This study investigates covert communication for a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted system with non-orthogonal multiple access (NOMA). In particular, a source first transmits a NOMA signal consisting of both public and covert information to an unmanned aerial vehicle relay, which then forwards it to both the covert and the public users via an STAR-RIS in the presence of a warden, which attempts to detect whether the covert information is forwarded to the covert user or not. Under practical communication scenarios, the warden may locate at different positions in order to detect the covert transmission in the dual-hop systems, which may lead to several different detecting scenarios. Out of those possible scenarios, in this study, three typical ones are considered: 1) The warden can only detect the signal from the source in the first-hop transmission; 2) Both the signals from the source and the relay in the first-hop and second-hop transmissions could be detected; 3) The warden can detect the direct as well as the reflected transmission via the STAR-RIS from the relay in the second-hop transmission. Taking those three scenarios into consideration, the analytical expressions of detection error probability of the warden are derived and verified with Monte-Carlo simulations. In addition, the optimal detection threshold as well as the maximum effective covert rate is presented and discussed.

Due to some transmission delays and channel estimation errors, it is always difficult to acquire perfect channel state information (CSI) in practical communications. Therefore, in this study, taking the practical scenario of imperfect CSI into consideration, a simultaneously transmitting and reflecting-reconfigurable intelligent surface (STAR-RIS)-assisted satellite-autonomous aerial vehicle (AAV)-terrestrial non-orthogonal multiple access (NOMA) network is investigated, in which the AAV is assumed to be randomly located in a spherical cap space while the users are randomly located in an inner circular and an outer annular plane on the ground. Specifically, a satellite source first sends NOMA signals to a AAV relay, which then forwards them to both ground users via an STAR-RIS through reflection as well as transmission. In addition, considering that the satellite-AAV link is subject to the shadowed-Rician distribution while the other links are subject to the Nakagami-m distributions, the cumulative distribution functions and probability density functions of channel gains in the presence of imperfect CSI are derived. Furthermore, in practical application scenarios, the positions of the AAV and both users may be randomly located in some regions. Therefore, three randomly distributed scenarios are considered: 1) The users are randomly located while the AAV’s position is fixed; 2) The AAV is randomly located while both users are in fixed locations; and 3) Both are randomly located. Under those three scenarios, both analytical and asymptotic expressions of outage probability (OP) for two users as well as the system OP are derived using the stochastic geometry approach, and their accuracy is confirmed with Monte-Carlo simulations.

In this work, the physical-layer security for a maritime relay-assisted hybrid satellite and underwater wireless optical communication system consisting of a satellite source, a maritime relay (R), an underwater destination, and an eavesdropper (E) is investigated. In addition, both R and E are with multiple antennas and randomly located. In real application scenarios, E may be different carriers, i.e., an aircraft in the air or a ship on the sea, with various scopes of activity. Therefore, two different randomly located scenarios are considered, such as randomly distributed in a three dimensional space and a two dimensional plane. Furthermore, the beam-angle information of the source is assumed to be unavailable to E, and E may be located outside of the beam coverage area. Therefore, the beam coverage probability (BCP) for E is also presented. Employing the stochastic geometry theory and the maximum ratio combining scheme, the analytical and asymptotic expressions of secrecy outage probability (SOP) are obtained. In addition, the overall system SOP is defined and presented by taking both SOP and BCP into account. Finally, Monte-Carlo simulations verify the accuracy of our analysis.

In this study, we investigate a non-orthogonal multiple access (NOMA)-assisted dual-hop underwater wireless optical communication-radio frequency (UWOC-RF) network. Specifically, an optical source forwards a NOMA signal to a marine relay, which first deploys either the heterodyne detection or intensity modulation direct detection to process the received signal, and then encodes and re-transmits a superimposed signal to two terrestrial destinations. Assuming the UWOC channel is subject to a unified mixture Exponential-Generalized Gamma (mEGG) distribution and the RF channels experience Nakagami-${m}$fading, the analytical as well as asymptotic expression for outage probability is obtained in the form of the Meijer’s$G$-function. Moreover, the average achievable rate is also investigated by utilizing the bivariate Fox’s$H$-function. In addition, the accuracy of those expressions is verified with Monte-Carlo simulations.

In this article, 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) criteria, 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, the Monte-Carlo simulation verifies the correctness of the theoretical derivations.

Energy harvesting (EH) is an effective approach that can not only extend the service life for those energy-constraint devices but also reduce the carbon footprint for wireless communication. Unlike the commonly used single EH scheme adopted in existing works, in this considered free-space optical-radio frequency (FSO-RF) system, a dual EH mode is considered, in which an EH relay is capable of harvesting energy from both the ambient RF signal in the environment and the direct-current component separated from the FSO signal simultaneously. In addition, in this considered system, a source node plans to communicate with two users equipped with multiple antennas via an EH relay on a non-orthogonal multiple access (NOMA) basis, and all the harvested energy of the relay is used to transmit the signal to both NOMA users. Moreover, assuming a selection-combining scheme is adopted at the user side, both the analytical and asymptotic expressions of outage probability for each NOMA user are presented, and the system performance with various atmospheric turbulence, detection technique, as well as pointing error is investigated. Finally, numerical results show that our proposed dual EH mode can lead to better outage performance than that with the single EH one.

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 study, a wireless powered dual‐hop relay system, which consists of a source, a destination and a relay that is equipped with ( ) receiving antennas and finite energy storage, is considered. Antenna selection scheme is employed for the relay to harvest energy from the received signals, under which the ( ) strongest received signals are combined for information processing and the other copies of signals are used for energy harvesting. Considering Nakagami‐ m fading scenarios, the authors first derive the closed‐form expressions for the probability density function and the cumulative distribution function of the signal‐to‐noise ratio with generalised selection combining (GSC), and then the exact and asymptotic analytical expressions for outage probability are derived, respectively. Finally, simulation results are presented to verify the proposed analysis model.

We consider a cooperative system consisting of a source node, a destination node, N ( N >1) wireless-powered relays, and an eavesdropper. Each relay is assumed to be with a nonlinear energy harvester, in which there exists a saturation threshold, limiting the level of the harvested power. For decode-and-forward and power splitting protocols, the K th best relay is selected to assist the source-relay-destination transmission. An analytical expression for the secrecy outage probability is derived, and also verified by simulation.

A maximum-likelihood (ML) receiver structure is proposed for fast frequency-hopped M-ary frequency-shift keying communication systems over Rayleigh-fading channels with the composite effects of multitone jamming (MTJ) and partial-band noise jamming (PBNJ). The corresponding semi-analytical bit-error rate (BER) expressions of the proposed ML receiver are also presented and validated by simulation results. Numerical results show that the BER performance with both MTJ and PBNJ is bounded by the two extreme cases, where only MTJ or PBNJ is present. It is also found that the proposed ML receiver can better counteract MTJ than PBNJ.

In this study, the effects of timing and frequency offsets on bit-error rate (BER) performance of fast frequency-hopped M-ary frequency-shift keying communication systems over Rician-fading channels with both multitone jamming (MTJ) and partial-band noise jamming (PBNJ) are investigated. Analytical BER expressions are derived for both linear-combining and product-combining receivers. Numerical results show that under both MTJ and PBNJ conditions, the BER performance falls between the two extreme cases, in which either MTJ or PBNJ presents. It is found that the BER performance is severely degraded as the timing or frequency offset increases. The product-combining receiver is found to be more sensitive to the timing and frequency offsets than the linear-combining receiver. It is also observed that for the linear-combining receiver, the optimum diversity order increases as the timing and frequency offset increases over both Rician-fading and Rayleigh-fading channels; however, the reverse is true for the product-combining receiver.

An optimum maximum-likelihood (ML) receiver structure for a fast frequency-hopped M-ary frequency-shift keying system over frequency-selective Rician-fading channels against partial-band-noise jamming and additive white Gaussian noise is proposed. In addition, we propose two suboptimum ML receivers and derive their corresponding analytical bit-error rate expressions.

Analytical expressions of bit-error rate (BER) are derived for a fast-frequency-hopping M-ary frequency-shift- keying (FFH/MFSK) spread-spectrum communication system over frequency-selective Rayleigh-fading channels with multitone jamming (MTJ) and additive white Gaussian noise. The FFH/MFSK systems employing a linear-combining receiver (LCR) and a product-combining receiver (PCR) are studied. The desired signal and MTJ are assumed to undergo independent fading. The characteristic function approach is used to derive the analytical BER expressions for higher diversity levels. The analytical BER expressions are validated by simulation results and the PCR is found to outperform the LCR when the jamming power is relatively strong.