  We present novel exact expressions and accurate closed-form approximations
for the level crossing rate (LCR) and the average fade duration (AFD) of the
double Nakagami-m random process. These results are then used to study the
second order statistics of multiple input multiple output (MIMO) keyhole fading
channels with space-time block coding. Numerical and computer simulation
examples validate the accuracy of the presented mathematical analysis and show
the tightness of the proposed approximations.
