  In this paper, we propose an algorithm for channel estimation, acquisition
and tracking, for orthogonal frequency division multiplexing (OFDM) systems.
The proposed algorithm is suitable for vehicular communications that encounter
very high mobility. A preamble sequence is used to derive an initial estimate
of the channel using least squares (LS). The temporal variation of the channel
within one OFDM symbol is approximated by two complex exponential basis
expansion models (CE-BEM). One of the Fourier-based BEMs is intended to capture
the low frequencies in the channel (slow variations corresponding to low
Doppler), while the other is destined to capture high frequencies (fast
variations corresponding to high Doppler). Kalman filtering is employed to
track the BEM coefficients iteratively on an OFDM symbol-by-symbol basis. An
interactive multiple model (IMM) estimator is implemented to dynamically mix
the estimates obtained by the two Kalman filters, each of which matched to one
of the BEMs. Extensive numerical simulations are conducted to signify the gain
obtained by the proposed combining technique.
