On the Throughput of Secure Hybrid-ARQ Protocols for Gaussian Block-Fading Channels

Abstract

The focus of this paper is an information-theoretic study of retransmission protocols for reliable packet communication under a secrecy constraint. The hybrid automatic retransmission request (HARQ) protocol is revisited for a block-fading wire-tap channel, in which two legitimate users communicate over a block-fading channel in the presence of a passive eavesdropper who intercepts the transmissions through an independent block-fading channel. In this model, the transmitter obtains a 1-bit ACK/NACK feedback from the legitimate receiver via an error-free public channel. Both reliability and confidentiality of secure HARQ protocols are studied by the joint consideration of channel coding, secrecy coding, and retransmission protocols. In particular, the error and secrecy performance of repetition time diversity (RTD) and incremental redundancy (INR) protocols are investigated based on good Wyner code sequences, which ensure that the confidential message is decoded successfully by the legitimate receiver and is kept in total ignorance by the eavesdropper for a given set of channel realizations. This paper first illustrates that there exists a good rate-compatible Wyner code family which ensures a secure INR protocol. Next, two types of outage probabilities, connection outage and secrecy outage probabilities are defined in order to characterize the tradeoff between the reliability of the legitimate communication link and the confidentiality with respect to the eavesdropper's link. For a given connection/secrecy outage probability pair, an achievable throughput of secure HARQ protocols is derived for block-fading channels. Finally, both asymptotic analysis and numerical computations demonstrate the benefits of HARQ protocols to throughput and secrecy.

ARQ-Based Secret Key Sharing

“…Each ARQ epoch is assumed to be contained in one coherence interval (i.e., fixed channel gains) and that different epochs correspond to independent coherence intervals (the same assumptions as [4]). We denote the constant rate used in each transmission frame by R 0 bits/channel use.…”

“…2) It is clear from (6) that a positive secret key rate is achievable under very mild conditions on the channels experienced by Bob and Eve. More precisely, unlike the approach proposed in [4], Theorem 1 establishes the achievability of a positive perfect secrecy rate by appropriately exploiting the ARQ feedback even when Eve's average SNR is higher than that of Bob. 3) Theorem 1 characterizes the fundamental limit on secret key sharing and not message transmission.…”

“…[3], [8]. Another frame of work [4] proposed using the well-known Hybrid ARQ protocols to facilitate the exchange of secure messages between Alice and Bob.…”

“…First, by distributing the key bits over multiple ARQ frames, we establish the achievability of a vanishing probability of secrecy outages [4] at the expense of a larger delay. Interestingly, using this approach, a non-zero perfectly secure key rate is achievable even when Eve is experiencing a more favorable average signal-to-noise ratio (SNR) than Bob (unlike the scheme Fig.…”

“…proposed in [4]). Second, we develop explicit constructions for secrecy ARQ coding that enjoy low implementation complexity.…”