Cooperation with an untrusted relay in broadcast channels

This paper studies secure communication over the broadcast channels with an untrusted relay. The relay helps users with transmission in broadcast channels, but we also wish to keep the information secret against the relay. Compress-and-forward relaying is a potential strategy to achieve secrecy. We obtain the achievable rate-equivocation regions by combining it with Marton´s coding scheme and Cover´s superposition coding scheme respectively. The secrecy rate region based on Marton´s coding scheme provides a general result, without knowing information regarding two users. If we know which user is better (i.e. receiving more information), we can design a simpler scheme with superposition encoding. Outer bound to the rate-equivocation regions is also provided in this paper.