Ergodic secrecy sum rate for multiuser MIMO downlink systems using block diagonalization

To date, the achievable ergodic secrecy sum rate has been studied for multiuser MIMO downlink systems with single-antenna receivers only. In this paper, we study multiuser MIMO downlink systems with multiple-antenna receivers and an external single-antenna eavesdropper to be kept ignorant of the transmitted messages. We adopt the block diagonalization technique at the transmitter to remove inter-user interference, and derive closed-form expressions of the achievable rate for the intended users and the information leakage rate for the eavesdropper. Furthermore, we analyze asymptotic characteristics of the ergodic secrecy sum rate for the system in the high and low SNR regime, and obtain a tight lower bound in the regime of large transmit and receive antennas. Finally, numerical results validate the accuracy of the theoretical analysis.