  We examine the impact of limited channel knowledge on the secondary user (SU)
in a cognitive radio system. Under a minimum signal-to-interference-and-noise
ratio (SINR) constraint for the primary user (PU) receiver, we determine the SU
capacity under five channel knowledge scenarios. We derive analytical
expressions for the capacity cumulative distribution functions and the
probability of SU blocking as a function of allowable interference. We show
that imperfect knowledge of the PU-PU channel gain by the SU-Tx often prohibits
SU transmission or necessitates a high interference level at the PU. We also
show that errored knowledge of the PU-PU channel is more beneficial than
statistical channel knowledge and imperfect knowledge of the SU-Tx to PU-Rx
link has a limited impact on SU capacity.
