The Two-Way MIMO Wire-Tap Channel

We introduce the two-way MIMO wire-tap channel and convey its potential to provide information-theoretic secure communications. We mainly address two challenges, namely the channel estimation and the single-user decodability issues. For the former, as the channel estimation becomes trickier owing to self-interference, we propose that users project their training signals on orthogonal subspaces to ensure their separability upon reception. For the latter, we suggest that each user uses, among the available antennas only the antenna subset that ensures both peers are not single-user decodable while maximizing their achievable secrecy sum-rate. Thus, unlike the result reported in literature about the two-way single-antenna wire-tap channel, we demonstrate that it is possible to achieve a positive secrecy rate even when a user is single-user decodable, provided that a sufficient number of transmit antennas be available. Finally, we observe that the proposed scheme, unlike secrecy schemes based on Wyner's degraded wire-tap channel, provides positive secrecy sum-rate even when the eavesdropper's channels incur as little noise as the legitimate users', and maintains the secrecy rate's growth in the high SNR regime, a fact we demonstrate by computer simulations. This makes the proposed scheme a better candidate for high-throughput secrecy applications.