Research On Channel Estimation Of OFDM System In High-speed Railway Environment

As the key technology of LTE, OFDM technology can overcome the channel frequency-selective fading through the reduction and elimination of Inter-Symbol Interference(ISI), and can improve the spectrum efficiency at the same time. OFDM technology not only can satisfy the higher demand for railway communications, but also lay a foundation for the future application of the LTE system in the high-speed railway. Compared with the cable channels, the environment of wireless channel is more complicated, and its channel characteristics continue to change over time. Especially when the speed of high-speed railway constantly is improved, the changes of channel characteristics will be much more intense, the velocity between the transmitting and receiving end is relatively faster, thus creating a larger Doppler shift, so the orthogonality between the subcarriers of OFDM system is destroyed and serious Inter-Carrier Interference(ICI)is produced. Serious Inter-Carrier Interference(ICI)significantly reduces the performance of OFDM system. Therefore, in the circumstance of the train moving at a high speed, how to improve the channel estimation accuracy so that the signal of receiving end can be detected correctly is the focus of current research.Aimed at the OFDM system in high-speed railway environment, channel estimation techniques based on comb pilot were studied in this paper, focusing on the two commonest time-varying models, including Linear Time Varying(LTV)model and Basis Expansion Model(BEM).Firstly, the complex wireless channel characteristics are analyzed, and the Doppler frequency shift effect and Multi-path effect are introduced, as well as the associated fading and expansion characteristics were also analyzed in detail, and a suitable channel model simulation environment for this study was given. It introduce the theoretical basis, the advantages, disadvantages of OFDM systems and study the generation of the Inter-Carrier Interference(ICI)under the fast time-varying environment.Secondly, the traditional channel estimation algorithm was studied, including the LS,LMMSE algorithm and its derivative simplifies algorithm. The Linear Time Varying(LTV)model was studied aimed at Inter-Carrier Interference(ICI), and the dual ICI cancellation algorithm based on the model was introduced, which was combined with the channel estimation algorithm. The results of simulation showed that the improved algorithm in this paper had smaller estimation error and better performance. Besides, compared the channel estimation error after combining with single ICI cancellation and dual ICI cancellation.The results showed that the errors were all further reduced after combining with ICI cancellation technology, while the errors reduced at a greater extent after combining with dual ICI cancellation technology.Finally, the Base Expansion Model( BEM) and the BEM based on different basisfunction were studied, including CE-BEM, P-BEM, KL-BEM and DPS-BEM. Meanwhile, in order to effectively reduce the ICI impact at the pilot, comb pilot construction based on group pilot was adopted. Then on this basis, a channel estimation algorithm combined with Base Extension Model ICI cancellation is studied. In this algorithm, firstly the interference term between the subcarriers was obtained using BEM channel estimation, and the ICI was eliminated to get feedback information,then the improved algorithm in this paper was used to make channel estimation. The simulation results showed that this algorithm can effectively reduce the channel estimation error.