User selection schemes for multiuser MISO downlink with eavesdropping

In this paper, the problem of secrecy sum-rate maximization is addressed for a multiuser multiple-input multiple-output wiretap channel. We consider the case of a single eavesdropper, when the information signals of the legitimate users are spatially multiplexed by using either zero forcing beamforming (ZFB) or Tomlinson-Harashima precoding (THP). By exploiting channel correlation of legitimate users with the eavesdropper, two low-complexity user selection schemes are investigated. In both schemes, the key idea is to extract multiuser diversity in order to protect information signals from eavesdropping rather than creating jamming interference. In the first scheme, user selection is performed in the null space of the eavesdropper´s channel and an iterative power allocation method is proposed. In the second scheme, the eavesdropper is considered to be a selected authorized user but its allocated power is forced to be zero. Numerical results are provided to illustrate the secrecy sum-rate performance of the proposed schemes.