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Due to flexibility in deployment and high mobility, unmanned aerial vehicles (UAVs) can improve the performance of cellular networks. In this paper, we focus on the UAV-assisted cooperative communication network where multiple UAVs serve as relays between a pair of ground users. Based on signal-to-noise ratio (SNR), we propose two UAV selection strategies namely best harmonic mean (HM) and best downlink SNR (BDS). Then, we derive the closed-form expressions for the outage probability, throughput and coverage probability of both the selection strategies. Furthermore, an optimization problem for maximizing the throughput is formulated, subject to the 3-D coordinates (i.e., x, y, and z coordinates) constraint of the selected UAV. The concavity of the problem is analyzed with respect to the horizontal placement of the selected UAV. Next, we propose algorithms to find optimal and suboptimal position/coordinates of the selected UAV. Computer simulations validate the accuracy of the derived expressions, and demonstrate that BDS selection strategy has a significant performance gain at low SNR values, whereas both the selection schemes attain a similar performance at high SNRs.INDEX TERMS Unmanned aerial vehicles (UAVs), UAV selection, harmonic mean, signal-to-noise ratio (SNR), throughput maximization.

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This paper investigates the performance of a new framework for low-outage downlink non-orthogonal multiple access (NOMA) using a coordinated direct and relay transmission (CDRT) scheme with direct links to both the nearuser (NU) and the far-user (FU). Both AF and DF relays are considered. In this framework, the NU combines the signals from BS and R at each stage of the successive interference cancellation (SIC) to attain good outage performance. For both NU and FU, the expressions for outage probability and throughput are derived in closed form. We also derive the high-SNR expressions for the outage probability to demonstrate that with the proposed framework, both users harness a diversity of two without feedback bits (this is the only framework to achieve this). We demonstrate that the choice of power allocation coefficient and target symbol rates is crucial to maximizing the NU throughput while ensuring a desired target FU throughput. We demonstrate that CDRT with the proposed framework outperforms known schemes in terms of outage probability, sum throughput, and energy efficiency. Moreover, we also show that optimal rate selection is important to maximize the EE. Monte Carlo simulations validate the accuracy of the derived analytical expressions.Index Terms-Non-orthogonal multiple access (NOMA), coordinated direct and relay transmission (CDRT), energy efficiency (EE), amplify-and-forward (AF), decode-and-forward (DF).

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