Research On Coherent Demodulation Of Drilling Fluid Pressure PSK Signals And Analysis On The Bit Error Rate

Measurement while drilling(MWD)is a kind of measurement technology,which can real-time measurement downhole information and transmit them to the ground by drilling fluid channel.MWD is one of the important auxiliary means of modern drilling technology by providing the engineering parameters and formation parameters for drilling process.This paper mainly relates to analysis of drilling fluid continuous pressure wave signals including coherent demodulation method research of drilling fluid pressure PSK(phase shift keying)signals and bit error rate(BER)analysis of reconstructed rotary valve control pulse.According to coherent demodulation mathematic model and reconstruction method of rotary valve control pulse of drilling fluid pressure DPSK(differential phase shift keying)signal,based on theoretical analysis of bit error rate in pulse code modulation system,a mathematical model of signal to noise rate(SNR)of reconstructed rotary valve control pulse was built for analyzing the impact of inherent interference generated by the DPSK signal demodulation system and noise within the signal frequency band on the pulse reconstruction.Research indicates that the inherent interference have a greater impact on bit error rate(BER)of the reconstructed pulse.By reasonable parameter design of the low pass filter in the demodulation system the inherent interference can be reduced to some extent,but not completely eliminated.The drilling fluid pressure QPSK(quadrature phase shift keying)signal can be demodulated only by coherent demodulation method according to analysing on modulation of the QPSK signal and code information of the downhole data can be discriminated by amplitude analysis of the reconstructed rotary valve control pulse.The parameter of low pass filter in the demodulation system and a special data processing method were applied to realiz the phase extraction and the pulse reconstruction in coherent demodulation of the QPSK signal.The SNR mathematical model of the reconstructed pulse was built to study the impact of inherent interference generated by the demodulation system and noise on bit error rate of the reconstructed pulse.Research results indicates that the random noise and inherent interference have a serious impact on the bit error rate of reconstructed pulse.In condition of the same SNR of input signal,the BER of reconstructed pulse with coherent demodulation of drilling fluid pressure QPSK signal is much higher than that of the DPSK signal.The research provides a theoretical basis for optimal design of the demodulation system and signal noise control.Theoretical calculation of bit error rate of reconstructed pulse is basically consistent with the numerical simulation.This shows that the mathematical model of signal to noise ratio of the reconstructed rotary valve control pulse is reasonable.This research provides a theoretical basis for integrity assessment of the signal transmission in drilling fluid channel.