Communication Systems, Doppler Frequency Shift Estimation Study

There is a serious decease in the performance because of the doppler frequency shift,caused by the relative motion between the transmitter and receiver,under the wireless communication environments,especially with the gradually increase of the applied carrier frequency.Therefore,it is necessary to estimate the doppler frequency shift,and eliminate it from the received sinals.The conventional phase lock loop has a good frequency estimation resolution,however,it has the limited acquisition range,when the residual doppler shift in the received signal exceed the acquisition range of the closed loop,it will be lead to too large acquisition time to applied to the practical application. The frequency offset estimation algorithm of non-closed loop,adopting the modern signal estimation theoretics,has large acquisition range and can rapidly track the change of frequency offset.Some of these typical estimation algorithm will be studied and simulated in this paper.In this paper,firstly,the influence of the Doppler shift on the spread-spectrum communication system and the orthogonal frequency division multiplexing technology have been studied.Then,some of the basic theory of the frequency estimation have been introduced to this paper,and a frequency offset estimation method of the residual doppler shift in the received QPSK signal has been researched.Finally,several estimation algorithm based on the different estimation rule have been researched,include the two-dimensional search algorithm,a approximate maximum likelihood esimation algorithm,the weighted phase average estimator,the data-aided frequency offset estimation algorithm.Furthermore,the phase estimator based on the stochastic nonlinear filter which has been applied to improved the performance of the weighted phase average estimator. In the mean time,computer simulation has applied to the various estimation algorithms.