Reverse piloting protocol for securing time varying wireless channels

We consider single-tone pilot based burst transmission over slow time varying flat fading wireless channels with no line of sight. We propose a reverse pilot signaling protocol, based on sending the pilot signal from the receiver to the transmitter instead of from the transmitter to the receiver and on channel tracking at the receiver. We show that the channel performs automatic symbol level encryption analogous to a shift cipher. We derive explicit relationships between channel and system parameters which guarantee secure transmission of the entire data burst for given channel conditions, and depict them for Rayleigh fading. The proposed protocol exhibits high key generating rates, and no overheads on key distribution, enciphering and deciphering. Its security strength increases with mobility. These properties make it a good candidate for mobile wireless systems where bandwidth and power resources are scarce, such as mobile wireless sensor networks and mobile ad-hoc networks.