In spectrum leasing, primary users (PUs) lease the part of their spectral resources to secondary users (SUs) in exchange for appropriate remuneration. In this paper, we consider spectrum leasing via cooperation for physical-layer security that the secondary cooperation exists for improving the primary secrecy rate while maintaining its quality of service (QoS). Unlike the previous researches with the perfect channel state information (CSI) at transmitter (CSIT) assumption, we study when no information regarding the eavesdropper is available at the transmitter. This imperfect CSIT makes the SU's transmission strategy limited. To find the optimal transmission technique for the SU, we formulate a problem appropriate for the imperfect CSIT case. By using the proposed problem, we design the optimal transmit beamforming for the SU. Also, we analyze the ergodic rate of the secondary link at high signal-to-noise ratio (SNR) when the secondary cooperation focuses only on maximizing the primary secrecy rate for the viable choice of the secondary QoS level. The numerical result shows that the primary secrecy rate by the proposed transmit beamforming is comparable to that based on the perfect CSIT.
