Optimal Spectrum Sensing for Cognitive Radio with Imperfect Detector

This paper investigates the sequential opportunistic channel sensing and access problem of an unlicensed or secondary user (SU) in presence of multiple licensed or primary users (PU). The SU senses one channel at a time for the presence of PU. The sensing is considered to be imperfect, i.e, the presence or absence of PU may not be detected correctly. The problem of selection of best channel amongst the sequentially scanned channels for the transmission of SU is modeled as an Optimal Stopping Problem (OSP) which also takes into account the cost of scan per channel. A simple threshold based policy is presented as a solution to OSP in which SU after sensing the channel to be free of PU activity checks the channel quality by comparing the channel gain with a threshold and if the channel gain is more than the threshold then SU selects this channel for transmission. The threshold is seen to be dependent on probabilities of channel occupancy and detection errors. The variation of threshold with respect to detection errors is analyzed and supported with plots. The plots for the variation are presented which match closely with the analytical results.