Research On MSK Modulation And Demodulation Method Of High Rate MWD System

In the field of oil and gas exploration,measurement while drilling technology is an important means to obtain real-time formation parameters during drilling,and mud pulse transmission technology is the most widely used information transmission technology in measurement while drilling.However,the domestic mud pulse transmission technology started later than foreign countries,the technology and process are not mature enough,and there is a big gap with foreign technology.In order to break through the monopoly of technology,this research topic is proposed.By discussing the principle of pulse signal generator while drilling,MSK with advantages in all aspects is selected as the modulation mode of transmission while drilling.Through discussing the technical schemes of mud pulse signal noise elimination,signal synchronization and demodulation,channel equalization,etc.,and set up the simulation system,and the whole modulation and demodulation solution is technically verified.The main research contents of this paper are as follows:1.For the choice of modulation method,the article analyzes the mechanical structure of the mud pulse signal generator and the principle of signal generation,and simulates the mud pulse signal for BPSK and MSK signals,and analyzes the power spectral density of the two modulation methods.Simultaneously perform performance simulation on analog signals,and choose MSK with better performance as the research direction of this article.2.Regarding the noise reduction of the signal,this article first analyzes the possible noise sources during drilling,and combines the noise signals collected by the actual drilling to summarize and analyze the noise characteristics of the mud pulse signal under the actual drilling.Two adaptive single-sensor denoising algorithm,and an adaptive dualsensor denoising algorithm.Finally,the BPSK modulated mud pulse signals collected from three real drilling scenes are denoised,and the advantages and disadvantages of the three denoising algorithms and applicable scenarios are analyzed.3.For the synchronization and demodulation of MSK signals,the article first introduces two kinds of orthogonal demodulation algorithms for MSK signals:frequency-shift quadrature demodulation and phase-shift quadrature demodulation.Two methods of timing synchronization and carrier synchronization of quadrature demodulation are introduced respectively.Finally,the optimal receiver structure based on phase-shifting quadrature demodulation is introduced,and the maximum likelihood sequence detector based on Viterbi algorithm is constructed to realize the optimal demodulation of MSK modulation signal.4.For signal equalization,the article first analyzes the transmission characteristics of the mud channel,and derives the correlation method to estimate the impulse response of the signal.After estimating the signal collected by the real drill,the channel frequency attenuation characteristics are analyzed,and it is concluded that the drilling fluid channel has strong frequency selectivity caused by channel multipath,which leads to serious intersymbol interference.According to the channel characteristics,an applicable frequency domain equalization algorithm and corresponding data frame structure are proposed,and the received signal is equalized to compensate for the attenuation of the signal by the channel.