Secure outage performance of zero-forcing beamforming with “torch nodes”

This paper proposes a novel usage of zero-forcing beamforming (ZFB) for secure transmission in multiple-input single-output (MISO) wiretap channels. When using ZFB for secure transmission, the difficulty lies in the legitimate transmitter (Alice) hardly knowing the channel to eavesdroppers (Eves). The knowledge of the channel to Eves is essential when Alice builds a ZFB precoder that could eliminate the information leakage to Eves. To overcome this difficulty, this paper introduces a torch node that is assumed to be close to an Eve and helps Alice to estimate the channel to the Eve though the estimation is prone to errors. An exact secrecy outage probability (SOP) of the proposed ZFB with torch nodes as well as its asymptotic analysis is provided in this paper. Due to the channel estimation errors, a torch node is not always beneficial to reducing SOP. For a typical problem with a single Eve, a sub-optimal threshold that supports the usage of a torch node is provided in close-form. Numerical results show that the proposed ZFB with torch nodes significantly improves SOP.