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Throughput enhancement of buffer-aided multi-hop UAV relaying wireless networks

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Abstract

Due to the advantages of high mobility, flexibility and fast reconfiguration, the interest of applying unmanned aerial vehicles (UAVs) to wireless communications is growing rapidly in the past decade. A multiple UAVs relaying wireless network is considered in this paper, where buffer-aided UAVs form a multi-hop aerial relaying network to provide the reliable wireless communication services for ground terminals where the direct communication link is unavailable. Considering the buffer constraints, the link selection constraints, the transmit power constraints and the UAV mobility constraints, the average throughput is maxiesqmized. Nevertheless, the formulated optimization problem is intractable because it involves mixed-integer non-convex programming. To track this problem, an iterative algorithm is proposed which can efficient obtain a suboptimal to the original problem. Numerical results validate that this algorithm is convergent and efficient. Furthermore, it is shown that the proposed design can greatly enhance the system performance as compared to benchmark schemes.

Introduction

With the rapid growth of demand for wireless services, unmanned aerial vehicles (UAVs) with such characteristics as high mobility, on-demand deployment and high quality communication links are gradually used in various applications to provide reliable and low-cost wireless communication services, such as remote monitoring, emergency rescue and package delivery, etc [1], [2]. UAVs have several well-known main promising applications in wireless communications. On the one hand, UAVs can be operated as aerial service platforms, known as aerial base stations and relays, by installing wireless communication transceivers to enhance the coverage and communication reliability of wireless networks [3], [4]. The other that UAVs can be served as aerial mobile terminals in cellular networks, known as cellular-connected UAVs, to implement various applications from data collection to remote monitoring [5], [6]. Recently, Qualcomm and AT&T are meaning to use UAVs to assist wireless communications for the upcoming fifth generation (5G). Actually, the existing UAVs can be classified into dynamic UAVs and static UAVs on the basis of whether the mobility of UAV is utilized or not.

The researches on static UAVs are mainly focused on the fact that UAVs can be exploited as aerial service platforms to obtain better communication conditions than ground terminals [7], [8], [9]. The authors in [7] first proposed to maximize the throughput via the joint design of the UAV deployment optimization and the communication resource allocation. In [8], the authors considered a communication system with a friendly UAV jammer, and maximized the intercept probability security region via jointly optimizing the jamming power and the deployment of the jammer. Similarly, a model that the UAV was used as a mobile base station to offer the reliable wireless services to a specific region in [9] and it found that the system sum-rate and coverage can be improved by the optimization of UAV’s altitude. With the rational control of the UAV’s mobility, the system performance can be greatly improved through joint trajectory optimization and communication designs [4], [10], [11]. A single-UAV relaying system and a multi-UAV relaying system were studied in [4], [10], respectively. These studies haven shown that higher throughput can be achieved through UAV trajectory optimization and transmit power allocation as compared with static relaying or non-optimized relaying schemes. Compared with the static relaying system, the mobile UAV relaying system can achieve better security performance as shown in [11]. Therefore, this paper takes advantage of the freedom brought by the UAV’s mobility to further improve the system throughput.

As we all know, due to the high mobility and energy limitation of UAVs, establishing effective communication links has always been a huge challenge for UAV-assisted wireless communications. Although air-to-ground (A2G) channels can often obtain better quality communication links, the low-altitude UAVs will be more or less affected by multi-path fading when communicating with ground terminals, which will cause serious signal fading and affect the communication quality [12]. Actually, the buffer-aided relaying technique can availably handle the negatively impact caused by the small scale fading [13]. The authors found that “load-carry-and-deliver” transmission mode can significantly improve the throughput of UAV-assisted wireless networks in [14], which was the first article to propose the application of buffer to the UAV relaying systems. In [15], [16], the authors studied the trajectory optimization problem based on the buffer constraints, and the simulation results shown that UAVs can communicate with other nodes in a more favorable situation thanks to the buffer and the UAV's mobility. Since multiple UAVs are considered in this paper, buffer-aided UAV relays will provide the feasibility for the communication scheduling.

Generally, a single UAV cannot provide reliable relaying communication services for ultra-long-distance ground terminals or complex working environment, such as densely populated cities or sparsely populated suburban area. However, a multi-UAV relaying system can provide more freedom to solve the problem of blocked communication links. Compared with satellite communications, UAVs can be easily deployed in the required applications. Therefore, a multi-hop UAV enabled wireless communication network is studied in this paper, where buffer-aided UAV relays provide the reliable wireless communication for ground terminals. In multi-hop UAV relaying systems, data transmission generally takes place in a continuous three dimensional (3D) space. Due to the asymmetry between the A2G links and the air-to-air (A2A) links, they cannot be modeled as the same channel model. Thus, the A2G channels and the A2A channels are modeled as Rician fading channels and light-of-sight (LoS) channels, respectively. To the best of our knowledge, little work has been done to optimize the multi-hop UAV relay systems. In [17], the authors considered multi-hop UAV wireless communications, where an aerial backhaul scheme is proposed, which utilizes UAVs as an on-demand flying network connecting the ground small base station and the core network. The proposed scheme seeks to form a multi-hop backhaul network by UAVs via a network formation game. In [10], [18], the authors considered the end-to-end throughput of the multi-hop UAV relaying system via joint trajectory and power allocation optimization for the UAVs. Although [10], [17], [18] considered multi-hop UAV wireless communication systems, they just adopted the pre-determined communication scheme, which did not consider the selection of communication links while optimizing the trajectory. In this paper, the data sent by the source is forwarded hop by hop between UAV relays. Combined the buffer constraints, the link selection constraints, the transmit power constraints and the mobility constraints, the system average throughput is maximized. The main contributions of this paper are summarized as follows:

  • Compared with the existing UAV relay literature, this paper exploits the buffer-aided relays, so the UAVs adopt “store-carry-forward” relaying mode. The buffer-assisted UAV relays break through the traditional relaying mode and UAVs have the ability to select channels flexibly. Taking the advantage of the UAV’s mobility, the link selection strategy is optimized.

  • Since the considered problem is non-convex, the optimization variables are closely coupled, and the number of constraints is semi-infinite, the proposed problem is difficult to solve optimally in general. To resolve this difficulty, an efficient algorithm is proposed based on the block coordinate descent (BCD) and successive convex optimization (SCA) techniques to obtain a local optimal solution.

  • The simulation results show that the proposed scheme can significantly improve the system throughput compared with benchmark schemes, thanks to the degree of freedom brought by the mobility of UAVs.

The remainder of this paper is organized as follows. In Section 2, the system model is characterized and the problem formulation is introduced. In Section 3, an efficient algorithm is proposed. Section 4 carries out the numerical results to testify the performance of the proposed design. Finally, Section 5 summarizes the whole paper.

Section snippets

System model

We design a multi-hop UAV relaying wireless network where K half-duplex UAV relays with buffer, denoted by the set k={1,,K}, are exploited to offer the reliable communication services for ground terminals which are consisted of a source S and a destination D, as shown in Fig. 1. It is supposed that the direct link between ground nodes is unavailable because of the small-scale fading and shadowing fading. In this design, all nodes can only communicate with the designated node, and the data

The proposed algorithm

We discuss the proposed algorithm in more detail which via iteratively optimizing the link selection A, the transmit power P and UAV trajectory Q by fixing other variables under the buffer constraints. Without loss of generality, a local optimal solution of problem (24) can be obtained by iteratively optimizing the above three blocks of variables.

Numerical results

Numerical results are given to demonstrated the proposed design. The main simulation parameters are provided in Table 1. To demonstrate the efficient of the proposed design, the following two benchmark schemes are considered:

  • Static UAV relaying scheme: the static UAVs locate at 600,0,100T and 1400,0,100T, respectively. The maximum average throughput is derived by optimizing the link selection and transmit power at the fixed location.

  • Equal power allocation (EPA) scheme: assuming that the

Conclusions

This paper studies the advantage of buffer-aided UAVs in facilitating the data transmission of multi-hop relaying systems. Specifically, the average throughput of the considered system is investigated by joint optimization of the link selection, transmit power and trajectory. The A2G channels are considered to obey the practical Rician fading model and the A2A channels are LoS channels. Nevertheless, the formulated optimization problem is non-convex. To tackle this intractability, an efficient

Contributions

Dongju Cao, Wendong Yang conceived of the main proposal of the design, proposed an efficient algorithm and derived numerical simulations. Dongju Cao and Wendong Yang wrote the manuscript. Ning Li provided considerable comments of the proposed design. Wendong Yang read and approved the final manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China under Grant Nos. 61671474 and 61771487.

Dongju Cao received the B.S. degree in electronic information engineering from Yancheng Institute of Technology, Yancheng, China, in 2018. She is currently working toward the M.S. degree in College of Communication Engineering, Army Engineering University of PLA. Her current research interests include cooperative communications, UAV communications and convex optimization techniques.

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Dongju Cao received the B.S. degree in electronic information engineering from Yancheng Institute of Technology, Yancheng, China, in 2018. She is currently working toward the M.S. degree in College of Communication Engineering, Army Engineering University of PLA. Her current research interests include cooperative communications, UAV communications and convex optimization techniques.

Wendong Yang received the B.S. degree in communications engineering and the Ph.D. degree in communications and information Systems both from College of Communications Engineering, PLA University of Science and Technology, Nanjing, China in 2004 and 2009 respectively. Since 2009, he has been with College of Communications Engineering, PLA University of Science and Technology. His current research interests include MIMO systems, OFDM systems, cooperative communications and cognitive radio.

Ning Li received the B.S. degree in microwave communications and the M.S. degree in satellite communication both from College of PLA Communications Engineering, Nanjing, China, in 1997 and 2000, respectively. Since 2000, he has been working in the school.

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