A method for electromagnetic wave command downlink based on multi-ary coded modulation

Abstract

The application discloses a kind of electromagnetic wave instruction downlink method based on multiple encoding modulation, first ground sending end uses channel encoder to carry out octal LDPC code encoding to the instruction signal that has passed through signal source coding, obtains information sequence;Second modulator carries out QPSK modulation to information sequence, obtains modulated signal, and it is transmitted to downhole receiving end;Then downhole receiving end receives the modulated signal of transmission, and using demodulator carries out coherent demodulation, recovers information sequence;Finally decoder adopts FFT-BP decoding algorithm to the multiple encoding modulation of information sequence, then source decoding is carried out, obtains original electromagnetic wave instruction signal.The application carries out octal LDPC code encoding and quaternary QPSK modulation to electromagnetic wave instruction signal, utilizes the characteristics of octal LDPC code high-speed information transmission advantage and QPSK modulation good anti-interference performance, carries out multiple encoding modulation processing to electromagnetic wave signal, can improve communication rate to meet the requirement of timeliness to 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 method based on multi-level coding modulation. Background Technology

[0002] Petroleum, often called the "lifeblood of industry," plays a decisive role in human production and daily life. As a non-renewable energy source, no complete substitute has yet been found for its applications. Therefore, increasing oil production and extraction rates is of significant practical importance. Early oil drilling techniques were simple, resulting in shallow drilling depths and low extraction efficiency. Against this backdrop, oil drilling technology has gained importance and has gradually become a focus of global attention and research in recent years.

[0003] Currently, oil drilling technology is moving towards automation and technological sophistication. During drilling, control commands need to be transmitted from the surface to the well in real time and quickly to control the drill bit's direction, position, and trajectory. Therefore, drilling communication technology is crucial for achieving automation and sophistication in the drilling process. Based on the signal transmission medium, drilling communication transmission methods are generally divided into wired and wireless signal transmission methods. Wired signal transmission includes cable transmission, fiber optic transmission, and smart drill pipe transmission, which features high information transmission rates and strong reliability. However, it can cause significant inconvenience to actual drilling operations and is subject to high wear and tear and difficult maintenance. Wireless signal transmission currently includes three methods: drilling mud pulse transmission, acoustic wave transmission, and electromagnetic wave transmission. Drilling mud pulse transmission and acoustic wave transmission have limited transmission speeds and are susceptible to attenuation.

[0004] Electromagnetic wave transmission technology is a newly developed drilling communication technology that uses low-frequency electromagnetic waves as a medium to transmit surface control commands to the bottom of the well. Compared with other drilling communication technologies, it has wide applicability and is stable and reliable. Major domestic and international oilfield service companies and research institutions have made it a research hotspot and achieved certain research results. However, most of them use binary coding modulation when processing electromagnetic wave signals, which leads to relatively low communication transmission rates. For example, the maximum signal transmission rate of electromagnetic measurement-while-drilling (EMWD) systems from international oilfield service companies such as Weatherford, Schlumberger, and Halliburton does not exceed 12 bps. In China, the maximum signal transmission rate of the DREMWD system from the China Petroleum Drilling Engineering Technology Research Institute is 11 bps; the maximum signal transmission rate of the CQ-EMWD system from the CNPC Sichuan-Chongqing Drilling and Production Research Institute is 5 bps; and the maximum signal transmission rate of the SEMWD-2000 system from the 22nd Research Institute of China Electronics Technology Group Corporation does not exceed 6.3 bps.

[0005] To address the issue of low communication transmission rates caused by the use of binary coding modulation of electromagnetic wave signals by the aforementioned companies or research institutions, this paper proposes an electromagnetic wave command downlink method based on multi-level coding modulation. This method first encodes the electromagnetic wave signal using octal parity-check code (LDPC) and then modulates it using quaternary quadrature phase shift keying (QPSK). This method can further improve the transmission rate of drilling communication while ensuring its reliability. Summary of the Invention

[0006] To address the aforementioned problems, the present invention aims to propose an electromagnetic wave command downlink method based on multi-level coding modulation. This method leverages the characteristic of multi-level coding modulation to improve communication speed, employing multi-level LDPC code encoding and QPSK modulation to process target control commands, thereby increasing the communication speed during the drilling communication process and further improving the timeliness and effectiveness of drilling communication.

[0007] To achieve the objectives of this invention, the invention is implemented through the following technical solution: a method for transmitting electromagnetic wave commands based on multi-level coding modulation, comprising the following steps:

[0008] Step 1: At the ground transmitter, the channel encoder performs multi-level encoding on the source-encoded electromagnetic wave command signal to obtain the information sequence;

[0009] Step 2: The modulator performs multi-level carrier modulation on the information sequence to obtain the modulated signal. The modulated signal is then transmitted through the channel to the downhole receiver using the transmitting antenna at the ground-based transmitter.

[0010] Step 3: The receiving antenna at the downhole receiving end receives the transmitted modulated signal and demodulates it using a demodulator to obtain the information sequence;

[0011] Step 4: The decoder performs multi-level decoding on the information sequence and then source decoding to obtain the original electromagnetic wave command signal.

[0012] A further improvement lies in the following: To ensure that the information sequence obtained during multi-base decoding is uncorrelated, thereby achieving a higher probability of correct decoding, the number of bases in the multi-base encoding is not less than the order of the multi-base modulation. Therefore, the multi-base encoding method used is octal LDPC code, and the multi-base modulation method used is quaternary QPSK modulation.

[0013] A further improvement lies in the following: The specific steps for performing octal LDPC encoding on the target electromagnetic wave command signal using a channel encoder in step one to obtain the information code are as follows:

[0014] Step 1: Construct a finite field GF(2)-based system. 3 The parity-check matrix H on ) M×NFirst, choose a suitable distance parameter d < N / W. r Where N is the length of the codeword; W r This represents the row weights of the parity-check matrix; then, an all-zero matrix is ​​constructed based on the codeword parameters; next, starting from the first column, W is randomly selected from each column. c After replacing each zero element with a non-zero element, and completing the zero element replacement in N columns, a parity check matrix H is constructed. M×N ;

[0015] The second step is to divide the parity check matrix H into two parts: H = [H I |H P ], H I It is the information part of the verification matrix; H P The parity check part of the parity check matrix;

[0016] Step 3: Calculate the information vector u = [u1, u2, ..., u... K The transpose of u T , by H·c T =0 to find the intermediate vector T;

[0017] Step 4: Calculate the parity check component H of the parity check matrix. P inverse matrix Multiply it by the intermediate vector T on the right to obtain the transpose P of the verification vector P. T ;

[0018] Step 5, through The check vector P is obtained and then appended to the information vector u through a multiplexer to obtain the codeword C = [u P]. The multiple codewords form the information code sequence.

[0019] A further improvement is that the multi-level carrier modulation in step two is QPSK modulation, specifically:

[0020] The first step is to convert the encoded octal information sequence into a binary sequence, and then group every two bits in the binary sequence to form a serial two-bit code stream.

[0021] The second step is to perform serial-to-parallel conversion on the serial two-bit code stream, converting it into a parallel two-bit stream with two paths: in-phase (I-path) and quadrature (Q-path).

[0022] The third step is to perform a unipolar / bipolar conversion on the two bitstreams, transforming the original unipolar dual-bitstream into a bipolar bitstream, i.e., logic 1 = +1 and logic 0 = -1. At this point, the bipolar dual-bitstream can constitute two baseband signals. and

[0023] Step 4: Combine the I-channel signal with the cosine carrier wave cosω generated by the local oscillator. c Multiplying by t yields the modulated signal of channel I. Phase shift of the cosine carrier generated by the local oscillator Convert to a sinusoidal carrier sinω c t is then multiplied by the Q-channel signal to obtain the Q-channel modulated signal.

[0024] Step 5: Add the I and Q modulation signals to obtain the QPSK signal.

[0025] A further improvement is made in that the demodulation method for the QPSK signal in step three is coherent demodulation, specifically:

[0026] The first step is to use a Costas ring to directly recover a carrier wave (cosω) from the received QPSK signal, which has the same frequency and phase as the original transmitted carrier wave. c t;

[0027] Step 2: QPSK signal The signal is divided into two paths, I and Q. The I-path signal is coupled with the recovered carrier wave cosω. c Multiplying by t yields a signal containing harmonic components. The Q-path signal and the recovered carrier after a 90° phase shift sinω c Multiply by t to get

[0028] The third step is to use a low-pass filter to filter both signals again, removing harmonic interference from the signals.

[0029] Step 4: Use a timer clock to make decisions on the two signals I′(t) and Q′(t) respectively. When I′(t) > 0, it is judged as 1; when I′(t) < 0, it is judged as 0. Similarly, when Q′(t) > 0, it is judged as 1, and when Q′(t) < 0, it is judged as 0. After the decision, two parallel dual bit streams are formed.

[0030] Step 5: Perform parallel-to-serial conversion on the parallel two-bit stream to convert it into the original binary serial two-bit stream; then perform a base conversion to convert it into the original octal LDPC code sequence.

[0031] The further improvement lies in the fact that the octal LDPC code decoding in step four uses the FFT-BP decoding algorithm, specifically:

[0032] The first step is to use channel likelihood information to obtain the channel state information of the symbol.

[0033] Step 2: Utilizing channel state information Initialize variable information and define variable node information. Verify node information

[0034] The third step is to perform permutation and rearrangement of the variable node information components based on the verification matrix, letting... Obtain the sequence

[0035] Step 4: Update the check node information. First, perform an FFT operation on the sequence obtained in Step 2. Then, update the information component of the nth variable node participating in the mth check equation with the product of the information components of the other variable nodes participating in the mth check equation. Finally, perform an IFFT transformation on the product to update the check node information components as follows:

[0036] Step 5: Verify the node information replacement, and process the message transmitted after the above transformation. Permutation and rearrangement processes yield the sequence.

[0037] Step 6: Update the variable node information by performing a multiplication operation on the permutation and rearrangement of the verification message to obtain the updated variable information.

[0038] Step 7: Calculate the posterior probability of the variable node's value as passed from the variable node to the verification node. Then according to A judgment is made, and the estimated result is expressed as follows: Finally, calculate the syndrome. If s is a vector of all zeros, the iteration process ends and decoding is complete; otherwise, the iteration process continues until decoding is successful.

[0039] The beneficial effects of this invention are as follows: This invention encodes the original electromagnetic wave signal with octal LDPC code and modulates it with quaternary QPSK. It utilizes the excellent codeword error correction performance and high-speed information transmission advantages of multi-level LDPC code and the good anti-interference performance of QPSK modulation to perform multi-level encoding and modulation processing on the electromagnetic wave signal. On the basis of meeting the reliability and accuracy of drilling communication, it further improves the communication rate to meet the timeliness requirements of drilling communication. Attached Figure Description

[0040] Figure 1 This is a flowchart of the method in Embodiment 1 of the present invention.

[0041] Figure 2 This is a flowchart of channel coding of the signal at the ground transmitter in Embodiment 2 of the present invention.

[0042] Figure 3This is a flowchart of the signal modulation process performed by the ground transmitter in Embodiment 2 of the present invention.

[0043] Figure 4 This is a flowchart of the signal demodulation process performed by the downhole receiver in Embodiment 2 of the present invention.

[0044] Figure 5 This is a flowchart of the signal decoding process at the downhole receiver in Embodiment 2 of the present invention. Detailed Implementation

[0045] 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.

[0046] Example 1

[0047] according to Figure 1 As shown, this embodiment provides a method for transmitting electromagnetic wave commands based on multi-level coded modulation, including the following steps:

[0048] To ensure that the information sequences obtained during multi-level decoding are uncorrelated, thereby achieving a higher probability of correct decoding, the method employs a multi-level encoding system with a base number no less than the order of the multi-level modulation. Therefore, the multi-level encoding method used is octal LDPC code, and the multi-level modulation method is quaternary QPSK modulation.

[0049] Step 1: At the ground transmitter, the channel encoder performs multi-level encoding on the source-encoded electromagnetic wave command signal to obtain the information sequence;

[0050] The encoding process of octal LDPC is as follows:

[0051] Step 1: Construct a finite field GF(2)-based system. 3 The parity-check matrix H on the )

[0052] The second step involves dividing the verification matrix H into two parts: H = [H I |H P ], H I This is the information part of the parity check matrix, H. P It is the parity check part of the parity check matrix;

[0053] The third step is to find the transpose u of the information vector u. T Multiply it by the information part H of the parity check matrix on the left. I Obtain the intermediate vector T;

[0054] Step 4: Calculate the parity check component H of the parity check matrix. P inverse matrix Multiply it by the intermediate vector T on the right to obtain the transpose P of the verification vector P. T;

[0055] Step 5, by P T The check vector P is obtained and then appended to the information vector u through a multiplexer to obtain the codeword C. The multiple codewords form an information sequence.

[0056] Step 2: The modulator performs multi-level carrier modulation on the information sequence to obtain the modulated signal. The modulated signal is then transmitted through the channel to the downhole receiver using the transmitting antenna at the ground-based transmitter.

[0057] Multi-level carrier modulation uses quaternary QPSK modulation, specifically:

[0058] The first step is to convert the encoded octal information sequence into a binary sequence, and then group every two bits in the binary sequence to form a serial two-bit code stream.

[0059] The second step is to perform serial-to-parallel conversion on the serial two-bit code stream, converting it into a parallel two-bit stream with half the rate of the in-phase path (I path) and the quadrature branch path (Q path).

[0060] The third step is to perform unipolar / bipolar conversion on the two bit streams, converting the original unipolar dual bit stream into a bipolar bit stream, forming two baseband signals I(t) and Q(t);

[0061] Step 4: Multiply the I-channel signal I(t) by the cosine carrier generated by the local oscillator to obtain the I-channel modulated signal; phase-shift the cosine carrier generated by the local oscillator. The signal is converted into a sinusoidal carrier wave and then multiplied with the Q-channel signal Q(t) to obtain the Q-channel modulated signal;

[0062] Step 5: Add the I and Q modulation signals together to obtain the QPSK signal.

[0063] Step 3: The receiving antenna at the downhole receiving end receives the transmitted modulated signal, and uses a demodulator to demodulate it to obtain the information sequence;

[0064] The demodulation method for QPSK signals is coherent demodulation, specifically:

[0065] The first step is for the carrier recovery circuit to recover a carrier with the same frequency and phase as the original transmitted carrier from the received QPSK signal. c t;

[0066] The second step is to split the received QPSK signal into two paths, I and Q. The I path signal is then compared with the recovered carrier wave cosω. c t is directly multiplied, and the Q-path signal is multiplied by the recovered carrier after a 90° phase shift, sinω. c Multiplying t by t yields two orthogonal BPSK signals;

[0067] The third step is to use a low-pass filter to filter out the harmonic interference in the signals.

[0068] Step 4: Use a timer clock to make decisions on the two signals respectively, and accurately recover the two parallel dual-bit streams;

[0069] Step 5: Perform a parallel-to-serial conversion on the parallel two-bit stream to convert it into the original binary serial two-bit stream;

[0070] Step 6: Convert the binary serial two-bit stream into octal and send it to the decoder.

[0071] Step 4: Perform multi-level decoding on the information sequence, and then perform source decoding to obtain the original electromagnetic wave command signal;

[0072] The decoding of octal LDPC codes uses the FFT-BP decoding algorithm, and the specific steps are as follows:

[0073] The first step is to use channel likelihood information to obtain the channel state information of the symbol.

[0074] Step 2: Utilizing channel state information Initialize variable node information and define variable node information. and verification node information

[0075] The third step is to perform permutation and rearrangement of the variable node information components based on the check matrix to obtain the sequence.

[0076] Step 4: Verify node information updates. Node m performs FFT and IFFT transformations on all the rearranged variable messages passed in;

[0077] Step 5: Verify the node information replacement, and process the message transmitted after the above transformation. Displacement rearrangement, this process is similar to The permutations are inverse;

[0078] Step 6: Update the variable node information by performing a multiplication operation on the rearranged verification message to obtain the updated variable message.

[0079] Step 7: Calculate the posterior probability of the variable node's value as passed from the variable node to the verification node, and then make a decision based on it.

[0080] Example 2

[0081] according to Figures 2-5As shown in the figure, this embodiment provides a detailed description of an electromagnetic wave command downlink method based on multi-level coding modulation technology, as follows.

[0082] Currently, to ensure the accuracy and reliability of drilling communications, major oil service companies and research institutions mostly employ binary coding and modulation methods when processing electromagnetic wave signals, such as binary LDPC and BCH codes, and binary frequency shift keying (2FSK) and binary phase shift keying (BPSK) modulation. However, due to the severe signal attenuation caused by the drilling communication channel, the above-mentioned coding and modulation methods result in low communication transmission rates, making it difficult to meet the timeliness requirements of drilling communications. Therefore, octal LDPC codes, which have excellent codeword error correction performance and high-speed information transmission advantages, and QPSK modulation, which has good anti-interference performance, can be used to encode and modulate electromagnetic wave signals, simultaneously meeting the reliability and timeliness requirements of drilling communications.

[0083] At the ground transmitter, the source-coded command signal is first encoded into an octal LDPC code, then modulated using QPSK modulation to obtain the modulated signal. The flowchart for channel coding of the command signal at the ground transmitter is attached to the instruction manual. Figure 2 The flowchart for modulating the encoded signal is shown in the attached instruction manual. Figure 3 No, the specific steps are as follows:

[0084] S1. The channel encoder performs octal LDPC encoding on the source-encoded electromagnetic wave command signal to obtain the information sequence. The specific steps are as follows:

[0085] S11, Construct a finite field GF(2) 3 The parity-check matrix H on ) M×N First, choose a suitable distance parameter d < N / W. r Where N is the length of the codeword, i.e., the parity check matrix H M×N The number of columns; W r This represents the row weights of the parity check matrix, i.e., the parity check matrix H. M×N The number of non-zero elements in each row is determined; then, an all-zero matrix is ​​constructed based on the codeword parameters; next, starting from the first column, W is randomly selected from each column. c Replace zero elements with non-zero elements, where the non-zero elements are from the finite field GF(2). 3 Randomly select from;

[0086] S12. Divide the parity check matrix H into two parts:

[0087] H = [H] I |H P ]

[0088] Where H IThis is the information part of the verification matrix, with dimensions M×K; H P The parity check part of the parity check matrix has a dimension of M×M;

[0089] S13. Find the information vector u = [u1, u2, ..., u] after source coding. K The transpose of u T , by H·C T =0 to find the intermediate vector T

[0090] T = H I ·u T =H P ·p T

[0091] Where C is the encoded codeword and C = [u P], and P is the required verification information sequence with a length of M;

[0092] S14. Find the parity check part H of the parity check matrix. P inverse matrix Multiply it by the intermediate vector T on the right to obtain the transpose P of the verification vector P. T

[0093]

[0094] S15, by P T The check vector P is obtained and then appended to the information vector u through a multiplexer to obtain the codeword C = [u P]. The multiple codewords form an information sequence.

[0095]

[0096] S2. The modulator uses the above information sequence to perform QPSK modulation on the carrier, specifically:

[0097] S21. Convert the encoded octal information sequence into a binary sequence, and group every two bits in the binary sequence into a serial two-bit code stream.

[0098] S22. Perform serial-to-parallel conversion on the serial two-bit code stream, converting it into a parallel two-bit stream with two paths (I-path) and two paths (Q-path) of in-phase path. At this time, the symbol rate is half of the original and the symbol width is twice the original.

[0099] S23. Perform a unipolar / bipolar conversion on the two bitstreams, transforming the original unipolar dual-bitstream into a bipolar bitstream, i.e., logic 1 = +1 and logic 0 = -1. The bipolar dual-bitstream can then form two baseband signals I(t) and Q(t), which are orthogonal to each other, i.e., 90° out of phase. The expressions for I(t) and Q(t) are:

[0100]

[0101]

[0102] Where A is the amplitude of the signal. A set of uniformly spaced phase values ​​controlled by a two-bit stream can be represented as:

[0103]

[0104] S24. Combine the I-channel signal I(t) with the cosine carrier wave cosω generated by the local oscillator. c Multiply by t to obtain the modulated signal I; phase shift the cosine carrier generated by the local oscillator. Convert to a sinusoidal carrier sinω c The modulated signal Q(t) is obtained by multiplying t by the Q-channel signal Q(t), and the expression is:

[0105]

[0106]

[0107] Where ω c For carrier frequency;

[0108] S25. Add the I and Q modulation signals to obtain the QPSK signal.

[0109]

[0110] S3. The above-mentioned modulated signal is transmitted to the underground receiving end via the channel using the ground transmitting antenna;

[0111] After receiving the modulated signal, the downhole receiving antenna first uses a demodulator to perform coherent demodulation, then uses a decoder to decode the demodulated signal using the FFT-BP decoding algorithm, and finally obtains the original command signal after source decoding. The demodulation process of the downhole receiver is shown in the attached manual. Figure 4 The flowchart for decoding the demodulated signal is shown in the attached instruction manual. Figure 5 As shown, the specific steps are as follows:

[0112] S4 and QPSK demodulation methods include coherent and non-coherent demodulation. While non-coherent demodulation is simpler to implement than coherent demodulation, coherent demodulation offers significantly better reliability and signal-to-noise ratio. In drilling communication environments with weak signals, high noise levels, and extremely high accuracy requirements, coherent demodulation is typically chosen. Coherent demodulation involves multiplying the received signal by a reference carrier of the same frequency and phase extracted at the receiver. Specifically:

[0113] S41, The carrier recovery circuit recovers a recovery carrier (cosω) from the received QPSK signal, which has the same frequency and phase as the original transmitted carrier. c t. Carrier recovery uses the direct recovery method, that is, the carrier is directly recovered from the modulated signal using the Costas loop method and the signal power is fully allocated to the carrier signal carrying baseband information;

[0114] S42, Receive the QPSK signal The signal is divided into two paths, I and Q. The I-path signal is coupled with the recovered carrier wave cosω. c Multiplying t directly yields I′(t); the Q-path signal is then multiplied by the recovered carrier after a 90° phase shift, sinω. c Multiplying t together yields Q′(t). I′(t) and Q′(t) can be viewed as two orthogonal BPSK signals.

[0115]

[0116]

[0117] S43. Next, use a low-pass filter to perform low-pass filtering on both signals to remove harmonic interference. At this time, I′(t) and Q′(t) can be expressed as

[0118]

[0119]

[0120] S44. Use a timer clock to make decisions on the two signals I′(t) and Q′(t) respectively. When I′(t) > 0, it is judged as 1; when I′(t) < 0, it is judged as 0. Similarly, when Q′(t) > 0, it is judged as 1, and when Q′(t) < 0, it is judged as 0. After the decision, two parallel dual bit streams are formed.

[0121] S45. Perform parallel-to-serial conversion on the parallel two-bit stream to convert it into the original binary serial two-bit stream, and then convert it into octal to restore the original octal LDPC code sequence.

[0122] S5. The demodulated LDPC code sequence is fed into the decoder and decoded using the FFT-BP decoding algorithm. The specific method is as follows:

[0123] S51. Obtain the channel state information of the symbol using channel likelihood information. The received channel likelihood information is defined as follows:

[0124]

[0125] Where y n,1 For the received vector y n If an element l ∈ {0, 1, ..., 7} is given, then the nth code character received from the channel is equal to GF(2). 3 The initial probability of the symbol 'a' on the ) is:

[0126]

[0127] S52, Utilizing channel state information Initialize variable information and define variable node information. and verification node information

[0128]

[0129]

[0130] S53. Perform permutation and rearrangement of the variable node information components based on the verification matrix: Let the variable information sent from variable node j to verification node m be... make Obtain the sequence

[0131]

[0132] S54. Update the verification node information. First, perform an FFT operation on the sequence obtained in the second step.

[0133]

[0134] Where W = [W i，j ] represents a Hadamard matrix of order q.

[0135] Then, update the information component of the nth variable node participating in the mth check equation with the product of the information components of the other variable nodes participating in the mth check equation.

[0136]

[0137] Where N(m)\n represents the set of variable nodes that participate in the m-th verification equation, excluding variable node n.

[0138] Finally, an IFFT transformation is performed on the multiplication result to update the information components of the check nodes. The transformation process is as follows:

[0139]

[0140] S55. Verify node information replacement, and perform the following steps on the message transmitted after the above transformation. Displacement rearrangement, this process is similar to The permutations are inverses, resulting in the sequence.

[0141]

[0142] S56. Update the variable node information by performing a multiplication operation on the rearranged verification message to obtain the updated variable message. Right now

[0143]

[0144] Where η mn As the normalization factor, the information vector of each variable node satisfies

[0145] S57. Calculate the posterior probability of the variable node's value passed from the variable node to the verification node.

[0146]

[0147] in This indicates that after completing this iteration, the nth code character is the posterior probability of symbol 'a', M(n) represents the set of all check equations in which the nth variable node participates, and β mn This is the normalization coefficient.

[0148] according to Perform a judgment estimation, and the estimation result is expressed as follows:

[0149]

[0150] That is, the estimation of the transmitted codeword is Finally, the judgment result is verified, that is, the concomitant expression is calculated. If the syndrome s is a vector of all zeros, the iteration process ends and decoding is complete; otherwise, the iteration process continues until decoding is successful.

[0151] 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 method for transmitting electromagnetic wave commands based on multi-level coded modulation, characterized in that, Includes the following steps: Step 1: At the ground transmitter, the channel encoder performs octal LDPC encoding on the source-encoded electromagnetic wave command signal to obtain the information sequence. Step 2: Convert the encoded octal information sequence into a binary sequence, and group every two bits in the binary sequence into a serial dual-bit stream; the modulator performs QPSK carrier modulation on the serial dual-bit stream to obtain the modulated signal, and uses the transmitting antenna of the ground transmitter to transmit the modulated signal through the underground electromagnetic wave channel to the underground receiver. Step 3: The receiving antenna at the downhole receiving end receives the transmitted modulated signal and uses a demodulator to demodulate it to recover the information sequence; Step 4: The decoder performs multi-level decoding on the information sequence based on the FFT-BP algorithm, and then performs source decoding to obtain the original electromagnetic wave command signal.

2. The electromagnetic wave command downlink method based on multi-level coded modulation according to claim 1, characterized in that... To ensure that the information sequence obtained by the multi-base decoder is uncorrelated, thereby obtaining a higher probability of correct decoding, the number of bases in the multi-base encoding is set to 8, and the order of the multi-base modulation is set to 4.

3. The electromagnetic wave command downlink method based on multi-level coded modulation according to claim 1, characterized in that: The multi-base encoding method in step one uses octal LDPC code encoding; specifically, it includes the following steps: Step 1: Construct the parity-check matrix H based on the finite field GF(23); The second step involves dividing the verification matrix H into two parts: This is the information part of the verification matrix. It is the parity check part of the parity check matrix; The third step is to find the transpose of the information vector u. Multiply it by the information part of the parity check matrix on the left. Obtain the intermediate vector T; Step 4: Calculate the parity check part of the parity check matrix. inverse matrix Then multiply it by the middle vector T on the right to obtain the transpose of the verification vector P. ; Step 5, from The check vector P is obtained and then appended to the information vector u through a multiplexer to obtain the codeword C. The multiple codewords form an information sequence.

4. The electromagnetic wave command downlink method based on multi-level coded modulation according to claim 1, characterized in that: The multi-level modulation method in step two is QPSK modulation with a modulation order of 4, which specifically includes the following steps: The first step is to convert the encoded octal information sequence into a binary sequence, and then group every two bits in the binary sequence into a serial two-bit stream. The second step is to perform serial-to-parallel conversion on the serial two-bit stream, converting it into two parallel two-bit streams with half the speed: an in-phase path (I path) and a quadrature branch (Q path). The third step is to perform unipolar / bipolar conversion on the two bitstreams, transforming the original unipolar dual bitstream into a bipolar bitstream, thus forming two baseband signals. and ; Step 4: Transfer the I-channel signal With the cosine carrier generated by the local oscillator Multiply to obtain the modulated signal in channel I; phase shift the cosine carrier generated by the local oscillator. Convert to sinusoidal carrier Then with Q-channel signal Multiplying them together yields the Q-channel modulated signal; Step 5: Add the modulated I and Q signals together to obtain the QPSK signal.

5. The electromagnetic wave command downlink method based on multi-level code modulation according to claim 1, characterized in that: The demodulation method for the QPSK signal in step three is coherent demodulation, and the specific steps are as follows: The first step is for the carrier recovery circuit to recover a carrier with the same frequency and phase as the original transmitted carrier from the received QPSK signal. ; The second step is to split the received QPSK signal into two paths, I and Q. The I path signal is then combined with the recovered carrier wave. Direct multiplication: the Q-channel signal and the recovered carrier with a 90° phase shift. Multiplying them yields two orthogonal BPSK signals; The third step is to use a low-pass filter to filter out the harmonic interference in the signals. Step 4: Use a timer clock to make decisions on the two signals respectively, and accurately recover the two parallel dual-bit streams; Step 5: Perform a parallel-to-serial conversion on the parallel two-bit stream to convert it into the original binary serial two-bit stream; Step 6: Convert the binary serial two-bit stream into octal and send it to the decoder.

6. The electromagnetic wave command downlink method based on multi-level coded modulation according to claim 1, characterized in that: The decoding method for the multi-level LDPC code in step four is the FFT-BP decoding algorithm, and the specific steps are as follows: The first step is to use channel likelihood information to obtain the channel state information of the symbol. ; Step 2: Utilizing channel state information Initialize variable messages and define variable node information. and verification node information ; The third step is to rearrange the variable node information components according to the check matrix to obtain the sequence. ; Step 4: Verify node information updates. Node m performs FFT and IFFT transformations on all the rearranged variable messages passed in. Step 5: Verify the node information replacement, and process the message transmitted after the above transformation. The permutation and rearrangement process, the permutation process of the verification node information is similar to... The permutations are inverse; Step 6: Update the variable node information by performing a multiplication operation on the rearranged verification message to obtain the updated variable message. ; Step 7: Calculate the posterior probability of the variable node's value that is passed from the variable node to the verification node, and make a decision estimate for it.

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