CN111786927B

CN111786927B - Orthogonal frequency division multiplexing while-drilling data transmission method, system, storage medium and application

Info

Publication number: CN111786927B
Application number: CN202010582849.XA
Authority: CN
Inventors: 郑重; 耿艳峰; 冯其涛
Original assignee: Qingdao Tuozhun Measurement And Control Technology Co ltd; China University of Petroleum East China
Current assignee: Qingdao Tuozhun Measurement And Control Technology Co ltd; China University of Petroleum East China
Priority date: 2020-06-23
Filing date: 2020-06-23
Publication date: 2021-09-17
Anticipated expiration: 2040-06-23
Also published as: CN111786927A

Abstract

The invention belongs to the technical field of petroleum exploration and discloses an orthogonal frequency division multiplexing while-drilling data transmission method, a system, a storage medium and application.A signal processor modulates data subjected to error control coding at a transmitting end, adds a cyclic prefix after fast inverse discrete Fourier transform and performs digital-to-analog conversion; the low-frequency radio frequency circuit performs power amplification after up-conversion on the analog signal, and the transmitting transducer converts the electric signal after power amplification into a sound wave signal to be transmitted in a drilling fluid channel; at a receiving end, a receiving transducer converts received sound wave signals into electric signals, and after matched filtering and down-conversion, data synchronization and fast discrete Fourier transform are carried out, and output complex vectors are de-mapped into bit data and subjected to structural decomposition. The invention improves the data transmission rate, reduces the bit error rate, effectively overcomes the intersymbol interference, improves the frequency spectrum utilization rate and realizes high-speed underground communication.

Description

Orthogonal frequency division multiplexing while-drilling data transmission method, system, storage medium and application

Technical Field

The invention belongs to the technical field of petroleum exploration, and particularly relates to an orthogonal frequency division multiplexing while-drilling data transmission method, an orthogonal frequency division multiplexing while-drilling data transmission system, a storage medium and application.

Background

At present, drilling is the link with the greatest risk in the process of oil field exploration and development. During drilling, petroleum geologists need to obtain sufficient information and data from downhole in time to monitor the downhole environment. The drilling data transmission systems most widely used at present mainly include two types, namely mud pulse and electromagnetic wave. Mud pulse and electromagnetic wave techniques are well established and widely used in the field of downhole data transmission, but due to the limited factors of limited bandwidth and large attenuation, the transmission rate of the methods is low, and the maximum transmission rate is not more than 40 bits per second. The problems of slow data uploading speed, long waiting time and low data precision in the underground are solved, and the efficiency and the yield of petroleum production are seriously restricted. In order to solve the problems, on the basis of accurately researching the channel characteristics of the drilling fluid, the acoustic wave transmission system for the data while drilling based on orthogonal frequency division multiplexing by taking the drilling fluid in the drill rod as a channel is invented. The while-drilling data acoustic wave transmission system based on orthogonal frequency division multiplexing solves the three key problems of limited transmission distance, low communication rate and weak anti-interference capability in the process of well drilling data transmission.

The underground mud pulse transmission technology takes drilling fluid as a transmission channel and generates a pulse signal according to the pre-coding pulse design. The high and low changes of the pulse signal cause synchronous changes of the drilling fluid pressure. The sensor decodes the drilling fluid pressure changes to obtain downhole measurement parameters. The mud pulse technology commonly used at present mainly comprises a positive pulse technology, a negative pulse technology and a continuous pressure wave modulation technology. The mud pulse data transmission technology uses a mud pulse generator with a special piston structure to generate discontinuous mud pressure pulse signals. The invention of the technology lays the rudiment of the mud pulse logging while drilling technology and plays a great role in promoting the development of the drilling industry. The mud pulse data transmission system of continuous sinusoidal pressure waves adopts a rotary valve to generate continuous pressure waves, and provides a mud pressure wave phase shift keying modulation scheme. Compared with the transmission rate of 0.5-1.5 bit/s of a positive pulse system, the transmission rate of the continuous pressure wave modulation system can reach 40 bit/s. The continuous wave mud pulse generator consists of motor, rotary valve rotor, stator, speed reducer, control circuit and ground pressure sensor. In the operation process, the motor drives the rotor to rotate relative to the stator, the flow area of slurry is changed, and pressure waves are generated through throttling. After being modulated, the pressure wave signal is transmitted to the ground through mud, and the underground transmission data is obtained through receiving, filtering, demodulating and decoding.

At present, the mud pulse measurement while drilling technology has become one of the industry standards for uploading downhole measurement data in real time, wherein a continuous pressure wave measurement while drilling system is the most important development trend. The continuous pressure wave generator generally adopts a motor to drive a rotor to modulate signals, and the signal frequency spectrum is moved to a frequency band with smaller noise frequency spectrum. The mud pulse system has high stability, but has strong dependence on the drilling fluid, high requirements on the mechanical properties of the drilling fluid and a mud pump, easy damage of a pulse valve and high drilling cost. In the aspect of signal modulation, because the mud pressure wave signal is generated by the rotation of a downhole mechanical component, more complex waveforms cannot be generated by using more advanced communication technology, the modulation mode is limited, and the data transmission rate is low.

Electromagnetic waves are the most important and most widely used waves in wireless communications. Electromagnetic waves are divided into analog waves, radio frequency waves and microwaves from low frequencies to high frequencies. The limited power and high attenuation of radio frequency waves (300KHz-300GHz) and microwaves (300MHz-3000GHz) limits their use in subterranean environments. The analog wave (less than 100Hz) antenna is too large to meet the requirements of small underground space and severely limits the available bandwidth and data transmission rate. Since the electromagnetic wave propagates in the formation, it is greatly affected by the formation resistivity, and the signal attenuation is severe. The ground receiving antenna detects through an electric field between electrodes, and a received signal is usually only dozens of microvolts. When the electromagnetic signal reaches a certain depth, it is difficult to detect a valid electromagnetic signal. In order to solve the problem, the technical measures adopted have three aspects, namely, the transmission power of electromagnetic wave signals is improved, the carrier frequency of the electromagnetic waves is reduced, and efficient detection and noise reduction technologies are adopted. The companies of schlumberger, harlebrand, samadeparallel and the like have introduced a series of electromagnetic wave telemetry systems while drilling. The Russian ZTS electromagnetic wave remote measurement while drilling system taking the stratum as a transmission channel is representative. Since the transmission distance is inversely proportional to the signal frequency, the lower the transmission frequency, the smaller the signal attenuation, and the correspondingly longer the transmission distance, the system uses low frequency signals of 10hz, 5hz, 2.5hz, and 1.25hz for transmission. The ZTS system is powered by a drilling fluid turbine generator, an impeller of the generator is driven by circulating drilling fluid, the circulating discharge capacity of the drilling fluid is required to be 30-75L/S, and the volume ratio of solid particles in the drilling fluid is required to be less than 3%. The power of the downhole turbine generator driven by the drilling fluid needs to reach over 350 watts.

Electromagnetic wave telemetry system while drilling has been widely used, but the transmission depth of the system is limited by factors such as signal frequency, downhole power, transmitting power, transmission efficiency, attenuation coefficient, drilling fluid flow, ground signal processing capability and the volume concentration of particulate matter in the drilling fluid. In order to penetrate through the stratum, the electromagnetic wave remote measuring system can only use low-frequency or ultralow-frequency signals not more than 10Hz, so that the problems of large antenna volume, low communication speed, limited transmission distance and the like exist. Digital chemical telemetry systems transmit information to a receiving end at the surface by releasing information-bearing particles downhole. The information carrying particles may be distinguished by shape, size, colour, composition, etc. In this system, different particles are stored in different downhole containers, respectively, and released according to a predetermined pattern under specific conditions. The system has the advantages of simple operation, low complexity, safety, reliability and the like. However, this method has the limitations of slow communication rate, drilling fluid flow requirement, limited amount of particles stored downhole, and the like.

Drilling is the most risky stage in oilfield exploration. In order to ensure the normal development of production, workers need to acquire information and data from the underground in time. At present, two types of drilling data transmission modes mainly comprise mud pulse and electromagnetic wave. Although the two data transmission technologies have been greatly developed and widely used, the bandwidth is narrow, the attenuation is large, the speed is slow, the precision is low, the maximum transmission rate does not exceed 40 bits per second, and the production efficiency and the yield of petroleum are severely limited. The reasons for these problems are twofold: (1) a mud pulse telemetry system. The system generally adopts a motor to drive a rotor to modulate a signal, and the frequency spectrum of the signal is shifted to a frequency band with smaller noise frequency spectrum. The mud pulse system has high stability, but has strong dependence on the drilling fluid, high requirements on the mechanical properties of the drilling fluid and a mud pump, easy damage of a pulse valve and high drilling cost. In the aspect of signal modulation, because the mud pressure wave signal is generated by the rotation of a downhole mechanical component, more complex waveforms cannot be generated by using more advanced communication technology, the modulation mode is limited, and the data transmission rate is low: (2) provided is an electromagnetic wave telemetry while drilling system. The transmission depth of the system depends on the restriction of factors such as signal frequency, downhole power, transmitting power, transmission efficiency, attenuation coefficient, drilling fluid flow, ground signal processing capacity, and the volume concentration of particulate matters in the drilling fluid. In order to penetrate through the stratum, the electromagnetic wave remote measuring system can only use low-frequency or ultralow-frequency signals not more than 10Hz, so that the problems of large antenna volume, low communication speed, limited transmission distance and the like exist.

Through the above analysis, the problems and defects of the prior art are as follows: at present, the drilling data transmission mode has narrow bandwidth, large attenuation, slow speed and low precision, the maximum transmission speed does not exceed 40 bits per second, and the production efficiency and the yield of petroleum are severely restricted.

The difficulty in solving the above problems and defects is: the signal of the mud pulse telemetry system is generated by the rotation of downhole mechanical components and cannot generate a complex modulation waveform. Because the attenuation of the signal of the electromagnetic wave remote measuring system is in direct proportion to the frequency, only the ultralow frequency signal which is not more than 10Hz can be adopted. Due to the limitation of bandwidth and other factors, the two modes cannot be fused with advanced communication technology with high transmission rate and low communication error.

The significance of solving the problems and the defects is as follows: compared with the prior art, the method has the advantages of being reflected in four aspects of transmission rate, interference resistance, transmission distance and convenience.

(1) The transmission rate is high. By utilizing the fourth generation and the fifth generation communication technologies based on orthogonal frequency division multiplexing and MIMO, data are transmitted in parallel in a frequency domain, and the utilization rate of frequency band resources is high. The data transmission rate of the acoustic communication can be improved to more than 1000 bits/second, and compared with pulse and electromagnetic wave communication modes based on PSK, FSK and AM, the data transmission rate is greatly improved.

(2) Anti-interference deviceThe capability is strong. Through frequency modulation, the noise frequency band (0-7kHz) generated by the drilling machinery can be flexibly and effectively avoided. By adopting the technologies of error correction coding, signal equalization, channel estimation, signal synchronization, peak-to-average power ratio reduction and the like, the interference of channel noise and multipath effect on the system can be reduced. Strong anti-interference ability, high precision, error rate at 10^-4The following.

(3) The distance transmission is far. By adopting communication technologies such as bit allocation, power amplification, reduction of peak-to-average power ratio and the like, the use efficiency of energy and the transmission distance of signals can be improved. Without signal relaying, a 30W transmission power can transmit acoustic signals with a center frequency of 100kHz over distances of more than 2000 meters in drill pipes in water-based drilling fluids, which are dominant in current applications.

(4) The implementation is convenient. The system combines the flexibility of software and the rapidity of hardware, and the USRP system based on the FPGA and the radio frequency module has the characteristics of high transmission speed and high precision. With the development of new technologies, sensors and circuits are being developed in the direction of small size, low cost, light weight, long life, high reliability, and the like. The system is convenient to use and suitable for drill rods with complex environments and limited space.

Disclosure of Invention

Aiming at the problems in the prior art, the invention provides an orthogonal frequency division multiplexing while-drilling data transmission method, an orthogonal frequency division multiplexing while-drilling data transmission system, a storage medium and application.

The invention is realized in such a way that an orthogonal frequency division multiplexing while-drilling data transmission method comprises the following steps:

at a transmitting end, a signal processor modulates the data subjected to error control coding, adds a cyclic prefix after fast inverse discrete Fourier transform and performs digital-to-analog conversion; the low-frequency radio frequency circuit performs power amplification after up-conversion on the analog signal, and the transmitting transducer converts the electric signal after power amplification into a sound wave signal to be transmitted in a drilling fluid channel;

at a receiving end, a receiving transducer converts received sound wave signals into electric signals, and after matched filtering and down-conversion, data synchronization and fast discrete Fourier transform are carried out, and output complex vectors are de-mapped into bit data and subjected to structural decomposition.

Further, the data frame at the sending end comprises a packet header, a data part, a check bit and a tail bit, the packet header of the data packet adopts a repeated sending method, the time domain OFDM symbols after IFFT are complex vectors, QAM mapping is adopted, and the cyclic prefix and the data symbols are used for eliminating intersymbol interference.

Further, the error control coding part of the orthogonal frequency division multiplexing while-drilling data transmission method uses cyclic redundancy check codes, the consistent relation between the data bits and the check bits is established through mathematical operation, CRC32 coding is a remainder of modulo-2 division, and a generating polynomial is x³²+x²⁶+x²³+x²²+x¹⁶+x¹²+x¹¹+x¹⁰+x⁸+x⁷+x⁵+x⁴+x²+ x +1, the generator polynomial determines the generator matrix and the whole cyclic code, and the signal after error control coding enters the signal modulation module.

Further, the signal modulation of the orthogonal frequency division multiplexing while-drilling data transmission method is completed by an FPGA-based USRP and a low-frequency radio frequency circuit, and the method comprises the following steps:

(1) the data packing module packs a data packet of a data link layer, and the data frame structure comprises four parts, namely a packet head, data, a 4-bit CRC check bit and a 1-bit tail part; the packet header comprises two kinds of information, namely a 4-bit whitening parameter and a 12-bit data packet length, the packet header of the data packet adopts a repeated sending method, and the structure and the packaging process of a data frame are divided into three steps; firstly, check bits using a CRC32 algorithm are added to data to be transmitted; secondly, the CRC check bit and the tail bit are whitened, and finally, the packet head of the data packet is added to complete the packaging process of the data link layer data packet;

(2) the constellation mapping module takes out a data packet from the queue, and after the data packet is converted into a parallel signal through serial data, the parallel signal is mapped into a complex vector of a frequency domain to complete constellation mapping of an IFFT input signal, a sequence head is added to the data after the constellation mapping to perform IFFT conversion, the IFFT input adopts a random complex vector of a 4QAM constellation {1+ j, 1-j, -1+ j, -1-j }, and the IFFT output is a corresponding complex vector x of a time domain; using an inverse fast discrete fourier transform, the upper case representing a frequency or discrete frequency domain variable and the lower case representing a time domain variable;

(3) the sign of the ofdm signal of the IFFT block describes the time domain sequence associated with an IFFT operation, the input of which is the complex vector X ═ X in the frequency domain₀ X₁ X₂ X₃ … X_N-1]^TEach element of X is a complex vector representing the frequency domain, and the number of complex vectors in the frequency domain is N. N is also the number of input frequency domain complex vectors per IFFT operation, each element of X represents the data to be carried on the corresponding subcarrier, X_kRepresenting the data carried on the kth subcarrier, the output of the IFFT is the time-domain complex vector x ═ x₀ x₁ x₂ x₃ … x_n-1]^T. Each element of x is a complex vector representing the frequency domain:

(4) the cyclic prefix module inserts a guard interval composed of cyclic prefixes between adjacent orthogonal frequency division multiplexing symbols, the length of the guard interval is greater than the maximum delay spread in a wireless channel, so that multipath components of one orthogonal frequency division multiplexing symbol do not interfere with the next symbol, and x (i) [ x ] x ═ x₀(i) x₁(i) x₂(i) x₃(i) … x_n-1(i)]^TAdding a cyclic prefix to the beginning of each time domain symbol before transmission for the output of the IFFT in the ith symbol period; copying the signal in the post Tg time of each symbol to the front of the symbol, wherein the transmission sequence is x_cp(i)＝[x_N-G(i) … x_N-1(i) x₀(i) x₁(i) x₂(i) x₃(i) … x_n-1(i)]^T；

(5) The up-conversion module uses the low-frequency radio frequency sub-board to realize the frequency shift of the baseband signal to a higher transmission frequency, after the cyclic prefix is added in the sending sequence, the digital signal is converted into an analog signal through digital-analog conversion, and then the signal is transmitted to the power amplification circuit after the frequency spectrum shift is carried out on the signal by the low-frequency radio frequency sub-board. The amplitude of the voltage of the signal output by the low-frequency radio frequency daughter board is less than 40mV, and the power amplification circuit completes the amplitude and power amplification of the small signal on the basis of ensuring the accuracy of the signal.

Further, the power amplification part of the orthogonal frequency division multiplexing while-drilling data transmission method amplifies the power of the output signal of the low-frequency radio frequency daughter board to realize the control and conversion of energy, and the power amplification part comprises a differential circuit, a proportional current source and an amplification circuit;

the differential circuit is used for offsetting the temperature drifts by using triodes with the same characteristics; the proportional current source outputs an amplified current, and the calculation formula of the output current is I₂＝I₁*R₂/R₁。

Further, the transmitting end uses a transmitting transducer which converts electric energy into sound energy, and the receiving end uses a receiving transducer which converts sound energy into electric energy;

the piezoelectric transducer realizes the mutual conversion of electric energy and sound energy by utilizing a piezoelectric effect and converts an electric signal into mechanical vibration of sound waves; the core component of the piezoelectric transducer is a piezoelectric crystal; the piezoelectric crystal is made of lead zirconate titanate ceramic materials by using the principle of electrostrictive effect and piezoelectric effect;

further, the signal demodulation of the ofdm while-drilling data transmission method includes:

(1) the matched filtering module removes noise of the received signal;

(2) the data synchronization module adopts a COX time frequency estimation method based on a PN sequence to complete window synchronization matching of received signals, and if the matching is successful, a mark is sent to the sampling module. P (d) is a sliding autocorrelation operation, r (d) is the energy within the sliding window, m (d) is the normalization of the sliding autocorrelation result:

(3) the frequency conversion module uses a numerical control oscillator and a mixer to remove carrier frequency to obtain baseband signals, the numerical control oscillator controls the frequency and phase of oscillation signals in a loop by using externally input signals to realize automatic tracking of input signal frequency, the mixer performs multiplication operation on a signal Acos (omega t + theta) received by a transducer and a signal cos (omega t + theta) transmitted by the numerical control oscillator module to perform coherent demodulation to obtain a required signal A, the whole process of carrier synchronization, namely frequency conversion is completed, A represents an OFDM signal required to be received, Acos (omega t + theta) represents the OFDM signal after up-conversion, and the key for realizing the whole frequency conversion link is that a receiving end generates a coherent carrier cos (omega t + theta) which is strictly in phase with the signal received by the transducer:

Acos(ωt+θ)cos(ωt+θ)＝A/2+A/2cos(2ωt+2θ)；

(4) the fast discrete Fourier transform module finishes FFT operation, processed signals are transmitted to the sampling link, the sampling link searches a data head part in an autocorrelation mode according to a matching mark signal of the synchronization module, the data head part is separated from data, boundary marks between the data head part and the data are given, and the system carries out FFT operation after removing a cyclic prefix in front of a data frame:

(5) the data frame acquisition module is carried out after fast discrete Fourier transform, known PN codes and received PN code sequences are compared to obtain channel gain, the data frames after gain correction are used for completing frame synchronization and equalization, and timing signals of the data frames are received in real time during the data frame acquisition period;

(6) the data packet extraction and demodulation module converts the complex vector output by the FFT into bit stream data of 0 and 1, completes the signal de-mapping process, and packs and sends the bit stream to the data packet structure decomposition module;

(7) and the data packet structure decomposition module is used for unpacking the demodulated data frame. Removing the whitening signal, and removing a packet head, a packet tail and CRC (cyclic redundancy check) data of the data packet to finally obtain actual useful data information;

after a feedback part of the orthogonal frequency division multiplexing while-drilling data transmission method receives a data packet of a data link layer or an OFDM signal is demodulated, a feedback signal is sent to the data link layer, whether the data packet is correctly received or demodulated is detected, a clock signal does not exist between the data link layer and a physical layer, synchronization is completed by a queue, and after one data packet is operated from the queue, a feedback signal is sent to the data link layer to perform operation of the next data packet;

an antenna part connecting system hardware and a signal-to-noise ratio estimation program; the signal-to-noise ratio estimation is to estimate a drilling fluid channel by using a second-order fourth-order matrix estimation algorithm, transmit the signal-to-noise ratio estimation value to a transmitting end, and perform self-adaptive adjustment by using the channel estimation value as a parameter through a self-adaptive selection function of the transmitting end of the system;

the error control decoding part restores the bit stream data information obtained by the data packet structure decomposition into the information represented by the bit stream data information, and the bit stream data information corresponds to the error control coding part of the sending end;

in a drilling fluid channel with water-based drilling fluid as a transmission medium, the transmission distance of sound waves is inversely proportional to the square of the center frequency of a transmitted signal, the center frequency of the up-converted signal and the signal bandwidth can be flexibly adjusted according to the transmission distance, and the center frequency of the signal can be reduced and the signal bandwidth can be reduced to increase the transmission distance.

It is another object of the present invention to provide a computer-readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the steps of:

Another object of the present invention is to provide an ofdm while-drilling data transmission system operating the ofdm while-drilling data transmission method, the ofdm while-drilling data transmission system including:

the transmitting end is used for realizing that the signal processor modulates the data after the error control coding, adds a cyclic prefix after the inverse fast discrete Fourier transform and performs digital-to-analog conversion; the low-frequency radio frequency circuit performs power amplification after up-conversion on the analog signal, and the transmitting transducer converts the electric signal after power amplification into a sound wave signal to be transmitted in a drilling fluid channel;

the receiving end is used for converting the received sound wave signals into electric signals by the receiving transducer, carrying out data synchronization and fast discrete Fourier transform after matched filtering and down-conversion, demapping the output complex vectors into bit data and carrying out structural decomposition;

the transmitting end comprises an error control coding part, a signal modulation part, a power amplification part and an electro-acoustic conversion part; the signal modulation part comprises data packing, constellation mapping, fast discrete Fourier inverse transformation, cyclic prefix and up-conversion; at a system sending end, the USRP signal processor is connected with a low-frequency sending circuit, the low-frequency sending circuit is connected with a power amplifier, and the power amplifier is connected with a sending transducer;

the electro-acoustic conversion part consists of a signal wire, a waterproof joint, an upper shell, a lower shell, epoxy resin glue, a cork plate and a lead zirconate titanate ceramic chip; the signal wire is connected with the upper shell through a waterproof joint, the upper shell is connected with the lower shell, the lead zirconate titanate ceramic sheet is arranged at the bottom of the lower shell, the outer side of the lead zirconate titanate ceramic sheet is wrapped by a cork plate, and epoxy resin adhesive is arranged on the outer side of the cork plate;

the data frame of the sending end comprises a header, a data part, a check bit and a tail bit; the power amplification circuit consists of a differential amplification circuit for eliminating zero drift, a proportional current source with temperature stability and a high-efficiency digital power amplification circuit; the piezoelectric transducer is composed of lead zirconate titanate ceramics, epoxy resin glue and a waterproof joint;

the receiving end comprises an acoustic-electric conversion part, a signal demodulation part, a feedback part, an antenna part and an error control decoding part; the receiving part is the most key link of the receiving end and comprises matched filtering, data synchronization, down conversion, sampling, fast discrete Fourier transform, data frame acquisition, data packet extraction and demodulation and data packet structure decomposition; at the system receiving end, the receiving transducer is connected with a low-frequency receiving circuit, and the low-frequency receiving circuit is connected with a signal processor.

The invention also aims to provide a communication system for drilling, oil production and well repair in oil exploration, which carries the orthogonal frequency division multiplexing while-drilling data transmission system.

By combining all the technical schemes, the invention has the advantages and positive effects that: in order to solve the problems of low data transmission rate, low data precision and long waiting time in the prior art, the invention adopts the technical scheme that sound waves are communicated in liquid, comprehensively considers the influence factors in the drilling process, and develops a sound wave data transmission system while drilling based on orthogonal frequency division multiplexing on the basis of deeply researching the volume concentration of suspended particles in drilling fluid, the well site environmental noise, the drill rod waveguide transmission line effect, the drilling fluid sound absorption coefficient, the multipath effect of the reflection and refraction of the inner wall of the drill rod and other factors. The system is based on the orthogonal frequency division multiplexing technology, improves the data transmission rate, reduces the bit error rate, effectively overcomes the intersymbol interference, improves the frequency spectrum utilization rate, realizes high-speed underground communication, and has the characteristics of strong anti-multipath interference capability and high data transmission rate.

According to the hearing frequency range of human ears, the sound waves can be divided into three types, namely infrasonic waves (less than 20Hz), threshold sound waves (20 Hz-20 KHz) and ultrasonic waves (more than 20 KHz). Infrasonic waves are characterized by low frequency and long wavelength and can be transmitted in drilling fluid for a long distance with small energy loss. However, due to the low infrasonic frequency, the narrow bandwidth, the small data transmission quantity and the large volume of the transceiving system, the use of the transceiving system in narrow drilling wells is limited. In the drilling fluid which takes water, oil and suspended particles as main components, the acoustic absorption rate of threshold acoustic waves and ultrasonic waves is low, the power consumption is moderate, the transmission at a long distance and a high speed can be realized, and the method is an ideal selection of drilling fluid channels.

The invention discloses a while-drilling sound wave data transmission system based on orthogonal frequency division multiplexing. The system uses a drilling fluid channel for sound wave transmission, combines the leading edge technologies of digital signal processing, orthogonal frequency division multiplexing, error control coding, power amplification and the like based on FPGA, which relate to multiple disciplines, software is compiled by Python and C language, and the system covers CRC error control coding, serial-parallel conversion, constellation mapping, cyclic prefix, digital-to-analog conversion, matched filtering, up-down frequency conversion, IFFT, FFT and signal synchronization modules. The hardware components comprise an FPGA-based USRP system, a low-frequency radio frequency circuit, a power amplification circuit and a piezoelectric lead zirconate titanate ceramic transducer.

The invention can improve the data transmission rate to more than 250 kbits per second and reduce the error rate to 10^-4Compared with the prior mud pulse and electromagnetic wave data transmission mode, the method has great improvement. The invention can effectively overcome the interference between data symbols, reduce the error rate, improve the data transmission rate, realize high-rate, low-error and long-distance communication, and effectively solve the four key problems of long-distance transmission, communication rate, anti-interference capability and implementation difficulty in the data communication while drilling.

Drilling is the most risky stage in oilfield exploration. In order to ensure the normal development of production, workers need to acquire exploration data from the underground in time. The system integrates multiple technologies of digital signal processing, orthogonal frequency division multiplexing, power amplification, error control coding and the like, software of a sending end and a receiving end is compiled by Python and C language, and hardware comprises core components such as a USRP system based on an FPGA, a low-frequency radio frequency circuit, a power amplification circuit, a piezoelectric ceramic transducer and the like. Through the mode, the invention can convert the data transmission rateThe bit error rate is reduced to 10 by increasing the bit error rate to more than 1000 bits per second^-4. Compared with the existing mud pulse and electromagnetic wave data transmission mode, the method is greatly improved. The system can effectively overcome the interference between data symbols, reduce the error rate, improve the data transmission rate and realize the high-rate, low-error and long-distance data communication while drilling.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments of the present application will be briefly described below, and it is obvious that the drawings described below are only some embodiments of the present application, and it is obvious for those skilled in the art that other drawings can be obtained from the drawings without creative efforts.

Fig. 1 is a flowchart of an orthogonal frequency division multiplexing while drilling data transmission method according to an embodiment of the present invention.

Fig. 2 is a schematic structural diagram of an orthogonal frequency division multiplexing while drilling data transmission system according to an embodiment of the present invention.

FIG. 3 is a schematic block diagram of a data transmission while drilling system according to an embodiment of the present invention.

Fig. 4 is a schematic diagram of a power amplifier circuit according to an embodiment of the present invention.

Fig. 5 is a schematic structural diagram of a piezoelectric transducer according to an embodiment of the present invention.

Fig. 6 is a schematic diagram of a time domain waveform of an OFDM signal according to an embodiment of the present invention.

Fig. 7 is a schematic diagram of a frequency domain waveform of an OFDM signal according to an embodiment of the present invention.

FIG. 8 shows an example of data transmission accuracy analysis (error rate less than 10)^-4) Schematic representation.

Fig. 9 is a waterfall diagram of an OFDM signal according to an embodiment of the present invention.

Figure 10 is a schematic diagram of a frequency response curve of a bandpass transducer provided by an embodiment of the present invention.

Fig. 11 is a schematic diagram of a data structure of a sender according to an embodiment of the present invention.

Fig. 12 is a schematic diagram of a sending-end software structure according to an embodiment of the present invention.

Fig. 13 is a schematic diagram of a receiving-end software structure according to an embodiment of the present invention.

Fig. 14 is a schematic diagram of an OFDM symbol structure according to an embodiment of the present invention.

In the figure: 1. a signal line; 2. a waterproof joint; 3. an upper shell; 4. a lower case; 5. epoxy resin glue; 6. a cork plate; 7. lead zirconate titanate ceramic chip; 8. a differential circuit; 9. a proportional current source; 10. an amplifying circuit; 11. a sending end; 12. and (4) receiving the data.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

In view of the problems in the prior art, the present invention provides a method, a system, a storage medium and an application for transmitting data while drilling by orthogonal frequency division multiplexing, and the present invention is described in detail below with reference to the accompanying drawings.

As shown in fig. 1, the method for transmitting data while drilling by orthogonal frequency division multiplexing provided by the present invention includes the following steps:

s101: at a transmitting end, a signal processor modulates the data subjected to error control coding, adds a cyclic prefix after fast inverse discrete Fourier transform and performs digital-to-analog conversion; the low-frequency radio frequency circuit performs power amplification after up-conversion on the analog signal, and the transmitting transducer converts the electric signal after power amplification into a sound wave signal to be transmitted in a drilling fluid channel;

s102: at a receiving end, a receiving transducer converts received sound wave signals into electric signals, and after matched filtering and down-conversion, data synchronization and fast discrete Fourier transform are carried out, and output complex vectors are de-mapped into bit data and subjected to structural decomposition.

The transmitting end includes an error control encoding section, a signal modulation section, a power amplification section, and an electro-acoustic conversion section. The signal modulation part is the most key link of a sending end, and the main modules comprise data packing, constellation mapping, inverse fast discrete Fourier transform, cyclic prefix and up-conversion. At a system sending end, the USRP signal processor is connected with a low-frequency sending circuit, the low-frequency sending circuit is connected with a power amplifier, and the power amplifier is connected with a sending energy converter.

The transmit data frame includes a header, a data portion, a parity bit, and a tail bit. In order to increase the reliability of the system, a method of repeatedly sending the packet header of the data packet is adopted. The time domain OFDM symbols after IFFT are complex vectors, adopt QAM mapping, and are formed by cyclic prefixes and data symbols for eliminating intersymbol interference. The power amplification circuit consists of a differential amplification circuit for eliminating zero drift, a proportional current source with temperature stability and a high-efficiency digital power amplification circuit. The piezoelectric transducer is composed of lead zirconate titanate ceramics, epoxy resin glue and a waterproof joint.

The receiving end comprises an acoustic-electric conversion part, a signal demodulation part, a feedback part, an antenna part and an error control decoding part. The receiving part is the most key link of the receiving end, and the main modules comprise matched filtering, data synchronization, down conversion, sampling, fast discrete Fourier transform, data frame acquisition, data packet extraction and demodulation and data packet structure decomposition. At the system receiving end, the receiving transducer is connected with a low-frequency receiving circuit, and the low-frequency receiving circuit is connected with a signal processor.

Those skilled in the art of the ofdm while-drilling data transmission method provided by the present invention may also use other steps to implement the method, and the ofdm while-drilling data transmission method provided by the present invention in fig. 1 is only one specific embodiment.

As shown in fig. 2, the ofdm while-drilling data transmission system provided by the present invention includes:

the transmitting end 11 is used for implementing signal modulation of the data after error control coding by the signal processor, adding a cyclic prefix after fast inverse discrete fourier transform and performing digital-to-analog conversion; the low-frequency radio frequency circuit performs power amplification after up-conversion on the analog signal, and the transmitting transducer converts the electric signal after power amplification into a sound wave signal to be transmitted in a drilling fluid channel.

And the receiving end 12 is configured to convert the received acoustic wave signal into an electrical signal by the receiving transducer, perform data synchronization and fast discrete fourier transform after matched filtering and down-conversion, demap the output complex vector into bit data, and perform structural decomposition.

The technical solution of the present invention is further described below with reference to the accompanying drawings.

As shown in FIG. 3, the acoustic wave while drilling data transmission system based on orthogonal frequency division multiplexing of the invention uses acoustic waves to upload exploration data from the bottom of a drilling well through a drilling fluid channel. The system comprises a signal transmitting end and a signal receiving end, and covers hardware development and software design. The hardware core components include an FPGA-based USRP signal processor, a low-frequency radio frequency circuit, a power amplifier, and piezoelectric transmitting and receiving transducers. The signal processing software of the sending end and the receiving end is written by Python and C language.

At a transmitting end, a signal processor modulates the data subjected to error control coding, adds a cyclic prefix after the data is subjected to inverse fast discrete Fourier transform, and performs digital-to-analog conversion. The low-frequency radio frequency circuit performs power amplification after up-conversion on the analog signal, and the transmitting transducer converts the electric signal after power amplification into a sound wave signal to be transmitted in a drilling fluid channel. At a receiving end, a receiving transducer converts received sound wave signals into electric signals, data synchronization and fast discrete Fourier transform are carried out after matched filtering and down-conversion, and finally output complex vectors are de-mapped into bit data and are subjected to structural decomposition.

The system sending end comprises an error control coding part, a signal modulation part, a power amplification part and an electro-acoustic conversion part. At a system sending end, the USRP signal processor is connected with a low-frequency sending circuit, the low-frequency sending circuit is connected with a power amplifier, and the power amplifier is connected with a sending energy converter.

The purpose of the error control coding part is to reduce the error rate of the received data. Widely used convolutional code, block code and cyclic redundancy check code with strong error detectionAnd (4) code. The present invention uses a cyclic redundancy check code. The consistent relation between the data bits and the check bits is established through mathematical operation, the calculation of the data check codes can be completed in a short time, errors in the information transmission process can be corrected quickly, and the method has the advantages of being strong in error detection capability, high in communication efficiency and low in detection cost. CRC32 encoding is the remainder of a modulo-2 division with a generator polynomial of x³²+x²⁶+x²³+x²²+x¹⁶+x¹²+x¹¹+x¹⁰+x⁸+x⁷+x⁵+x⁴+x²+ x + 1. The generator polynomial determines the generator matrix and the entire cyclic code. The signal after error control coding enters a signal modulation module.

The signal modulation part is the most key link of the transmitting end, and the signal processing is completed by the USRP based on the FPGA and the low-frequency radio frequency circuit in the module. The main modules of the signal modulation part comprise data packing, constellation mapping, fast discrete Fourier inversion, cyclic prefix and up-conversion.

(1) And the data packing module is used for packing the data packet of the data link layer. The data frame structure of the system comprises four parts, namely a packet header, data, a 4-bit CRC check bit and a 1-bit tail. The header includes two kinds of information, one is a whitening parameter of 4 bits, and the other is a data packet length of 12 bits. The header of the data packet adopts a repeated transmission method to increase the reliability of the system. The structure and packing process of the data frame are divided into three steps. First, the data to be transmitted is added with check bits using the CRC32 algorithm. Second, in order to make the data have a random uniform distribution characteristic, both the CRC check bits and the tail bits are whitened. And finally, adding the packet head of the data packet to complete the packaging process of the data link layer data packet.

(2) The constellation mapping module takes out the data packet from the queue, and after the data packet is converted into parallel signals through serial data, the parallel signals are mapped into complex vectors of frequency domains, and constellation mapping of IFFT input signals is completed. The constellation mapping has various modes such as QPSK, 16QAM, 64QAM, etc. And adding a sequence header to the data subjected to constellation mapping, and performing IFFT transformation. To reduce the error rate, the input of the IFFT uses a random complex vector of a 4QAM constellation {1+ j, 1-j, -1+ j, -1-j }. The output of the IFFT is the corresponding time-domain phasor x. The invention uses an inverse fast discrete fourier transform, with the upper case representing the frequency or discrete frequency domain variable and the lower case representing the time domain variable.

(3) The fast discrete Fourier transform module is the core of the signal modulation part. The symbols of the orthogonal frequency division multiplexing signal describe the time domain sequence associated with one IFFT operation. The input of the IFFT is the complex vector X ═ X in the frequency domain₀ X₁ X₂ X₃… X_N-1]^TEach element of X is a complex vector representing the frequency domain, and the number of complex vectors in the frequency domain is N. N is also the number of input frequency domain complex vectors that are subjected to the IFFT operation each time. Each element of X represents data to be carried on a corresponding subcarrier, X_kRepresenting the data carried on the k-th subcarrier. The output of the IFFT is the time-domain complex vector x ═ x₀ x₁ x₂ x₃ … x_n-1]^T. Each element of x is a complex vector representing the frequency domain:

(4) the cyclic prefix module can effectively resist multipath time delay expansion, eliminate multipath interference of a drilling fluid channel to the maximum extent, and insert a protective interval consisting of cyclic prefixes between adjacent orthogonal frequency division multiplexing symbols. The length of the guard interval is greater than the maximum delay spread in the wireless channel so that multipath components of one orthogonal frequency division multiplexing symbol do not interfere with the next symbol. x (i) ═ x₀(i) x₁(i) x₂(i) x₃(i) … x_n-1(i)]^TIs the output of the IFFT in the ith symbol period. A cyclic prefix is added at the beginning of each time domain symbol prior to transmission. The method used is to copy the signal in the post Tg time of each symbol to the front of the symbol. The transmission sequence is x_cp(i)＝[x_N-G(i) … x_N-1(i) x₀(i) x₁(i) x₂(i) x₃(i) … x_n-1(i)]^T。

(5) The up-conversion module realizes the frequency shift of the baseband signal to a higher transmission frequency by using the low-frequency radio frequency daughter board. After the cyclic prefix is added to the sending sequence, the digital signal is converted into an analog signal through digital-analog conversion, and the signal is subjected to frequency spectrum shifting through the low-frequency radio frequency sub-board and then is transmitted to the power amplification circuit. The amplitude of the voltage of the signal output by the low-frequency radio frequency daughter board is less than 40mV, and the power amplification circuit completes the amplitude and power amplification of the small signal on the basis of ensuring the accuracy of the signal.

The power amplification part is used for carrying out power amplification on the output signal of the low-frequency radio frequency sub-board, so that energy control and conversion are realized, and undistorted amplification is ensured in a high-temperature high-pressure and strong-noise drilling environment. The power amplification circuit design suitable for the drilling environment comprises three key links. The method comprises the following steps: a differential circuit 8, a proportional current source 9 and an amplifying circuit 10.

The differential circuit 8 uses transistors having the same characteristics to cancel out the temperature drift. The design can eliminate the zero drift phenomenon caused by temperature change, wherein the input voltage is zero and the output voltage is not zero. The proportional current source 9 outputs an amplified current having higher temperature stability under the downhole severe environment. The calculation formula of the output current is I₂＝I₁*R₂/R₁. The amplifier circuit 10 has high efficiency, small volume and strong reliability.

The electro-acoustic conversion part mainly uses a piezoelectric transducer to realize mutual conversion of electric energy and acoustic energy. The piezoelectric lead zirconate titanate ceramic band-pass transducer used by the invention is composed of a signal line 1, a waterproof joint 2, an upper shell 3, a lower shell 4, epoxy resin glue 5, a cork plate 6 and a lead zirconate titanate ceramic plate 7. The signal line 1 is connected with the upper shell 3 through the waterproof connector 2, the upper shell 3 is connected with the lower shell 4, the lead zirconate titanate ceramic chip 7 is arranged at the bottom of the lower shell 4, the outer side of the lead zirconate titanate ceramic chip 7 is wrapped by the cork plate 6, and the outer side of the cork plate 6 is epoxy resin glue 5. The transmitting end uses a transmitting transducer that converts electrical energy into acoustic energy, and the receiving end uses a receiving transducer that converts acoustic energy into electrical energy. The main design criteria of the transducer are the operating frequency, the frequency bandwidth, the impedance at the resonance frequency, the directivity and the sensitivity. The energy converter has different requirements on performance parameters, and the energy converter at the transmitting end has higher output power and higher energy conversion efficiency. The transducer at the receiving end has higher sensitivity and resolution.

The piezoelectric transducer realizes the mutual conversion of electric energy and sound energy by utilizing a piezoelectric effect and converts an electric signal into mechanical vibration of sound waves. The core component of a piezoelectric transducer is a piezoelectric crystal. The piezoelectric crystal is made of lead zirconate titanate ceramic materials by using the principle of electrostrictive effect and piezoelectric effect. The lead zirconate titanate ceramic piezoelectric crystal has high electro-acoustic conversion efficiency, low price, mature technology, convenient manufacture and difficult aging.

The system receiving end is mainly divided into an acoustic-electric conversion part, a signal demodulation part, a feedback part, an antenna part and a decoding part. At the system receiving end, the receiving transducer is connected with a low-frequency receiving circuit, and the low-frequency receiving circuit is connected with a signal processor.

The acoustoelectric conversion part mainly uses a piezoelectric transducer to convert a received sound wave signal into an electric signal which can be processed.

The signal demodulation part is the most critical link of the receiving end. The signal demodulation part comprises main modules of matched filtering, data synchronization, down conversion, fast discrete Fourier transform, sampling, data frame acquisition, data packet extraction and demodulation and data packet structure decomposition.

(1) The matched filtering module is mainly used for removing noise of the received signal and improving the signal-to-noise ratio of the received signal. Of the amplitude-frequency characteristic and the phase-frequency characteristic of the system, the amplitude-frequency characteristic more represents the frequency characteristic, and the phase-frequency characteristic more represents the time characteristic. The matched filter ensures that the signal passes as much as possible and the noise passes as little as possible in both the time and frequency domains, thereby achieving an output with the greatest signal-to-noise ratio.

(2) The data synchronization module adopts a COX time frequency estimation method based on a PN sequence, and has the main functions of completing window synchronization matching of received signals and sending a mark to the sampling module if the matching is successful. P (d) is a sliding autocorrelation operation, r (d) is the energy within the sliding window, m (d) is the normalization of the sliding autocorrelation result:

(3) the down-conversion module mainly uses a numerical control oscillator and a mixer to remove carrier frequency and obtain baseband signals. The digital controlled oscillator is a typical feedback control circuit, functions as a phase-locked loop, and utilizes an externally input signal to control the frequency and phase of an internal oscillation signal of the loop, so as to realize automatic tracking of the frequency of the input signal. The mixer multiplies the signal Acos (ω t + θ) received by the transducer by the signal cos (ω t + θ) transmitted by the numerically controlled oscillator module, performs coherent demodulation to obtain a required signal a, and completes the whole process of carrier synchronization, i.e. down conversion. A denotes an OFDM signal to be received, and Acos (ω t + θ) denotes an OFDM signal after up-conversion. The key for realizing the whole down-conversion link is that a receiving end generates a coherent carrier cos (ω t + θ) which has the same frequency and phase as the signal received by the transducer strictly:

Acos(ωt+θ)cos(ωt+θ)＝A/2+A/2cos(2ωt+2θ)；

(4) and the fast discrete Fourier transform module completes FFT operation. The processed signals are transmitted to a sampling link, the sampling link searches a data head part in an autocorrelation mode according to the matching mark signals of the synchronization module, the data head part is separated from the data, and a boundary mark of the data head part and the data head part is given. The system removes the cyclic prefix in front of the data frame and then carries out FFT operation:

(5) the data frame acquisition module is performed after the fast discrete fourier transform. And comparing the known PN code with the received PN code sequence to obtain channel gain, and completing frame synchronization and equalization by using the data frame after gain correction. The timing signal of the data frame is received in real time during data frame acquisition.

(6) The data packet extraction and demodulation module converts the complex vector output by the FFT into bit stream data of 0 and 1, completes the signal de-mapping process, packs the bit stream and sends the packed bit stream to the data packet structure decomposition module.

(7) And the data packet structure decomposition module is used for unpacking the demodulated data frame. And removing the whitening signal, and removing a packet head, a packet tail and CRC (cyclic redundancy check) data of the data packet to finally obtain actual useful data information.

The feedback part sends a return signal to the data link layer after receiving the data packet of the data link layer or demodulating the OFDM signal, and detects whether the data packet is received or demodulated correctly. There is no clock signal between the data link layer and the physical layer, and the synchronization is completed by the queue. After a packet is processed from the queue, a feedback signal must be sent to the data link layer to perform the next packet.

The antenna section connects the system hardware and the signal-to-noise ratio estimation procedure. And the signal-to-noise ratio estimation is to estimate a drilling fluid channel by using a second-order fourth-order matrix estimation algorithm and transmit the signal-to-noise ratio estimation value to a transmitting end. The adaptive selection function of the system sending end carries out adaptive adjustment by taking the channel estimation value as a parameter.

The error control decoding section restores the bit stream data information obtained through the packet structure decomposition into information represented by it, corresponding to the error control encoding section at the transmitting end.

In a drilling fluid channel with water-based drilling fluid as a transmission medium, the transmission distance of sound waves is inversely proportional to the square of the center frequency of a transmitted signal. In the invention, the center frequency and the bandwidth of the signal after up-conversion can be flexibly adjusted according to the transmission distance, and the center frequency of the signal can be reduced and the bandwidth of the signal can be reduced to increase the transmission distance. The method can be widely applied to various communication occasions such as drilling, oil extraction, well repair and the like in petroleum exploration.

The technical effects of the present invention will be described in detail with reference to experiments.

1. Pipeline simulation experiment parameters

TABLE 1

Experimental parameters	Parameter value
		Number of transmitting and receiving transducers	Single-transmitting single-receiving
Center frequency	500kHz
		Bandwidth of signal	100kHz
Range of passband	450kHz-550kHz
		Modulation system	QAM
Length of FFT operation	512
		Length of data	200
Length of cyclic prefix	128
		Channel estimation method	MMSE
Signal synchronization method	SchmidlCox algorithm
		FPGA operation rate	64M
FPGA decimation factor	256
		Data transmission rate	250kbit/s
Error control coding mode	CRC32
		Inner diameter of drill rod	100mm
Outside diameter of drill rod	115mm
		Solid content of water-based drilling fluid	10％

Note: parameters such as the center frequency, the signal bandwidth, the passband range, the modulation mode, the IFFT operation length, the data length, the cyclic prefix length and the like of the system can be flexibly adjusted according to the actual needs of drilling.

2. Results of pipeline simulation experiments

FIG. 6 is an OFDM signal time domain waveform; FIG. 7 is a frequency domain waveform of an OFDM signal; FIG. 8 data Transmission accuracy analysis (error Rate < 10)^-4) (ii) a Fig. 9 an OFDM signal waterfall diagram; figure 10 band pass transducer frequency response curves.

Parameters such as the center frequency, the signal bandwidth, the passband range, the modulation mode, the IFFT operation length, the data length, the cyclic prefix length and the like of the system can be flexibly adjusted according to the actual needs of drilling. The OFDM symbols in the experiment adopt QAM mapping, the length of the symbol is 512, the length of the cyclic prefix is 128, and the length of the data is 200. The MMSE algorithm is adopted for channel estimation, the Schmidl Cox algorithm is adopted for synchronization, and the CRC32 algorithm is adopted for error control coding. The inner diameter of the drill rod is 100mm, the outer diameter of the drill rod is 115mm, and the solid content of the water-based drilling fluid is 10%. The center frequency of the up-converted signal is 500KHz, the bandwidth of the signal is 100KHz, the passband range is 450kHz-550kHz, the operation rate of the FPGA is 64M, the extraction factor is 256, and the transmission rate of the data is 250 Kbit/s.

3. Pipeline simulation experiment flow chart

FIG. 11 is a diagram of a data structure at the sender; FIG. 12 is a diagram illustrating the software architecture of the sender; FIG. 13 is a diagram of a receiver software architecture; fig. 14 is a schematic diagram of an OFDM symbol structure.

In the description of the present invention, "a plurality" means two or more unless otherwise specified; the terms "upper", "lower", "left", "right", "inner", "outer", "front", "rear", "head", "tail", and the like, indicate orientations or positional relationships based on the orientations or positional relationships shown in the drawings, are only for convenience in describing and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed in a particular orientation, and be operated, and thus, should not be construed as limiting the invention. Furthermore, the terms "first," "second," "third," and the like are used for descriptive purposes only and are not to be construed as indicating or implying relative importance.

It should be noted that the embodiments of the present invention can be realized by hardware, software, or a combination of software and hardware. The hardware portion may be implemented using dedicated logic; the software portions may be stored in a memory and executed by a suitable instruction execution system, such as a microprocessor or specially designed hardware. Those skilled in the art will appreciate that the apparatus and methods described above may be implemented using computer executable instructions and/or embodied in processor control code, such code being provided on a carrier medium such as a disk, CD-or DVD-ROM, programmable memory such as read only memory (firmware), or a data carrier such as an optical or electronic signal carrier, for example. The apparatus and its modules of the present invention may be implemented by hardware circuits such as very large scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, or programmable hardware devices such as field programmable gate arrays, programmable logic devices, etc., or by software executed by various types of processors, or by a combination of hardware circuits and software, e.g., firmware.

The above description is only for the purpose of illustrating the present invention and the appended claims are not to be construed as limiting the scope of the invention, which is intended to cover all modifications, equivalents and improvements that are within the spirit and scope of the invention as defined by the appended claims.

Claims

1. An orthogonal frequency division multiplexing while-drilling data transmission method is characterized by comprising the following steps:

at a receiving end, a receiving transducer converts a received sound wave signal into an electric signal, performs data synchronization and fast discrete Fourier transform after matched filtering and down-conversion, and de-maps an output complex vector into bit data and performs structural decomposition;

the data frame of the sending end comprises a packet header, a data part, a check bit and a tail bit, wherein the packet header of the data packet adopts a repeated sending method, a time domain OFDM symbol after IFFT transformation is a complex vector, QAM mapping is adopted, and a cyclic prefix and a data symbol are formed for eliminating intersymbol interference;

the error control coding part of the orthogonal frequency division multiplexing while-drilling data transmission method uses cyclic redundancy check codes and passes throughThe mathematical operation establishes a consistent relationship between the data bits and the check bits, and the CRC32 encoding is the remainder of the modulo-2 division, resulting in a polynomial of x³²+x²⁶+x²³+x²²+x¹⁶+x¹²+x¹¹+x¹⁰+x⁸+x⁷+x⁵+x⁴+x²+ x +1, generating a polynomial to determine a generating matrix and the whole cyclic code, and enabling a signal subjected to error control coding to enter a signal modulation module;

the signal modulation of the orthogonal frequency division multiplexing while-drilling data transmission method is completed by an FPGA-based USRP and a low-frequency radio frequency circuit, and the method comprises the following steps:

(3) the sign of the ofdm signal of the IFFT block describes the time domain sequence associated with an IFFT operation, the input of which is the complex vector X ═ X in the frequency domain₀ X₁ X₂ X₃…X_N-1]^TEach element of X is a complex vector representing the frequency domain, the number of complex vectors of the frequency domain is N,n is also the number of input frequency domain complex vectors per IFFT operation, each element of X represents the data to be carried on the corresponding subcarrier, X_kRepresenting the data carried on the kth subcarrier, the output of the IFFT is the time-domain complex vector x ═ x₀ x₁ x₂ x₃…x_n-1]^TEach element of x is a complex vector representing the frequency domain:

(4) the cyclic prefix module inserts a guard interval composed of cyclic prefixes between adjacent orthogonal frequency division multiplexing symbols, the length of the guard interval is greater than the maximum delay spread in a wireless channel, so that multipath components of one orthogonal frequency division multiplexing symbol do not interfere with the next symbol, and x (i) [ x ] x ═ x₀(i) x₁(i) x₂(i) x₃(i)…x_n-1(i)]^TAdding a cyclic prefix to the beginning of each time domain symbol before transmission for the output of the IFFT in the ith symbol period; copying the signal in the post Tg time of each symbol to the front of the symbol, wherein the transmission sequence is x_cp(i)＝[x_N-G(i)…x_N-1(i) x₀(i) x₁(i) x₂(i) x₃(i)…x_n-1(i)]^T；

(5) The up-conversion module is used for realizing the shift of a baseband signal to a frequency with higher transmission frequency by using the low-frequency radio frequency daughter board, after the cyclic prefix is added in a sending sequence, a digital signal is converted into an analog signal through digital-analog conversion, and the signal is subjected to frequency spectrum shift by using the low-frequency radio frequency daughter board and then is transmitted to the power amplification circuit, the amplitude of the voltage of the signal output by the low-frequency radio frequency daughter board is less than 40mV, and the power amplification circuit completes the amplitude and power amplification of a small signal on the basis of ensuring the accuracy of the signal;

the power amplification part of the orthogonal frequency division multiplexing while-drilling data transmission method amplifies the power of an output signal of a low-frequency radio frequency daughter board to realize the control and conversion of energy, and the power amplification part comprises a differential circuit, a proportional current source and an amplification circuit;

the differential circuit is used for offsetting the temperature drifts by using triodes with the same characteristics; the proportional current source outputs an amplified current, and the calculation formula of the output current is I₂＝I₁*R₂/R₁；

The transmitting end uses a transmitting transducer which converts electric energy into acoustic energy, and the receiving end uses a receiving transducer which converts acoustic energy into electric energy;

the signal demodulation of the orthogonal frequency division multiplexing while-drilling data transmission method comprises the following steps:

(1) the matched filtering module removes noise of the received signal;

(2) the data synchronization module adopts a COX time frequency estimation method based on a PN sequence to complete window synchronization matching of a received signal, if the matching is successful, a mark is sent to the sampling module, P (d) is sliding autocorrelation operation, R (d) is energy in a sliding window, and M (d) is normalization processing of a result of the sliding autocorrelation:

Acos(ωt+θ)cos(ωt+θ)＝A/2+A/2cos(2ωt+2θ)；

(7) the data packet structure decomposition module is used for unpacking the demodulated data frame; removing the whitening signal, and removing a packet head, a packet tail and CRC (cyclic redundancy check) data of the data packet to finally obtain actual useful data information;

2. A computer readable storage medium storing a computer program which, when executed by a processor, causes the processor to perform the orthogonal frequency division multiplexing while drilling data transmission method of claim 1.

3. An orthogonal frequency division multiplexing while-drilling data transmission system for operating the orthogonal frequency division multiplexing while-drilling data transmission method according to claim 1, wherein the orthogonal frequency division multiplexing while-drilling data transmission system comprises:

4. A communication system for drilling, oil production and well repair in oil exploration, which is characterized in that the communication system for drilling, oil production and well repair in oil exploration is provided with the orthogonal frequency division multiplexing while-drilling data transmission system of claim 3.