Research On Channel Estimation Based On Pilot In OFDM System

Orthogonal frequency multiplexing technique(OFDM)has been widely used in modern wireless communication area due to the high spectrum efficiency,simplicity to implementation,etc.Many wireless communication standards including digital audio and video broadcasting(DAB,DVB), wireless LAN standard IEEE802.11a and 802.16,have adopted OFDM as one of the main techniques. and this makes it a promising technique in the next generation communication system. Channel estimation plays a significant role in OFDM systems.Via channel estimation,we can carry out coherent detection which can help to maintain low BER performance even in low SNR environments. among which pilot symbol aided modulation(PSAM)is a practical channel estimation method in tracking the fast fading channel with low complexity and high accuracy.First,wireless channel is introduced with some primary characters,such as path loss,shadow fading,multi-path fading and Doppler frequency shift,etc. Especially,we expound the multipath fading and Doppler effect in detail. Then we give some general background information of OFDM algorithm,such as QPSK modulation,IFFT,cyclic prefix,D/A converter and so on.The principles of OFDM parameter selection are presented after analyzing its advantages and disadvantages.Secondly, the general situation of the development of the SNR estimation algorithms and the definition of SNR is introduced.After that, two classic SNR estimation algorithms ---Boumard's and Improvd Boumard's algorithms that applies to OFDM system are studied and simulated.Thirdly, Deeply studies various channel estimation algorithms based on pilot and their performance in OFDM system, LS,MMSE,LMMSE algorithm and Linear interpolation,Gaussian interpolation,Lagrange interpolation,Cubic interpolation,insert in transform domain algorithm based on pilot are researched in the thesis.After studying in-depth of channel estimation method based on decision direct of training sequence and interpolation of pilot,a linear interpolation with compensation is proposed for OFDM system.then compared these algorithms through simulation.Finallly, Adaptive modulation is an efficiency methods to provide a high spectral efficiency, combined with channel coding and multi-level coded modulation,it can maintain an acceptable transmission quality and make a good advantage of the scares spectral resourse,which will play an important role in the future mobile communication system.On the base of the discussion of OFDM adaptive modulating technique ahead,the Matlab simulation model is constructed and the simulation of OFDM baseband system is carried. The results signified that by using adaptive modulation we can use a higher modulation by adaptive modulation technique when the channel condition is good ,and use lower modulation when the channel condition is bad. So, The overall system frequency utilization efficiency can be improved greatly,and the system capacity can be doubled.