text: This paper considers the exploitation of unmanned aerial vehicles (UAVs) in wireless networking, with which communication-enabled robots operate as flying wireless relays to provide connectivity or a capacity boost to a ground user. We focus on the particular problem of (automatic) UAV positioning, which greatly affects the end-to-end throughput performance. While existing methods rely on propagation distance minimiza- tion and statistical models for the presence or absence of a line-of-sight (LOS), we propose an approach capable of leveraging local topological information so as to offer better performance guarantees. The proposed method allows to strike a trade-off between minimizing distance path loss and discovering (near) LOS opportunities at locations away from the base station (BS)-user axis. Furthermore, the algorithm is shown to find the global optimal UAV position, although it only requires a local exploration of a signal strength map and the length of search trajectory is only linear to the geographical scale. Hence, it lends itself to online implementation. Significant throughput gains are found when compared to other positioning approaches based on LOS statistical models.
  @cite_14: This paper investigates the problem of communication relay between ground units using an unmanned aerial vehicle (UAV). The positions of the ground units are considered to be unavailable to the UAV and the objective is to drive the vehicle in real-time to the optimal placement maximizing the strength of communication signals from ground units. To this end, a novel non model-based navigation law is proposed that is solely based on signals strength and their angles of arrival. The stability of the proposed navigation law is proved in two scenarios, driving the UAV to the optimal placement and returning it back to the launch base. Simulation and experimental results are given to show the effectiveness of the proposed approach.
  @cite_24: We investigate the maneuvering control of a UAV-based relay for efficient communication with mobile ground nodes. The objective of the maneuvering control is to maximize the minimum ergodic link capacity which is achieved by any ground node. We propose an efficient maneuvering control scheme of a UAV-based relay which does not require any location or direction information of ground nodes. The proposed control scheme exploits the received signal strengths (RSSs) from ground nodes but it dose not need a channel model to estimate the location of ground nodes. Therefore, the proposed control scheme significantly reduces the communication overhead and or computational complexity while achieving similar performance, compared with that of an optimal scheme which requires the location information of ground nodes.
  @cite_19: In this paper, we examine the resource optimization problem in a broadcast relay network where a source broadcasts signals to the user receivers, which are distributed over a service region. The source does not have direct line-of-sight to the service area and the information is delivered through a relay. The objective of this paper is to jointly optimize the relay position as well as the power allocation between the source and relay so that the outage probability of the signal received at the user nodes is minimized. Two different optimization criteria, which respectively minimize the worst-case outage probability and the average outage probability, are used. The analyses are verified by simulation results.
  @cite_5: We consider a collection of single-antenna ground nodes communicating with a multi-antenna unmanned aerial vehicle (UAV) over a multiple-access ground-to-air communications link. The UAV uses beamforming to mitigate inter-user interference and achieve spatial division multiple access (SDMA). First, we consider a simple scenario with two static ground nodes and analytically investigate the effect of the UAV's heading on the system sum rate. We then study a more general setting with multiple mobile ground-based terminals, and develop an algorithm for dynamically adjusting the UAV heading to maximize the approximate ergodic sum rate of the uplink channel, using a prediction filter to track the positions of the mobile ground nodes. For the common scenario where a strong line-of-sight (LOS) channel exists between the ground nodes and UAV, we use an asymptotic analysis to find simplified versions of the algorithm for low and high SNR. We present simulation results that demonstrate the benefits of adapting the UAV heading in order to optimize the uplink communications performance. The simulation results also show that the simplified algorithms provide near-optimal performance.
  @cite_25: Abstract Recently, unmanned aerial vehicles (UAVs) acting as relay platforms have attracted considerable attention due to the advantages of extending coverage and improving connectivity for long-range communications. Specifically, in the scenario where the access point (AP) is mobile, a UAV needs to find an efficient path to guarantee the connectivity of the relay link. Motivated by this fact, this paper proposes an optimal design for beamforming (BF) and UAV path planning. First of all, we study a dual-hop amplify-and-forward (AF) wireless relay network, in which a UAV is used as relay between a mobile AP and a fixed base station (BS). In the network, both of the AP and the BS are equipped with multiple antennas, whereas the UAV has a single antenna. Then, we obtain the output signal-to-noise ratio (SNR) of the dual-hop relay network. Based on the criterion of maximizing the output SNR, we develop an optimal design to obtain the solution of the optimal BF weight vector and the UAV heading angle. Next, we derive the closed-form outage probability (OP) expression to investigate the performance of the dual-hop relay network conveniently. Finally, computer simulations show that the proposed approach can obtain nearly optimal flying path and OP performance, indicating the effectiveness of the proposed algorithm. Furthermore, we find that increasing the antenna number at the BS or the maximal heading angle can significantly improve the performance of the considered relay network.
summary: One essential issue for positioning the UAV is to predict the air-to-ground channel, since a precise UAV-user channel knowledge is usually not available before the UAV is sent to a target position. As a way to circumvent this problem, some prior works simply assumed LOS propagation from the UAV to the ground user, and hence the channel gain is merely an explicit continuous function of the UAV position. Using the LOS model, @cite_25 @cite_19 @cite_24 @cite_14 focused on UAV navigation problems, and @cite_5 studied the mimo channel to jointly optimize the UAV position and the orientation of a ula mounted at the UAV for the best spatial channel to multiple users. However, LOS models overestimate the channel gain in urban scenarios, especially in low altitude UAV applications, because there the air-to-ground signal is likely blocked by obstacles surrounding the user, and such a user-side shadowing effect significantly dominates the relay performance.