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This paper investigates the secrecy performance of a cognitive relay non-orthogonal multiple access system based on an unmanned aerial vehicle (UAV). In particular, a UAV works as an aerial secondary relay to assist in securing transmissions from a terrestrial secondary source to secondary users in the presence of terrestrial eavesdroppers by sharing the licensed wireless spectrum designated for terrestrial primary systems. The minimum average secrecy rate (MASR) of the secondary users is taken as the main criterion to evaluate the security of the system under the assumption that the location information of the eavesdroppers and successive interference cancellations are imperfect. To maximize MASR, an efficient iterative algorithm is proposed to optimize key system parameters while keeping interference powers within tolerable limits. Since the total MASR problem is non-convex, the successive convex approximation technique is applied to find efficient approximate solutions. Simulation results demonstrate that the proposed algorithm substantially improves the MASR of the system in comparison to benchmark schemes and the orthogonal multiple access system.

This article analyzes the performance of a multiantenna unmanned aerial vehicle (UAV)-aided nonorthogonal multiple access (NOMA) relay system to support the finite-blocklength communications between multiple user pairs. To achieve better performance, at the multiantenna UAV, the maximal-ratio combining and maximal-ratio transmission are deployed, and the greedy algorithm is utilized to select the optimal user pair. Based on the approximate Gaussian–Chebyshev quadrature and the incomplete Gamma function, we successfully derive the exact and asymptotic closed-form expression for the average block error rate (BLER), throughput, and goodput of the investigated multiantenna UAV-aided NOMA relay system, taking the impacts of the line-of-sight probability, communication environments, and UAV velocity into consideration. Moreover, we also study other metrics, such as reliability, age of information (AoI), and average packet arrival rate. Simulation results verify the accuracy of the analytical framework. Numerical results indicate that the investigated system meets the requirements of ultrareliable and low-latency communications. Notably, the results showed that the multiantenna UAV-aided NOMA relay system has a reliability higher than 99.99% and an AoI of less than 1 s. Moreover, the optimal UAV altitude, blocklength, and number of transmission bits can be found to maximize the throughput and goodput and minimize the BLER and AoI of the system.

This article investigates the performance of a multi-antenna unmanned aerial vehicle (UAV)-aided backscatter short packet communication (SPC) system. The energy and radio frequency (RF) for the operations of backscatter devices are supported by UAV, and finite block length data are transmitted via uplink. The block error rate (BLER), throughput, goodput, reliability, latency, and age of information (AoI) are used to evaluate the proposed system performance, where the closed-form BLER expressions are approximated by the Gaussian-Chebyshev quadrature with line-of-sight (LoS) and non-LoS (NLoS) probabilities. We formulate optimization problems to maximize the throughput subject to the number of transmit bits and the UAV altitude and to minimize BLER subject to the UAV altitude, then solve them by using the one-dimensional search method. Monte Carlo simulations verify the accuracy of analysis results. The results show that using multi-antennas and maximal-ratio transmission (MRT) significantly improves the BLER. In addition, choosing a suitable altitude of UAV and relevant transmission bit can provide the best BLER and throughput or goodput. Finally, the impacts of UAV velocity, the operating environments, and LoS probability on the system performance are significant.