  A spectrum-sharing communication system where the secondary user is aware of
the instantaneous channel state information (CSI) of the secondary link, but
knows only the statistics and an estimated version of the secondary
transmitter-primary receiver (ST-PR) link, is investigated. The optimum power
profile and the ergodic capacity of the secondary link are derived for general
fading channels (with continuous probability density function) under average
and peak transmit-power constraints and with respect to two different
interference constraints: an interference outage constraint and a
signal-to-interference outage constraint. When applied to Rayleigh fading
channels, our results show, for instance, that the interference constraint is
harmful at high-power regime in the sense that the capacity does not increase
with the power, whereas at low-power regime, it has a marginal impact and
no-interference performance corresponding to the ergodic capacity under average
or peak transmit power constraint in absence of the primary user, may be
achieved.
