  Understanding the performance of cognitive radio systems is of great
interest. To perform dynamic spectrum access, different paradigms are
conceptualized in the literature. Of these, Underlay System (US) has caught
much attention in the recent past. According to US, a power control mechanism
is employed at the Secondary Transmitter (ST) to constrain the interference at
the Primary Receiver (PR) below a certain threshold. However, it requires the
knowledge of channel towards PR at the ST. This knowledge can be obtained by
estimating the received power, assuming a beacon or a pilot channel
transmission by the PR. This estimation is never perfect, hence the induced
error may distort the true performance of the US. Motivated by this fact, we
propose a novel model that captures the effect of channel estimation errors on
the performance of the system. More specifically, we characterize the
performance of the US in terms of the estimation-throughput tradeoff.
Furthermore, we determine the maximum achievable throughput for the secondary
link. Based on numerical analysis, it is shown that the conventional model
overestimates the performance of the US.
