Secrecy rate balancing for the downlink multiuser MISO system with independent confidential messages

Consider secure transmissions over the downlink of a multiuser MISO system with independent confidential messages, each of which is intended for one of the users and should keep secret from others. We are interested in a scenario where users have individual secrecy rate requirements. This scenario is practical but has drawn little attention in the literature so far. In this paper, we propose a secrecy rate balancing algorithm which tries to fulfill the secrecy rate requirements of users, and maximizes the minimum margin which is defined as the difference between the available secrecy rate and the required rate for each user. This algorithm balances out all the secrecy rate margins until an equilibrium is reached. We derive a necessary condition for the optimal solution. Finally, theoretical results are illustrated by numerical simulations.