Study On Frequency Estimation Technique For Orthogonal Frequency Division Multiplexing

OFDM is an effective way to combat the negative effects of fading and multipath .The ideal of OFDM is to split the data stream to be transmitted into N parallel streams of reduced data rate and to transmit each of them on a separate sub-carrier. These carriers are made orthogonal by appropriately choosing the frequency spacing between them. Therefore, spectral overlapping among sub-carriers is allowed, giving a better spectral efficiency.One of the principal disadvantage of OFDM is sensitivity to frequency offset. The main problem with frequency offset is that it induces interference among the carriers, resulting in the decrease of carrier power and the increase of noise power. So frequency offset must be removed prior to demodulation.First ,this paper describes the OFDM principle and presents the modem realization based on FFT technique .Then, the effect on carrier- to- noise ratio with frequency offset is discussed. Next, the paper describes a maximum likelihood estimation (MLE) technique to estimate frequency offset using the guard interval, and give an analysis on this estimate, which shows that the estimate is unbiased and its variance depends both on the total power of samplyings and noise power .In order to validate the performance of estimation further, a computer analysis is done. The result of simulation indicates that the estimate is accurate even in case of multipath. Because of the limitation that the MLE algorithm is invalid if the frequency offset exceeds 1/2 carrier spacing, an initial acquisition making use of correlation algorithm is presented.Also, a system for the test of frequency estimation algorithm is established. The result indicates that the frequency estimation technique is feasibleAt last ,a conclusion is given.