HARQ with Quantized 1-Bit CSI Feedback for Block Fading Wiretap Channels

We propose a hybrid automatic repeat request (HARQ) scheme with quantized channel state information (CSI) feedback for secure transmission from agent Alice to agent Bob over block fading channels. Bob feeds back to Alice a one-bit information on his CSI and she decides whether to transmit at a fixed rate or stay idle. If Bob fails to decode the secret message, the feedback, encoding and transmission processes are iterated. In order to prevent information leakage to a passive eavesdropper agent Eve, a different wiretap code is used at each iteration. We derive the secrecy outage probability and the number of bits per channel use that are reliably delivered to Bob while not being leaked to Eve. Such an opportunistic use of the channel leads to the reduction of the number of transmissions, with a) a higher secure throughput and b) a reduction of energy consumption (per transmitted bit).