Research On Timing And Frequency Synchronization Algorithm In OFDM System

OFDM(orthogonal frequency division multiplexing) splits a high rate datastream into a number of low rate streams which are transmitted simultaneously over a series of subcarriers. It has become a key technology for 4G systems for its high spectrum efficiency and robust capability of resisting multipath interference.But OFDM system is very sensitive to the timing and frequency offset because of its high requirement for the subcarriers' .orthogonal feature. The main task of this thesis is the timing and frequency offset estimation for OFDM system.First of all, this paper depicts the main principle of OFDM system, then describes the effect of the timing offset and frequency offset to the system in detail, and then analyses two classic frequency and timing synchronization algorithms: the ML(maximum likelihood) algorithm based on the CP and the Schmidl's algorithm based on the PN training sequence.At last, the paper proposes three improving algorithms based on the former algorithms. Fist is the improving ML algorithm which uses the ML algorithm first to make a coarse timing estimation and then employs the mass pilot symbols to make a robust evaluation. After observing the results by mathlab, it is obviously that the proposed algorithm improved the timing offset estimation efficiently on the condition of the multi-path channel by making the precision of the estimation arrives at the 10-3 rate. The second algorithm is based on the Schmidl's algorithm. By designing a pseudo PN sequence which has a random phase, we get an estimation curve which has the feature of impulse response and eliminate the plateau of the Schmidl's algorithm, we also get a smaller variance error when using the same pseudo PN sequence to make a fraction frequency estimation. The third improving algorithm is also based on the Schmidl's algorithm. This algorithm uses a short conjugate symmetry repeated PN sequence. First we make use of the conjugate symmetry feature to make the timing offset estimation , and after compensating the timing offset we make the frequency offset estimation by employing the repeated feature, in order to improving the precision, we employ the iterative algorithm. After simulation , the algorithm shows that it also eliminates the timing plateau, and the frequency estimation result is improving with the times of iterative increasing, the estimation precision can reach 10-4 rate when the SNR comes to 20dB.