Modulation and demodulation communication method and system suitable for ELF communication

Abstract

The invention discloses a modulation-demodulation communication method suitable for ELF communication. The method comprises the following steps: carrying out information source data coding on underground measurement-while-drilling information, carrying out DQPSK modulation by adopting a phase selection method, carrying out demodulation and decoding at a receiving end by adopting a correlation method after channel transmission, recovering decoded data into measurement information, and finally carrying out terminal display on a computer. The method has the beneficial effects that (1) the signal transmission capability is enhanced; and (2) the communication efficiency is improved. And (3) the anti-interference capability is enhanced. And (4) the system power consumption is reduced. And (5) the system reliability is improved. And (6) multiple modulation modes are supported. And (7) multi-field application is promoted.

Description

Technical Field

[0001] The invention belongs to the field of electromagnetic wave communication while drilling, and in particular relates to a modulation and demodulation communication method and system suitable for ELF (extremely low frequency) communication in the field. Background Art

[0002] With the rapid development of electronic technology and the growing demand for high-precision, long-distance communications, extremely low frequency communications are constantly being optimized in design. The use of high-sensitivity receiving antennas, combined with low-noise analog front ends, high-performance digital processing technology, and continuously optimized modulation and demodulation schemes can achieve efficient signal capture, precise analysis, and reliable transmission, further promoting scientific and technological progress and development in related fields.

[0003] Extremely low frequency (ELF) signals usually refer to signals with a frequency range of 3Hz to 30Hz. Such signals have the following significant characteristics:

[0004] Good penetration: Extremely low frequency signals have good penetration into metal, seawater, soil and other media, which makes them have unique application value in some specific scenarios.

[0005] Long wave propagation: Due to the low frequency, the wavelength of the extremely low frequency signal is longer, which can form a waveguide effect between the earth's surface and the ionosphere, thereby achieving long-distance propagation.

[0006] Strong anti-interference ability: Compared with high-frequency signals, ultra-low frequency signals are less affected by electromagnetic interference, so they have better stability in some scenarios with complex electromagnetic environments.

[0007] Technologies related to extremely low frequency communications are currently mainly used in the following areas:

[0008] Submarine communication: Extremely low frequency signals are widely used for communication between submarines and surface ships or land command posts because they can penetrate seawater. This communication method overcomes the difficulty of high-frequency radio waves to penetrate seawater, and enables submarines to communicate reliably over long distances underwater.

[0009] Long-distance communication: Since the wavelength of ELF signals is extremely long and the attenuation is extremely low, attenuating only 1 to 2 dB per thousand kilometers, a single transmitter has the potential to communicate around the world. Therefore, in some special occasions, ELF signal receivers are used to achieve long-distance communication across the ocean.

[0010] Electromagnetic wave engineering communication while drilling: Extremely low frequency electromagnetic wave communication while drilling is a technology that uses electromagnetic waves to transmit information between the underground and the ground during the drilling process. This technology has the advantages of fast transmission speed, stable signal, and little influence by drilling fluid. It has been widely used in drilling operations in the oil, natural gas and other industries.

[0011] In the field of electromagnetic wave communication while drilling, the modulation and demodulation methods mostly choose pulse position modulation (PPM) and frequency shift keying modulation (FSK). These two modulation methods can meet the needs of extremely low frequency engineering communication to a certain extent, but they also have some disadvantages.

[0012] Disadvantages of PPM modulation:

[0013] (1) High synchronization requirements: PPM modulation has high requirements for synchronization because information is transmitted through the position of pulses. In extremely low frequency communications, due to the long propagation distance and large attenuation of signals, it may be more difficult to obtain and maintain synchronization signals, which will affect the reliability and stability of communications.

[0014] (2) Low bandwidth utilization: PPM modulation may have lower bandwidth utilization than other modulation methods, such as QAM (Quadrature Amplitude Modulation). In extremely low frequency communications, low bandwidth utilization may become a problem due to the preciousness of spectrum resources.

[0015] (3) Susceptibility to interference: ELF communications themselves may be affected by various natural and man-made interferences, such as ionospheric disturbances, electromagnetic noise, etc. PPM modulation may be more susceptible to interference in such an environment, resulting in a decrease in communication quality.

[0016] Disadvantages of FSK modulation:

[0017] Low spectrum utilization: FSK modulation transmits information by changing the frequency of the carrier, but this method usually leads to low spectrum utilization. In extremely low frequency communications, spectrum resources are limited, and FSK modulation may occupy more spectrum resources.

[0018] High requirements for carrier frequency stability: FSK modulation requires the carrier frequency to be stable in order to accurately transmit information. However, in extremely low frequency communications, due to the long propagation distance and large attenuation of signals, the stability of the carrier frequency may be affected, thus affecting the communication quality.

[0019] High demodulation complexity: Although FSK modulation can use non-coherent demodulation at the receiving end to simplify the receiver design, in some cases, in order to obtain better communication performance, it may be necessary to use coherent demodulation, which will increase the complexity and cost of the receiver. Summary of the invention

[0020] The technical problem to be solved by the present invention is to provide a modulation and demodulation communication method and system suitable for ELF communication.

[0021] The present invention adopts the following technical solution:

[0022] A modulation and demodulation communication method suitable for ELF communication, the improvement of which is:

[0023] The downhole measurement while drilling information is encoded as source data, and then DQPSK modulation is performed using the phase selection method. After channel transmission, the correlation method is used to demodulate and decode the data at the receiving end, and the decoded data is restored to measurement information, and finally displayed on the computer terminal.

[0024] Before modulation, the baseband signal is coded and filtered;

[0025] During modulation, the measured information is converted into binary and converted into an input signal sequence, which then passes through a serial-to-parallel converter, a code converter, a four-phase carrier generator, a four-to-one phase selector and a signal amplification drive circuit in sequence to obtain a modulated DQPSK square wave signal with downhole measurement information;

[0026] Before demodulation, the received signal is filtered to remove noise and interference signals, and then amplified by a low-noise amplifier;

[0027] During demodulation, a correlation method of non-coherent demodulation is used to perform demodulation, and the received modulated carrier signal is restored to baseband measurement information.

[0028] Furthermore, the encoding process includes adding a frame header synchronization sequence and error correction coding.

[0029] Furthermore, the correlation method adopted by the demodulator is:

[0030] Assume the original signal S(t) is:

[0031] S(t)=x(t)+N(t)=Asin(ωt+θ+φ)+N x (t)

[0032] The reference signal is:

[0033] z(t)=Bsin(ωt+θ)

[0034] z1(t)=Bcos(ωt+θ)

[0035] Then the phase φ of S(t) is obtained as:

[0036]

[0037] In the above formula,

[0038] Furthermore, a fast algorithm based on normalized cross-correlation coefficient is used to perform waveform matching during the demodulation process, and the data start segment of the modulated signal is determined according to the maximum cross-correlation coefficient of the waveform matching.

[0039] Furthermore, bit synchronization and frame synchronization processing are performed during the demodulation process; Manchester decoding or differential Manchester decoding is used to achieve bit synchronization, and a Barker code identifier or a small m sequence detector is used to achieve frame synchronization.

[0040] Furthermore, error correction decoding is performed on the demodulated signal.

[0041] A modulation and demodulation communication system suitable for ELF communication, the improvement of which is that it comprises a transmitting end and a receiving end, the transmitting end comprises a signal excitation source, a modulator, a power amplifier and a transmitting antenna which are electrically connected in sequence; the receiving end comprises a receiving antenna, a demodulator, a signal processing unit and a data terminal which are electrically connected in sequence; the modulator adopts a square wave carrier of a phase selection method to perform DQPSK modulation, and the demodulator adopts a correlation method of non-coherent demodulation to perform demodulation.

[0042] Furthermore, the modulator implements DQPSK modulation using a switching circuit or a digital signal processor. When the baseband signal is at a high level, the switching circuit is closed and the transmitter outputs a square wave carrier signal; when the baseband signal is at a low level, the switching circuit is disconnected and no carrier signal is output.

[0043] Furthermore, the transmitting antenna adopts an asymmetric dipole insulating antenna.

[0044] Furthermore, a switching transformer is used to control the power supply voltage input in a time-sharing manner, and a modulated signal is output after oscillation, modulation, and rectification.

[0045] The beneficial effects of the present invention are:

[0046] (1) Enhanced signal transmission capability: Extremely low frequency communication has the characteristics of strong penetration and long transmission distance, but the signal strength is relatively weak. The present invention can effectively improve the signal transmission capability by optimizing the modulation and demodulation technology, so that the signal can be transmitted stably in complex environments and reduce signal attenuation and distortion.

[0047] (2) Improving communication efficiency: The present invention can fully utilize spectrum resources in extremely low frequency bands through efficient signal conversion and modulation methods, improve data transmission rate and channel utilization, and thus meet higher communication requirements.

[0048] (3) Enhanced anti-interference capability: In extremely low frequency communications, there are many interference sources, such as electromagnetic interference, environmental noise, etc. The present invention can effectively suppress and filter out interference signals by adopting advanced modulation and demodulation technology and signal processing technology, thereby improving the performance, reliability, anti-interference capability and stability of the communication system.

[0049] (4) Reducing system power consumption: The power consumption of extremely low frequency communication systems is usually high. However, the present invention can reduce the overall power consumption of the system by optimizing the system design and modulation and demodulation algorithms, thereby extending the service life of the equipment and reducing operating costs.

[0050] (5) Improving system reliability: The present invention improves the reliability and stability of the system by adopting technical means such as redundant design and backup mechanism, and can ensure the continuity of communication and the integrity of data even when some devices or links fail.

[0051] (6) Support for multiple modulation modes: The present invention supports multiple modulation modes, such as traditional BPSK, QPSK, etc., as well as more advanced modulation technologies. This flexibility enables the system to select the most appropriate modulation mode according to different communication requirements and environmental conditions, further improving communication efficiency and reliability.

[0052] (7) Promote multi-field applications: Extremely low frequency communication has broad application prospects in the fields of ocean exploration, underwater communication, geological exploration, etc. This invention will promote technological progress and application development in these fields and provide strong support for the innovative development of related industries. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 It is a flow chart of the communication method disclosed in the present invention;

[0054] Figure 2 It is a flow chart of DQPSK modulation using the phase selection method;

[0055] Figure 3 It is a comparison chart of DQPSK coded square wave signal and DQPSK coded sinusoidal signal of a specific carrier frequency;

[0056] Figure 4 It is a flowchart of the demodulation signal of the correlation algorithm;

[0057] Figure 5 It is a flowchart of DQPSK modulation normalized cross-correlation detection synchronous communication. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0059] Embodiment 1, this embodiment discloses a modulation and demodulation communication method suitable for ELF communication, focusing on receiving and processing extremely low frequency signals in the range of 3Hz to 30Hz. Extremely low frequency has shown unique application value in the fields of submarine communication, geological exploration, pipeline detection, tunnel communication, etc. due to its good penetration, long wave propagation characteristics and anti-interference ability.

[0060] like Figure 1 As shown, the downhole measurement while drilling information is encoded as source data, and then DQPSK modulation is performed using the phase selection method. After channel transmission, the correlation method is used to demodulate and decode the data at the receiving end, and the decoded data is restored to measurement information. Finally, the terminal is displayed on the computer to guide the operation of on-site construction personnel.

[0061] Before modulation, the baseband signal is coded and filtered to improve the signal's anti-interference ability and transmission reliability, and to ensure that the signal quality meets the transmission requirements. Coding includes adding a frame header synchronization sequence and error correction coding.

[0062] The inherent disadvantages of the orthogonal modulator are local oscillator leakage and sideband suppression, which can easily cause the carriers of the I(t) and Q(t) signals to be out of sync, ultimately making demodulation difficult. Due to the special communication environment of the electromagnetic wave transmission measurement project while drilling, it is impossible to synchronize the carriers of both ends of the modulation and demodulation for a long time using conventional methods. Therefore, unlike conventional IQ orthogonal modulation, the present invention adopts a modulation method based on the phase selection method. This modulation scheme is simple and reliable, requires fewer hardware units, and is easy to implement in harsh downhole engineering environments. The carrier is selected as a square wave because the transmitter square wave signal is easier to implement in hardware than the sine and cosine signal.

[0063] like Figure 2 As shown, during modulation, the measured information is converted into binary and converted into an input signal sequence, and then sequentially passes through a serial-to-parallel converter, a code converter, a four-phase carrier generator, a four-to-one phase selector and a signal amplification drive circuit to obtain a modulated DQPSK square wave signal with downhole measurement information;

[0064] Figure 3 The figure is a comparison of DQPSK coded square wave signal and DQPSK coded sinusoidal signal of a specific carrier frequency;

[0065] Before demodulation, the received signal is filtered to remove noise and interference signals, and then amplified by a low-noise amplifier;

[0066] During demodulation, a correlation method of non-coherent demodulation is used to perform demodulation, and the received modulated carrier signal is restored to baseband measurement information.

[0067] In the demodulation of DQPSK coded signals, the key to coherent demodulation is carrier synchronization, otherwise the decoded information will have a high bit error rate. Different from coherent demodulation, the present invention uses correlation decoding to decode the modulated signal. This demodulation method does not require carrier synchronization, as long as the signal synchronization decoding position is found. Related literature research shows that the higher the frequency of the ELF-EM (electromagnetic wave) signal, the faster the transmission attenuation in the stratum. Therefore, for the modulated DQPSK square wave signal, the stratum is equivalent to an "invisible low-pass filter", and the higher harmonics of the square wave signal have different degrees of attenuation. The present invention uses a sine signal and a cosine signal with the same frequency as the main frequency signal to perform correlation detection according to a specific step size or signal width to obtain its phase information. The specific derivation is as follows:

[0068] Autocorrelation function R x (τ) and the cross-correlation function R xy (τ) is defined as follows:

[0069]

[0070] Where x(t) and y(t) are:

[0071] x(t)=A0 sin(ωt+θ)

[0072] y(t)=B0 sin(ωt+θ-φ)

[0073] Then the estimated value of the cross-correlation function within time T is for:

[0074]

[0075] Suppose there are two measured signals of the same frequency, S(t) and Y(t), with a phase difference of φ. A and B are the amplitudes of signals x(t) and y(t), respectively. N x (t) and N y (t) are the noise signals in S(t) and Y(t) respectively.

[0076] S(t)=x(t)+N x (t) = Asin(ωt+θ+φ)+N x (t)

[0077] Y(t)=y(t)+N y (t) = B sin (ωt + θ) + N y (t)

[0078] Then the estimated value of the correlation function between the measured signals S(t) and Y(t) is:

[0079]

[0080] It can be further deduced that:

[0081]

[0082] in:

[0083]

[0084] Based on the above inference, the present invention uses an improved correlation algorithm to calculate the amplitude and phase of the unknown signal. The details are as follows:

[0085] Assume the original signal S(t) is:

[0086] S(t)=x(t)+N(t)=Asin(ωt+θ+φ)+N x (t)

[0087] The reference signal is:

[0088] z(t)=Bsin(ωt+θ)

[0089] z1(t)=Bcos(ωt+θ)

[0090] Then the phase φ of S(t) is obtained as:

[0091]

[0092] In the above formula,

[0093] The following uses the waveform encoded with "10" as an example to use the relevant algorithm for decoding. Figure 4 As shown, the square wave signal of the underground transmitter is passed through the stratum and the FIR bandpass filter to obtain the corresponding sinusoidal frequency signal, and then its phase information is calculated according to the relevant algorithm, and finally the corresponding decoding information is obtained.

[0094] Different from the traditional synchronization scheme, the present invention sets a pilot data in the front section of the modulated extremely low frequency electromagnetic signal, and adopts a fast algorithm based on the normalized cross correlation (NCC) coefficient to perform waveform matching during the demodulation process, and determines the data start section and synchronization position of the modulated signal according to the maximum value of the cross correlation coefficient of the waveform matching. Taking the extremely low frequency DQPSK communication system while drilling as an example, Figure 5 As shown, the modulated DQPSK signal received on the ground enters the synchronization analysis and detection stage after filtering, and the pilot synchronization point is detected by the fast algorithm of the NCC coefficient and then enters the DQPSK signal demodulation and decoding data output.

[0095] During the demodulation process, bit synchronization and frame synchronization are performed to ensure correct signal recovery; Manchester decoding or differential Manchester decoding is used to achieve bit synchronization, and a Barker code identifier or a small m sequence detector is used to achieve frame synchronization.

[0096] The demodulated signal is subjected to error correction decoding processing to correct errors that may occur during the transmission process.

[0097] This embodiment also discloses a modulation and demodulation communication system suitable for ELF communication. According to the characteristics of the while-drilling electromagnetic wave communication project, a DQPSK modulation method different from the conventional IQ orthogonal modulation is designed (i.e., phase selection modulation is adopted). The modulation scheme is simple and reliable, requires fewer hardware units, and is convenient for implementation in harsh underground engineering environments. In terms of the excitation method, a square wave signal that is easy to implement is selected as the communication carrier, and the demodulation method selects the correlation decoding method to demodulate and solve the phase. Compared with the traditional method, the frequency band utilization rate and the information transmission rate are improved. It can be applied to deep earth communication, submarine communication, while-drilling EM communication, resource exploration and other fields.

[0098] It includes a transmitting end and a receiving end. The transmitting end includes a signal excitation source, a modulator, a power amplifier and a transmitting antenna which are electrically connected in sequence. The modulated signal is amplified by the power amplifier to increase the signal transmission power and transmission distance. The receiving end includes a receiving antenna, a demodulator, a signal processing unit and a data terminal which are electrically connected in sequence. The demodulated data is output to the data terminal or further processed. The modulator uses a square wave carrier of a phase selection method for DQPSK modulation, and the demodulator uses a correlation method of incoherent demodulation for demodulation. The system architecture can ensure that the entire process from signal generation to reception is efficient and stable.

[0099] The modulator uses a switching circuit or a digital signal processor to implement DQPSK modulation. When the baseband signal is at a high level, the switching circuit is closed and the transmitter outputs a square wave carrier signal; when the baseband signal is at a low level, the switching circuit is disconnected and no carrier signal is output.

[0100] The transmitting antenna uses an asymmetric dipole insulated antenna to ensure that the signal can be effectively radiated to the receiving antenna at the receiving end. The receiving antenna receives the signal from the transmitting antenna and minimizes the attenuation and interference of the signal during transmission.

[0101] A switching transformer using nonlinear synthesis technology controls the power supply voltage input in a time-sharing manner, and outputs a modulated signal after oscillation, modulation, and rectification.

Claims

1. A modulation and demodulation communication method suitable for ELF communication, characterized in that: The downhole measurement while drilling information is encoded as source data, and then DQPSK modulation is performed using the phase selection method. After channel transmission, the correlation method is used to demodulate and decode the data at the receiving end, and the decoded data is restored to measurement information, and finally displayed on the computer terminal. Before modulation, the baseband signal is coded and filtered; During modulation, the measured information is converted into binary and converted into an input signal sequence, which then passes through a serial-to-parallel converter, a code converter, a four-phase carrier generator, a four-to-one phase selector and a signal amplification drive circuit in sequence to obtain a modulated DQPSK square wave signal with downhole measurement information; Before demodulation, the received signal is filtered to remove noise and interference signals, and then amplified by a low-noise amplifier; During demodulation, a correlation method of non-coherent demodulation is used to perform demodulation, and the received modulated carrier signal is restored to baseband measurement information.

2. The modulation and demodulation communication method suitable for ELF communication according to claim 1, characterized in that: The encoding process includes adding a frame header synchronization sequence and error correction coding.

3. The modulation and demodulation communication method suitable for ELF communication according to claim 1, characterized in that: The correlation method used by the demodulator is: Assume the original signal S(t) is: S(t)=x(t)+N(t)=Asin(ωt+θ+φ)+N x (t) The reference signal is: z(t)=Bsin(ωt+θ) z1(t)=Bcos(ωt+θ) Then the phase φ of S(t) is obtained as: In the above formula, 4. The modulation and demodulation communication method suitable for ELF communication according to claim 1, characterized in that: In the demodulation process, a fast algorithm based on normalized cross-correlation coefficient is used to perform waveform matching, and the data start segment of the modulated signal is determined according to the maximum cross-correlation coefficient of waveform matching.

5. The modulation and demodulation communication method suitable for ELF communication according to claim 1, characterized in that: In the demodulation process, bit synchronization and frame synchronization processing are performed; Manchester decoding or differential Manchester decoding is used to achieve bit synchronization, and a Barker code identifier or a small m sequence detector is used to achieve frame synchronization.

6. The modulation and demodulation communication method suitable for ELF communication according to claim 1, characterized in that: The demodulated signal is subjected to error correction decoding processing.

7. A modulation and demodulation communication system suitable for ELF communication, characterized in that: It includes a transmitting end and a receiving end, wherein the transmitting end includes a signal excitation source, a modulator, a power amplifier and a transmitting antenna which are electrically connected in sequence; the receiving end includes a receiving antenna, a demodulator, a signal processing unit and a data terminal which are electrically connected in sequence; the modulator uses a square wave carrier of a phase selection method to perform DQPSK modulation, and the demodulator uses a correlation method of incoherent demodulation to perform demodulation.

8. The modulation and demodulation communication system suitable for ELF communication according to claim 7, characterized in that: The modulator uses a switching circuit or a digital signal processor to implement DQPSK modulation. When the baseband signal is at a high level, the switching circuit is closed and the transmitter outputs a square wave carrier signal; when the baseband signal is at a low level, the switching circuit is disconnected and no carrier signal is output.

9. The modulation and demodulation communication system suitable for ELF communication according to claim 7, characterized in that: The transmitting antenna adopts an asymmetric dipole insulated antenna.

10. The modulation and demodulation communication system suitable for ELF communication according to claim 7, characterized in that: A switching transformer is used to control the power supply voltage input in a time-sharing manner, and a modulated signal is output after oscillation, modulation and rectification.

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