钻井液连续波传输的OQPSK调制原理与性能分析

Analysis on OQPSK modulation principle and system performance of continuous pressure wave signal in drilling fluid channel

基于无线随钻测量对高传输速率和高可靠性的需要,提出钻井液连续波信号的OQPSK调制方式。分析QPSK的延迟正交调制与相干解调原理,研究输入比特序列相位矢量图的变化;依据连续波的产生原理,分析转阀调整相位的转动机制、调整相位时信号波形与转速变化的关系、OQPSK连续波系统的信息速率、频带利用率和误码率,并进行仿真。结果表明:OQPSK消除了π弧度的相位变化,属于恒包络调制,适合低信噪比的钻井液信道,同时简化了相位调整的复杂度,降低了发生器的设计难度;对于π/2和3π/2的相位变化转阀分别适合采用'减速-加速'和'加速-减速'的转动方式调整;采用位置闭环与速度闭环相结合的电机控制方式,既可获得相位稳定的信号,又可减小电机的动态扭矩;在相同载频和信噪比条件下,OQPSK的信息速率和频带利用率是二相键控系统的两倍,可改善无线随钻测量系统的传输性能。

Based on the needs for high transmission speed and high reliability of wireless measurement while drilling （MWD）, the OQPSK modulation of drilling fluid continuous wave signal was proposed. The principles of delay orthogonal modulation and coherent demodulation of OQPSK were analyzed, and the changes in the phase vector of the input bit sequence were studied. According to the principle of continuous wave generation, the phase adjustment mechanism of rotary value and the relationship between the signal waveform and the rotational speed adjusting the phase were studied. The information transmission rate, band efficiency and bit error rate of the OQPSK continuous wave communication system were analyzed and simulated. The results show that OQPSK eliminates the phase transition of π, which belongs to constant envelope modulation, and is suitable for the transmission under low SNR drilling fluid channel, meanwhile simplifies the phase adjustment and reduces generator design. The rotary valve forms of ＂slow down-speed up＂ and ＂speed up-slow down＂ are suitable for π/2 and π/2 phase jump respectively. The motor control modes which combine closed loop position with closed loop velocity can achieve phase-stable signal and reduce the dynamic torque of the motor. In the same carrier frequency and SNR, OQPSK information rate and bandwidth efficiency are twice as the two-phase shift keying system, and can improve the wireless transmission performance of MWD system.