A method for electromagnetic wave command downlink modulation based on DSSS technology

Abstract

The application discloses a kind of electromagnetic wave instruction downlink modulation and demodulation method based on DSSS technology, including step one, the original electromagnetic wave instruction signal is encoded and spread spectrum modulated to obtain composite code by ground transmitting terminal, step two, composite code is modulated into modulated signal by carrier wave and is transmitted to downhole receiving terminal, step three, the modulated signal received by downhole receiving terminal is processed synchronously, step four, the modulated signal after synchronization is recovered to obtain original narrowband signal, step five, baseband demodulation obtains original signal;The application is expanded by direct sequence spread spectrum technology using pseudo-random sequence m sequence to original signal sequence, then BPSK modulation is carried out, reduce the attenuation of signal caused by absorption of rock debris, mud and other electromagnetic wave signals when downlink in wellbore, and multipath fading caused by the shielding of drill pipe and other obstacles, enhance the anti-noise interference and anti-multipath interference ability of electromagnetic wave signal, further improve the stability, accuracy and reliability of drilling communication.

Description

Technical Field

[0001] This invention relates to the field of electromagnetic wave drilling communication technology, and in particular to an electromagnetic wave command downlink modulation and demodulation method based on DSSS technology. Background Technology

[0002] Oil is an extremely important strategic resource and a crucial factor influencing the rapid development of various sectors of a nation. For countries seeking to enhance their oil competitiveness, a primary concern is increasing oil production and recovery rates, with drilling communication technology being a critical element. Currently, drilling communication can be broadly categorized into wired and wireless transmission based on different data transmission methods. Wired transmission includes cable transmission, special drill pipe transmission, and fiber optic transmission. Wireless transmission can be divided into mud pulse transmission, acoustic wave transmission, and electromagnetic wave transmission. Cable transmission involves complex cable manufacturing processes and often interferes with normal drilling operations; special drill pipes are costly to manufacture, difficult to maintain, and the continuous circuitry at the joints is challenging, resulting in poor reliability; optical fibers are thin and easily worn away in drilling mud, limiting their use to short periods. Wireless transmission methods such as mud pulse transmission and acoustic wave transmission have poor anti-interference capabilities and significant signal attenuation.

[0003] One of the most widely used modulation methods in traditional drilling communication is BPSK. This method has strong anti-interference capabilities and a smaller useful signal bandwidth than FSK, which uses the same frequency as the carrier. However, its energy is not concentrated, and the power spectrum sidelobes decrease slowly. In drilling communication, it is easily affected by multipath interference, which causes signal attenuation and is prone to errors during demodulation. Direct sequence spread spectrum (DSSS) technology can not only effectively resist multipath interference, but also reduce the system's signal-to-noise ratio requirements. Therefore, this invention proposes an electromagnetic wave command downlink modulation and demodulation method based on DSSS technology to solve the problems existing in the prior art. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to propose an electromagnetic wave command downlink modulation and demodulation method based on DSSS technology. This DSSS-based method uses direct sequence spread spectrum (DSSS) technology to extend the original signal sequence with a pseudo-random m-sequence before BPSK modulation. This reduces signal attenuation caused by rock debris and mud absorption during downlink transmission within the drilling wellbore, as well as multipath fading caused by obstructions such as drill pipes. It enhances the electromagnetic wave signal's resistance to noise and multipath interference, further improving the stability, accuracy, and reliability of drilling communication.

[0005] To achieve the objectives of this invention, the invention is implemented through the following technical solution: an electromagnetic wave command downlink modulation and demodulation method based on DSSS technology, comprising the following steps:

[0006] Step 1: The original electromagnetic wave command signal is encoded to obtain the information code. Then, the information code and the spreading code are spread spectrum modulated to generate the composite code c(t).

[0007] Step 2: Modulate the obtained composite code with a carrier wave to obtain a modulated signal, and transmit the modulated signal through the channel to the underground receiver using the ground transmitter.

[0008] Step 3: After receiving the transmitted modulated signal, the downhole receiver performs pseudo-random code synchronization processing to obtain the synchronized modulated signal.

[0009] Step 4: Restore the synchronized modulated signal to the original narrowband signal, i.e., despreading, to obtain a carrier modulated signal containing only information data modulation.

[0010] Step 5: Perform baseband demodulation on the despread modulated signal to obtain the original signal.

[0011] The further improvement lies in the following: the spread spectrum modulation in step one adopts direct sequence spread spectrum technology; the spread spectrum code used is a pseudo-random sequence, and the pseudo-random sequence is selected as the longest linear shift register sequence with certain randomness, autocorrelation and periodicity, denoted as m sequence.

[0012] A further improvement is that the m-sequence is a linear spread spectrum sequence determined by the original characteristic polynomial of the n-order linear feedback shift register structure, and the polynomial is represented by the following equation.

[0013]

[0014] The further improvement lies in: the spread spectrum modulation in step one is specifically as follows:

[0015] The first step is to encode the original electromagnetic wave command signal into an information code, denoted as d(t);

[0016] The second step is to use a pseudo-random sequence generator to generate an m-sequence, denoted as m(t);

[0017] The third step is to perform a modulo-2 addition operation on d(t) and m(t) to obtain the composite code after spreading the spectrum, denoted as c(t), and its expression is c(t) = d(t)m(t).

[0018] The further improvement lies in the fact that the carrier modulation in step two adopts binary phase shift keying, specifically...

[0019] Step 1: Generate a binary phase shift keying signal using the phase selection method, and modulate the phase wave using the formula... Indicated; where ω0 is the carrier frequency, The phase modulation index;

[0020] Step 2: Modulate the carrier wave using the spread spectrum modulated composite code sequence c(t), that is, shift the signal onto the carrier wave;

[0021] Step 3: The output modulated signal is represented as follows

[0022] The further improvement lies in the fact that the pseudo-random code synchronization processing in step three is specifically as follows:

[0023] Step 1: First, the receiving end searches for the received signal, and then judges the magnitude of the phase difference between the received signal and the local pseudo-random sequence.

[0024] Step 2: If the difference between transmit and receive signals is greater than one symbol, the acquisition requirement is not met. Adjust the clock and search again until the difference between transmit and receive signals is less than one symbol.

[0025] Step 3: The captured signal is then tracked, and the phase difference between transmission and reception is further reduced to the required error range to meet the demodulation requirements of the system.

[0026] Step 4: Continuously detect the synchronization signal. When the synchronization information is lost, immediately switch to the initial acquisition stage to carry out a new synchronization process.

[0027] A further improvement lies in the following: In step four, the despreading process specifically involves first using the autocorrelation of the spreading sequence to perform a modulo-2 addition operation again on the received modulated signal and the local pseudo-random sequence m′(t) at the receiving end, thus obtaining the narrowband signal s′(t), expressed by the formula.

[0028] A further improvement is that the local pseudo-random sequence m′(t) of the receiving end is the same as the pseudo-random sequence code used in the spread spectrum of the ground transmitting end, and the code elements are synchronized, so m(t)=m′(t),m(t)m′(t)=1.

[0029] A further improvement is made in that: the despread modulated signal in step five is demodulated using a Costas loop demodulator, specifically...

[0030] S1. Divide the signal generated by the voltage-controlled oscillator (VCO) into two paths to obtain the local reference signals of the I branch and the Q branch.

[0031] S2. Multiply the binary phase shift keying modulated signal with the local reference signal of the I branch generated by the voltage-controlled oscillator (VCO) to obtain the I-channel signal. The local reference signal of the Q branch is phase shifted by π / 2 and then multiplied with the binary phase shift keying modulated signal to obtain the Q-channel signal.

[0032] S3. Finally, the I-channel signal is processed by a low-pass filter and output to obtain the original signal.

[0033] The beneficial effects of this invention are as follows: This invention uses direct sequence spread spectrum technology to expand the original signal sequence with a pseudo-random m-sequence before performing BPSK modulation. This overcomes the problems of energy concentration and slow power spectrum sidelobe descent in BPSK modulation, reduces signal attenuation caused by absorption by rock debris and mud during transmission in the drilling wellbore, and reduces multipath fading caused by obstruction by obstacles such as drill pipes. This enhances the electromagnetic signal's resistance to noise and multipath interference, further improving the stability, accuracy, and reliability of drilling communication. Attached Figure Description

[0034] Figure 1 This is a flowchart of the method in Embodiment 1 of the present invention.

[0035] Figure 2 This is a flowchart of the signal processing at the ground transmitter in Embodiment 2 of the present invention.

[0036] Figure 3 This is a flowchart of the downhole receiving end signal processing in Embodiment 2 of the present invention.

[0037] Figure 4 This is a schematic diagram of the process of performing pseudo-random code synchronization processing on the modulated signal in Embodiment 2 of the present invention.

[0038] Figure 5 This is a schematic diagram of the demodulation of the Costas ring demodulator in Embodiment 2 of the present invention. Detailed Implementation

[0039] To enhance understanding of the present invention, the present invention will be further described in detail below with reference to embodiments. These embodiments are only used to explain the present invention and do not constitute a limitation on the scope of protection of the present invention.

[0040] Example 1

[0041] according to Figure 1 As shown, this embodiment provides an electromagnetic wave command downlink modulation and demodulation method based on DSSS technology, including the following steps:

[0042] Step 1: The original electromagnetic wave command signal is encoded to obtain the information code. Then, the information code and the spreading code are spread spectrum modulated to generate the composite code c(t). The spreading spectrum modulation uses direct sequence spread spectrum technology.

[0043] The spreading code used is a pseudo-random sequence, and the longest linear shift register sequence with certain randomness, autocorrelation and periodicity is selected and denoted as m sequence;

[0044] The m-sequence is a linear spread spectrum sequence determined by the intrinsic characteristic polynomial of the n-order linear feedback shift register structure, which is represented by the following equation;

[0045]

[0046] Spread spectrum modulation specifically refers to:

[0047] The first step is to encode the original electromagnetic wave command signal into an information code, denoted as d(t);

[0048] The second step is to use a pseudo-random sequence generator to generate an m-sequence, denoted as m(t);

[0049] The third step is to perform a modulo-2 addition (XOR) operation on d(t) and m(t) to obtain the composite code after spreading the spectrum, denoted as c(t), and its expression is c(t) = d(t)m(t).

[0050] Step 2: Modulate the obtained composite code with a carrier wave to obtain a modulated signal, and transmit the modulated signal through the channel to the underground receiver using the ground transmitter.

[0051] The carrier modulation employs binary phase shift keying (BPSK), specifically:

[0052] Step 1: Generate a binary phase shift keying signal using the phase selection method, and modulate the phase wave using the formula... Indicated; where ω0 is the carrier frequency, The phase modulation index;

[0053] Step 2: Modulate the carrier wave using the spread spectrum modulated composite code sequence c(t), that is, shift the signal onto the carrier wave;

[0054] Step 3: The output modulated signal is represented as follows

[0055] Step 3: After receiving the transmitted modulated signal, the downhole receiver performs pseudo-random code synchronization processing to obtain the synchronized modulated signal.

[0056] The pseudo-random code synchronization process is as follows:

[0057] Step 1: First, the receiving end searches for the received signal, and then judges the magnitude of the phase difference between the received signal and the local pseudo-random sequence.

[0058] Step 2: If the difference between transmit and receive signals is greater than one symbol, the acquisition requirement is not met. Adjust the clock and search again until the difference between transmit and receive signals is less than one symbol.

[0059] Step 3: The captured signal is then tracked, and the phase difference between transmission and reception is further reduced to the required error range to meet the demodulation requirements of the system.

[0060] Step 4: Continuously detect the synchronization signal. When the synchronization information is lost, immediately switch to the initial acquisition stage to carry out a new synchronization process.

[0061] Step 4: Restore the synchronized modulated signal to the original narrowband signal, i.e., despreading, to obtain a carrier modulated signal containing only information data modulation.

[0062] The despreading process specifically involves first using the autocorrelation of the spreading sequence to perform a modulo-2 addition operation again on the received modulated signal and the local pseudo-random sequence m′(t) at the receiver, thus obtaining the narrowband signal s′(t), expressed by the formula.

[0063] If the local pseudo-random sequence m′(t) at the receiver is the same as the pseudo-random sequence code used in the spread spectrum at the ground transmitter, and the code elements are synchronized, then m(t) = m′(t) and m(t)m′(t) = 1.

[0064] Step 5: Perform baseband demodulation on the despread modulated signal to obtain the original signal; the despread modulated signal is demodulated using a Costas loop demodulator, specifically...

[0065] S1. Divide the signal generated by the voltage-controlled oscillator (VCO) into two paths to obtain the local reference signals of the I branch and the Q branch.

[0066] S2. Multiply the binary phase shift keying modulated signal with the local reference signal of the I branch generated by the voltage-controlled oscillator (VCO) to obtain the I-channel signal. The local reference signal of the Q branch is phase shifted by π / 2 and then multiplied with the binary phase shift keying modulated signal to obtain the Q-channel signal.

[0067] S3. Finally, the I-channel signal is processed by a low-pass filter and output to obtain the original signal.

[0068] Example 2

[0069] according to Figures 2-5 As shown in the figure, this embodiment provides a detailed description of an electromagnetic wave command downlink modulation and demodulation method based on DSSS technology, as detailed below.

[0070] Because BPSK modulation suffers from energy dispersion and slow power spectrum sidelobe descent, it is susceptible to multipath interference and signal attenuation during drilling communications. To overcome this problem, direct sequence spread spectrum (DSSS) technology can be used to expand the original information code before BPSK modulation. The theoretical basis of DSS is Shannon's channel formula.

[0071]

[0072] In the formula, C is the channel capacity in bits per second (b / s), B is the channel bandwidth in Hz, and S and N are the signal and noise power in watts (W), respectively. If the information transmission rate remains constant, bandwidth and signal-to-noise ratio (SNR) are interchangeable. That is, increasing bandwidth allows for reliable information transmission at the same information transmission rate even with a lower SNR. Even when the signal is obscured by noise, increasing the signal bandwidth can still maintain reliable communication.

[0073] At the ground transmitter, the encoded electromagnetic wave command signal is first spread-spectrum processed, then modulated using BPSK modulation, finally obtaining the modulated signal. The flowchart of the signal processing at the ground transmitter is shown in the appendix of the instruction manual. Figure 2 As shown, the specific steps are as follows:

[0074] S1. The specific steps for obtaining the spreading code sequence using direct sequence spreading technology are as follows:

[0075] S11. The encoded information code of the original electromagnetic wave command signal is d(t), and the symbol rate is R. a The symbol width is T a Its expression is

[0076]

[0077] In the formula a n For information codes, it can be represented as

[0078]

[0079] g a (t) is a gate function, which can be expressed as:

[0080]

[0081] S12. The pseudo-random sequence generator produces an m-sequence, which is derived from the primitive characteristic polynomial of an n-order linear feedback shift register structure. The determined linear spreading sequence, possessing certain randomness, autocorrelation, and periodicity, is denoted as m(t), and its symbol rate is R. c The symbol width is T c The expression is

[0082]

[0083] In the formula m n For pseudo-random symbols, take +1 or -1, g c (t) is the gate function;

[0084] S13. Performing a modulo-2 addition (XOR) operation on d(t) and m(t) yields the spread spectrum signal code c(t), with a pseudo-random code rate R. c Wall information rate R a Much larger, generally R c / R a The ratio is an integer, and R c / R a ≥1, so the extended rate is still the pseudo-random code rate, expressed as:

[0085]

[0086] Where c n The expression is .

[0087]

[0088] S2. Using the aforementioned signal code c(t), perform binary phase shift keying (BPSK) modulation on the carrier wave, specifically as follows:

[0089] S21. Generate a BPSK signal using the phase selection method, with the phase-modulated wave being...

[0090]

[0091] Where ω0 is the carrier frequency. The phase modulation index;

[0092] S22. Modulate the carrier wave with the extended signal code sequence c(t), that is, shift the signal onto the carrier wave;

[0093] S23, The output modulated signal is.

[0094]

[0095] S3. The above-mentioned modulated signal is transmitted to the downhole receiving end through the channel;

[0096] After receiving the modulated signal, the downhole receiver sequentially processes it through synchronization, despreading, and demodulation to finally obtain the original command signal. The signal processing flow of the downhole receiver is shown in the attached instruction manual. Figure 3 As shown, the specific steps are as follows:

[0097] S31. The receiving end first searches the received signal and judges the magnitude of the phase difference between the received signal and the local pseudo-random sequence.

[0098] S32. If the difference between transmit and receive phases is greater than one symbol, the capture requirement is not met. Adjust the clock and search again until the difference between transmit and receive phases is less than one symbol.

[0099] S33. Then, the captured signal is tracked, and the phase difference between transmission and reception is further reduced to the required error range to meet the demodulation requirements of the system;

[0100] S34. At the same time, the synchronization signal is continuously detected. Once the synchronization information is lost, the initial acquisition phase is immediately entered to start a new synchronization process.

[0101] S4. Restore the synchronized modulated signal to the original narrowband signal, i.e., despread. The specific method is as follows:

[0102] S41. After synchronization, the local pseudo-random code generator at the underground receiver generates a pseudo-random sequence m′(t) that is in phase and frequency with the pseudo-random sequence generated at the ground transmitter.

[0103] S42. By utilizing the autocorrelation of the spread spectrum sequence, and XORing the received modulated signal with the local pseudo-random sequence m′(t) at the receiver, a narrowband signal can be obtained, expressed as follows:

[0104]

[0105] Since the pseudo-random sequence generated by the underground receiver is the same as the code pattern generated by the ground transmitter and the code elements are synchronized, then m(t) = m′(t), m(t)m′(t) = 1, and the above expression can be simplified to:

[0106]

[0107] S43. After further processing by a low-pass filter, a narrowband signal can be obtained.

[0108] S5. The downhole receiver demodulates the despread BPSK modulated signal using a Costas ring demodulator. The procedure is as shown in the attached manual. Figure 5 As shown, the specific steps are as follows:

[0109] S51. The signal generated by the voltage-controlled oscillator (VCO) is divided into two paths to obtain the local reference signals of the I branch and the Q branch, and their expressions are shown below.

[0110]

[0111]

[0112] S52. Multiply the BPSK modulated signal with the local reference signal of branch I generated by the voltage-controlled oscillator (VCO) to obtain the I-channel signal. The local reference signal of branch Q is phase-shifted by π / 2 and then multiplied with the BPSK modulated signal to obtain the Q-channel signal. The expression for the I-channel signal is:

[0113]

[0114] The expression for the Q-channel signal is:

[0115]

[0116] The I-channel signal, which contains phase shift keying information, and the Q-channel signal, which contains carrier information, are multiplied in a multiplier and then low-pass filtered to obtain a control signal to control the voltage-controlled oscillator (VCO) so that it tracks the input carrier.

[0117] The S53 and I signals are obtained by passing through a low-pass filter to obtain the original signal.

[0118] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A modulation and demodulation method for electromagnetic wave command downlink based on DSSS technology, characterized in that, Includes the following steps: Step 1: The ground transmitter encodes the original electromagnetic wave command signal and performs spread spectrum modulation by adding the information code and the pseudo-random spread spectrum code modulo-2 to obtain the composite code. Step 2: After mapping the composite code to a bipolar non-return-to-zero code, multiply it with the carrier, obtain the modulated signal through BPSK modulation, and transmit it to the downhole receiver. Step 3: The downhole receiver performs pseudo-random code synchronization processing on the received modulated signal, which includes four stages: Step 1: Search for the signal and determine the phase difference between the local pseudo-random code and the pseudo-random code in the received signal; Step 2: If the phase difference is greater than one symbol, adjust the clock phase and search again until the phase difference is less than one symbol; Step 3: Track the signal and reduce the phase difference to within the preset error range; Step 4: Continuously detect the synchronization signal, and if it is lost, switch back to Step 1. Step 4: Despread the synchronized modulated signal: By performing a modulo-2 addition operation on the modulated signal and the local pseudo-random sequence with the same frequency and phase generated downhole, and then filtering out high-frequency components through a low-pass filter, the narrowband BPSK signal is restored. Step 5: The downhole receiver performs baseband demodulation on the despread signal using a Costas ring demodulator: Two quadrature reference signals, I and Q, are generated using a voltage-controlled oscillator. These signals are multiplied by the despread signal and then low-pass filtered. The frequency of the voltage-controlled oscillator is controlled based on the product of the two outputs to achieve phase locking and output demodulation commands.

2. The electromagnetic wave command downlink modulation and demodulation method based on DSSS technology according to claim 1, characterized in that: The spread spectrum modulation uses direct sequence spread spectrum technology; the spread spectrum code used is a pseudo-random sequence, which is the longest linear shift register sequence with certain randomness, autocorrelation and periodicity, denoted as m sequence.

3. The electromagnetic wave command downlink modulation and demodulation method based on DSSS technology according to claim 2, characterized in that: The m-sequence is a linear spread spectrum sequence determined by the intrinsic characteristic polynomial of the n-order linear feedback shift register structure, which is expressed by the following equation: .

4. The electromagnetic wave command downlink modulation and demodulation method based on DSSS technology according to claim 1, characterized in that: The spread spectrum modulation in step one specifically refers to... The first step is to encode the original electromagnetic wave command signal into an information code, denoted as d(t); The second step is to use a pseudo-random sequence generator to generate an m-sequence, denoted as m(t); The third step is to perform a modulo-2 addition operation on d(t) and m(t) to obtain the composite code after spreading the spectrum, denoted as c(t), whose expression is c(t)=d(t)m(t).

5. The electromagnetic wave command downlink modulation and demodulation method based on DSSS technology according to claim 1, characterized in that: In step two, the carrier modulation employs binary phase shift keying (BPSK), specifically... Step 1: Generate a binary phase shift keying signal using the phase selection method. The phase-modulated wave is obtained from the equation s(t) = Acos(ω0t + φ). s ) represents; where ω0 is the carrier frequency, φ s The phase modulation index; Step 2: Modulate the carrier wave using the composite code sequence c(t) after spread spectrum modulation, that is, shift the signal onto the carrier wave; Step 3: The modulated output signal is represented as s(t) = Ac(t)cos(ω0t + φ). s =Ad(t)m(t)cos(ω0t+φ) s ).

6. The electromagnetic wave command downlink modulation and demodulation method based on DSSS technology according to claim 1, characterized in that: The pseudo-random code synchronization process in step three is specifically as follows: Step 1: First, the receiving end searches for the received signal, and then judges the magnitude of the phase difference between the received signal and the local pseudo-random sequence. Step 2: If the difference between transmit and receive signals is greater than one symbol, the acquisition requirement is not met. Adjust the clock and search again until the difference between transmit and receive signals is less than one symbol. Step 3: The captured signal is then tracked, and the phase difference between transmission and reception is further reduced to the required error range to meet the demodulation requirements of the system. Step 4: Continuously detect the synchronization signal. When the synchronization information is lost, immediately switch to the initial acquisition stage to carry out a new synchronization process.

7. The electromagnetic wave command downlink modulation and demodulation method based on DSSS technology according to claim 1, characterized in that: In step four, the despreading process specifically involves first using the autocorrelation of the spreading sequence to perform a modulo-2 addition operation again on the received modulated signal and the local pseudo-random sequence m´(t) at the receiving end, thus obtaining the narrowband signal s´(t), which can be expressed as s´(t) = Ad(t)m(t)m´(t)cos(ω0t+φ). s )=Ad(t)cos(ω0t+φ s ).

8. The electromagnetic wave command downlink modulation and demodulation method based on DSSS technology according to claim 7, characterized in that: The local pseudo-random sequence m´(t) of the receiving end is the same as the pseudo-random sequence code used by the ground transmitting end for spreading, and the code elements are synchronized, so m(t)=m´(t) and m(t)m´(t)=1.

9. The electromagnetic wave command downlink modulation and demodulation method based on DSSS technology according to claim 1, characterized in that: In step five, the despread modulated signal is demodulated using a Costas ring demodulator. Specifically: S1. Divide the signal generated by the voltage-controlled oscillator (VCO) into two paths to obtain the local reference signals of the I branch and the Q branch. S2. Multiply the binary phase shift keying modulated signal with the local reference signal of the I branch generated by the voltage-controlled oscillator (VCO) to obtain the I-channel signal. The local reference signal of the Q branch is phase shifted by π / 2 and then multiplied with the binary phase shift keying modulated signal to obtain the Q-channel signal. S3. Finally, the I-channel signal is processed by a low-pass filter and output to obtain the original signal.

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