  In the spectrum-sharing technology, a secondary user may utilize the primary
user's licensed band as long as its interference to the primary user is below a
tolerable value. In this paper, we consider a scenario in which a secondary
user is operating in the presence of both a primary user and an eavesdropper.
Hence, the secondary user has both interference limitations and security
considerations. In such a scenario, we study the secrecy capacity limits of
opportunistic spectrum-sharing channels in fading environments and investigate
the optimal power allocation for the secondary user under average and peak
received power constraints at the primary user with global channel side
information (CSI). Also, in the absence of the eavesdropper's CSI, we study
optimal power allocation under an average power constraint and propose a
suboptimal on/off power control method.
