Research On Receivers Of Continuous Phase Modulation Systems

Continuous phase modulation signal with its constant envelope property and higher band utilization,has become the mainstream in the field of remote sensing system,as the key part of long-distance monitoring equipment,telemetering receiver in launch vehicle,unmanned aircraft,satellites and other fields,still has broad prospects for development and application of its study has the very vital significance.Firstly,to solve the problem of error accumulation in the initial phase of the code element in direct digital frequency modulation(DDFM),a baseband modulation scheme in which the code element rate and the modulation index can vary continuously is proposed.The scheme separates the initial phase calculation of the code element from the phase modulation in the code element,eliminates the influence of operational error in the modulation process on the initial phase of the code element,makes the initial phase of each code element more accurate,and ensures the phase continuity of the modulated signal.Next to baseband many symbol detection technology problems such as high complexity,storage resource use,this paper proposes a multiple symbol detection technology,based on CORDIC algorithm in observation symbol for three symbols,each symbol under the condition of the eight sampling,storage of the first two symbols related to value,reducing the required number of complex multiplication each symbol decision to the original,the implementation scheme complexity is lower and less storage resources.The influence of the length of the observed symbol,the sampling rate and the accuracy of the synchronization algorithm on the multi-symbol detection technology is analyzed by MATLAB simulation.Again,in order to reduce the influence of signal amplitude change on synchronization accuracy and demodulation performance,a digital automatic gain control loop with dynamic range better than 45d B was designed.Finally,in order to further improve the demodulation performance of the multi-symbol detection technology,the methods of frequency synchronization and bit synchronization are studied in this paper.A simple and easy frequency synchronization loop is realized by averaging the frequency discrimination results as frequency error in symbol period.The bit synchronization loop was constructed by using Farrow interpolation filter,in-phase mid-phase timing error detector,random wandering filter and NCO.By reasonably setting the parameters of the random wandering filter,the bit synchronization loop still had good stability under low SNR,making the demodulation loss caused by bit synchronization negligible.For CPFSK signal modulation,multi-symbol detection demodulation,synchronization technology and other modules using Verilog HDL language FPGA correlation implementation.The test results show that the multi-symbol detection technology based on CORDIC algorithm has a sensitivity of about-93.5d B at bit error rate of1×10^-4and symbol rate of 4Mbps,which meets the design requirements.