Research On Sychronization, Channel Estimation And Joint Algorithm For OFDM System

Due to the high spectrum efficiency,the robustness to multipath fading,the high transmission speed and the ability of flexible modulation on subcarriers in accordance with channel conditions,Orthogonal Frequency Division Multiplexing(OFDM) has found its wide applications in high speed data transmission wireless systems and plays a key role in the forth generation mobile communication systems.However,OFDM is much more vulnerable to errors of both synchronization and channel estimation,and how to obtain timing,carrier frequency offset and channel estimation efficiently is highly critical to OFDM systems.In recent years,many algorithms only estimate one parameter,either synchronization or channel.In order to effectively enhance the capability of OFDM systems,synchronization and channel should be estimated jointly. This thesis investigates a variety of issues involving synchronization,channel estimation and joint algorithm in OFDM system:Firstly,the effect of synchronization shift to OFDM system performance is investigated via Matlab simulations.ML estimation of time and frequency offset algorithm is investigated comprehensively,some improved algorithms are also presented,and simulation results illustrate the performance of algorithms.In addition, typical timing synchronization algorithms based on training sequence,that is, Schmidl&Cox,Minn,and Park,are detailed and compared.Three improved algorithms are proposed to avoid timing metric plateau in Schmidl&Cox's method, high value and slow change of the sidelobes in Minn's method,and sidelobes in Park's method respectively.The simulation results show that each of them has an impulse-shaped timing metric,which offers a more accurate timing offset estimation. Secondly,decision feedback channel estimation algorithm is detailed and analyzed.A combination of time domain filter and decision feedback channel estimator is proposed.Considering the relevance of adjacent channel,weighted factor is introduced.The simulation of this improved algorithm shows that the error rate of system can be further reduced and about 2-4dB improvement can be obtained.Thirdly,a novel joint carrier frequency synchronization and channel estimation algorithm for OFDM using block-type pilot is proposed.Each pilot symbol is composed of repeated short pseudo-noise sequences.By using this kind of pilot,the estimation of carrier frequency offset is performed into two steps,an acquisition step and a tracking step.During the acquisition step,the estimation range and precision can be adjusted.Making full use of the relation between carrier frequency and channel coefficient,the tracking step is realized based on the decision feedback algorithm. Furthermore,an improved scheme is presented to reduce the noise and achieve a more accurate estimation,which regards L_f OFDM symbols as a group.Simulation results show that the proposed algorithms have good performance in both AWGN and multipath fading channel.And the improved joint algorithm has significantly better performance than joint algorithm in slow time-varying channel.Since the fast fading channel has been transmitted into several slow fading channels,the proposed algorithms overcome the disadvantage that block-type pilot can be only applied to the slow fading channel,and restrain the error accumulation propagation effectively.Finally,a novel iterative algorithm for joint carrier frequency offset and channel estimation is proposed.This algorithm focuses on the carrier frequency offset estimation problem where the initial carrier frequency offset is assumed to be less than one half of the subcarrier spacing.Once the assumption holds,it can estimate carrier frequency offset and channel coefficient iteratively and alternately in the frequency domain.The algorithm uses only part of the pilot to obtain the initial channel estimation.It requires neither pilot in iterative processing nor observation over a number of OFDM symbols.It is suitable for tracking a time-varying carrier frequency offset.The performance of the algorithm is well demonstrated by simulations.