  We define a class of Z-interference channels for which we obtain a new upper
bound on the capacity region. The bound exploits a technique first introduced
by Korner and Marton. A channel in this class has the property that, for the
transmitter-receiver pair that suffers from interference, the conditional
output entropy at the receiver is invariant with respect to the transmitted
codewords. We compare the new capacity region upper bound with the
Han/Kobayashi achievable rate region for interference channels. This comparison
shows that our bound is tight in some cases, thereby yielding specific points
on the capacity region as well as sum capacity for certain Z-interference
channels. In particular, this result can be used as an alternate method to
obtain sum capacity of Gaussian Z-interference channels. We then apply an
additional restriction on our channel class: the transmitter-receiver pair that
suffers from interference achieves its maximum output entropy with a single
input distribution irrespective of the interference distribution. For these
channels we show that our new capacity region upper bound coincides with the
Han/Kobayashi achievable rate region, which is therefore capacity-achieving. In
particular, for these channels superposition encoding with partial decoding is
shown to be optimal and a single-letter characterization for the capacity
region is obtained.
