  Interactive, immersive and critical applications demand ultra-reliable
low-latency communication (URLLC). To build wireless communication systems that
can support these applications, understanding the characteristics of the
wireless medium is paramount. Although wireless channel characteristics and
dynamics have been extensively studied, it is important to revisit these
concepts in the context of the strict demands of low latency and
ultra-reliability. In this paper, we bring a modeling approach from robust
control to wireless communication -- the wireless channel characteristics are
given a nominal model around which we allow for some quantified uncertainty. We
propose certain key "directions" along which to bound model uncertainty that
are relevant to URLLC. For the nominal model, we take an in-depth look at
wireless channel characteristics such as spatial and temporal correlations
based on Jakes' model. Contrary to what has been claimed in the literature, we
find that standard Rayleigh fading processes are not bandlimited. This has
significant implications on the predictability of channels. We also find that
under reasonable conditions the spatial correlation of channels provide a
fading distribution that is not too far off from an independent spatial fading
model. Additionally, we look at the impact of these channel models on
cooperative communication based systems. We find that while
spatial-diversity-based techniques are necessary to combat the adverse effects
of fading, time-diversity-based techniques are necessary to be robust against
unmodeled errors. Robust URLLC systems need to operate with both an SNR margin
and a time/repetition margin.
