Secrecy in Gaussian MIMO bidirectional broadcast wiretap channels: Transmit strategies

In this work, we investigate the Gaussian MIMO bidirectional broadcast wiretap channel, from the vantage point of optimal transmit strategy design, to fulfill a certain secrecy constraint. An achievable secrecy rate region is computed and the gap to the capacity region of the same channel without the secrecy constraint is upper-bounded. Moreover, for the case of single receive antennas at all terminals, beamforming optimality is proved for non-orthogonal channels to the legitimate receivers. Finally, a strategy to reduce the eavesdropper´s rate without compromising the users´ by artificial-noise transmission is proposed, and its optimization with respect to the quality of the relay´s knowledge about the eavesdropper´s channel is characterized.