Secure degrees of freedom of the multiple access wiretap channel with no eavesdropper CSI

We consider the K-user Gaussian multiple access wiretap channel (MAC-WT), where no eavesdropper channel state information (CSI) is available at the transmitters. We show that the exact sum secure degrees of freedom (s.d.o.f.) of this channel model is K-1/K . This result shows that, under the condition of no eavesdropper CSI, the MAC-WT acts like a single-transmitter K - 1 helper wiretap channel. We further show that, when a subset of the transmitters have eavesdropper CSI, then higher sum s.d.o.f. can be achieved, and the system can be operated as a MAC-WT for the users with eavesdropper CSI, with the remaining users acting as helpers. In particular, if m of the K transmitters have eavesdropper CSI, we show that m(K-1)/m(K-1)+1 sum s.d.o.f. can be achieved, showing the benefits of having the eavesdropper CSI at the transmitters.