SNR Estimation And Modulation Recognition Of OFDM Signals In Complex Environment

Orthogonal frequency division multiplexing(OFDM)is used to distribute a single data stream to subchannels where each subchannel's spectrum is overlapped.Having high data transmission rate and high spectrum efficiency,OFDM is commonly used in modern wireless communication systems.OFDM is also a commonly used digital multi-carrier modulation scheme.The modulation type of each subcarrier signal in the OFDM system can be used to provide important prior information for the subsequent parameter estimation and demodulation process.The underlay spectrum sharing technology improves the spectrum utilization with the primary user and secondary user sharing the spectrum resources,and can achieve higher spectral efficiency combined with OFDM technology.In underlay mode,the new secondary user needs to know the signal-to-noise ratio(SNR)to determine the transmission strategy before accessing the licensed band.Therefore,this paper mainly studies the SNR estimation of time-frequency overlapped OFDM signals in underlay cognitive networks and the subcarrier identification of OFDM signals with fractional frequency offset in multipath channel.The main contents of this paper are as follows:1.In this paper,a signal-to-noise ratio estimation method based on the received signal's cyclic autocorrelation function is proposed.In this method,the cyclic autocorrelation function of the received signal is firstly calculated.And the total power of the component OFDM signals is estimated by the discrete spectral line of the received signal's cyclic autocorrelation function whose time delay less than the slice time;Then the total power of the received signal is estimated by the value of it's cyclic autocorrelation function whose time delay and cyclic frequency are zero;Finally,based on the total power of the component signals and the total power of the received signal,the signal-to-noise ratio of the received signal is estimated.Because of there is no research on the SNR estimation of time-frequency overlapping OFDM signals in the existing domestic and foreign literatures,the Cramer-Rao bound(CRB)of SNR estimation of time-frequency overlapping OFDM signals is deduced as an evaluation index for the proposed method in this paper.2.In this paper,a subcarrier modulation identification method for OFDM signal with carrier frequency offset in multipath channel is proposed,which is based on subcarrier's statistics.This paper firstly obtains the subcarrier signal by stripping the cyclic prefix and taking fast Fourier transform of the received OFDM signal with carrier frequency in multipath channel.Then it calculates the second-order moment of the subcarrier with carrier frequency offset to preferentially identify the virtual subcarrier and pilot subcarrier signal.Finally,the data subcarrier is recognized by calculating the fourth-order cumulant and the combination of the fourth and sixth order cumulants of the subcarrier with carrier frequency offset.Thus,the modulation recognization of OFDM subcarrier with carrier frequency offset in multipath channel is realized.3.Some experiments are conducted to verify the proposed subcarrier recognition method of OFDM signal and blind SNR estimation method of time-frequency overlapped OFDM signal under certain simulation conditions.The simulation results show that the proposed blind SNR estimation method can effectively estimate the SNR of time-frequency overlapped OFDM signals and it's estimated performance is close to the CRB with the increase of SNR.The SNR estimation method in this paper is robust to the spectrum overlapped ratio,the power ratio and the oversampling rate.The subcarrier identification method in this paper can effectively identify the subcarrier signals of OFDM under complex conditions.Compared with the traditional method,it has lower computational complexity and better recognition performance at low SNR.