  We study the performance of a cognitive system modeled by one secondary and
one primary link and operating under statistical quality of service (QoS) delay
constraints. We analyze the effective capacity (EC) to quantify the secondary
user (SU) performance under delay constraints. The SU intends to maximize the
benefit of the feedback messages on the primary link to reduce SU interference
for primary user (PU) and makes opportunistic use of the channel to transmit
his packets. We assume that SU has erroneous access to feedback information of
PU. We propose a three power level scheme and study the tradeoff between
degradation in EC of SU and reliability of PU defined as the success rate of
the transmitted packets. Our analysis shows that increase in error in feedback
access causes more interference to PU and packet success rate decreases
correspondingly.
