Research on High Speed Communication Method of ELF-EM While Drilling Based on Adaptive Combined Filtering Algorithm

In terms of the electromagnetic wave measurement while drilling, the extremely low frequency electromagnetic(ELF-EM) signal below 20Hz is usually chosen as the downhole data carrier. To improve the transmission rate of ELF-EM signal and the signal to noise ratio (SNR) at the receiving end, the DQPSK modulation was proposed as the modulation method for the communication of electromagnetic wave system. Different from the traditional IQ orthogonal modulation and coherent demodulation methods, the proposed phase selection modulation and correlation algorithm demodulation are easier to implement and more practical. With regard to the communication synchronization, a fast algorithm, which based on the normalized cross-relation number, was used for waveform matching, and the maximum point of the correlation coefficient was used as the starting point of communication synchronization. The communication simulation results show that the proposed DQPSK modulation signal based on the adaptive combined filtering algorithm has better terminal error rate and transmission rate than the traditional modulation method. Under the same carrier frequency and code width, the transmission rate of DQPSK modulation is 4 to 5 times and 2 times that of PPM modulation and 2DPSK modulation respectively. The communication modulation and demodulation modes as well as the decoding algorithm with combined adaptive filter proposed in this paper can effectively solve practical engineering problems.