Towards Enhanced Security for Two-Way Untrusted Relaying Systems: A Constellation Overlapping Scheme

This paper proposes a constellation overlapping scheme to secure two-way untrusted relaying systems, where the relay acts as both a helper facilitating data transmission and an eavesdropper intercepting users' messages. A truncated-channel-inversion based approach is developed to make the signals transmitted from two users experience the same equivalent channel, thereby realizing full constellation overlapping at the relay. Consequently, it is extremely difficult for the relay to recover the users' individual signals, and data confidentiality is thus protected. The achieved error floor level at the untrusted relay is analyzed, and the truncation thresholds are optimized to maximize the sum rate for end-to-end information exchange. Simulation results demonstrate the superiority of our scheme in terms of security and transmission efficiency compared with the existing alternatives.