Maximum Doppler Spread Estimation by Tracking the Delay-Subspace for LOFDM Systems in Doubly Dispersive Fading Channels

In this work, a novel maximum Doppler spread estimation algorithm is specifically designed for Lattice-OFDM (LOFDM) systems with the optimal 2D cluster pilot pattern in doubly dispersive channels. The LMMSE channel estimation is adopted instead of LS method for accurate original channel estimation, which is used to generate two auto-correlation matrices with different lags. The maximum Doppler spread is acquired by inverting the time correlation function. Since the noise term will bias the estimator, the estimated channel frequency responses (CFR's) are projected onto the delay subspace of the channel to reduce the bias term as well as the computation complexity. Furthermore, the subspace tracking algorithm is adopted to automatically update the delay subspace. Simulation results demonstrate the proposed algorithm can quickly converge to the true values for a wide range of SNR's and Doppler spreads in doubly dispersive channels.