Energy-Aware Multiuser Scheduling for Physical-Layer Security in Energy-Harvesting Underlay Cognitive Radio Systems

In this paper, we consider an energy-harvesting underlay cognitive radio (CR) system, which is comprised of multiple cognitive users (CUs), a common cognitive base station (CBS), and multiple passive eavesdroppers (Es). When CUs transmit to CBS, Es may overhear the confidential transmissions from CUs. In the aforementioned system, all CUs are considered to be equipped with energy harvesters to collect energy from their surrounding environments. Meanwhile, the transmit power of CUs shall be limited by a tolerable interference temperature to guarantee the quality-of-service of a primary user. In order to improve the physical-layer security of energy-harvesting CR systems, we propose an Energy-AwaRe mulTiuser scHeduling (EARTH) scheme, which takes into account both the interference temperature constraint and residual energy level harvested from the ambient surroundings. We consider the use of security-reliability tradeoff (SRT) as a metric to evaluate the performance of proposed EARTH scheme. To be specific, we derive closed-form expressions of outage probability (OP) and intercept probability (IP) for the proposed scheme over Rayleigh fading channels. For the purpose of comparison, the SRT analysis of round-robin scheduling (RRS) and conventional multiuser scheduling (CMUS) methods is also carried out. Numerical results show that the SRT of proposed EARTH scheme is better than that of RRS and CMUS methods, demonstrating the advantage of our proposed scheme against eavesdropping attacks. Additionally, it is illustrated that the sum of IP and OP can be minimized for RRS and CMUS as well as EARTH schemes through a time allocation between the energy harvesting and information transmission phases.