  One of the main challenges in high-speed mobile communications is the
presence of large Doppler spreads. Thus, accurate estimation of maximum Doppler
spread (MDS) plays an important role in improving the performance of the
communication link. In this paper, we derive the data-aided (DA) and
non-data-aided (NDA) Cramer-Rao lower bounds (CRLBs) and maximum likelihood
estimators (MLEs) for the MDS in multiple-input multiple-output (MIMO)
frequency-selective fading channel. Moreover, a lowcomplexity NDA-moment-based
estimator (MBE) is proposed. The proposed NDA-MBE relies on the second- and
fourth-order moments of the received signal, which are employed to estimate the
normalized squared autocorrelation function of the fading channel. Then, the
problem of MDS estimation is formulated as a non-linear regression problem, and
the least-squares curvefitting optimization technique is applied to determine
the estimate of the MDS. This is the first time in the literature when DAand
NDA-MDS estimation is investigated for MIMO frequency-selective fading channel.
Simulation results show that there is no significant performance gap between
the derived NDA-MLE and NDA-CRLB even when the observation window is relatively
small. Furthermore, the significant reduced-complexity in the NDA-MBE leads to
low root-mean-square error (NRMSE) over a wide range of MDSs when the
observation window is selected large enough.
