On Frequency Offset Estimation Techniques In TD-SCDMA System

In the wireless communication system, the frequency offset always exists between carrier frequency and local reference, caused by the frequency difference between transmitter and receiver and impact of the Doppler shift. Therefore, to ensure the reliable data transmission, the frequency between transmitter and receiver must be synchronized. This paper mainly focuses on the frequency synchronization techniques. Based on the requirements of TD-SCDMA system, the frequency synchronization techniques for TD-SCDMA system are discussed.Firstly, the fundamental knowledge about frequency offset, the source of it, the impact on the signal caused by frequency offset and the importance of frequency offset estimation and compensation are introduced. Based on which, the fundamental principle of the frequency synchronization techniques and the criterion of their performance evaluation are generally discussed. Then, the impact on the practical communication systems caused by the precision of frequency offset estimation and compensation is also discussed, considering the criterion of signal quality evaluation in TD-SCDMA system.Secondly, the frequency offset estimation algorithms in AWGN channel are analyzed and compared. The results of analysis and comparing among the Fitz algorithm, LRE algorithm and the most likelihood estimation algorithm show that the algorithms based on the most likelihood method have the advantages of low computational load, fast computing speed and good performance in low SNR condition. However, they also have constraint on estimation range caused by the accumulation of the signal phase. Moreover, there exists mutual effect between accuracy and estimation range. The increase of estimation accuracy would cause the decline of the estimation range, vice versa. To promote the performance of frequency offset estimation, this paper proposes a phase difference based frequency offset estimation algorithm. The research indicates that, by effectively utilizing two groups of signal samples, the phase difference algorithm can largely promote the performance of frequency estimation in the high SNR condition. In addition, the phase difference algorithm is implemented in the vector signal analysis of practical TD-SCDMA system, which indicates that it can satisfy the requirements of practical communication system.Afterwards, the frequency offset estimation algorithms in fading channel are discussed. Different from AWGN channel, the fading process brings the multiplicative interference to the signal. The algorithms in AWGN channel are no longer suitable for fading channel. The analysis on KF, MKF, uMMV and NLS algorithm shows that all algorithm need to reduce the interference of the fading channel by using the channel information, expect NLS algorithm. However, although the NLS algorithm has a good performance, it does not have a close form solution, which would require large computation load. Based on which, we apply the phase difference algorithm to the fading channel. The research indicates that good performance can also be obtained via phase difference algorithm. Additionally, a fading reference compensation based algorithm, which can reduce the interference caused by the channel, is presented. The simulation shows that it can promote the performance of frequency estimation if the parameters are properly chosen.