Joint Trajectory and Communication Design for UAV-Enabled Multiple Access

Unmanned aerial vehicles (UAVs) have attracted significant interest recently in wireless communication due to their high maneuverability, flexible deployment, and low cost. This paper studies a UAV-enabled wireless network where the UAV is employed as an aerial mobile base station (BS) to serve a group of users on the ground. To achieve fair performance among users, we maximize the minimum throughput over all ground users by jointly optimizing the multiuser communication scheduling and UAV trajectory over a finite horizon. The formulated problem is shown to be a mixed integer non-convex optimization problem that is difficult to solve in general. We thus propose an efficient iterative algorithm by applying the block coordinate descent and successive convex optimization techniques, which is guaranteed to converge to at least a locally optimal solution. To achieve fast convergence and stable throughput, we further propose a low-complexity initialization scheme for the UAV trajectory design based on the simple circular trajectory. Extensive simulation results are provided which show significant throughput gains of the proposed design as compared to other benchmark schemes.Comment: Submitted for possible publicatio

New Periodic Solutions of Singular Hamiltonian Systems with Fixed Energies

By using the variational minimizing method with a special constraint and the direct variational minimizing method without constraint, we study second order Hamiltonian systems with a singular potential $V\in C^2(R^n\backslash O,R)$ and $V\in C^1(R^2\backslash O,R)$ which may have an unbounded potential well, and prove the existence of non-trivial periodic solutions with a prescribed energy. Our results can be regarded as some complements of the well-known Theorems of Benci-Gluck-Ziller-Hayashi and Ambrosetti-Coti Zelati and so on

Securing UAV Communications Via Trajectory Optimization

Unmanned aerial vehicle (UAV) communications has drawn significant interest recently due to many advantages such as low cost, high mobility, and on-demand deployment. This paper addresses the issue of physical-layer security in a UAV communication system, where a UAV sends confidential information to a legitimate receiver in the presence of a potential eavesdropper which are both on the ground. We aim to maximize the secrecy rate of the system by jointly optimizing the UAV's trajectory and transmit power over a finite horizon. In contrast to the existing literature on wireless security with static nodes, we exploit the mobility of the UAV in this paper to enhance the secrecy rate via a new trajectory design. Although the formulated problem is non-convex and challenging to solve, we propose an iterative algorithm to solve the problem efficiently, based on the block coordinate descent and successive convex optimization methods. Specifically, the UAV's transmit power and trajectory are each optimized with the other fixed in an alternating manner until convergence. Numerical results show that the proposed algorithm significantly improves the secrecy rate of the UAV communication system, as compared to benchmark schemes without transmit power control or trajectory optimization.Comment: Accepted by IEEE GLOBECOM 201

An Investigation into American University Students’ Motivation for Chinese Learning: A Case Study

The study investigates the motivation behind learners of the Chinese language as a second language at the university level in the United States of America. The study uses Dornyei’s (1994a) three-level framework of L2 motivation and Dornyei’s (2001a) demotivation theory, as the theory guidelines. In addition, the study takes both quantitative and qualitative approaches and uses questionnaires and interviews for data collection. First, the study explores the various constructions of motivation for students learning Chinese at a public university in the New England region. All students who were enrolled in the elementary, intermediate, and advanced Chinese courses at the University in the fall semester of 2019 participated in this study. 108 valid questionnaires have been collected for this investigation. The data from these questionnaires were then analyzed in Excel and the results indicate that the highest motivation is instrumental motivation. The second-highest motivation is achievement motivation, the third is integrative motivation, and the fourth is positive learning attitudes and experiences motivation. While social milieu motivation and language requirement motivation are low. Second, the students were separated based on the students’ gender, ethnic backgrounds, and learning levels. The participants were divided into the following groups: male and female learners, learners of the Chinese heritage and non-heritage, and learners in the beginning, intermediate, and advanced level. The data from the questionnaires were then analyzed in SPSS, with results showing that there are significant differences pertaining to motivation between the female and the male groups as female students’ motivation is higher than that of male students. There were no significant differences found with the amount of motivation between the Chinese heritage and the non-heritage groups. However, the Chinese heritage students displayed much higher integrative and social milieu motivation than the non-heritage students. Furthermore, students of non-heritage background displayed significantly better achievement motivation than students of Chinese heritage. No significant differences with motivational levels between students of different learning levels were found. But in spite of that, there are considerable disparities between these three groups in terms of integrative motivation, language requirement motivation, and social milieu motivation. This study also explores factors that cause students’ demotivation to learn Chinese. The six highest demotivation factors are reported in the following: “Chinese is too difficult for me, tones, characters, grammar, and phrases, etc.”; “my exam grades cannot show my actual Chinese level”; “I feel upset because I put great effort in studying Chinese while making little progress”; “class activities are far too few and boring”; “I cannot understand my Chinese teacher because he/she speaks too fast and difficult for me”; “I do not like using the Chinese textbook”; and “too much homework.” Furthermore, students also complain about the curriculum being too fast-paced with too many exams and homework. Finally, after the analysis and discussion of the data and findings, corresponding pedagogical implications are suggested and explained