Secure Analog Network Coding With Wireless Energy Harvesting Under Multiple Eavesdroppers

In this paper, we consider a two-way relay system in which two users exchange data via a relay based on analog network coding (ANC) when multiple eavesdroppers exist. The relay replenishes energy from the two users’ signal and utilizes it to forward the ANC signal to each user. Each user removes its own signal from the received signal through self-interference cancellation and decodes the desired data signal. On the other hand, the eavesdroppers overhear the relaying signal and attempt to recover the data of two users. For this ANC-based two-way relay network with multiple eavesdroppers, we propose two secure ANC protocols: power splitting-ANC (PS-ANC) and time switching-ANC (TS-ANC), in which the relay determines the power splitting ratio ( $\alpha $ ) and time switching ratio ( $\beta $ ), respectively, to balance between the energy harvesting and the data reception. The optimal $\alpha $ and $\beta $ for each protocol are obtained analytically to maximize the minimum achievable secrecy rate ( $S_{\text {min}}$ ) in consideration of multiple eavesdroppers. Analytical results show that both the PS-ANC and TS-ANC protocols with the optimal $\alpha $ and $\beta $ accomplish the near-optimal $S_{\text {min}}$ irrespective of the locations and number of eavesdroppers. The comparison of these two protocols in various network environments exhibits that PS-ANC has a better performance than TS-ANC when the network conditions are unfavorable for wiretapping by the eavesdroppers.