Wireless-Powered Two-Way Relaying Protocols for Optimizing Physical Layer Security

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 (p) and time switching ratio (α), in order to achieve a balance between the data receiving and the energy harvesting. The optimal values of p and α for each protocol are found analytically to maximize the minimum guaranteed secrecy capacity (C
_S
^min
) 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 p and α achieve the near-optimal C
_S
^min
no matter how many eavesdroppers exist anywhere. Comparisons of the two protocols in various scenarios also show that PS-TWR achieves better C
_S
^min
than TS-TWR because PS-TWR inherently has a shorter vulnerable time for eavesdropping than TS-TWR.