The time-varying impulse response of rapidly fading mobile communication channels is expanded over a basis of complex exponentials that arise due to Doppler effects encountered with multipath propagation. Blind methods are reviewed for estimating the bases' parameters and the model orders. Existing second-order methods are critiqued and novel algorithms are developed for blind identification, direct, zero-forcing equalization and minimum mean square error (MMSE) equalization by combining channel diversity with temporal (fractional sampling) and/or spatial diversity which becomes available with multiple receivers. Illustrative simulations are also presented.

In wireless communications, the relative strength of the direct and scattered components of the received signal, as expressed by the Ricean factor, provides an indication of link quality. Accordingly, efficient and accurate methods for estimating are of considerable interest. In this paper, we propose a general class of moment-based estimators which use the signal envelope. This class of estimators unifies many of the previous estimators, and introduces new ones. We derive, for the first time, the asymptotic variance (AsV) of these estimators and compare them with the Cramér-Rao bound (CRB). We then tackle the problem of estimating from the in-phase and quadrature-phase (I/Q) components of the received signal and illustrate the improvement in performance as compared with the envelope-based estimators. We derive the CRBs for the I/Q data model, which, unlike the envelope CRB, is tractable for correlated samples. Furthermore, we introduce a novel estimator that relies on the I/Q components, and derive its AsV even when the channel samples are correlated. We corroborate our analytical findings by simulations. Index Terms-Detection and estimation, propagation and channel characterization.

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Estimation of the maximum Doppler spread or, equivalently, the vehicle velocity, is useful in improving handoff algorithms and necessary for the optimal tuning of parameters for systems that adapt to changing channel conditions. We provide a novel velocity estimator based on the spectral moments of the in-phase and the quadrature phase components or the envelope of the received signal. We characterize the joint effects of the Ricean factor, the directivity and the angle of nonisotropic scattering, and the effects of additive white noise on our estimator and other covariance-based velocity estimators analytically. We also prove the mean-square consistency of the covariance-based velocity estimators under some assumptions on the angle of arrival distribution. Simulations illustrate our approach and compare with existing techniques.

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