Secure Degrees of Freedom of the Gaussian MIMO Wiretap and MIMO   Broadcast Channels with Unknown Eavesdroppers
  We investigate the secure degrees of freedom (SDoF) of the wiretap and the K user Gaussian broadcast channels with multiple antennas at the transmitter, the legitimate receivers and an unknown number of eavesdroppers each with a number of antennas less than or equal to a known value NE. The channel matrices between the legitimate transmitter and the receivers are available everywhere, while the legitimate pair have no information about the eavesdroppers' channels. We provide the exact sum SDoF for the considered system. A new comprehensive upperbound is deduced and a new achievable scheme based on utilizing jamming is exploited. We prove that cooperative jamming is SDoF optimal even without the eavesdropper CSI available at the transmitters. 
Mohamed Amir, Tamer Khattab, Tarek Elfouly, Amr Mohamed
cs.IT math.IT