Non-orthogonal multiple access (NOMA) and ambient backscatter communication (AmBC) have gained a lot of interest as key enabling technologies in wireless communications systems. NOMA is a key player for enhancing spectrum utilization either by multiplexing multiple messages for the same user or allowing multi-users access, while AmBC shows great potential for enhancing both spectrum and energy efficiency for battery-limited devices. In this paper, the performance of a downlink NOMA multiplexing based symbiotic-radio ( $SR$ ) AmBC system is analyzed over Nakagami-\textit{m} fading channels. New closed-form expressions for the exact and asymptotic outage probabilities are derived. Moreover, we analyze the diversity order and the influence of the system parameters on the outage performances. Besides, we proposed a power allocation optimization technique to achieve an outage-optimal performance. Through representative Monte-Carlo simulations, we have verified the analytical results. Finally, we compared the performance of the proposed system against a benchmark OMA-based system.

The mixture of wireless and power line communications (PLC) is vital for implementing new applications in smart grid and vehicular communications. In this work, we investigate the performance of non-orthogonal multiple access (NOMA) based dual-hop hybrid communication systems with decode-and-forward relay. The wireless channel is characterized by Nakagami-\textit{m} fading under an additive white gaussian noise (AWGN), while the PLC channels are characterized by Log-normal distribution with Bernoulli Gaussian noise including both background and impulsive noise components. New closed-form expressions for the outage probability, the asymptotic outage probability and ergodic capacity are derived and verified via extensive representative simulations. For more insights on the outage performance, we analyze the diversity order. Additionally, we proposed a power allocation optimization technique to achieve an outage-optimal performance. The results show that the system outage probability improves as the impulsive noise index and the arrival probability of the impulsive component of the PLC additive noise decrease, while their effect is negligible on the ergodic capacity. Finally, the performance of the proposed system is compared against a benchmark OMA-based system.

The hybrid combination between underwater optical wireless communication (UOWC) and radio frequency (RF) is a vital demand for enabling communication through the air–water boundary. On the other hand, non-orthogonal multiple access (NOMA) is a key technology for enhancing system performance in terms of spectral efficiency. In this paper, we propose a downlink NOMA-based dual-hop hybrid RF-UOWC with decode and forward (DF) relaying. The UOWC channels are characterized by exponential-generalized Gamma (EGG) fading, while the RF channel is characterized by Rayleigh fading. Exact closed-form expressions of outage probabilities and approximated closed-form expressions of ergodic capacities are derived, for each NOMA individual user and the overall system as well, under the practical assumption of imperfect successive interference cancellation (SIC). These expressions are then verified via Monte-Carlo simulation for various underwater scenarios. To gain more insight into the system performance, we analyzed the asymptotic outage probabilities and the diversity order. Moreover, we formulated and solved a power allocation optimization problem to obtain an outage-optimal performance. For the sake of comparison and to highlight the achievable gain, the system performance is compared against a benchmark orthogonal multiple access (OMA)-based system.

Vehicle-to-infrastructure (V2I) communication facilitates seamless interaction between vehicles and the surrounding infrastructure. Recently, there has been a notable surge in research interest in ambient backscatter communications (AmBC), primarily due to its ability to enable battery-free communication. Concurrently, reconfigurable intelligent surfaces (RISs) have garnered attention as a promising technology, particularly for the advancement of cellular systems beyond 5G. Additionally, non-orthogonal multiple access (NOMA) assumes a pivotal role in enhancing spectrum utilization. This paper proposes a new RIS-enhanced NOMA-AmBC system where all channels are characterized by Nakagami-m fading and with the main objective of assessing the performance of such system. To this end, new closed-form expressions for the outage probabilities (OPs) were derived under the practical assumption of imperfect successive interference cancellation (SIC). In addition, to gain a deep insight into the system’s performance, we derived asymptotic, upper-bound, and lower-bound expressions for the OPs. Furthermore, we proposed a power allocation optimization problem to achieve an outage-optimal performance. To validate the analytical results, we conducted extensive investigations using Monte Carlo simulations, which indicates a high degree of consistency. Moreover, our results underscore the remarkable performance improvements achieved by the RIS-assisted AmBC-NOMA system when compared to both the traditional benchmark AmBC-NOMA system and the RIS-assisted orthogonal multiple access (AmBCOMA) counterparts.

We investigate enhancing the sum secrecy rate (SSR) of a reconfigurable intelligent surface (RIS)-assisted integrated sensing and communication (ISAC) system in a multi-cluttered environment with multiple eavesdropping targets. We formulate an SSR maximization problem by jointly optimizing the ISAC base station (BS) beamformer, the RIS phase shifts, and artificial noise (AN) covariance matrix used by the ISAC BS to disrupt the eavesdropping targets. A novel low-complexity manifold-based approach is proposed to solve this problem, incorporating signal-to-clutter-plus-noise ratio (SCNR) and signal-to-interference-plus-noise ratio (SINR) as constraints for sensing and communications, respectively. The proposed alternating optimization (AO) algorithm on Riemannian manifolds achieves high secrecy rates with a reduced complexity when compared to classical optimization approaches.

Non-orthogonal multiple access (NOMA) and ambient backscatter communication (AmBC) play major roles to enhance spectrum efficiency in wireless communication systems. Besides, the AmBC provides good reinforcement for the current trend towards dispensing batteries for battery-free Internet-of-Things (IoT) devices. In this paper, we propose a symbiotic battery-free IoT system, that exploits the downlink transmission of a NOMA multiplexing enabled cellular network, to permit an IoT spectrum-efficient uplink communication. The IoT backscatter device (BD) performs a symbiotic radio (SR) relation with the cellular source to power its communication by intelligently reflecting the received power. We derive a closed-form expression of the ergodic capacity (EC) of the BD transmission and tight approximations of the ECs of the cellular source transmission, where all channels undergo Nakagami-m fading. Additionally, we validate the analytical results obtained using Monte-Carlo simulations. The influences of several system parameters such as power allocation factor, reflection coefficient, and channels’ severity factors have been investigated. Finally, the performance of the proposed system is compared with a benchmark OMA-based system to highlight the achievable performance improvement.