  With emerging technologies like 6G, many new applications like autonomous
systems evolve which have strict demands on the reliability of data
communications. In this work, we consider a system with multiple slowly fading
channels that constitute a diversity system. We show that the joint
distribution of the fading coefficients has a tremendous impact on the outage
performance of a communication system. In particular, we investigate the
zero-outage capacity (ZOC) and characterize the joint fading distributions with
a strictly positive ZOC. Interestingly, the set of joint distributions, that
lead to positive ZOCs, is larger than a singleton in general. We also derive
expressions for the maximum ZOC with respect to all possible joint
distributions for selection combining (SC) as diversity combining technique at
the receiver. For maximum ratio combining (MRC), we characterize the maximum
ZOC within a finite number of bits. Finally, the results are evaluated
explicitly for the special cases of Rayleigh fading and Nakagami-$m$ fading in
order to quantify the ZOCs for common fading models.
