  In this paper, detection of the primary user (PU) signal in an orthogonal
frequency division multiplexing (OFDM) based cognitive radio (CR) system is
addressed. According to the prior knowledge of the PU signal known to the
detector, three detection algorithms based on the Neyman-Pearson philosophy are
proposed. In the first case, a Gaussian PU signal with completely known
probability density function (PDF) except for its received power is considered.
The frequency band that the PU signal resides is also assumed known. Detection
is performed individually at each OFDM sub-carrier possibly interfered by the
PU signal, and the results are then combined to form a final decision. In the
second case, the sub-carriers that the PU signal resides are known.
Observations from all possibly interfered sub-carriers are considered jointly
to exploit the fact that the presence of a PU signal interferers all of them
simultaneously. In the last case, it is assumed no PU signal prior knowledge is
available. The detection is involved with a search of the interfered band. The
proposed detector is able to detect an abrupt power change when tracing along
the frequency axis.
