| WLAN is an important branch of wireless communication.It has attracted more and more attention and applications due to its ease of use,convenient installation,networking flexible and easy expansion.With the explosive growth of mobile terminal devices,unstoppable trend of wirelesszation and the increasing use of WLAN appliacations,people have new expectations for breakthrough WLAN technologies.Therefore,IEEE802.11 ac has also emerged.The physical layer of IEEE802.11 ac uses MIMO-OFDM technology,which can achieve high-density modulation and very high throughout and ultra-high transmission rate.However,there are still many defects.MIMO-OFDM is sensitive to timing errors and carrier frequency deviations that will cause ISI and ICI,which seriously affects the reliability of data transmission and degrades the performance of communication system finally.Therefore,it needs to synchronize the commnunication system.This thesis focuses on IEEE802.11 ac,studies existing MIMO-OFDM synchronization techniques and then proposes new solutions.The specific work is as follows:Firstly,this thesis analyzes the development process of IEEE802.11 ac,explore the research status of synchronous technology at home and abroad,further analyzes the IEEE802.11 ac physical layer specification and key technology—MIMO-OFDM.Secondly,analyze the typical MIMO-OFDM timing synchronizaiton algorithm is analyzed.Combine with WLAN characteristics based on IEEE802.11 ac,and then reaseach the MIMO-OFDM timing synchronizaiton algorithm including frame synchronization and symbol timing synchronizaiton algorithm.The triangular peak generated in the previous frame synchronization algorithm is not sharp enough to affect the initial position of the data frame.Therefore,this thesis.This propose a new algorithm.This algorithm devides the L-STF field into 5 parts and carries out two-to-two conjugate correlation,and then it sums to obtain the time metric function.Finally,the starting position of data frame is accurately found.In addition,the problem of timing inaccuracy caused by clutter interference caused by previous symbol timing synchronization algorithms is solved.This thesis presents a new symbol timing algorithm.This algorithm makes full use of the strong correclation of L-LTF and uses the product of auto-correclation function and cross-correlation function as a new metric function.Then the time metric function is used to find out the starting position of OFMD symbols.Thirdly,this thesis research typical MIMO-OFDM frequency synchronization algorithm: pilot-based Moose and training sequence-based S&C algorithm.Based on this,this thesis propose new MIMO-OFDM frequency synchronization algorithms for IEEE802.11 ac.Use the antenna diversity technology and correlation window function of CRS algorithm to obtain the estimated value of the coarse frequency offset.Then compensate the offset to complete the coarse frequency synchronization.Continue to use the antenna diversity technology,L-LTF and CP to obtain the precision frequency offset,and then complete fine frequency synchronizaiton after compensation.The frequency synchronizaiton is complete after both steps are implemented the frequency.Finally,According to the synchronization process,test and verify the improved synchronization algorithm one by one in the AWGN and Rayleigh channels.Through the simulation and comparison,the improved timing synchronization algorithm makes the triangular peak generated by the time metric function sharper and the value more obvious;the improved algorithm has a higher detection probability and ultimately finds the starting position of the data frame and the OFDM symbol more accurately.The frequency offsets estimated by the improved coarse and fine frequency synchronization algorithms are 1 and 0.5 times the subcarrier spacing respectively.In addition,the improved algorithm's MSE value is obviously smaller than the traditional algorithm the estimation accuracy is higher and it is more adaptable to the MIMO-OFDM system. |
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