Achieving secrecy without any instantaneous CSI: polar coding for fading wiretap channels

This paper presents a polar coding scheme for fading wiretap channels that achieves reliability as well as security without the knowledge of instantaneous channel state information at the transmitter. Specifically, a block fading model is considered for the wiretap channel that consists of a transmitter, a receiver, and an eavesdropper; and only the information regarding the statistics (i.e., distribution) of the channel state information is assumed at the transmitter. For this model, a coding scheme that hierarchically utilizes polar codes is presented in order to address channel state variation. In particular, on polarization of different binary symmetric channels over different fading blocks, each channel use (corresponding to a possibly different polarization) is modeled as an appropriate binary erasure channel over fading blocks. Polar codes are constructed for both coding over channel uses for each fading block and coding over fading blocks for certain channel uses. In order to guarantee security, message bits are transmitted such that they can be reliably decoded at the receiver, and random bits are introduced to exhaust the observations of the eavesdropper. It is shown that this coding scheme, without instantaneous channel state information at the transmitter, is secrecy capacity achieving for the corresponding fading binary symmetric wiretap channel.