Secrecy Outage Performance of Cognitive Radio Network with Selection Combining at Eavesdropper
Authors: Chopra Khyati , Bose Ranjan , Joshi Anupam 
Published: August 2019

Background:
Based on the idea of cooperative communication, recently a lot of attention has
been drawn to cooperative spectrum access for the secure information transmission in a Cognitive Radio
Network (CRN). Security is one of the most important aspects of these networks, as due to their open and
dynamic nature, they are extremely vulnerable to malicious behavior. Cooperative cognitive radio has
emerged as a dynamic spectrum access technique, where an unlicensed (secondary) user is allowed to simultaneously
access the licensed channels dedicated to a Primary User (PU), as long as the Quality of Service
(QoS) of primary communication is not affected.

Method:
This paper investigates the secrecy outage performance of threshold-based cognitive decode-andforward
relay network, with interference constraints from primary licensed user. Threshold-based relaying is
considered where; the source message is successfully decoded by the relay, only if the received SNR satisfies
the particular threshold. Outage probability expressions have been derived for the worst-case scenario,
where only the eavesdropper can achieve the advantage of diversity. The Selection Combining (SC) diversity
scheme is employed only at the secondary eavesdropper.

Results:
The system secrecy performance is better for SC diversity scheme at the eavesdropper than Maximal
Ratio Combining (MRC) diversity scheme, as MRC has better diversity performance than SC. We have
shown that the improvement in desired secrecy rate, predetermined threshold, eavesdropper channel quality
and interference constraints affect the secrecy performance of the cognitive radio system. The outage probability
decreases accordingly with an increase in the maximum tolerable interference level at primary destination.
The outage probability of Optimal relay Selection (OS) scheme is derived for a multi-relay system,
when either the Instantaneous Channel State Information (ICSI) or the Statistical Channel State Information
(SCSI) is available. We have shown that the secrecy performance of the OS with ICSI of the system is better
than with SCSI. Also, the OS improves the performance of the multi-relay system, when the number of relays
is increased.

Conclusion:
The secrecy outage probability of threshold-based DF underlay cognitive relay network is
evaluated. Both interference and maximum transmit power constraints are considered at secondary source
and secondary relay. Also, the relay can successfully decode the message, only if it meets the pre-defined
threshold. We have investigated the performance of MRC and SC diversity schemes at the secondary eavesdropper
and have shown that the system secrecy performance is better for SC than MRC, as MRC has better
diversity performance than SC. We have shown that the system secrecy performance is significantly affected
by the required secrecy rate, pre-defined threshold, interference constraints and choice of diversity scheme
(MRC/SC) at the eavesdropper. The outage probability of OS scheme is derived for a multi-relay system,
when either the ICSI or the SCSI is available. We have shown that the secrecy performance of the OS with
ICSI of the system is better than with SCSI. Also, the OS improves the performance of the multi-relay system,
when the number of relays is increased.