Research on the ICI Elimination Technology Based on TD-LTE Standard under High Speed Mobile Scenario
|School||Beijing Jiaotong University|
|Course||Communication and Information System|
|Keywords||OFDM high-speed scene ICI doppler shift pilot pattern cycleiteration|
ABSTRACT:As we all know, OFDM is a multi-carrier system which simply and effectively eliminates the influence of the ISI (Inter-Symbol Interference) and multipath fading. Moreover, its spectral efficiency is high, because of the orthogonality between subcarriers. However, the OFDM system will have serious Doppler shift and spectral expansion, caused by leakage of Carrier spectrum when the speed is up to350km/h and the channel varys rapidly. Since OFDM is extremly sensitive to the frequency deviation, the OFDM system suffers from serious Inter-carrier interference when the subcarriers are no longer orthogonal. Thus, whether implement ICI’s cancellation can directly decrease the performance of the OFDM system, and then affect the performance of the entire TD-LTE at high speed scene.In this paper, the ICI elimination is based on OFDM system in the high-speed mobile environment. The main content includes classic ICI cancellation algorithm, pilot studies, iteration algorithm design and so on. Specifically, the paper first introduces the key technologies of TD-LTE system. By summing up various techniques, it is known that, compared to other technologies, performance of OFDM system is more likely to be affected in the fast-moving scenes. In addition, the OFDM system has very bad level of ICI, so its orthogonality becomes very fragile. Then, we make a summary on the reasons of the ICI, carrying out a detailed analysis of the mathematical expression of system model, and directly give three reasons of the ICI. After that, a research on previous ICI cancellation algorithms is made, including self-cancellation algorithm, time-domain windowing algorithm, and frequency domain equalization algorithm, together with the simulation of those algorithms. Through theoretical analysis of the pilot density in the fast-moving scenes, we reveal the simulation of quick-changing channel under the fast-moving situation, and make a detail research on the channel estimation and sampling points in a sub frame at high speed scenes. Combining simulation and theory, I give my own pilot pattern design. Then I make a comparison between the standard pilot pattern and new pilot pattern under different Doppler shift. Simulations show that the new pilot pattern in the elimination of error platform has a good effect, which can effectively reduce the error platform, when SNR is relatively high. With the channel Doppler spread increased, the performance advantages of the new pilot pattern reflect more clearly, that is the system achieve significant gain even in low SNR. Finally, based on the research above and analysis of linear time-varying channel, we innovatively propose an iterative loop ICI cancellation algorithm that can be used in fast-moving scenes. And a detailed mathematical derivation for the channel model and algorithm implementation process is showed. Then we implement the simulation under different conditions for the new algorithm, proving that:regardless of a high speed scene or a low speed scenario, the new proposed ICI cancellation algorithm can effectively eliminate the interference and improve system performance, especially in the fast-moving scenes.