The Anti-Doppler Technology Of LTE Downlink System In High-Speed Railway Scenario

Compared with other transportation modes, high-speed railway has been developing rapidly in recent years for its obvious advantages. However, increasing the speed puts forward a higher requirement to railway mobile communication system. Consequently, researching suitable communication technologies has become an important issue. When applying LTE downlink to high-speed railway environment, the doppler frequency shift and doppler spread generated by the high moving speed may cause severe ICI to OFDM symbols and deteriorate system performance. The thesis focuses on the anti-doppler technology of LTE downlink system in high-speed railway scenario.First of all, the key technologies of LTE and the downlink physical layer protocol related to anti-doppler algorithms are expatiated, and then LTE downlink system simulation platform is built by MATLAB software.On the basis of researches on wireless propagation characteristics, the paper lays stress on the characteristics and the modeling methods of high-speed railway channel next. The simulation model is built by MATLAB and the results show that the statistical properties of the model can fit the theoretical characteristics very well.Due to the carrier frequency offset of the received signal caused by the doppler effect, the receiver must perform carrier frequency synchronization to restore the frequency. In high-speed railway scenario, since the channel presents rician fading and doppler frequency shift of the LOS path can be considered constant within 1ms, if frequency synchronization can be completed in a sub-frame, it is possible to track the variation of the LOS path doppler frequency. CP is utilized to capture doppler frequency shift coarsely first, and then pilots are used to estimate further. The mean square error curves of frequency estimation show that the pilot based method can achieve higher estimation accuracy.On the other hand, in order to eliminate doppler spread of the received signal, fast fading time-varying channel estimation methods which contain two steps are adopted. In the first step, pilots inserted in LTE downlink signals are used to estimate channel impulse response at the center time point of the pilots containing OFDM symbol duration. There are LS and MMSE algorithms and their simplification. Simulation results indicate that the performances of simplified algorithms are the same in a single path channel. In the second step, for the purpose of tracking the varying channel, basis expansion models are used to interpolate channel impulse response for every time point, converting estimation of the time varying channel response into the constant coefficient of the model. Traditional models are CE-BEM model and its modification. However, when the two models are applied under the condition that the rician channel LOS path has a large doppler frequency, because carrier frequency recovery previously will change spectrum characteristics of the channel to be estimated, modeling may be inaccurate. Another further modified model is proposed, so that modeling is consistent with the channel characteristics after frequency recovery and the performance is improved. As long as the channel response matrix being obtained, zero forcing or MMSE equalization is used to restore the transmitted signal. Finally, MATLAB simulation results of channel estimation performances using different BEMs and bit error rate results after equalization are given and analyzed.