Rate-adaptive secure HARQ protocol for block-fading channels

This paper analyzes the achievable secrecy throughput in incremental redundancy secure HARQ protocols for communication over block-fading wiretap channels (WTC). The transmitter has no instantaneous channel state information (CSI) but can receive an outdated version of CSI from both legitimate receiver and eavesdropper through reliable multi-bit feedback channels. Using outdated CSI, the transmitter can adapt the coding rates. Since the transmitter cannot adapt the coding rates to the instantaneous channel conditions, we consider the outage performance of secure HARQ protocols. We show how to find the optimal rate-adaptation policies to maximize the secrecy throughput under constraints on outage probabilities. Numerical results for a Rayleigh-fading WTC show that the rate-adaptation using multilevel feedbacks provides important gains in secrecy throughput comparing to the non-adaptive model. The fact that the eavesdropper also feedbacks information may seem unrealistic, but obtained results can be understood as an upper limit of the possible secrecy throughput improvements.