial data stream; Using a transmitting antenna to transmit the signal after the first filtering process to a magnetic induction through-the-earth communication channel; At the signal receiving end, Using a receiving antenna to obtain a signal in a magnetic induction through-the-earth communication channel; performing a second filtering process on the signal; Performing serial-to-parallel conversion on the signal after the second filtering process to generate a second parallel data stream; Separating the second serial data stream into original OFDM subcarriers by FFT; Demodulate the signals separated into the original OFDM subcarriers to recover the parallel data streams; Performing parallel-to-serial conversion on the parallel data stream to generate a second serial data stream; converting the second serial data stream back into binary data; Perform source decoding on the converted binary data to restore the digital signal; Convert the recovered digital signal to analog form and output the original voice information.

2. According to the OFDM-based magnetic induction through-the-earth communication method of claim 1, it is characterized in that: The transmitting antenna adopts a single-axis multi-turn air coil made of copper wire, with a radius of 1 meter, 1000 turns, and a copper wire diameter of 0.4 mm.

3. The OFDM-based magnetic induction through-the-earth communication method according to claim 1, characterized in that: The receiving antenna is made of a three-axis orthogonal multi-turn air coil made of copper wire, with a radius of 10 cm, 200 turns, and a copper wire diameter of 0.14 mm.

4. The OFDM-based magnetic induction through-the-earth communication method according to claim 3, characterized in that: In the receiving antenna, based on the magnitude of the magnetic flux on each receiving coil, the signal in the receiving coil with the largest magnetic flux is selected for output.

5. The OFDM-based magnetic induction through-the-earth communication method according to claim 1, characterized in that: Parameter design for the signal transmitter and the signal receiver, including: Construct a magnetic induction through-the-ground communication model between the transmitting antenna and the receiving antenna, as well as a receiving antenna attitude model; Determine the equivalent circuits of the signal transmitter and the signal receiver; The signal transmitting end equivalent circuit includes: the transmitting antenna internal resistance R t , transmitting antenna inductance L t , transmitting circuit current i t , EMF of the transmitting circuit u t ;in, I0 represents the current i t The effective value of , e represents the base of the natural logarithm, j represents the imaginary unit, π represents the circumference of a circle, f represents the frequency of the alternating magnetic field generated by the transmitting antenna coil, and t represents the time variable; The signal receiving end equivalent circuit includes: the receiving antenna internal resistance R r , receiving antenna inductance L r , receiving loop current i r 、Receiving circuit electromotive force u e , compensation capacitor C L , load resistance R L and load electromotive force u r ; Based on the magnetic induction through-the-ground communication model and the receiving antenna attitude model, as well as the equivalent circuits of the signal transmitter and the signal receiver, the average power P of the signal transmitter is determined. t and load resistance R L The received power P r ;in, P t =I0 2 R t P r =i r 2 R L Based on the average power P of the signal transmitting end t and load resistance R L The received power P r , determine the equivalent path loss L(f) of magnetic induction through-the-ground communication; where, Based on the equivalent path loss L(f), a curve of path loss versus transmission current frequency is constructed to determine the available communication bandwidth; The number of OFDM subcarriers and the cyclic prefix length are determined based on the coherence time ΔT and coherence bandwidth ΔB of the magnetic induction through-the-earth communication channel.

6. The OFDM-based magnetic induction through-the-earth communication method according to claim 5, characterized in that: The number of OFDM subcarriers is 8, and the corresponding multi-ary codewords are octal codewords.

7. The OFDM-based magnetic induction through-the-earth communication method according to claim 5, characterized in that: The cyclic prefix length is 0, and when generating OFDM symbols, the IFFT result without adding the cyclic prefix is ​​directly output.

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