Secrecy-Enhancing Signaling Schemes for Fast-Varying Wiretap Channels With Only CSI at the Transmitter

This paper proposes secrecy-enhancing signaling schemes to exploit the advantages of having only channel state information (CSI) at the transmitter, but no (or limited) CSI at the receiver and the eavesdropper in fast-fading wiretap channels and also in wiretap channels with Gaussian interference. With only CSI at the transmitter, the transmit signaling can be designed to precompensate for the distortion or the interference on the main channel to facilitate decoding at the receiver while leaving the eavesdropper confused by the uncertainties of its own channel. In fading wiretap channels, truncated channel-inversion and phase-compensation schemes are proposed to achieve this task. Here, truncation is used so that transmission occurs only when the main channel is sufficiently reliable. In wiretap channels with Gaussian interference, a truncated interference-nulling scheme is proposed to enable interference precancellation at the transmitter. The proposed signaling schemes exploit the advantages of having only CSI at the transmitter without employing more complex encoding schemes. The achievable secrecy rates are derived using Gaussian input, and approximate expressions are proposed for the optimization of system parameters. The effectiveness of the proposed transmission schemes and the advantages of having only CSI at the transmitter are demonstrated through numerical simulations.