When direct source-destination communications are in outage, relay selection is a preferable solution to improve reliability for this communications. However, such a relay selection makes the eavesdropper better overhear source data through both source-relay and relay-destination communication hops, losing data security. To improve both reliability and security, this paper proposes a relay selection-and-jamming (RaJ) scheme to select one intermediate node as a conventional relay and another node as a jammer. To enhance energy efficiency, all intermediate nodes harvest radio frequency energy in source signals for their operations with nonlinear energy harvesting (NL-EH). The security and reliability of the RaJ scheme are assessed through suggested rigorous/asymptotic expressions and are significantly better than two benchmark schemes without neither jamming nor both relay selection and jamming. Additionally, they can be optimized with reasonable selection of specifications. Moreover, the NL property of the energy harvesters dramatically affects the reliability but negligibly degrades the security for the RaJ scheme. Furthermore, the linear EH (L-EH) is more reliable but less secure than the NL-EH.

This paper recommends a simultaneous jamming-and-transmitting scheme for spectrum-sharing relaying networks with nonlinear energy scavenging. More specifically, spectrum-sharing relaying networks include a secondary source which expects to communicate with a secondary destination but impossible due to communication blockage between them, a secondary relay which helps the source overcome this blockage, a primary receiver, and a wiretapper which steals secret messages from both source and relay. To motivate assistance, the relay scavenges radio frequency energy from the source with practical nonlinear energy scavenger (NL-ES) instead of linear energy scavenger (L-ES) as in previous publications. Additionally, both source and relay perform simultaneous jamming-and-transmitting to secure their communication. The intercept and outage probabilities of the recommended scheme are evaluated through exact closed-form formulas, which are corroborated by Monte-Carlo simulations. Illustrative results show that the proposed scheme offers the reliability-security trade-off yet suffers error floor at large maximum transmit/interference power. Moreover, its performance can be optimally set with appropriate system parameters. Notably, the proposed scheme can guarantee absolute security with proper parameter setting. Furthermore, the NL-ES is practical but performs significantly worse than the L-ES.