This letter proposes several relay selection policies for secure communication in cognitive decode-and-forward relay networks, where a pair of cognitive relays is opportunistically selected for security protection against eavesdropping. The first relay transmits the secrecy information to the destination, and the second relay, as a friendly jammer, transmits the jamming signal to confound the eavesdropper. We present new exact closed-form expressions for the secrecy outage probability. Our analysis and simulation results strongly support our conclusion that the proposed relay selection policies can enhance the performance of secure cognitive radio. We also confirm that the error floor phenomenon is created in the absence of jamming.

In this paper, we investigate the end-to-end performance of dual-hop proactive decode-and-forward relaying networks with Nth best relay selection in the presence of two practical deleterious effects: i) hardware impairment and ii) co-channel interference. In particular, we derive new exact and asymptotic closed-form expressions for the outage probability and average channel capacity of Nth best partial and opportunistic relay selection schemes over Rayleigh fading channels. Insightful discussions are provided. It is shown that, when the system cannot select the best relay for cooperation, the partial relay selection scheme outperforms the opportunistic method under the impact of the same co-channel interference (CCI) . In addition, without CCI but under the effect of hardware impairment, it is shown that both selection strategies have the same asymptotic channel capacity. Monte Carlo simulations are presented to corroborate our analysis.

In this paper, the performance of a wireless system, which is beneficial from utilization of multiple reconfigurable intelligent surfaces (RISs) and the direct link between the source node and the mobile user, is analyzed. Especially, the effect of transceiver hardware impairments (HIs) are taken into consideration. We successfully derive the outage probability (OP), throughput, achievable data rate (ADR), and symbol error probability (SEP) expressions of this multiple-RIS assisted wireless systems with and without HIs (referred as RIS-HI and RIS-ID system, respectively) over Nakagami-m fading channels. The above performance factors are also benchmarked with the case that only a point-to-point (P2P) link without RIS is considered (denotes as as P2P-HI and P2P-ID systems for HI and ideal hardware conditions, respectively). All obtained formulas are verified by Monte-Carlo simulations. It is observed that the RIS-HI system significantly outperforms the P2P-HI system at low transmit power regime, while the performance of the RIS-ID system is always better than that of P2P-ID system. On the other hand, HIs show more severe effect for higher data transmission rates or high modulation orders.

In this paper, the advantages of multiple reconfigurable intelligent surfaces (RISs) assisted wireless systems and direct link between two terminals are exploited by considering a cooperative communication system where reflecting links created by multiple RISs and the direct link are combined at the receiver. We mathematically analyze the performance of the cooperative multiple RISs - direct link (RIS-D) system over Nakagami- $m$ fading channels by obtaining the symbol error probability (SEP) expression. In addition, the SEP of the single-input single-output (SISO) system without RISs (i.e., there is only direct link between two terminals) is also obtained for convenience in comparison the performance of the cooperative RIS-D and SISO systems. We demonstrate the correctness of the derived expressions via Monte-Carlo simulations. Numerical results show that the SEP of the cooperative RIS-D system significantly lower than that of the SISO system for a certain transmission power of the transmitter. Also, for a certain SEP target, the cooperative RIS-D system can greatly save energy consumption in comparison with the SISO system. Specifically, increasing the number of RISs or the number of reflecting elements (REs) in the RISs can remarkably reduce the SEP of the cooperative RIS-D system. On the other hand, the impacts of the system parameters such as the number of RISs, the number of REs, and the Nakagami- $m$ fading channels are fully investigated to clearly indicate their influences on the SEP of the cooperative RIS-D system.

This study considers a dual‐hop cognitive inter‐vehicular relay‐assisted communication system where all communication links are non‐line of sight ones and their fading is modelled by the double Rayleigh fading distribution. Road‐side relays (or access points) implementing the decode‐and‐forward relaying protocol are employed and one of them is selected according to a predetermined policy to enable communication between vehicles. The performance of the considered cognitive cooperative system is investigated for K th best partial and full relay selection (RS) as well as for two distinct fading scenarios. In the first scenario, all channels are double Rayleigh distributed. In the second scenario, only the secondary source to relay and relay to destination channels are considered to be subject to double Rayleigh fading whereas, channels between the secondary transmitters and the primary user are modelled by the Rayleigh distribution. Exact and approximate expressions for the outage probability performance for all considered RS policies and fading scenarios are presented. In addition to the analytical results, complementary computer simulated performance evaluation results have been obtained by means of Monte Carlo simulations. The perfect match between these two sets of results has verified the accuracy of the proposed mathematical analysis.

Energy-harvesting-based physical layer security (PLS) has become a promising technique, as it not only secures information from eavesdropping without upper layer data encryption, but it also improves the energy efficiency of wireless networks. However, it imposes new challenges, as adversary parties can overhear the transmission of confidential information between the source and destination via a relay. Therefore, the transmit power of the signals must be large enough for energy harvesting, but it must also be small enough to avoid eavesdropping. This is even more challenging with multi-hop multi-path wireless networks. Motivated by these observations, this paper proposes three innovative protocols, namely, the shortest path selection (SPS) protocol, random path selection (RPS) protocol, and best path selection (BPS) protocol. These will enhance the security of multi-hop multi-path randomize-and-forward (RF) cooperative wireless sensor networks (WSNs) under the presence of eavesdroppers and hardware impairment, wherein the source node and relay nodes are capable of harvesting energy from beacon for data transmission. Furthermore, we derive exact closed-form expressions and the asymptotic outage probability for each protocol under multiple eavesdropping attacks. The simulation results validate the theoretical results.