Secure space-time communication

Network security is important for information protection in open, secure, or covert communications. One such requirement is to achieve high-rate communications between clients, e.g., terminals or sensors, in the network while hiding information about the transmitted symbols, signal activity, or other sensitive data from an unintended receiver, e.g., an eavesdropper. For wireless links, the single-user capacity advantages of deployment of multiple antennas at the transmitter is well known. One of the principal conclusions of this paper is that proper exploitation of space-time diversity at the transmitter can also enhance information security and information-hiding capabilities. In particular, we show that significant gains are achievable when the transmitter and the client receiver are both informed about their channel while the transmitter and eavesdropper receiver are uniformed about their channel. More generally, we compare capacity limits for both informed and uninformed transmitter and informed receiver scenarios subject to low probability of intercept (LPI) and low probability of detection (LPD) constraints. For several general cases, we can characterize the LPI- and LPD-optimal transmitted source distributions and compare them to the standard optimal source distribution satisfying a power constraint. We assume the standard quasi-static flat Rayleigh-fading channel model for the transmitter-receiver pairs. This paper is a step toward answering the fundamental question: what are the qualitative and quantitative differences between the information-carrying capabilities of open space-time channels versus secure space-time channels?