  The outage probability is an important performance measure for cooperative
diversity schemes. However, in mobile environments, the outage probability does
not completely describe the behavior of cooperative diversity schemes since the
mobility of the involved nodes introduces variations in the channel gains. As a
result, the capacity outage events are correlated in time and second-order
statistical parameters of the achievable information-theoretic capacity such as
the average capacity outage rate (AOR) and the average capacity outage duration
(AOD) are required to obtain a more complete description of the properties of
cooperative diversity protocols. In this paper, assuming slow Rayleigh fading,
we derive exact expressions for the AOR and the AOD of three well-known
cooperative diversity protocols: variable-gain amplify-and-forward,
decode-and-forward, and selection decode-and-forward relaying. Furthermore, we
develop asymptotically tight high signal-to-noise ratio (SNR) approximations,
which offer important insights into the influence of various system and channel
parameters on the AOR and AOD. In particular, we show that on a
double-logarithmic scale, similar to the outage probability, the AOR
asymptotically decays with the SNR with a slope that depends on the diversity
gain of the cooperative protocol, whereas the AOD asymptotically decays with a
slope of -1/2 independent of the diversity gain.
