—This paper investigates power allocation algorithms
for OFDM-based cognitive radio systems, where the intra-system
channel state information (CSI) of the secondary user (SU) is
perfectly known. However, due to loose cooperation between the
SU and the primary user (PU), the inter-system CSI is only
partially available to the SU transmitter. Two types of PUs are
considered to have different capabilities. One is a dumb (Peak
Interference-Power tolerable) system that can tolerate a certain
amount of peak interference at each subchannel. The other
is a more sophisticated (Average Interference-Power tolerable)
system that can tolerate the interference from the SU as long
as the average interference over all subchannels is within a
certain threshold. Accordingly, we introduce an interference
power outage constraint, with which the outage is maintained
within a target level. The outage is here defined as the probability
that peak or average interference power to the PU is greater
than a given threshold. With both this interference-power outage
constraint along with a transmit-power constraint, we propose
optimal and suboptimal algorithms to maximize the capacity of
the SU. We evaluate the spectral efficiency through extensive
simulations and show that the SU can achieve higher performance
(up to two times) with the more sophisticated PU than with the
dumb PU.