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

Dept. of Electr. & Comput. Eng., Rutgers Univ., Brunswick, NJ
IEEE Transactions on Information Theory (Impact Factor: 2.62). 01/2008; DOI: 10.1109/TIT.2009.2013043
Source: arXiv

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.

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    ABSTRACT: Secure transmission between two agents, Alice and Bob, over block fading channels can be achieved similarly to conventional hybrid automatic repeat request (HARQ) by letting Alice transmit multiple blocks, each containing an encoded version of the secret message, until Bob informs Alice about successful decoding by a public error-free return channel. In existing literature each block is a differently punctured version of a single codeword generated with a Wyner code that uses a common randomness for all blocks. In this paper instead we propose a more general approach where multiple codewords are generated from independent randomnesses. The class of channels for which decodability and secrecy is ensured is characterized, with derivations for the existence of secret codes. We show in particular that the classes are not a trivial subset (or superset) of those of existing schemes, thus highlighting the novelty of the proposed solution. The result is further confirmed by deriving the average achievable secrecy throughput, thus taking into account both decoding and secrecy outage.

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