Secrecy performance for underlay cognitive multi-relaying MISO-RF/SIMO-FSO networks with outdated CSI

This study evaluates the physical-layer security for an underlay cognitive radio-based radio frequency (RF)–free space optical (FSO) network. In particular, a multi-antenna secondary source (S) communicates with a multi-aperture secondary destination (D) via M (M ≥ 1) relays with each equips with both a single antenna and a single aperture in the presence of a N (N ≥ 1)-aperture eavesdropper. In addition, a transmit antenna selection scheme is adopted at S with a channel feedback delay, and an equal gain combining protocol is performed at D to combine all the received signals. Moreover, a general Kth relay selection criterion is adopted to select a relay to assist the transmission through a decode-and-forward protocol. Considering the RF links are subject to Nakagami-m fading and the FSO channels are with Gamma–Gamma distributions, the analytical and asymptotic expressions for the lower bound of secrecy outage probability are obtained. Moreover, Monte-Carlo simulations are used to verify those expressions.