Research On The Equalization Technique Of The Receiver In The Continuous Phase Modulation System

For continuous phase modulation(CPM),the envelope of its signal waveform is constant,which leads to a very low peak-to-average power ratio(PAPR),so the cheap nonlinear amplifier can be used at the receiving end.Besides,the phase of the signal waveform is continuous,which leads to more compact frequency spectrum compared to other modulation methods,so its frequency band utilization is relatively high.Because of these advantages of CPM signals,continuous phase modulation is widely used in military and commercial wireless communication systems.One of the parameters in the CPM signal is the memory length.Therefore,the CPM signal has memory,which leads to a very complicated equalization of the CPM signal.Furthermore,the orthogonal frequency division multiplexing technology that is common in linear system cannot be used in the transmitting terminal,because the continuous phase modulation is non-linear.Therefore,the study of low-complexity equalization techniques for CPM signals is one of the important topics in continuous phase modulation systems.The content of this paper mainly includes the following aspects:(1)we mainly study the CPM frequency domain equalization iterative detection algorithm under the new frame structure,and use soft output Viterbi algorithm to reduce the system complexity.In the tilted phase model,a new frame structure is constructed,that is,a training sequence is used to replace the cyclic prefix frame structure.At the same time,we also make a comparison about the similarities and differences between the training sequence and the cyclic prefix.The advantage of using the training sequence is to provide convenience for channel estimation and synchronization.Meanwhile,the Laurent decomposition of binary CPM signal is also introduced to reduce the complexity.In the iterative detection,a low-complexity soft-decision SOVA algorithm(Soft Output Viterbi Algorithm)is used.The results verify the performance of the algorithm.(2)we mainly study the improvement of the traditional double-iterative frequency domain equalization algorithm.Based on Laurent decomposition,the differential precoding technique is applied in the transmitting end.Because we use these two techniques,the memory of the CPM signal is eliminated.Therefore,the frequency domain equalizer and the SISO decoder can directly constitute an iterative systems.The pre-coding CPM iterative frequency domain equalization algorithm proposed in this chapter can achieve better performance with lower complexity.The system does not require complex CPM soft demodulators in iterative equalization,which reduces system complexity.Because the frequency domain equalizer and the SISO decoder can directly an iterative systems,the two directly transmit information.The problem of error information propagation in the traditional double-iterative frequency domain equalization algorithm is eliminated,and the performance is also improved.The author makes a detailed derivation of the algorithm and compares the complexity of the low-complexity pre-coding CPM iterative frequency domain equalization algorithm with the traditional double-iterative frequency domain equalization algorithm.(3)Unlike the previous part,the CPM signal used in this part does not have a constant modulation index.Besides the CPM signal has multiple modulation index CPM signals.we mainly studies the frequency domain equalization of the multiple modulation index CPM signal and extends the modulation order from binary to Quaternary.Two kinds of frequency domain equalization algorithms are used: minimum mean square error equalization algorithm and zero-forcing equalization algorithm.At the same time,we make an analysis of the influence of different modulation parameters of CPM signal on the equalization algorithm.Finally,two equalization algorithms are analyzed through simulation results.