Achievable Information-Theoretic Secrecy in the Presence of a Radar

As the demand for spectrum increases, co-existence between radar and communication systems has been of increased interest. However, the effect of a radar signal on the secrecy capacity of a communication system operating in the environment has not been addressed. Particularly, for military operations, the limited dynamic range of real receivers is important to consider because the radar signal is often large enough to send the receivers into nonlinearity. In this paper, information-theoretic secrecy is considered when a friendly radar is present in the wiretap channel: the intended receiver Bob can cancel the radar signal before recording the signal based on a key sequence which is shared by the radar with the transmitter Alice and with Bob. However, due to the non-linear conversion operation in the eavesdropper Eve's receiver, Eve is inhibited from recovering the transmitted signal even if she obtains the key after storing the received signal. We analyze the information-theoretic secrecy rate of a wiretap channel co-existing with the radar signal without Gaussian assumptions on the radar signal. This analysis is made difficult relative to prior work due to the complex structure of the radar signal and the correlation between the two dimensions. We show that a positive secrecy rate can be achieved, even when the eavesdropper is closer to the transmitter than the intended receiver.