  In this paper, we examine the physical-layer security for a spectrum sharing
system consisting of multiple source-destination pairs, which dynamically
access their shared spectrum for data transmissions in the presence of an
eavesdropper. We propose a source cooperation (SC) aided opportunistic jamming
framework for protecting the transmission confidentiality of the spectrum
sharing system against eavesdropping. Specifically, when a source node is
allowed to access the shared spectrum for data transmissions, another source is
opportunistically selected in the spectrum sharing system to transmit an
artificial noise for disrupting the eavesdropper without affecting the
legitimate transmissions. We present two specific SC aided opportunistic
jamming schemes, namely the SC aided random jammer selection (RJS) and optimal
jammer selection (OJS), which are referred to as the SC-RJS and SC-OJS,
respectively. We also consider the conventional non-cooperation as a baseline.
We derive closed-form intercept probability expressions for the
non-cooperation, SC-RJS and SC-OJS schemes, based on which their secrecy
diversity gains are determined through an asymptotic intercept probability
analysis in the high signal-to-noise ratio (SNR) region. It is proved that the
conventional non-cooperation exhibits a secrecy diversity of zero, whereas the
proposed SC-RJS and SC-OJS achieve a higher secrecy diversity of one. This also
surprisingly means that no additional secrecy diversity gain is achieved by the
optimal jammer selection compared to the random selection strategy. In
addition, numerical results show that the intercept probability performance of
the SC-OJS is always better than that of the SC-RJS and non-cooperation, even
when the legitimate channel is worse than the eavesdropping channel.
