  In this letter, we consider a multi-antenna downlink network where a secure
user (SU) coexists with a passive eavesdropper. There are two design
requirements for such a network. First, the information should be transferred
in a secret and efficient manner. Second, the quality of service (QoS), i.e.
delay sensitivity, should be take into consideration to satisfy the demands of
real-time wireless services. In order to fulfill the two requirements, we
combine the physical layer security technique based on switched beam
beamforming with an energy-efficient power allocation. The problem is
formulated as the maximization of the secrecy energy efficiency subject to
delay and power constraints. By solving the optimization problem, we derive an
energy-efficient power allocation scheme. Numerical results validate the
effectiveness of the proposed scheme.
