Channel Estimation Based On OFDM System For Complex Multipath Environment In Tunnels

OFDM, serving as the core technology of the next generation mobile communication LTE, has undergone much leaping development and is favored by more and more people. At the same time its application has been expanded into many fields widely. The propagation characteristics of a radio wave in tunnel is much different from that of the traditional mobile communication, the reason is that tunnel is a special communication environment. The waves reflected by the wall of tunnel through propagation can cause multi-path effects at the receiving terminal. The effects have a bad influence on the demodulation of the received signal, and restrict the application of wireless communication system in tunnels. OFDM has advantages of resisting to multi-path effects effectively in tunnels and improving the quality of communication, so it becomes the first choice for wireless communication in tunnels. The technology of channel estimation, as one of the key technologies in OFDM, plays a very important role in OFDM communication system. By the OFDM channel estimation technology, the information of wireless channel can be acquired by the receiving terminal, and compensated further by an equalizer. Furthermore the technology can enhance the ability of resistance to fading caused by wireless channels, and improve the reliability of a communication system.This article will focus on the channel estimation for complex multipath environment in tunnels, and will carry out the following works:1) Firstly, the propagation characteristics including the transmission of radio waves, loss of radio waves and small scale loss will be researched. Based on theories of free-space propagation, the characteristics of radio propagation in tunnel environment will be deeply studied. After the power loss expression of radio propagation in tunnels is presented, the deterministic and statistical wireless channel models under tunnel environment will be established and simulated for the purpose of researching channel estimation based on OFDM system in tunnels.2) Before doing researches into channel estimations in tunnels, the basic theory and key implementation technology of OFDM technology is carried out as the foundation of exploring channel estimation algorithms for OFDM systems. Based on the baseband model of OFDM system and the wireless channel models set up for straight tunnels and curved tunnels, further study and simulations obtained by Matlab for the estimation performance of traditional OFDM channel estimation algorithms including of LS, MMSE, SVD and LS based wavelet de-noising will be done, and so as the channel interpolation algorithms like constant interpolator, linear interpolator, quadratic polynomial interpolator, spline interpolator and interpolator based on DFT algorithms for straight tunnels.3) With the deep research on the wireless channel estimation algorithm based on DFT and on the basic of the simulation and analysis for two zeros-padding methods-zeros padding in tails and de-noising in time domain, the improved channel estimator based on the traditional DFT estimator will be carried out. The improved estimator can lessen the noise attached to the channel impulses through twice de-noising process, the de-noising process works with two thresholds set by noise and signals. Compared with the traditional one, the new way can effectively weaken LS estimator’s disadvantage which is being fragile to noise, and the simulation result shows that, the improved estimator can acquire about 3 to 6dB extra SNR gains.