Research on a TCM-based Transmission Approach for EM-MWD by Combining Phase Modulation and Convolutional Coding

In practical applications of electromagnetic measurement while drilling (EM-MWD) in the underground coal mine, the signal-to-noise ratio (SNR) of the receiver cannot always meet the requirements of reliable communication conditions due to the earth-attenuation, interfering signal from a well site, etc. Traditional digital communication systems use independent design coding and modulation techniques to improve system performance. The coding is mainly achieved by introducing redundant bits and the improvement of error performance is at the expense of information rate. Aimed to solve this problem, we use a coding technique based on trellis coded modulation (TCM) to maximize the minimum distance between modulated output sequences and achieve significant coding gain. Simulation and experiments show that the system can improve the anti noise performance of the system by obtaining a coding gain without reducing the transmission rate. Compared to the uncoded quadrature phase shift keying (QPSK), TCM can achieve a coding gain of at least 3 dB under the same transmitting rate. At that time, as the number of TCM system states or trace back depth increases, the coding gain is further enhanced. The TCM code modulation method can be used in an EM-MWD system to improve the system performance.