Secure Communication With Outdated Channel State Information via Untrusted Relay Capable of Energy Harvesting

Abstract

In this paper, we consider secure communication via a wireless-powered untrusted relay with outdated channel state information (CSI). The low security clearance of the relay means that both the source and destination wish to keep information confidential from it. To prevent the relay from decoding the source information, the destination emits a jamming signal to the relay during data transmission by the source. The wireless-powered relay uses both the data and the jamming signals as valuable sources of energy for the amplify-and-forward relaying process. In addition, it can adopt either a power splitting or a time switching policy to determine the proportions of energy harvesting and information processing. However, when the CSI of the relay-to-destination link is outdated, the destination cannot remove the jamming signal completely from the relaying signal due to imperfect channel reciprocity. To evaluate the effect of outdated CSI on the secrecy performance, we derive the analytical expressions of two performance metrics, outage probability ( $P_{out}$ ) for delay-limited transmission and ergodic secrecy rate ( $\bar{R}$ ) for delay-tolerant transmission. The results of the analysis also allow us to find the jamming power level and energy harvesting ratio to optimize $P_{out}$ and $\bar{R}$ , respectively. Numerical simulations are used to verify the precision of our analysis, and confirm that the proposed secure relaying protocols accomplish near-optimal secrecy performances. Our results indicate that there is a need to reduce the jamming power level to ensure secure communication when the channel is severely outdated, while outdated CSI has a relatively low impact on the energy harvesting ratio.