User selection scheme with secrecy capacity in MIMO downlink systems

For multiuser Multiple-Input-Multiple-Output (MIMO) downlink systems where some receivers exist, several user selection schemes have been recently proposed. However, these user selection schemes do not take security into consideration. One possible scenario of wireless communication systems is that a transmitter sends confidential messages to multiple receivers in the presence of eavesdroppers. In this paper, we propose a user selection scheme with secrecy capacity for multiuser MIMO downlink system. We assume a system with a transmitter, some legitimate users and eavesdroppers where the transmitter wants to send data securely against eavesdroppers. In the proposed scheme, the transmitter selects a user set so that their secrecy capacities among legitimate users and eavesdroppers are larger than zero. Thus, the transmitter can send confidential messages to users securely. The proposed scheme can be applied to conventional scheduling schemes, such as maximum capacity (MAX) scheduling and proportional fairness (PF) scheduling. Simulation results show that the proposed scheme provides secure transmissions and has little degradation of performance of system capacity and fairness when applied to the conventional scheduling schemes. Hence, we show that the proposed scheme is an effective user selection scheme to guarantee secrecy of communication.