Study And FPGA Implementation Of Demodulation Algorithm For Continuous Phase Modulation Systems

Continuous Phase Modulation (CPM) is a bandwidth and energy efficient digitalmodulation scheme due to its continuous phase and constant envelope, and it has beenwidely used in wireless communications and military communication systems. In thispaper, the principles of CPM modulation and demodulation algorithms were stuied, andthe performance of coherent and non-coherent algorithms were compared. Moreover,we have designed a non-coherent demodulator based on FPGA. The main contributionsof this thesis are listed as follows.1. The modulation theory of CPM and the representation of tilt-phase based trelliswere investigated. Based on Maximum Likelihood method, this paper studys threecoherent demodulation algorithms, including Viterbi coherent, reduced state coherentand Laurent decomposition based CPM coherent algorithm. Combined with optimumnon-coherent detection theory, we have studied two non-coherent demodulationalgorithms, including Viterbi non-coherent and reduced state non-coherent algorithms.We have simulated the BER performace and compared the complexity of differentdemodulation algorithms.2. We have also investigated the influence of coherent and non-coherentdemodulation algorithm when introduce frequency offset. Simulation results show thatcoherent algorithm is sensitive to frequency offset, while non-coherent algorithms aretolerant to specific frequency offset. Combined non-coherent demodulation algorithmand frequency estimation technique, we can constrcut a demodulator which canwithstand large frequency offset.3. Based on FPGA and soft output non-coherent CPM demodulation algorithm, wehave designed and implemented a CPM demodulator in serial pipeline mode. Thesynthesis results indicate that the CPM demodulator can work at85MHz, while thehardware overhead is only a little.4. Additionally, we have proposed a modified slide window non-coherentdemodulation algorithm. The proposed algorithm divides a long frame into many shortwindows, and the demodulation algorithm is based on the small windows. It can reducethe latency of demodulation output and save the memory size.