Research On Joint Time-frequency Synchronization Algorithm For OFDM Systems Based On Preamble Sequences

Orthogonal frequency division multiplexing(OFDM)technology has been widely used in 4G and 5G due to its faster transmission rate while ensuring data transmission quality.However,OFDM systems are very sensitive to symbol timing offsets and carrier frequency offsets.During information transmission,synchronization errors might introduce inter symbol interference(ISI)and inter carrier interference(ICI),which leads to the degradation of OFDM system performance.Therefore,this thesis focuses on the synchronization techniques in OFDM systems.The main work is as follows:1)Firstly,the research background and significance of OFDM synchronization technology are analyzed,and the current research status of OFDM synchronization technology at home and abroad is summarized.Then,the system model of OFDM is provided,and the impact of symbol timing offset and carrier frequency offset on OFDM system are analyzed.Finally,the principles of classical synchronization algorithms based on preamble sequences are introduced in detail,and the advantages and disadvantages of each synchronization algorithm are analyzed through simulation.2)Aiming at the problems of poor timing synchronization performance under low SNR conditions and sensitivity to frequency offset of existing timing synchronization algorithms in OFDM systems,an improved timing synchronization algorithm is proposed.Based on the property that the cross correlation values of CAZAC sequences with different root values are close to zero,a preamble sequence with central symmetry characteristics is designed.Based on the structural characteristics of the preamble sequence,a timing metric function that is insensitive to frequency offset is designed using symmetric correlation instead of delay correlation.Finally,timing synchronization is achieved by capturing the peak value of the timing metric function.The results indicate that,compared with the existing algorithms,the proposed algorithm has a strong anti-frequency offset capability and better timing synchronization performance under low signal-to-noise ratio(SNR)conditions,and is suitable for communication systems with low-SNR and frequency offsets.3)A novel time-frequency joint synchronization algorithm is proposed,which ensures good timing synchronization performance while achieving accurate frequency offset estimation with low complexity.An improved preamble sequence structure is proposed by using the good correlation property of CAZAC sequence.According to the preamble sequence structure characteristics,a timing metric function is designed to realize timing synchronization.The received signal is corrected using the timing estimation value to find the starting position of the sequence at the receiver end.Further,based on the preamble sequence structure characteristics,a frequency offset estimation function is designed to perform the first fractional frequency offset estimation.To improve the estimation accuracy,the second fractional frequency offset estimation is realized by combining the cyclic prefix and cyclic suffix,Finally,the integer frequency offset is estimated by using the time-domain sliding correlation operation.Simulation results indicate that the proposed algorithm has good timing synchronization performance and frequency offset estimation performance under Gaussian channels.4)In order to further improve the synchronization performance of synchronization algorithms,a weighted time frequency synchronization algorithm based on Golomb polyphase sequence is proposed.Based on the ZC sequence in the CAZAC sequence,a two-stage preamble structure is designed,and further use the weighted sequence constructed by the Golomb polyphase sequence to weight it.According to the preamble structure characteristics,a timing metric function is designed to achieve timing synchronization.In order to improve the performance of frequency offset estimation,based on the first fractional frequency offset estimation,a frequency offset estimation algorithm weighted by cyclic prefix and cyclic suffix is combined to complete the fractional frequency offset estimation.The proposed algorithm has been verified through simulation to have good timing synchronization performance and frequency offset estimation performance in both Gaussian and multipath channels.