  In this paper, the second-order statistics of the instantaneous mutual
information are studied, in time-varying Rayleigh fading channels, assuming
general non-isotropic scattering environments. Specifically, first the
autocorrelation function, correlation coefficient, level crossing rate, and the
average outage duration of the instantaneous mutual information are
investigated in single-input single-output (SISO) systems. Closed-form exact
expressions are derived, as well as accurate approximations in low- and
high-SNR regimes. Then, the results are extended to multiple-input
single-output and single-input multiple-output systems, as well as
multiple-input multiple-output systems with orthogonal space-time block code
transmission. Monte Carlo simulations are provided to verify the accuracy of
the analytical results. The results shed more light on the dynamic behavior of
the instantaneous mutual information in mobile fading channels.
