Research On Passive Demodulation Of OFDM Signal

OFDM?Orthogonal Frequency Division Multiplexing?technology divides the channel into multiple sub-channels,and the different sub-channels are orthogonal to each other.So that the high-speed data stream is converted to a lower-speed data stream for transmission,which has an efficient spectrum utilization rate.OFDM technology has been widely used in civil communications and military strategies,and the research on passive demodulation of OFDM signals has practical significance.This paper proposes a complete set of passive OFDM signal demodulation process.This process includes OFDM signal related parameter estimation and time-frequency synchronization to achieve passive demodulation of OFDM signal.The main tasks are as follows:1.Firstly,the basic principle of OFDM signal is introduced,the cyclic prefix structure is analyzed,and the basic flow of the OFDM signal transmitting end is given.Combined with the characteristics of non-cooperative communication,a general flow of OFDM passive demodulation is proposed;2.The relevant parameters of the OFDM signal are estimated,including the OFDM signal carrier frequency,effective symbol length,total symbol length,and total number of subcarriers.The correlation of the cyclic prefix is used to estimate the effective symbol length.The cyclic stationary characteristics of the OFDM signal are analyzed,and the total symbol length is estimated by using the cyclic autocorrelation function.Simulation analysis of parameter estimation in the presence of sampling deviation shows that the estimation algorithm has good performance;3.The time-frequency synchronization of OFDM signals is studied,including symbol timing synchronization,carrier frequency synchronization and sampling rate synchronization.Using the phase rotation caused by the symbol timing deviation,a fine synchronization algorithm for OFDM signal symbols is proposed.Simulation results show that the algorithm can effectively estimate the symbol timing position.The carrier frequency deviation is estimated using a maximum likelihood synchronization algorithm,and the remaining carrier frequency deviation estimated to be corrected is phase-tracked.Aiming at the sampling rate synchronization,a method of simultaneously completing the sampling rate synchronization and downsampling is proposed by using the relationship between the number of sampling points of the receiving end and the transmitting end within the same number of symbols.Simulation results show that the method can effectively complete the sampling rate synchronization;4.Finally,the performance simulation analysis of the overall passive demodulation process of the OFDM signal is given.The simulation results show that when the sampling deviation is6.25?10^-4 and the signal-to-noise ratio is greater than 12dB,the demodulation bit error rate is lower than10^-3 and the system demodulation performance is good.