Interactive secret key generation over reciprocal fading channels

We study a two-terminal secret-key generation problem over a two-way, approximately reciprocal, block-fading channel. The channel gains between the legitimate terminals are not revealed to any terminal, whereas the channel gains of the eavesdropper are revealed perfectly to the eavesdropper. We study a separation based scheme that involves a training phase followed by a communication phase within each block. The training phase generates correlated estimates of the channel state sequence between the two terminals, whereas the communication phase generates correlated source sequences between the two terminals. A portion of the secret-key is generated from the correlated channel state sequences by creating omniscience between the legitimate terminals and the remainder of the secret-key is generated from the correlated source sequences. An upper bound on the secret-key capacity is also established by reducing the setup to a coherent channel by providing genie-aided side information to the receivers. We observe that the difference between the upper and lower bound decreases as 1/T in the high signal-to-noise-ratio (SNR) regime. Numerical results indicate that the proposed scheme achieves significant gains over the commonly used training-only schemes even for moderate SNR.