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

TL;DR

This paper analyzes the throughput and secrecy performance of secure HARQ protocols over Gaussian block-fading channels, introducing new coding schemes and outage probability metrics to optimize reliable and confidential communication.

Contribution

It introduces a secure INR protocol with a rate-compatible Wyner code family and characterizes the tradeoff between reliability and secrecy using new outage metrics.

Findings

HARQ protocols improve throughput and secrecy in block-fading channels

A secure INR protocol with a good Wyner code family is feasible

Tradeoff between connection and secrecy outage probabilities is characterized

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.