  We consider a cognitive radio (CR) network consisting of a secondary
transmitter (ST), a secondary destination (SD) and multiple secondary relays
(SRs) in the presence of an eavesdropper. We rely on careful relay selection
for protecting the ST-SD transmission against the eavesdropper with the aid of
both single-relay and multi-relay selection. To be specific, only the "best" SR
is chosen in the single-relay selection for assisting the secondary
transmission, whereas the multi-relay selection invokes multiple SRs for
simultaneously forwarding the ST's transmission to the SD. We analyze both the
intercept probability and outage probability of the proposed single-relay and
multi-relay selection schemes for the secondary transmission relying on
realistic spectrum sensing. We also evaluate the performance of classic direct
transmission and artificial noise based methods for the purpose of comparison
with the proposed relay selection schemes. It is shown that as the intercept
probability requirement is relaxed, the outage performance of the direct
transmission, the artificial noise based and the relay selection schemes
improves, and vice versa. This implies a trade-off between the security and
reliability of the secondary transmission in the presence of eavesdropping
attacks, which is referred to as the security-reliability trade-off (SRT).
Furthermore, we demonstrate that the SRTs of the single-relay and multi-relay
selection schemes are generally better than that of classic direct
transmission. Moreover, as the number of SRs increases, the SRTs of the
proposed single-relay and multi-relay selection approaches significantly
improve. Finally, our numerical results show that as expected, the multi-relay
selection scheme achieves a better SRT performance than the single-relay
selection.
