Research Of Pilot-Aided Channel Estimation In OFDM System

Orthogonal Frequency Division Multiplexing (OFDM) technology is the core technology fourth generation mobile communication systems, which has a high spectral efficiency, strong resistance to multipath fading and narrowband interference. However, broadband wireless channel multipath effects can cause frequency selective, time-varying characteristics of the channel would bring inter-carrier interference, which can cause distortion of the signal through the channel, then lead to signal amplitude deviation and phase shift. In order to recover the signal accurately, the demodulator need to obtain channel state information by channel estimation. In this paper, OFDM channel estimation technique has been mainly researched, the main work done as follows:Firstly, compressed sensing channel estimation has taken the sparsity of wireless channel into consideration, which has greatly reduced the number of pilot or training sequence and improved the spectrum efficiency. The smoothed l0algorithm (SLO) has utilized the convex optimization to recover the signal. But the selection of step in this algorithm is an experienced value, so the estimated results are not accurate. In this paper, improved SLO algorithm has been proposed, it utilized the hyperbolic tangent function as the smooth function to approximate l0norm, then used quasi-Newton method to recover signal. In this improved algorithm, the selection of step could be adaptive, meanwhile the convergence rate and the accuracy has also been improved. Numerical simulations results illustrate the performance gains achieved by the improved method.Secondly, Doppler shift occurs when channel changes rapidly, then the channel impulse response can have a significant change in an OFDM symbol duration, the traditional channel estimation cannot get accurate results. Basis expansion model (BEM) is a common method for such fast varying channel estimation. In this paper, a method of making better use of discrete prolate spheroidal sequence (DPSS) is proposed, it has an optimized parameter in a mathematical relationship with Doppler shift, then using inverse reconstruction method to recover the BEM coefficients of the channel taps. Simulation results have confirmed the validity of this improvement and also proved an enhanced use of DPSS than other basis functions in high mobility environments.