Comparative Analysis on OQPSK and QDPSK of Continuous Pressure Wave System in Drilling Fluid Channel

Abstract

Based on the urgent needs for high transmission speed and high reliability of wireless MWD, the OQPSK modulation of the drilling fluid continuous wave signal is proposed in this article. The Principle of delay orthogonal modulation and coherent demodulation of OQPSK are studied, and the changes in the phase vector of the inputted bit sequence are analyzed; according to the principle of continuous wave generation, the phase adjustment mechanism of rotary value are analyzed and the relationship between the signal waveform, as well as the rotational speed when adjusting the phase, is studied; the information transmission rate, band efficiency and bit error rate of the OQPSK and QDPSK continuous wave communication systems are comparatively analyzed and simulated. The results show that OQPSK eliminates the phase transition of π, which belongs to constant envelope modulation, is suitable for the transmission under low SNR drilling fluid channel, meanwhile simplifies the phase adjustment and reduces generator design. QDPSK can simplify the computational complexity of phase adjustment, and reduces the design difficulty of the continuous-wave generator’s control circuit. The rotary valve forms of "slow down - speed up" and " speed up - slow down " are suitable for π / 2 and 3π / 2 phase jump respectively; the motor control modes which combine closed loop position and closed loop velocity can achieve phase stable signal and reduce the dynamic torque of the motor; in the same carrier frequency and SNR, the information rate and bandwidth efficiency of both OQPSK’s and QDPSK’s are twice as those of the two-phase shift keying system’s, and they can improve the wireless transmission performance of MWD system.