In this paper, the secrecy performance of a wireless-powered multi-antenna relaying system, which consists of a source $\boldsymbol {(S)}$ , a destination $\boldsymbol {(D)}$ , $\boldsymbol {N (N>1)}$ relays (Rs), a power beacon $\boldsymbol {(B)}$ , and an eavesdropper $\boldsymbol {(E)}$ , is studied in the presence of imperfect channel state information at Rs. Both S and Rs are assumed to be with nonlinear energy harvesters, and harvest energy from radio-frequency signals from B through a time-switching harvesting scheme. Assuming Rs deploy decode-and-forward scheme, a general $\boldsymbol {K}$ th best relay in terms of instantaneous secrecy capacity for S-R-D link is selected for information transmission. An analytical expression for secrecy outage probability (SOP), validated via Monte Carlo simulation, is derived when all channels experience independent Rayleigh fading. Numerical results show that a higher correlation coefficient between the estimated and real channel values leads to better SOP performance when the ratio of the channel gains of R-D and R-E links is in the high region for the first hop or in the low-to-medium region for the second hop. Furthermore, there exists a special value of the saturation threshold of the energy harvester at S or R, and SOP performance cannot be improved when the saturation threshold is beyond that value.

In this paper, the secrecy performance of a mixed radio frequency-free space optical (RF-FSO) communication system, which consists of a source (S), a relay (R), a destination (D) and an eavesdropper (E), is studied. In this system, it is assumed that E can overhear the FSO link between R and D as it is close to D. It is further assumed that RF and FSO links experience Rayleigh and Gamma-Gamma fading, respectively. Considering atmospheric turbulence, pointing errors, two types of detection techniques as well as decode-and-forward scheme, the closed-form expressions for the lower bound of secrecy outage probability and the probability of strictly positive secrecy capacity are derived. Monte Carlo simulation results are also presented to validate the accuracy of the derivations.

This article investigates the secrecy performance for an unmanned aerial vehicle (UAV)-aided nonorthogonal multiple access (NOMA) network in the context of an underlay cognitive radio. Specifically, a multiantenna secondary source ( $S$ ) transmits a mixed signal incorporated with NOMA to two multiantenna secondary destinations ( $D_{i}$ ) via a UAV-enabled decode-and-forward relay in the presence of an eavesdropper ( $E$ ). In addition, both a transmit antenna selection scheme at $S$ and a maximal ratio combining protocol at ${D_{i}}$ and $E$ are taken into account. Considering all the links undergo Nakagami- $m$ fading with path loss, we analyze the secrecy performance in terms of secrecy outage probability for both users. Moreover, we also use Monte-Carlo simulations to verify the accuracy of those derived expressions.

In this paper, we consider a non-orthogonal multiple access (NOMA) based mixed free space optical–radio frequency (FSO-RF) system with imperfect channel state information (CSI) and evaluate the secrecy performance in terms of secrecy outage probability (SOP). Specifically, a source ( S ) first forwards an optical NOMA signal to a relay ( R ) via an FSO link, and then R performs an optical–electrical conversion and broadcasts the converted signal to users over the RF links. Both the FSO and RF links are incorporated with the NOMA technique, and the CSI imperfection in both links is considered. Assuming both users have multiple antennas, and adopting the maximal ratio combining diversity schemes, we derive the analytical and asymptotic expressions for SOP when the FSO link undergoes gamma–gamma fading with pointing error and the RF links are subject to Rayleigh fading. In addition, both decode-and-forward and amplify-and-forward protocols are taken into account and compared. Monte Carlo simulations are also used to verify the accuracy of the derived results.

In this paper, we investigate the outage performance for a dual-hop free space optical-radio frequency communication system, in which a source node communicates with two users on a non-orthogonal multiple access basis via a single-aperture and multi-antenna relay, which can harvest energy from a power beacon under a nonlinear energy harvesting mode. In particular, two transmitting antenna selection (AS) criterions, namely the random-maximum AS (RA-AS) and the minimum-maximum AS (IA-AS) schemes, are considered to improve the outage performance of the system. In addition, an adaptive power allocation algorithm, which allocates the power to each user according to the instantaneous channel power gain, is adopted for the RA-AS scheme. Considering the Gamma-Gamma distribution and the Rayleigh fading, the closed-form expressions of the outage probability for each user under both the RA-AS and IA-AS schemes are derived. Moreover, Monte-Carlo simulation verifies the correctness of the theoretical derivations.

In this paper, we investigate the physical layer security for an asymmetric dual-hop radio frequency-free space optical (RF-FSO) communication system, in which a source transmits information to a multi-aperture destination (D) through a best relay under the wiretapping of a multi-aperture eavesdropper (E). The best relay is selected by a partial relay selection scheme in terms of maximum instantaneous channel gain for the RF link. Moreover, equal gain combining scheme is also used to process the multiple copies of receiving signal at D and E. Assuming all the RF channels are subject to independent and identical distributed (i.i.d.) Nakagami-m fading and all the FSO channels are modeled by i.i.d. Gamma-Gamma process, the secrecy performance in terms of secrecy outage probability and the probability of strictly positive secrecy capacity are studied, and the accuracy of the analytical results is verified by Monte-Carlo simulations.