Research On Carrier Frequency Offset Estimation And Directional Interference Suppression In Wireless MIMO System

With the development of wireless communication technologies and theemergence of all kinds of new services,the demands for improving capacity arebooming.It is an impotant task to achieve higher system capacity with limited spectrumresources in wireless communication.MIMO (Multiple Input Multiple Output)communication system,using multiple transmit and receive antennas,is considered asan effective system to mitigate fading effect and to improve system capacity.Sychronization is necessary in MIMO system.To ensure better carriersynchronization,the different channels in tramsimter/receiver are suggested to use thesame carrier generated from one ossilator.However,the carriers in transmitter andreceiver are not synchronous.In the presence of a carrier frequency offset,theperformance of MIMO system may dramatically degrade.So accurate frequency offsetestimation is important for MIMO system to achieve good performance.In realenvironments,the MIMO system will be faced with interferences.Especially in militaryapplication,the MIMO system may receive strong interference coming from certaindirection.In this case,the MIMO system will be blocked by the strong interference,andsuppression of directional interference is necessary to ensure MIMO communication.The elemental topics on system capatity and space-time coding in MIMO systemare firstly described in this dissert.Then,the main topics on carrier frequency offsetestimation and directional interference suppression in wireless MIMO system arestudied in in this dissert.Frequency offset estimation in MIMO system is analysed,andmethod to extend the operating range of frequency offset estimation in MIMO system isstudied.Frequency offset estimation in time-varing fading channels is also investigated.Suppression of directional interference is studied.And the frequency offset estimationin MIMO flat-fading channels with directional interference is studied.Finally,the studyon MIMO system simulation is given in this dissert.Main contents of this dissert are as follows:1) Introduce to MIMO system.The elemental topics on system capatity andspace-time coding in MIMO system are described. 2) Research on carrier frequency offset estimation for MIMO system.Maximumlikelihood frequency offset estimation and sub-optimal closed-form estimation arestudied.The computational complexity of maximum likelihood frequency offsetestimation is very high.Closed-form estimation is computational efficient,but theoperating range for frequency offset is limited.In the case of periodical orthogonaltraining sequence,recursive frequency offset is derived in detail based on sometheoretical analysis.With the recursive frequency offset estimation,the operating rangecan be extended.In the case of arbitrary training sequence,a corrected recursivefrequency offset estimation is presented.With the corrected recursive frequency offsetestimation,the operating range can be extended further.3) Research on carrier frequency offset estimation for MIMO system intime-varying channels.In time-varying channels,the carrier frequency offset isestimated jointly with the statistical characteristic of channel.Utilizing the property thatthe statistical correlation of complex-valued channel is real-valued,a modifiedfrequency offset estimation is presented,which can improve the MSE performance intime-varying channels.4) Research on the suppression of directional interference in MIMO system.Thebackground of MIMO system with directional interference is firstly analyzed.Then,technique on the suppression of directional interference in antenna array is analyzed.Arecursive interference suppression algorithm with varied matrix dimension based onbeam space decomposition is presented.Compared with the direct beam spacedecomposition algorithm,the computational complexity can be decreased from O(M~2·L)to O(M·L~3/2) in the case that the number of interferences is far less than that ofantennas.Maximum likelihood interference suppression for MIMO system is studied,and a simplified maximum likelihood interference suppression for MIMO system isgiven.Finally directional interference suppression for MIMO system based onbeam-forming is researched.And simulation result shows that it is valid to suppress thedirectional interference in MIMO system.5) Research on the frequency offset estimation for MIMO system with directionalinterference.The maximum likelihood frequency offset estimation for MIMO system inthe presence of directional interference is derived in detail.Due to the appearance ofcovariance matrix of directional interference,it is hard to find sub-optimal closed-form frequency offset estimation.As a sub-optimal solution,an iterative frequency offsetestimation is presented.The directional interference is treated as spatially uncorrelatedin the initial estimation.The by the iterative estimation,the accuracy of frequency offsetestimation is improved gradually which thanks to the gradual improvement on theknowledge of covariance characteristic of directional interference.Finally thecompensation for frequency offset in the presence of directional interference is alsostudied.6) Research on the simulation of MIMO system with car ier frequency offsetestimation and directional interference suppression.The diagram of simulation platformfor MIMO system with carrier frequency offset estimation and directional interferencesuppression is given.Simulation of the transmission of a static picture is carried out onthe platform,and the BER performance is also carried out on the platform.Thefrequency offset estimation and directional interference suppression presented in thisdissert are verified through different simulations on the platform.