CN104113501A - Modulator, demodulator, modulation method and demodulation method for low-frequency magnetic induction communication - Google Patents

Abstract

The present invention provides a modulator, a demodulator, a modulation method and a demodulation method for low-frequency magnetic induction communication, which are used to solve the mutual limitation of bandwidth and Q value of the transmitting end of low-frequency magnetic induction communication, and the low-frequency communication rate is low. The hardware demodulation method at the receiving end has low integration and flexibility, and the software demodulation method has the problem of large amount of calculation. In the modulator provided by the present invention, user data is sequentially input into a first series resonant circuit after channel coding, BPSK digital modulation, D/A conversion and power amplification, and the phase of the loop current signal in the first series resonant circuit The timing of the change physically reverses the voltage across the first capacitor in the resonant circuit to avoid attenuation of the circuit current signal. This modulator can greatly increase the bandwidth of the modulated signal and increase the communication rate while maintaining a relatively high Q value.

Description

Modulator, demodulator, modulation method and demodulation method of low-frequency magnetic induction communication

technical field

The invention belongs to the modulation and demodulation technology in the communication field, and in particular relates to a modulator, a demodulator, a modulation method and a demodulation method for low-frequency magnetic induction communication.

Background technique

At present, low-frequency magnetic induction communication is widely used in underwater communication and underground communication because of its small antenna size, and it is not affected by multipath effect, propagation delay and fading during propagation. However, the channel under the low-frequency band is a low-rate channel, and how to make the most effective use of the communication channel is a difficult point in the current low-frequency communication technology.

In the prior art, the common low-frequency resonant circuit in the transmitting end of low-frequency magnetic induction communication generally requires a trade-off between the bandwidth and the Q value. To increase the resonant bandwidth must be at the cost of reducing the Q value of the circuit, and vice versa. Therefore, the appropriate bandwidth and Q value are generally selected after weighing to meet the communication conditions, and the bandwidth and Q value cannot be maximized at the same time. In addition, when the receiving end of the low-frequency magnetic induction communication performs signal acquisition, the correlation operation between the local copy and the preamble also reduces the real-time performance of the communication, and the existing hardware demodulation method has low hardware integration and low demodulation flexibility. In order to solve this problem, a software demodulation solution is proposed in the prior art, and the complex demodulation circuit is replaced by a single-chip system, which not only improves the integration of the hardware system but also increases the flexibility of demodulation, but compared to Traditional circuit-based hardware demodulation, the disadvantage of existing software demodulation is that the amount of calculation is large, and a long processing time is required.

To sum up, the mutual limitation of the bandwidth and Q value of the existing low-frequency magnetic induction communication transmitter leads to low low-frequency communication rate, and the hardware demodulation method at the receiving end is low in integration and flexibility, and the software demodulation method exists computationally intensive problem.

Contents of the invention

The present invention provides a modulator, a demodulator, a modulation method and a demodulation method for low-frequency magnetic induction communication, which are used to solve the problem of low frequency communication rate due to the mutual limitation of the bandwidth and Q value of the transmitting end of the existing low-frequency magnetic induction communication. The hardware demodulation method at the receiving end has low integration and flexibility, and the software demodulation method has the problem of large amount of calculation.

For solving the above problems, the present invention at first provides a kind of modulator of low-frequency magnetic induction communication, comprising: the first ARM processor, phase-shifting full-bridge BPSK modulation circuit and low-frequency magnetic induction transmitting antenna; the input of described phase-shifting full-bridge BPSK modulation circuit The end is connected with the first ARM processor through the FSMC bus, and the output end is connected with the low-frequency magnetic induction transmitting antenna; the first ARM processor performs channel coding to the user data, obtains the code element sequence to be sent and sends the code The element sequence is sent to the phase-shifting full-bridge BPSK modulation circuit through the FSMC bus; the phase-shifting full-bridge BPSK modulation circuit adds the preamble sequence 01010101 before the received symbol sequence and modifies the received symbol sequence: Every time 0101010, a 0 is inserted thereafter, and the modified symbol sequence is sequentially subjected to BPSK digital modulation, D/A conversion and power amplification to obtain the first analog modulation signal; the ports that output the first analog modulation signal are sequentially A first series resonant circuit is composed of a first capacitor and the low-frequency magnetic induction transmitting antenna, and the phase-shifted full-bridge BPSK modulation circuit judges in real time whether the phase of the loop current signal of the first series resonant circuit will undergo a 180-degree shift change, if so, at the moment when the phase of the loop current signal of the first series resonant loop changes, the voltage at both ends of the first capacitor is physically reversed; the low-frequency magnetic induction transmitting antenna converts the current signal flowing through itself Send for low frequency magnetic field signal.

Preferably, the phase-shifted full-bridge BPSK modulation circuit includes: BPSK digital modulation module, D/A conversion module, power amplifier, first switch, second switch, third switch, fourth switch, switch circuit control module, non- A gate logic device and the first capacitor; the input end of the BPSK digital modulation module is connected with the first ARM processor through the FSMC bus, and the output end is connected with the input end of the D/A conversion module; the BPSK The digital modulation module adds a preamble sequence 01010101 before the received symbol sequence and modifies the received symbol sequence: insert a 0 after every 0101010; and perform BPSK digital modulation on the modified symbol sequence; The output end of the D/A conversion module is connected to the input end of the power amplifier; the output end of the power amplifier is connected to one end of the first switch and the third switch at the same time, and the other end of the first switch and the other end of the third switch are connected through the first capacitor; the connecting end of the first switch and the first capacitor is also connected to one end of the second switch, and the other end of the second switch is connected to the low frequency One end of the coil of the magnetic induction transmitting antenna is connected, and the other end of the coil of the low-frequency magnetic induction transmitting antenna is grounded; the connecting end of the third switch and the first capacitor is also connected with one end of the fourth switch, and the other end of the fourth switch is One end is connected to the second switch and the coil connection end of the low-frequency magnetic induction transmitting antenna; the switch circuit control module is connected to the BPSK digital modulation module, and its control output terminal is simultaneously connected to the input terminal of the NOT gate logic device , the control terminal of the first switch and the input terminal of the fourth switch are connected, and the output terminal of the NOT gate logic device is connected with the control terminal of the second switch and the control terminal of the third switch at the same time; the switch circuit control module According to the modulation situation of the symbol sequence to be transmitted by the BPSK digital modulation module, at the moment when the phase of the loop current signal of the first series resonant circuit changes by 180 degrees, the digital signal output to the NOT gate logic device is processed. 0-1/1-0 switching, to control the first switch, the second switch, the third switch, and the fourth switch to change the current switch state.

Preferably, the BPSK digital modulation module and the switch circuit control module are realized by FPGA.

Preferably, the phase-shifted full-bridge BPSK modulation circuit further includes a first resistor, an isolation amplifier circuit, a first A/D conversion module, and a current zero point detection module; the low-frequency magnetic induction transmitting antenna is not connected to the second switch The other end of the coil is grounded through the first resistor, the input end of the isolation amplifier circuit is connected to the coil of the low-frequency magnetic induction transmitting antenna and the connection end of the first resistor, and the output end is connected to the input of the first A/D conversion module end; the output end of the first A/D conversion module is connected to the input end of the current zero point detection module, and the output end of the current zero point detection module is connected to the switch circuit control module; the current zero point detection module passes The first A/D conversion module, the isolation amplifier circuit, the first resistor and the circuit connected to the low-frequency magnetic induction transmitting antenna detect the direction of the loop current of the first series resonant circuit, and the loop current of the first series resonant circuit is about to The switch circuit control module is notified at the zero-crossing moment; the switch circuit control module judges the current signal symbol to be sent and just Whether the last symbol sent is the same, if not, the digital signal output to the NOT gate logic device is switched 0-1/1-0 at the moment when the loop current of the first series resonant loop crosses zero.

The present invention also provides a low-frequency magnetic induction communication modulator corresponding to the above low-frequency magnetic induction communication modulator, including: a low-frequency magnetic induction receiving antenna, a second capacitor, an impedance matching transformer, an active filter, a program-controlled amplifier circuit, and a second A /D conversion module, BPSK demodulation module and the second ARM processor; the alternating magnetic field formed by the low-frequency magnetic induction receiving antenna according to the low-frequency magnetic field signal sent by the modulator of the low-frequency magnetic induction communication induces the second analog current modulation signal; The low-frequency magnetic induction receiving antenna, the second capacitor, and the primary coil of the impedance matching transformer are connected in series to form a second series resonant circuit, and the equivalent impedance of the impedance matching transformer in the second series resonant circuit is purely resistive and is compatible with the low-frequency The internal resistance of the magnetic induction receiving antenna is equal, and the second series resonant circuit is used for frequency selection of the second analog current modulation signal output by the low-frequency magnetic induction receiving antenna; the third analog current output by the secondary coil of the impedance matching transformer The modulated signal is sequentially filtered by the active filter, amplified by the program-controlled amplifier circuit, converted into a digital modulated signal by the second A/D conversion module, and demodulated by the BPSK demodulation module into a digital demodulated signal The modulated signal is sent to the second ARM processor for error detection, and the second ARM processor provides the correct digital demodulated signal to the user as user data; wherein, the BPSK demodulation module is used to demodulate the preamble For the signal whose code is 01010101, the BPSK demodulation module is realized by means of FPGA.

Preferably, the BPSK demodulation module in the demodulator includes: a carrier recovery unit, a correlation demodulation unit, a preamble identification unit, a clock sampling unit and a threshold judgment unit; the carrier recovery unit adopts a costa ring calculation method , correcting the carrier phase in real time through the phase difference between the local carrier and the signal carrier of the digitally modulated signal output by the second A/D conversion module, and sending the corrected carrier to the correlation demodulation unit; the correlation The demodulation unit includes a multiplier and an FIR low-pass filter, and the multiplier is respectively connected to the output of the second A/D conversion module, the output of the carrier recovery unit, and the input of the FIR low-pass filter terminal connection, the multiplier multiplies the carrier signal output by the carrier recovery unit and the digital modulation signal output by the second A/D conversion module and sends it to the FIR low-pass filter, and the FIR low-pass filter Filter the input signal to obtain a baseband signal and output it to the preamble identification unit; the preamble identification unit identifies the preamble of the baseband signal output by the FIR low-pass filter, so as to obtain the phase of the sampling clock and provide For the clock sampling unit and the threshold judgment unit; the clock sampling unit is connected to the carrier recovery unit, and is used for, according to the phase of the sampling clock sent by the preamble identification unit, for the carrier output by the carrier recovery unit The signal is clock sampled to obtain the frequency of the sampling clock and provide it to the threshold judging unit; the threshold judging unit uses the phase of the sampling clock and the frequency of the sampling clock to obtain the sampling clock and use this clock to Threshold judgment is performed on the baseband signal, and a digital demodulated signal is demodulated.

Corresponding to the modulator for low-frequency magnetic induction communication provided by the present invention, the present invention also provides a modulation method for low-frequency magnetic induction communication, the method comprising steps:

S11: Perform channel coding on user data to obtain a symbol sequence to be sent;

S12: Perform BPSK digital modulation on the symbol sequence to be sent, and set the preamble of the modulated digital signal to 01010101 during the modulation process, and modify the received symbol sequence: every time 0101010, the Then insert a 0 to get the BPSK digital modulation signal;

S13: Perform D/A conversion on the BPSK digital modulation signal to obtain a BPSK analog modulation signal;

S14: Perform power amplification on the BPSK analog modulation signal to obtain a first analog modulation signal;

S15: Input the first analog modulation signal into a first series resonance circuit composed of a port outputting the first analog modulation signal, a first capacitor and a low-frequency magnetic induction transmitting antenna, and judge the first series resonance in real time Whether the phase of the loop current signal of the loop will change by 180 degrees, and if so, physically reverse the voltage at both ends of the first capacitor when the phase of the loop current signal changes;

S16: The low-frequency magnetic induction transmitting antenna converts the current signal flowing through itself into a low-frequency magnetic field signal and sends it.

Preferably, in the modulation method of the low-frequency magnetic induction communication, the method for judging whether the phase of the loop current signal of the first series resonant loop will change by 180 degrees in S15 is: judging the current signal symbol to be sent Whether it is the same as the last symbol just sent, if not, the loop current signal of the first series resonant circuit has a phase shift at the current zero-crossing point near the moment when the symbol changes.

Corresponding to the demodulator for low-frequency magnetic induction communication provided by the present invention, the present invention also provides a demodulation method for low-frequency magnetic induction communication, comprising the steps of:

S21: The low-frequency magnetic induction receiving antenna induces a second analog current modulation signal according to the alternating magnetic field formed by the low-frequency magnetic field signal sent by the low-frequency magnetic induction communication modulator;

S22: A third analog current modulation signal obtained by frequency-selecting the second analog current modulation signal;

S23: Perform active filtering, signal amplification, and A/D conversion on the third analog current modulation signal in sequence to obtain a digital modulation signal with a preamble of 01010101;

S24: Perform BPSK demodulation on the digital modulation signal whose preamble code is 01010101, and modify the demodulated symbol sequence: remove the last 0 every time 01010100 is encountered to obtain a digital demodulation signal;

S25: Perform error detection on the digital demodulated signal, and provide the correct digital demodulated signal as user data to the user.

Preferably, in the above-mentioned demodulation method, the method for BPSK demodulation of the digitally modulated signal whose preamble is 01010101 described in S24 is:

S241: Using the costa loop calculation method, the carrier phase is corrected in real time through the phase difference between the local carrier and the signal carrier of the digital modulation signal;

S242: Recover the baseband signal by multiplying the digital modulation signal and the carrier signal obtained in S241, and performing FIR low-pass filtering on the result of the multiplication;

S243: Identify the preamble 01010101 of the baseband signal, and remove the last 0 every time 01010100 is encountered, and finally obtain the phase of the sampling clock, and perform clock sampling on the carrier signal obtained in S241 to obtain the frequency of the sampling clock ;

S244: Obtain a sampling clock by using the phase and frequency of the sampling clock, and use the obtained sampling clock to perform threshold judgment on the baseband signal, modify symbols, and demodulate a digital demodulated signal.

Preferably, in the S243, the finite automatic state machine is used to identify the preamble 01010101 of the baseband signal, and the specific identification method is:

S31: Set the initial state value of the state machine to 0, indicating that the preamble is to be recognized, and enter state 1 if the rising edge of the baseband signal is recognized in this state;

S32: Under the premise of state 1, if it is recognized that the next transition edge of the baseband signal is a falling edge and the time interval from the previous rising edge is between (1-m)T and (1+m)T, then Enter state 2, if the time interval from the previous rising edge exceeds (1+m)T and no falling edge occurs, return to state 0; where m is a fixed value and 0<m<1/2, T is a symbol cycle;

S33: Under the premise of state 2, if it is recognized that the next transition edge of the baseband signal is a rising edge and the time interval from the previous falling edge is between (1-m)T and (1+m)T, enter State 3, if the time interval from the previous falling edge exceeds (1+m)T and no rising edge occurs, return to state 0; where m is a fixed value and 0<m<1/2, T is a symbol cycle;

S34: Repeat the process of steps S32-S33, the state value is incremented by 1 every time the detection is passed, otherwise the state value returns to 0, until entering state 7, the preamble of the baseband signal is identified successfully.

Preferably, the method for identifying the transition edge of the baseband signal in the process of S31-S34 is: mark the data point of the baseband signal that needs to be identified currently as the nth point, if the (n-N/2)th point to the ( Points between n-k) points are negative and lower than the set negative threshold, and points between (n+k) point and (n+N/2) point are positive and higher than the set positive threshold , it is considered that there is a rising edge at the data point of the baseband signal that needs to be identified currently, and the identification of the jump edge of the baseband signal is suspended in the next 1/2 symbol period; where k is a constant and 0<k<N/ 4. N is the number of sampling points corresponding to one symbol period.

The beneficial effects of above-mentioned technical scheme of the present invention are as follows:

In the modulator and modulation method for low-frequency magnetic induction communication provided by the present invention, the low-frequency magnetic induction transmitting antenna, the first capacitor and the loop resistance form a series resonant circuit, and the resonant capacitor (i.e. the first capacitor) is located in the "H" bridge switch circuit. When the code element is changed, that is, when the signal phase shifts, the "H" bridge switch circuit can control the first capacitor to reverse in the circuit, so that the direction of the signal voltage is consistent with the discharge direction of the first capacitor, thereby avoiding the attenuation of the current signal , so that the modulation signal is always maintained in the maximum resonance state in the resonant circuit, the bandwidth of the modulation signal can be greatly increased under the premise of maintaining a high Q value, the rate of low-frequency communication is effectively improved, and the existing low-frequency magnetic induction communication is solved. The bandwidth of the transmitting end and the value of the Q value limit each other and the problem of low communication rate. In addition, the corresponding low-frequency magnetic induction communication demodulator and demodulation method provided by the present invention adopt digital demodulation mode for demodulation, which can quickly and accurately capture the preamble, ensure the real-time performance of demodulation, increase the flexibility of demodulation and The integration level of the hardware is improved, and the preamble is captured by pattern recognition in the demodulation, and the calculation amount is much lower than that of the traditional correlation capture, which is beneficial to the low power consumption design of the whole system.

Description of drawings

FIG. 1 is a schematic structural diagram of a modulator for low-frequency magnetic induction communication provided by an embodiment of the present invention;

Figure 2 is a schematic diagram of a preferred real-time structure of a modulator for low-frequency magnetic induction communication provided by an embodiment of the present invention

Fig. 3 is the schematic diagram of phase-shifting full-bridge BPSK modulation process in the circuit shown in Fig. 2;

Fig. 4 is another preferred implementation structure diagram of the modulator of low-frequency magnetic induction communication;

FIG. 5 is a flowchart of a modulation method for low-frequency magnetic induction communication provided by the present invention;

FIG. 6 is a structural schematic diagram of a demodulator for low-frequency magnetic induction communication provided by the present invention;

Fig. 7 is a schematic diagram of a preferred real-time structure of the demodulator of the low-frequency magnetic induction communication shown in Fig. 6;

FIG. 8 is a flow chart of the demodulation method for low-frequency magnetic induction communication provided by the present invention.

[Description of marking of main components in the attached drawing]

1. The first ARM processor;

2. Phase-shifted full-bridge BPSK modulation circuit;

3. Low frequency magnetic induction transmitting antenna;

4. BPSK digital modulation module;

5. D/A conversion module;

6. Power amplifier;

7. Switch circuit control module;

8. NOT gate logic device;

9. Isolation amplifier circuit;

10. The first A/D conversion module;

11. Current zero point detection module;

12. Low frequency magnetic induction receiving antenna;

13. Impedance matching transformer;

14. Active filter;

15. Program-controlled amplifier circuit;

16. The second A/D conversion module;

17. BPSK demodulation module;

18. The second ARM processor;

19. Carrier recovery unit;

20. Related demodulation unit;

21. Preamble identification unit;

22. Clock sampling unit;

23. Threshold judgment unit;

24. Multiplier;

25. FIR low-pass filter;

K1, the first switch;

K2, the second switch;

K3, the third switch;

K4, the fourth switch;

C1, the first capacitor;

R1, the first resistor;

C2, the second capacitor.

Detailed ways

In order to make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the following will describe in detail with reference to the drawings and specific embodiments.

Figure 1 is a schematic structural diagram of a modulator for low-frequency magnetic induction communication provided by an embodiment of the present invention. As shown in Figure 1, the modulator includes: a first ARM processor 1, a phase-shifted full-bridge BPSK modulation circuit 2 and Low frequency magnetic induction transmitting antenna 3. Wherein, the input end of the phase-shifted full-bridge BPSK modulation circuit 2 is connected to the first ARM processor 1 through the FSMC bus, and the output end is connected to the low-frequency magnetic induction transmitting antenna 3 . The first ARM processor 1 performs channel coding on the user data, obtains the symbol sequence to be sent and sends the symbol sequence to the phase-shifted full-bridge BPSK modulation circuit 2 via the FSMC bus, and is responsible for the interface protocol between the modulator and other devices . The phase-shifting full-bridge BPSK modulation circuit 2 adds the preamble sequence 01010101 before the received symbol sequence and modifies the received symbol sequence: insert a 0 after every 0101010 to avoid the same sequence as the preamble , and perform BPSK digital modulation, D/A conversion, and power amplification on the modified symbol sequence to obtain the first analog modulation signal; the port that outputs the first analog modulation signal passes through a first capacitor and low-frequency magnetic induction transmitting antenna 3 in turn The first series resonant circuit is formed, and the phase-shifted full-bridge BPSK modulation circuit 2 judges in real time whether the phase of the loop current signal of the first series resonant circuit will change by 180 degrees, and if so, the loop current signal of the first series resonant circuit The moment when the phase of the phase changes physically reverses the voltage across the first capacitor. The low-frequency magnetic induction transmitting antenna 3 converts the current signal flowing through itself into a low-frequency magnetic field signal for transmission.

In the traditional BPSK modulation, due to the limitation of the bandwidth of the resonant circuit at the transmitting end, the current in the resonant circuit will be attenuated at the 180° phase shift point, that is, during the working process of the series resonant circuit, the energy is transferred back and forth between the capacitor and the inductor. At the moment when the loop current crosses zero, all the energy is stored in the capacitor of the resonant circuit, and the capacitor will be discharged at the next moment. The direction of the discharge current is opposite to that before the zero crossing. In the BPSK modulation mode, the signal will generate 180° at the moment when the current zero crosses. The phase shift causes the signal voltage output by the power amplifier to be in the opposite direction to the voltage of the capacitor, and the loop current signal will attenuate after the two cancel each other out. In the present invention, the voltage at both ends of the first capacitor is physically reversed at the moment of signal phase shift, so that the voltage of the first capacitor is consistent with the direction of the signal voltage, so that the phase of the signal can be changed rapidly without attenuation. The method of modulating in a physical way can greatly increase the bandwidth of the modulated signal under the premise of maintaining a relatively high Q value.

Figure 2 is a schematic diagram of a preferred real-time structure of the modulator for low-frequency magnetic induction communication provided by the embodiment of the present invention. As shown in Figure 2, in the modulator for low-frequency magnetic induction communication, the phase-shifted full-bridge BPSK modulation circuit includes : BPSK digital modulation module 4, D/A conversion module 5, power amplifier 6, first switch K1, second switch K2, third switch K3, fourth switch K4, switch circuit control module 7, NOT gate logic device 8 and The first capacitor C1.

Wherein, the input end of BPSK digital modulation module 4 is connected with the first ARM processor 1 by FSMC bus, and the output end is connected with the input end of D/A conversion module 5; BPSK digital modulation module 4 adds before the symbol sequence that receives The preamble sequence is 01010101 and the received symbol sequence is modified: a 0 is inserted after every 0101010 to avoid the same sequence as the preamble, and the modified symbol sequence is BPSK digitally modulated, and the modulated The final digital signal is sent to the D/A conversion module 5. The output end of D/A conversion module 5 is connected with the input end of power amplifier 6; The other end of K3 is connected through the first capacitor C1; the connecting end of the first switch K1 and the first capacitor C1 is also connected with one end of the second switch K2, and the other end of the second switch K2 is connected with one end of the coil of the low-frequency magnetic induction transmitting antenna 3 , the other end of the coil of the low-frequency magnetic induction transmitting antenna 3 is grounded; the connecting end of the third switch K3 and the first capacitor C1 is also connected to one end of the fourth switch K4, and the other end of the fourth switch K4 is connected to the second switch K2 and the low-frequency magnetic induction transmitting Coil connection terminal of antenna 3. The switch circuit control module 7 is connected with the BPSK digital modulation module 4, and its control output terminal is connected with the input terminal of the NOT gate logic device 8, the control terminal of the first switch K1, and the input terminal of the fourth switch K4 at the same time, and the NOT gate logic device The output terminal of 8 is connected with the control terminal of the second switch K2 and the control terminal of the third switch K3 at the same time. The switch circuit control module 7 is connected with the BPSK digital modulation module 4, and according to the modulation situation of the symbol sequence to be sent by the BPSK digital modulation module 4, when the phase of the loop current signal of the first series resonant circuit changes by 180 degrees, the output The digital signal to the NOT gate logic device 8 is switched 0-1/1-0 to control the first switch K1, the second switch K2, the third switch K3, and the fourth switch K4 to change the current switch state. Wherein, the power amplifier 6 is a low-noise and low-frequency power amplification circuit, which is used to amplify the power of the BPSK modulation signal output by the D/A conversion module 5 to drive the transmitting antenna loop.

As can be clearly seen from FIG. 2 , since the control ends of the first switch K1 and the fourth switch K4 are connected to the input of the NOT logic device 8, and the second switch K2 and the third switch K3 are connected to the output of the NOT logic device 8 Therefore, the switching states of the first switch K1 and the fourth switch K4 are always consistent, and the switching states of the second switch K2 and the third switch K3 are always consistent and opposite to those of the first switch K1 and the fourth switch K4. Different from the traditional resonant circuit, the first capacitor in Fig. 2 is located in the "H" bridge switch circuit, therefore, the first, second, third, The "H" bridge switch circuit composed of the fourth switch realizes the rapid reversal of the first capacitor C1 when the current of the series resonant circuit crosses zero, and reverse discharge of the first capacitor C1 to achieve a 180-degree phase shift of the loop current.

FIG. 3 is a schematic diagram of a phase-shifted full-bridge BPSK modulation process in the circuit shown in FIG. 2 . It can be seen from Figure 2 that the resonant capacitor in normal resonance will undergo four stages of forward charging, reverse discharging, reverse charging, and forward discharging in sequence. In the circuit shown in Figure 2, the initial state of the four electronic switches is K1 , K4 are turned on, K2, K3 are turned off, and at this moment, the first capacitor C1 is forwardly connected to the circuit. Since the first symbol change occurs at the current zero-crossing moment after the reverse charging of the first capacitor is completed, the output voltage of the power amplifier 6 is shifted by 180° at this time, and the "H" bridge switch circuit is controlled to switch to K1 and K4 at this time. When disconnected, K2 and K3 are turned on, so that the first capacitor C1 is reversely connected to the circuit. Through this physical inversion operation, the phase of the loop current is shifted by 180°, thereby artificially allowing the resonance to continue normally.

Preferably, as shown in FIG. 2 , the BPSK digital modulation module 4 and the switch circuit control module 7 are implemented by means of FPGA, and can be implemented in the same FPGA coprocessor. For example, the FPGA coprocessor can use Altera's cyclone IV series chips.

Preferably, in order to enable the switch circuit control module 7 to more accurately control the switch switching action in the "H" bridge switch circuit, auxiliary control can be performed by detecting the direction of the loop current of the series resonant circuit, as shown in Figure 4 As shown in the structural diagram of another preferred implementation of the modulator for low-frequency magnetic induction communication, the phase-shifted full-bridge BPSK modulation circuit also includes a first resistor R1, an isolation amplifier circuit 9, a first A/D conversion module 10 and a current zero point detection module 11. Wherein, the other end of the coil that is not connected with the second switch K2 of the low-frequency magnetic induction transmitting antenna 3 is grounded through the first resistor R1, and the input end of the isolation amplifier circuit 9 is connected to the coil of the low-frequency magnetic induction transmitting antenna 3 and the connection end of the first resistor R1, and the output terminal is connected to the input end of the first A/D conversion module 10; the output end of the first A/D conversion module 10 is connected to the input end of the current zero point detection module 11, and the output end of the current zero point detection module 11 is connected to the switch circuit control module 7 . In Fig. 4, the current zero detection module 11 detects the direction of the loop current of the first series resonant circuit through the circuit connected to the first A/D conversion module 10, the isolation amplifier circuit 9, the first resistor R1 and the low-frequency magnetic induction transmitting antenna 3, and The switch circuit control module 7 is notified when the loop current of the first series resonant loop is about to cross zero. The switch circuit control module 7 is about to pass zero notification according to the loop current of the first series resonant circuit sent by the current zero detection module 11, and judges whether the current signal symbol to be sent is the same as the last symbol just sent, if not, Then, at the moment when the loop current of the first series resonant loop crosses zero, the digital signal output to the NOT logic device 8 is switched 0-1/1-0.

Corresponding to the modulation end of the low-frequency magnetic induction communication provided by the present invention, the present invention also provides a modulation method for low-frequency magnetic induction communication, as shown in Figure 5, the modulation method for low-frequency magnetic induction communication provided by the present invention includes steps:

S11: Perform channel coding on user data to obtain a symbol sequence to be sent;

S12: Perform BPSK digital modulation on the symbol sequence to be sent, and set the preamble of the modulated digital signal to 01010101 during the modulation process, and modify the received symbol sequence: every time 0101010, insert a 0, get BPSK digital modulation signal;

S13: performing D/A conversion on the BPSK digital modulation signal to obtain the BPSK analog modulation signal;

S14: Perform power amplification on the BPSK analog modulation signal to obtain a first analog modulation signal;

S15: Input the first analog modulation signal into a first series resonant circuit composed of a port for outputting the first analog modulation signal, the first capacitor and the low-frequency magnetic induction transmitting antenna, and judge the loop current signal of the first series resonant circuit in real time Whether the phase will change by 180 degrees, and if so, physically reverse the voltage across the first capacitor when the phase of the loop current signal changes;

S16: The low-frequency magnetic induction transmitting antenna converts the current signal flowing through itself into a low-frequency magnetic field signal for transmission.

Preferably, in the modulation method of the above-mentioned low-frequency magnetic induction communication, the method of judging whether the phase of the loop current signal of the first series resonant loop will change by 180 degrees in step S15 is: judging the current signal symbol to be sent and the signal symbol just sent Whether the last symbol of the symbol is the same, if not, the loop current signal of the first series resonant circuit is phase-shifted at the current zero-crossing point near the moment when the symbol changes.

Corresponding to the modulator for low-frequency magnetic induction communication provided in the embodiment of the present invention, the embodiment of the present invention also provides a demodulator for low-frequency magnetic induction communication as shown in FIG. 6, the demodulator includes: a low-frequency magnetic induction receiving antenna 12, a Two capacitors C2 , an impedance matching transformer 13 , an active filter 14 , a program-controlled amplifier circuit 15 , a second A/D conversion module 16 , a BPSK demodulation module 17 and a second ARM processor 18 .

In the low-frequency magnetic induction communication demodulator shown in FIG. 6 , the low-frequency magnetic induction receiving antenna 12 induces the second analog current modulation signal according to the alternating magnetic field formed by the low-frequency magnetic field signal sent by the low-frequency magnetic induction communication modulator. The low frequency magnetic induction receiving antenna 12, the second capacitor C2 and the primary coil of the impedance matching transformer 13 are connected in series to form a second series resonant circuit. The second series resonant circuit is used to select the frequency of the second analog current modulation signal output by the low-frequency magnetic induction receiving antenna 12, so as to suppress part of the noise. The equivalent impedance of the impedance matching transformer 13 in the second series resonant circuit is purely resistive and It is equal to the internal resistance of the low-frequency magnetic induction receiving antenna 12, so as to achieve impedance matching between the amplifying circuit and the resonant circuit, thereby coupling the received signal to the subsequent stage circuit with maximum power. The third analog current modulation signal output by the secondary coil of the impedance matching transformer 13 is filtered by the active filter 14 in sequence, amplified by the program-controlled amplifier circuit 15 and then sent to the second A/D conversion module 16 for A/D conversion. This is because in actual communication, as the transmission power and communication distance change, the received signal power will also change. If A/D conversion is performed directly, the weaker the signal, the greater the impact of the quantization error. , in order to ignore the quantization error, the signal should be amplified to a certain stable amplitude before the A/D conversion, so a programmable amplifying circuit 15 is set between the active filter 14 and the second A/D conversion module 16 here. Subsequently, the second A/D conversion module 16 converts the input analog signal into a digital modulation signal, and then demodulates it into a digital demodulation signal through the BPSK demodulation module 17 and then sends it to the second ARM processor 18 for error detection. The second ARM processor 18 provides the correct digital demodulated signal to the user as user data. Wherein, the BPSK demodulation module 17 is used to demodulate the signal whose preamble is 01010101, preferably, the BPSK demodulation module 17 adopts the FPGA coprocessor mode to realize, and the FPGA coprocessor can also adjust the program control in real time according to the current signal amplitude The signal amplification factor of the amplifying circuit 15 is to stabilize the signal amplitude within a certain range.

FIG. 7 is a schematic diagram of a preferred real-time structure of a demodulator for low-frequency magnetic induction communication shown in FIG. Unit 22 and threshold judging unit 23. in,

The carrier recovery unit 19 uses the costa loop calculation method to correct the carrier phase in real time through the phase difference between the local carrier and the signal carrier of the digitally modulated signal output by the second A/D conversion module 16, and sends the corrected carrier to the correlation solution Tuning unit 20.

Correlation demodulation unit 20 comprises multiplier 24 and FIR low-pass filter 25, multiplier 24 and the output end of second A/D conversion module 16, the output end of carrier recovery unit 19 and the input of FIR low-pass filter 25 respectively terminal connection, the multiplier 24 sends to the FIR low-pass filter 25 after multiplying the carrier signal output by the carrier recovery unit 19 and the digital modulation signal output by the second A/D conversion module 16, and the FIR low-pass filter 25 is to the input signal The baseband signal is obtained by filtering and output to the preamble identification unit 21 .

The preamble identifying unit 21 identifies the preamble of the baseband signal output by the FIR low-pass filter 25 to obtain the phase of the sampling clock and provide it to the clock sampling unit 22 and the threshold judging unit 23 .

The clock sampling unit 22 is connected with the carrier recovery unit 19, and is used for clock sampling the carrier signal output by the carrier recovery unit 19 according to the phase of the sampling clock sent by the preamble identification unit 21, and obtains the frequency of the sampling clock and provides it to Threshold judging unit 23 .

The threshold judging unit 23 obtains the sampling clock by using the phase and frequency of the sampling clock received, and performs threshold judgment on the baseband signal with this clock, and demodulates the digital demodulated signal.

Corresponding to the demodulator for low-frequency magnetic induction communication provided by the present invention, an embodiment of the present invention also provides a demodulation method for low-frequency magnetic induction communication, as shown in FIG. 8 is a flow chart of the demodulation method for low-frequency magnetic induction communication, including steps :

S21: The low-frequency magnetic induction receiving antenna induces a second analog current modulation signal according to the alternating magnetic field formed by the low-frequency magnetic field signal sent by the low-frequency magnetic induction communication modulator;

S22: a third analog current modulation signal obtained by frequency-selecting the second analog current modulation signal;

S23: sequentially perform active filtering, signal amplification, and A/D conversion on the third analog current modulation signal to obtain a digital modulation signal with a preamble of 01010101;

S24: Perform BPSK demodulation on the digital modulation signal whose preamble code is 01010101, and modify the symbol sequence obtained by demodulation: remove the last 0 every time 01010100 is encountered, and obtain the digital demodulation signal;

S25: Perform error detection on the digital demodulated signal, and provide the correct digital demodulated signal to the user as user data.

Preferably, in the method shown in Fig. 8, the method for carrying out BPSK demodulation to the digital modulation signal that preamble is 01010101 in step S24 is:

S241: Using the costa loop calculation method, the carrier phase is corrected in real time through the phase difference between the local carrier and the signal carrier of the digital modulation signal;

S242: Recover the baseband signal by multiplying the digital modulation signal and the carrier signal obtained in S241, and performing FIR low-pass filtering on the result of the multiplication;

S243: Identify the preamble 01010101 of the baseband signal, and remove the last 0 every time 01010100 is encountered, and finally obtain the phase of the sampling clock, and perform clock sampling on the carrier signal obtained in S241 to obtain the frequency of the sampling clock;

S244: Obtain a sampling clock by using the phase and frequency of the sampling clock, and perform threshold judgment on the baseband signal with the obtained sampling clock, and demodulate a digital demodulated signal.

Preferably, in step S243, the finite automatic state machine is used to identify the preamble 01010101 of the baseband signal, and the specific identification method includes the following steps:

S31: Set the initial state value of the state machine to 0, indicating that the preamble is to be recognized, and enter state 1 if the rising edge of the baseband signal is recognized in this state;

S32: Under the premise of state 1, if it is recognized that the next transition edge of the baseband signal is a falling edge and the time interval from the previous rising edge is between (1-m)T and (1+m)T, then Enter state 2, if the time interval from the previous rising edge exceeds (1+m)T and no falling edge occurs, return to state 0; where m is a fixed value and 0<m<1/2, T is a symbol cycle;

S33: Under the premise of state 2, if it is recognized that the next transition edge of the baseband signal is a rising edge and the time interval from the previous falling edge is between (1-m)T and (1+m)T, enter State 3, if the time interval from the previous falling edge exceeds (1+m)T and no rising edge occurs, return to state 0; where m is a fixed value and 0<m<1/2, T is a symbol cycle;

S34: Repeat the process of steps S32-S33, the state value is incremented by 1 each time the detection is passed, otherwise the state value returns to 0, until state 7 is entered, and the preamble of the baseband signal is identified successfully.

Preferably, the method for identifying the transition edge of the baseband signal in the process of S31-S34 is: mark the data point of the baseband signal that needs to be identified currently as the nth point, if the (n-N/2)th point to the (n-k)th point The points between the points are negative and lower than the set negative threshold, and the points between the (n+k)th and (n+N/2)th points are positive and higher than the set positive threshold, then It is considered that there is a rising edge at the data point of the baseband signal that currently needs to be identified, and the identification of the jumping edge of the baseband signal is suspended in the next 1/2 symbol period; where k is a constant and 0<k<N/4, N is the number of sampling points corresponding to one symbol period.

In the modulator and modulation method for low-frequency magnetic induction communication provided by the present invention, the low-frequency magnetic induction transmitting antenna, the first capacitor and the loop resistance form a series resonant circuit, and the resonant capacitor (i.e. the first capacitor) is located in the "H" bridge switch circuit. When the code element is changed, that is, when the signal phase shifts, the "H" bridge switch circuit can control the first capacitor to reverse in the circuit, so that the direction of the signal voltage is consistent with the discharge direction of the first capacitor, thereby avoiding the attenuation of the current signal , so that the modulation signal is always maintained in the maximum resonance state in the resonant circuit, the bandwidth of the modulation signal can be greatly increased under the premise of maintaining a high Q value, the rate of low-frequency communication is effectively improved, and the existing low-frequency magnetic induction communication is solved. The bandwidth of the transmitting end and the value of the Q value limit each other and the problem of low communication rate. In addition, the corresponding low-frequency magnetic induction communication demodulator and demodulation method provided by the present invention adopt digital demodulation mode for demodulation, which can quickly and accurately capture the preamble, ensure the real-time performance of demodulation, increase the flexibility of demodulation and The integration level of the hardware is improved, and the preamble is captured by pattern recognition in the demodulation, and the calculation amount is much lower than that of the traditional correlation capture, which is beneficial to the low power consumption design of the whole system.

The above description is a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the principle of the present invention, some improvements and modifications can also be made, and these improvements and modifications can also be made. It should be regarded as the protection scope of the present invention.

Claims (12)

1. a modulator of low-frequency magnetic induction communication, it is characterized in that, comprises: the first ARM processor, phase-shifting full-bridge BPSK modulation circuit and low-frequency magnetic induction transmitting antenna; The input end of described phase-shifting full-bridge BPSK modulation circuit passes FSMC The bus is connected to the first ARM processor, and the output end is connected to the low-frequency magnetic induction transmitting antenna; The first ARM processor performs channel coding to user data, obtains a symbol sequence to be sent and sends the symbol sequence to the phase-shifting full-bridge BPSK modulation circuit through the FSMC bus; The phase-shifting full-bridge BPSK modulation circuit adds a preamble sequence 01010101 before the received symbol sequence and modifies the received symbol sequence: every time 0101010, a 0 is inserted thereafter, and the modified symbol sequence Sequentially perform BPSK digital modulation, D/A conversion and power amplification to obtain the first analog modulation signal; the port outputting the first analog modulation signal sequentially forms the first series resonance through a first capacitor and the low-frequency magnetic induction transmitting antenna loop, and the phase-shifted full-bridge BPSK modulation circuit judges in real time whether the phase of the loop current signal of the first series resonant loop will change by 180 degrees, and if so, the loop current signal of the first series resonant loop Physically inverting the voltage across the first capacitor at the moment when the phase changes; The low-frequency magnetic induction transmitting antenna converts the current signal flowing through itself into a low-frequency magnetic field signal for transmission. 2. the modulator of low-frequency magnetic induction communication as claimed in claim 1, is characterized in that, described phase-shifting full-bridge BPSK modulation circuit comprises: BPSK digital modulation module, D/A conversion module, power amplifier, the first switch, the second A second switch, a third switch, a fourth switch, a switch circuit control module, a NOT gate logic device, and the first capacitor; The input end of described BPSK digital modulation module is connected with described first ARM processor by FSMC bus, and output end is connected with the input end of described D/A conversion module; Described BPSK digital modulation module is in the symbol sequence that receives Add the preamble sequence 01010101 before and modify the received symbol sequence: insert a 0 after every 0101010; and perform BPSK digital modulation on the modified symbol sequence; The output end of the D/A conversion module is connected to the input end of the power amplifier; the output end of the power amplifier is connected to one end of the first switch and the third switch at the same time, and the other end of the first switch and the other end of the third switch are connected through the first capacitor; the connecting end of the first switch and the first capacitor is also connected to one end of the second switch, and the other end of the second switch is connected to the low frequency One end of the coil of the magnetic induction transmitting antenna is connected, and the other end of the coil of the low-frequency magnetic induction transmitting antenna is grounded; the connecting end of the third switch and the first capacitor is also connected with one end of the fourth switch, and the other end of the fourth switch is One end is connected to the second switch and the coil connection end of the low-frequency magnetic induction transmitting antenna; The switch circuit control module is connected to the BPSK digital modulation module, and its control output terminal is connected to the input terminal of the NOT gate logic device, the control terminal of the first switch, and the input terminal of the fourth switch at the same time. The output terminal of the gate logic device is connected to the control terminal of the second switch and the control terminal of the third switch at the same time; According to the modulation situation of the symbol sequence to be transmitted by the BPSK digital modulation module, the switch circuit control module will output to the NOT gate when the phase of the loop current signal of the first series resonant loop changes by 180 degrees. The digital signal of the logic device performs 0-1/1-0 switching to control the first switch, the second switch, the third switch, and the fourth switch to change the current switch state. 3. The modulator of low frequency magnetic induction communication as claimed in claim 2, characterized in that, said BPSK digital modulation module and switch circuit control module are realized by FPGA. 4. the modulator of low-frequency magnetic induction communication as claimed in claim 3, is characterized in that, described phase-shifting full-bridge BPSK modulation circuit also comprises first resistance, isolation amplifier circuit, first A/D conversion module and current zero point detection module; the other end of the coil of the low-frequency magnetic induction transmitting antenna that is not connected to the second switch is grounded through the first resistor, and the input end of the isolation amplifier circuit is connected to the coil of the low-frequency magnetic induction transmitting antenna and the first resistor connection terminal, the output terminal is connected to the input terminal of the first A/D conversion module; the output terminal of the first A/D conversion module is connected to the input terminal of the current zero detection module, and the output of the current zero detection module The terminal is connected with the switch circuit control module; The current zero point detection module detects the direction of the loop current of the first series resonant circuit through the first A/D conversion module, the isolation amplifier circuit, the first resistor and the circuit connected to the low-frequency magnetic induction transmitting antenna, and in the first Notifying the switching circuit control module when the loop current of the series resonant loop is about to cross zero; The switch circuit control module judges whether the current signal symbol to be sent is the same as the last symbol just sent according to the loop current of the first series resonant circuit sent by the current zero point detection module is about to cross zero notification, if If not, the digital signal output to the NOT logic device is switched 0-1/1-0 when the loop current of the first series resonant loop crosses zero. 5. A demodulator for low-frequency magnetic induction communication, comprising: low-frequency magnetic induction receiving antenna, second capacitor, impedance matching transformer, active filter, program-controlled amplifying circuit, second A/D conversion module, BPSK solution tuning module and a second ARM processor; The low-frequency magnetic induction receiving antenna induces a second analog current modulation signal according to the alternating magnetic field formed by the low-frequency magnetic field signal sent by the modulator of the low-frequency magnetic induction communication; The low-frequency magnetic induction receiving antenna, the second capacitor, and the primary coil of the impedance matching transformer are connected in series to form a second series resonant circuit, and the equivalent impedance of the impedance matching transformer in the second series resonant circuit is purely resistive and is identical to the The internal resistance of the low-frequency magnetic induction receiving antenna is equal, and the second series resonant circuit is used to select the frequency of the second analog current modulation signal output by the low-frequency magnetic induction receiving antenna; The third analog current modulation signal output by the secondary coil of the impedance matching transformer is sequentially filtered by the active filter, amplified by the program-controlled amplifying circuit, and converted into a digital signal by the second A/D conversion module. The modulated signal is sent to the second ARM processor for error detection after being demodulated into a digital demodulation signal by the BPSK demodulation module, and the second ARM processor provides the correct digital demodulation signal as user data to user; Wherein, the BPSK demodulation module is used to demodulate the signal whose preamble is 01010101, and the BPSK demodulation module is realized by FPGA. 6. the demodulator of low-frequency magnetic induction communication as claimed in claim 5, is characterized in that, described BPSK demodulation module comprises: carrier recovery unit, correlation demodulation unit, preamble identification unit, clock sampling unit and threshold judgment unit ; The carrier recovery unit uses a costa loop calculation method to correct the carrier phase in real time through the phase difference between the local carrier and the signal carrier of the digital modulation signal output by the second A/D conversion module, and sends the corrected carrier to The relevant demodulation unit; The correlation demodulation unit includes a multiplier and an FIR low-pass filter, and the multiplier is respectively connected to the output end of the second A/D conversion module, the output end of the carrier recovery unit, and the FIR low-pass filter The input end of the device is connected, and the multiplier multiplies the carrier signal output by the carrier recovery unit and the digital modulation signal output by the second A/D conversion module and sends it to the FIR low-pass filter, and the FIR low filter the input signal with a pass filter to obtain a baseband signal and output it to the preamble identification unit; The preamble identification unit identifies the preamble of the baseband signal output by the FIR low-pass filter, so as to obtain the phase of the sampling clock and provide it to the clock sampling unit and the threshold judgment unit; The clock sampling unit is connected to the carrier recovery unit, and is used to perform clock sampling on the carrier signal output by the carrier recovery unit according to the phase of the sampling clock sent by the preamble identification unit, to obtain the frequency of the sampling clock and providing it to the threshold judging unit; The threshold judging unit obtains the sampling clock by using the phase and frequency of the sampling clock received, and performs threshold judgment on the baseband signal with this clock, and demodulates a digital demodulated signal. 7. A modulation method for low-frequency magnetic induction communication, comprising the steps of: S11: Perform channel coding on user data to obtain a symbol sequence to be sent; S12: Perform BPSK digital modulation on the symbol sequence to be sent, and set the preamble of the modulated digital signal to 01010101 during the modulation process, and modify the received symbol sequence: every time 0101010, the Then insert a 0 to get the BPSK digital modulation signal; S13: Perform D/A conversion on the BPSK digital modulation signal to obtain a BPSK analog modulation signal; S14: Perform power amplification on the BPSK analog modulation signal to obtain a first analog modulation signal; S15: Input the first analog modulation signal into a first series resonance circuit composed of a port outputting the first analog modulation signal, a first capacitor and a low-frequency magnetic induction transmitting antenna, and judge the first series resonance in real time Whether the phase of the loop current signal of the loop will change by 180 degrees, and if so, physically reverse the voltage at both ends of the first capacitor when the phase of the loop current signal changes; S16: The low-frequency magnetic induction transmitting antenna converts the current signal flowing through itself into a low-frequency magnetic field signal and sends it. 8. The modulation method of low-frequency magnetic induction communication as claimed in claim 7, characterized in that, the method for judging whether the phase of the loop current signal of the first series resonant circuit in S15 will change by 180 degrees is: judging Whether the current signal symbol to be sent is the same as the last symbol just sent, if not, the current zero-crossing point of the loop current signal of the first series resonant circuit is shifted near the moment when the symbol changes. 9. A demodulation method for low-frequency magnetic induction communication, characterized in that, comprising steps: S21: The low-frequency magnetic induction receiving antenna induces a second analog current modulation signal according to the alternating magnetic field formed by the low-frequency magnetic field signal sent by the low-frequency magnetic induction communication modulator; S22: A third analog current modulation signal obtained by frequency-selecting the second analog current modulation signal; S23: Perform active filtering, signal amplification, and A/D conversion on the third analog current modulation signal in sequence to obtain a digital modulation signal with a preamble of 01010101; S24: Perform BPSK demodulation on the digital modulation signal whose preamble code is 01010101, and modify the demodulated symbol sequence: remove the last 0 every time 01010100 is encountered to obtain a digital demodulation signal; S25: Perform error detection on the digital demodulated signal, and provide the correct digital demodulated signal as user data to the user. 10. the demodulation method of low-frequency magnetic induction communication as claimed in claim 9, it is characterized in that, the method for carrying out BPSK demodulation to the described digital modulation signal of 01010101 to preamble described in S24 is: S241: Using the costa loop calculation method, the carrier phase is corrected in real time through the phase difference between the local carrier and the signal carrier of the digital modulation signal; S242: Recover the baseband signal by multiplying the digital modulation signal and the carrier signal obtained in S241, and performing FIR low-pass filtering on the result of the multiplication; S243: Identify the preamble 01010101 of the baseband signal, and remove the last 0 every time 01010100 is encountered, and finally obtain the phase of the sampling clock, and perform clock sampling on the carrier signal obtained in S241 to obtain the frequency of the sampling clock ; S244: Obtain a sampling clock by using the phase and frequency of the sampling clock, and use the obtained sampling clock to perform threshold judgment on the baseband signal, modify symbols, and demodulate a digital demodulated signal. 11. The demodulation method of low-frequency magnetic induction communication as claimed in claim 10, is characterized in that, utilizes finite automatic state machine to identify the preamble 01010101 of described baseband signal in the described S243, and concrete identification method is: S31: Set the initial state value of the state machine to 0, indicating that the preamble is to be recognized, and enter state 1 if the rising edge of the baseband signal is recognized in this state; S32: Under the premise of state 1, if it is recognized that the next transition edge of the baseband signal is a falling edge and the time interval from the previous rising edge is between (1-m)T and (1+m)T, then Enter state 2, if the time interval from the previous rising edge exceeds (1+m)T and no falling edge occurs, return to state 0; where m is a fixed value and 0<m<1/2, T is a symbol cycle; S33: Under the premise of state 2, if it is recognized that the next transition edge of the baseband signal is a rising edge and the time interval from the previous falling edge is between (1-m)T and (1+m)T, enter State 3, if the time interval from the previous falling edge exceeds (1+m)T and no rising edge occurs, return to state 0; where m is a fixed value and 0<m<1/2, T is a symbol cycle; S34: Repeat the process of steps S32-S33, the state value is incremented by 1 every time the detection is passed, otherwise the state value returns to 0, until entering state 7, the preamble of the baseband signal is identified successfully. 12. The demodulation method of low-frequency magnetic induction communication as claimed in claim 11, it is characterized in that, the method for identifying the transition edge of baseband signal in the described S31-S34 process is: the data point mark of the baseband signal that needs identification at present For the nth point, if the point between the (n-N/2)th point and the (n-k)th point is negative and lower than the set negative threshold, and the (n+k)th point to the (n+Nth) /2) If the point between the points is positive and higher than the set positive threshold, it is considered that there is a rising edge at the data point of the baseband signal that needs to be identified currently, and the identification of the baseband will be suspended in the next 1/2 symbol period The transition edge of the signal; wherein, k is a constant and 0<k<N/4, and N is the number of sampling points corresponding to one symbol period.