  Direct-conversion transceivers are in demand, due to low implementation cost
of analog front-ends. However, these transmitters or receivers introduce
imperfections such as in-phase and quadrature-phase (I/Q) imbalances. In this
paper, we first investigate the effect of I/Q imbalance on the performance of
primary system, and show that these impairments can severely degrade the
performance of cognitive radio system that are based on orthogonal frequency
division multiplexing (OFDM) multiple access scheme. Next, we design a new
four-level hypothesis blind detector for spectrum sensing in such cognitive
radio system, and show that the proposed detector is less vulnerable to I/Q
imbalance than conventional two-level detectors.
