Paths complex gain tracking algorithms for OFDM receiver in slowly-varying channels

This paper deals with channel estimation for Orthogonal Frequency Division Multiplexing (OFDM) systems over time-varying fading channels. In conventional methods, the least-squares (LS) estimate is obtained over the pilot subcarriers, and next interpolated over the entire frequency grid. Those methods only exploit the frequency-domain correlation of the channel. In this paper, we propose to exploit both the time-domain correlation and the specific features of the wireless radio channel. Assuming the availability of delay related information, we propose to track the variation of the paths complex amplitudes by means of on-line recursive algorithms. We developed two simple sub-optimal algorithms based on second-order loops which exhibit a reduced complexity compared to that of the widely popular Kalman algorithm. The error signal is based on the LS estimate of the path complex gains for the first loop, and on the steepest-descent method of the same LS cost function for the second loop. For each algorithm, we give derivations to correctly tune the loop coefficients. Simulation results over slow Rayleigh fading channel with Jakes´ spectrum show that our algorithms outperform the conventional methods. Moreover, the Mean Square Error (MSE) of the first algorithm is closer to the Bayesian Cramer Rao Bound than that of a Kalman filter based on a first-order AutoRegressive approximation of the channel.