  Fundamental capacity limits are studied for the two-user wireless ergodic
fading IC with partial Channel State Information at the Transmitters (CSIT)
where each transmitter is equipped with an arbitrary deterministic function of
the channel state (this model yields a full control over how much state
information is available). One of the main challenges in the analysis of fading
networks, specifically multi-receiver networks including fading ICs, is to
obtain efficient capacity outer bounds. In this paper, a novel capacity outer
bound is established for the two-user ergodic fading IC. For this purpose, by a
subtle combination of broadcast channel techniques (i.e., manipulating mutual
information functions composed of vector random variables by Csiszar-Korner
identity) and genie-aided techniques, first a single-letter outer bound
characterized by mutual information functions including some auxiliary random
variables is derived. Then, by novel arguments the derived bound is optimized
over its auxiliaries only using the entropy power inequality. Besides being
well-described, our outer bound is efficient from several aspects.
Specifically, it is optimal for the fading IC with uniformly strong
interference. Also, it is sum-rate optimal for the channel with uniformly mixed
interference. More importantly, it is proved that when each transmitter has
access to any amount of CSIT that includes the interference to noise ratio of
its non-corresponding receiver, the outer bound differs by no more than one bit
from the achievable rate region given by Han-Kobayashi scheme. This result is
viewed as a natural generalization of the ETW to within one bit capacity result
for the static channel to the wireless ergodic fading case.
