A Lower Bound on Secrecy Capacity for MIMO Wiretap Channel Aided by a Cooperative Jammer with Channel Estimation Error

We consider a multiple-input multiple-output wiretap channel with one transmitter, one receiver, one cooperative jammer, and one eavesdropper whereby each node is equipped with multiple antennas. Eigenbeam-space division multiplexing (E-SDM) and cooperative jamming are known as techniques for improving secrecy rate. E-SDM enables the transmitter and the legitimate receiver to communicate with each other in parallel using channel state information known to them to maximize the legitimate receiver's mutual information. On the other hand, cooperative jamming is to jam only the eavesdropper to degrade the eavesdropper's mutual information. However, co-channel interference and residual interference from the cooperative jammer are caused by channel estimation errors, which degrade the legitimate receiver's mutual information. How these interferences affect the secrecy capacity has not been clarified. In this paper, we derive a lower bound on secrecy capacity using the legitimate receiver's mutual information in the presence of channel estimation errors and the mutual information of the eavesdropper who uses a minimum mean square error receiver. We analyze the effect of co-channel interference and residual interference from the cooperative jammer on the secrecy rate.