Research And Implementation Of Signal Demodulation And Test Algorithm In OFDM System

Orthogonal frequency division multiplexing(OFDM)technology has been widely applied for its advantages of nearly 50% band utilization,strong anti-jamming capability,modulating flexibility and high efficiency transmission.The thesis intends to design and implement a fast demodulation module for OFDM signals based on the software defined radio architecture and digital demodulation logic algorithm.Furthermore,the thesis intends to design evaluation algorithms for demodulation index of OFDM signal,and completes the evaluation of relevant technical indicators.Firstly,the thesis simulates and optimizes the key algorithm by studying the demodulation algorithms of OFDM signal and analyzing the difficulty of hardware implementation.In the packet detection algorithm,delay correlation and length are combined to solve the problem of dynamic judgement threshold in dynamic packet detection.In the carrier synchronization algorithm,the carrier frequency offset can be calculated directly without the need to convert the data to the frequency domain by using the correlation between the adjacent short training sequences.In the symbol synchronization algorithm,the end point of the short training sequence is confirmed by the correlation between the local short training sequence and the received one.In the channel estimation and equalization algorithm,the frequency response of the entire channel is obtained by using the known received long training sequence.In the phase tracking algorithm,the phase rotation factor and the amplitude adjustment factor are obtained by comparing the known pilot and the received pilot.Secondly,in the hardware demodulation module,the above algorithm is implemented by logic design.After demodulating the OFDM symbols,the symbols are then transmitted to MATLAB software for parameter testing.The validity of the demodulation module is proved by comparing the demodulation output of the hardware demodulation module with the MATLAB simulation output.Finally,for the OFDM modulation signal demodulation index evaluation,the complementary cumulative distribution function(CCDF),the cumulative distribution function(CDF),error vector magnitude(EVM),error vector time(EVT)and other parameter measurement algorithms are designed by the use of MATLAB.The demodulated data can be transferred to a computer evaluated directly by MATLAB software.In the research process,a MATLAB+FPGA-based solution was adopted.Firstly,MATLAB software was used for system simulation and completing the digital design of OFDM modulation signal and demodulation algorithm.Then the optimized simulation algorithm was optimized by logic and the demodulation module was completed by designing and programming the hardware logic code in the FPGA.Finally,the demodulation output of the demodulation module is transmitted back to the MATLAB software,and the signal is evaluated by the MATLAB software.The thesis intends to complete the demodulation test of the OFDM signal on the hardware demodulation module,and verify the correctness of the thesis' s design scheme.