Artificial Noise Assisted Communication in the Multiuser Downlink: Optimal Power Allocation

We consider the problem of secure communication in the multiuser downlink with an external eavesdropper. The transmitter simultaneously transmits concurrent information signals to the users and an artificial noise (AN) to the eavesdropper. We derive a closed-form expression for the secrecy sum-rate in the large system limit and use it as the objective function to optimize the power allocation between the information signals and the AN to maximize the secrecy sum-rate. We obtain a simple approximation for the optimal power allocation in the high signal-to-noise ratio regime, which proves to be a generic and near-optimal strategy. Analytical and simulation results show that more power needs to be allocated to AN if the system serves fewer users and the eavesdropper has more antennas.