  In this paper, we investigate the physical-layer security of a multi-user
multi-eavesdropper cognitive radio system, which is composed of multiple
cognitive users (CUs) transmitting to a common cognitive base station (CBS),
while multiple eavesdroppers may collaborate with each other or perform
independently in intercepting the CUs-CBS transmissions, which are called the
coordinated and uncoordinated eavesdroppers, respectively. Considering multiple
CUs available, we propose the round-robin scheduling as well as the optimal and
suboptimal user scheduling schemes for improving the security of CUs-CBS
transmissions against eavesdropping attacks. Specifically, the optimal user
scheduling is designed by assuming that the channel state information (CSI) of
all links from CUs to CBS, to primary user (PU) and to eavesdroppers are
available. By contrast, the suboptimal user scheduling only requires the CSI of
CUs-CBS links without the PU's and eavesdroppers' CSI. We derive closed-form
expressions of the secrecy outage probability of these three scheduling schemes
in the presence of the coordinated and uncoordinated eavesdroppers. We also
carry out the secrecy diversity analysis and show that the round-robin
scheduling achieves the diversity order of only one, whereas the optimal and
suboptimal scheduling schemes obtain the full secrecy diversity, no matter
whether the eavesdroppers collaborate or not. In addition, numerical secrecy
outage results demonstrate that for both the coordinated and uncoordinated
eavesdroppers, the optimal user scheduling achieves the best security
performance and the round-robin scheduling performs the worst. Finally, upon
increasing the number of CUs, the secrecy outage probabilities of the optimal
and suboptimal user scheduling schemes both improve significantly.
