Message-Prioritization Based Unequal Secrecy Protection for Untrusted Two-Way Relaying Networks

This paper proposes a message-prioritization based unequal secrecy protection framework for untrusted two- way relaying networks, where two terminal users communicate bidirectionally with the assistance of an untrusted relay. Each user is assumed to have two messages with distinct priorities: high priority and low priority. The high priority messages (HPM) are first transmitted, for which we devise a constellation overlapping method such that the received signals at the relay overlap with each other and a high error floor is created to prevent the untrusted relay from deciphering the information. Upon the completion of HPM exchange, users transmit their low priority messages (LPM) using a noise aggregation approach. To be specific, each user superposes its LPM onto the previously decoded HPM from the other user. By exploiting the difference between the error patterns for HPM at the terminal users and the relay, channel noises in various time slots can be aggregated at the relay to secure the LPM transmission. It is shown from simulation results that HPM is guaranteed to have greater reliability and higher secrecy level than that of LPM. However, the transmission of LPM enjoys lower implementation complexity and reduced system overhead, which fully demonstrates that the proposed unequal secrecy protection scheme can realize a good performance-complexity tradeoff.