Practical Secret Sharing in Fast Fading Channels based on Reliability-Based Hybrid ARQ

Consider a wireless communication system in which Alice wishes to transmit a message to Bob in the presence of an eavesdropper, Eve. We consider the design of a communication technique that allows Alice to send a message such that Bob can decode it but Eve can get little information about the message. This scenario is a classic example of the wiretap channel, for which several researchers have begun to develop practical schemes. However, we consider a challenging version in which the channel from Alice to Eve may be as good or better than the channel from Alice to Bob. There have been no practical secrecy coding techniques that have been developed for this scenario. The key to allow secret communication in such a scenario is feedback from Bob to Alice (which Eve can also observe). The scheme we develop uses reliability-based hybrid ARQ (RB-HARQ) over a fast fading channel to provide a decoding advantage at Bob. Bob requests information about received symbols that suffer from deep fades. The retransmitted symbols improve Bob’s ability to decode the message faster than at Eve, for whom a set of bits with random reliabilities is retransmitted. We evaluate the proposed scheme by performance measures such as the probability of successful decoding and the equivocation rate at Eve.