This paper considers a two-way relay network, in which two sources exchange data through a relay and a cooperative jammer transmits an artificial noise (AN) while a number of nearby eavesdroppers overhear to recover data from both sources. The relay harvests energy from the two source signals and the AN, and then, uses this harvested energy to forward the received signals to the two sources. Each source eliminates its own signal from the relaying signal by self-cancellation and then decodes the data signal received from the other source. For this wireless-powered two-way relay system, we propose two secure relay protocols based on power splitting and time switching techniques. The two protocols are power splitting-based two-way relaying (PS-TWR) and time switching-based two-way relaying (TS-TWR), in which the relay respectively controls the power splitting ratio (ρ) and time switching ratio (α), in order to achieve a balance between the data receiving and the energy harvesting. The optimal values of ρ and α for each protocol are found analytically to maximize the minimum guaranteed secrecy capacity (C min S ) considering multiple eavesdroppers in high signal-to-noise ratio environments. Numerical results show that both the PS-TWR and TS-TWR protocols using the optimized values of ρ and α achieve the near-optimal C min S no matter how many eavesdroppers exist anywhere. Comparisons of the two protocols in various scenarios also show that PS-TWR achieves better C min S than TS-TWR because PS-TWR inherently has a shorter vulnerable time for eavesdropping than TS-TWR. Index Terms—Energy harvesting, physical layer security, secrecy capacity, two-way relay, power splitting, time switching.

INTRODUCTION

The demand for high-speed mobile communications continues to increase rapidly with recent emphasis on the emergence of real-time multimedia services. According to a recent report [1], mobile data traffic is expected to grow at a compound annual growth rate (CAGR) of 46% between 2016 and 2021 and global video traffic is predicted to account for 82% of total Internet traffic by 2021. As a result, improved spectral efficiency is currently considered one of the main objectives for future mobile systems. There has been extensive investigation of relay networks to meet the performance requirements of next-generation wireless systems. Typically, these are based on one-way relaying, in which the relay can forward a single message at a time. However, with recent advances in self-interference cancellation techniques, two-way relaying has begun to attract considerable attention, as a means of improving the spectral efficiency of one-way relay networks [2]–[۴]. A two-way relay forwards received signals from each of two transceivers at the same time, and each transceiver is able to recover the desired message from the relaying signal via self-interference cancellation. Given that two independent messages are transferred at a time, a two-way relay network approximately provides a two-fold increase in spectral efficiency compared with conventional one-way relaying. Wireless communication security is also one of main issues in the development of the Internet of things (IoT) because wireless channels are inherently vulnerable to eavesdropping; this problem likely to become more severe as the number of wireless devices increases. As one of the more promising techniques enabling secure wireless communication, physical layer security has been widely investigated [5]–[۸]. Considering the fact that relays are more susceptible to eavesdropping than any other node, physical layer security techniques in two-way relay networks were recently considered in [9]–[۱۴]. Relays can be classified into two different types; in the first, trusted relays are authorized to facilitate secure communications between sources [9]–[۱۱]. In the second, untrusted relays are not permitted to decode confidential information even though they are required to forward source signals [12]–[۱۴]. In [9], a joint trusted relay and jammer selection was proposed under the constraint of secrecy rate in two-way cooperative networks with multiple intermediate nodes. Moreover, in twoway trusted relay networks in which all nodes are equipped with multi-antennas, the impact of three different antenna selection schemes on the trade-off between secrecy and system complexity was analysed [10], and precoding designs for user signal and jamming signals were proposed for secure communication [11].