Research On Key Techniques Of Demodulation For High-order QAM Signals In MIL-STD-188-110C

Quadrature amplitude modulation(QAM) is increasingly widely used in high speed short wave communication because of its attractive bandwidth efficiency. The MIL-STD-188-110 C announced in 2011 firstly adopted high-order QAM as a method of modulation in U.S military standard for short wave communication. It brings greater challenges to the demodulation techniques of the uncooperative receiver. This thesis studies the key technologies of blind demodulation including blind equalization, time synchronization and carrier synchronization for the high-order QAM signals in MIL-STD-188-110 C. The work involved in this thesis is part of a project undertaken by the laboratory the author works in.The main work and innovative contributions obtained in this thesis can be summarized as follows:1. The characteristics of the signals in the standard and HF channels are analyzed. Based on the analysis of the physical condition and the task plan, the key technical steps of demodulating the high-order QAM signals in the standard under the project background are obtained.2. In regard to blind equalization, the characteristics of constellation of the signals in the standard is analyzed firstly. Aiming at equalizing the circle high-order QAM signals in the standard, a region-based hybrid blind equalization algorithm is proposed. The algorithm improves convergence rate and decreases residual error by redividing the region and defining new error function. In order to improve its practicability, data reusing technique is brought in to reduce the number of symbols. In order to remit the performance deterioration of symbol rate equalization in the multipath deep-fading channels, a blind equalization algorithm with good convergence is proposed. It combines fractionally spaced structure and predictive decision feedback equalization, and updates feedforward and feedback filter coefficients respectively by dual mode multi-modulus algorithm based on determined error. Simulation results prove the feasibility of the new algorithm for high order QAM signals in the standard in multipath deep-fading channels.3. With regard to blind time synchronization for high order QAM signals in the standard, several classical time synchronization algorithms are analyzed and summarized. In order to avoid the bad performance of the classical algorithms in HF channels, timing error estimation algorithms based on cyclostational statistics are studied, and a joint algorithm for carrier and time synchronization parameters estimation based on high order cyclic cumulation in frequency-selective fading channels is proposed, which can realize joint estimation of carrier frequency offset, phase offset and timing error. Experimental results indicate that the algorithm proposed improves the estimation accuracy of timing error compared with the algorithm based on second-order cyclostational statistics and frequency offset compared with the algorithm which estimates the parameter by simply searching the cycle frequency, and has good performance on phase estimation for high order QAM signals in the standard in bad HF channels with low SNR.4. In regard to carrier synchronization for high order QAM signals in the standard, in consideration of the real environment of short wave communication, time varying frequency offset is taken into account, and a joint carrier synchronization parameters estimation algorithm based on quadratic phase function is proposed. The algorithm proposed estimates the Doppler frequency rate, frequency offset and phase offset at once and has a better estimation performance than several classical algorithms in the environment with time varying frequency offset. In respect of feedback algorithms, a modified multi-states adaptive extended Kalman filter carrier synchronization algorithm is proposed. Compared to the existing Kalman filter carrier synchronization algorithms, the algorithm proposed increases the convergence speed, expands the catching scope and improves the performance in the environment with time varying frequency offset by detailing the stages that the filter works and changing the filter parameter adaptively. At last, the Kalman synchronization algorithm is combined with equalization algorithm to realize joint blind demodulation in multipath fading channels and enhance the practicability.