This paper studies an unmanned aerial vehicle (UAV)-enabled multicasting system, where a UAV is dispatched to disseminate a common file to a number of geographically distributed ground terminals (GTs). Our objective is to design the UAV trajectory to minimize its mission completion time, while ensuring that each GT is able to successfully recover the file with a high probability required. We consider the use of practical random linear network coding (RLNC) for UAV multicasting, so that each GT is able to recover the file as long as it receives a sufficiently large number of coded packets. However, the formulated UAV trajectory optimization problem is non-convex and difficult to be directly solved. To tackle this issue, we first derive an analytical lower bound for the success probability of each GT's file recovery. Based on this result, we then reformulate the problem into a more tractable form, where the UAV trajectory only needs to be designed to meet a set of constraints each on the minimum connection time with a GT, during which their distance is below a designed threshold. We show that the optimal UAV trajectory only needs to constitute connected line segments, thus it can be obtained by determining first the optimal set of waypoints and then UAV speed along the lines connecting the waypoints. We propose practical schemes for the waypoints design based on a novel concept of virtual base station (VBS) placement and by applying convex optimization techniques. Furthermore, for given set of waypoints, we obtain the optimal UAV speed over the resulting path efficiently by solving a linear programming (LP) problem. Numerical results show that the proposed UAV-enabled multicasting with optimized trajectory design achieves significant performance gains as compared to benchmark schemes. Fig. 1: UAV-enabled information multicasting. and final locations, under various practical constraints such as collision avoidance with other UAVs and/or terrain obstacles [18]-[21]. There have been a handful of works recently on the UAV trajectory design dedicated to optimizing the communication performance. For example, by assuming that the UAV is equipped with multiple antennas and flies with a constant speed, the authors in [22] proposed an algorithm to dynamically adjust the UAV's heading to maximize the ergodic sum rate of the uplink communications from the GTs to the UAV. In [14], for UAV-enabled mobile relaying systems, a design framework for jointly optimizing the communication power/rate allocation and the UAV trajectory, including both the flying direction and speed, was proposed to maximize the communication throughput. For the nonconvex UAV trajectory optimization, [14] proposed the use of successive convex optimization technique to find efficient suboptimal solutions. This technique has been later adopted for UAV trajectory optimization in various other setups, including the energy efficiency maximization for UAV-enabled communication [23], throughput maximization for UAVenabled multi-user downlink communication [24], an...

Abstract-The spectrum usage by a secondary user often happens in a certain geographical region and in a certain time interval, and the requests often come in an online fashion. Considering the selfish behaviors of primary users and secondary users, it is imperative to design online double spectrum auction methods. The most significant challenge is how to make the online double auction economic-robust (truthful in particular). Unfortunately, existing designs either do not consider the online requests or become untruthful when applied to scenarios when both primary users and secondary users could be selfish.In this paper, we address this problem by proposing TODA, a general framework for truthful online double auction for spectrum allocation. We assume that there is a central auctioneer, and the arrivals of secondary users' requests follow Poisson distribution. Upon receiving online spectrum requests, the central auctioneer will decide immediately which secondary and primary users will win the auction, and match winning primary users and secondary users, as well as decide how much secondary users should pay and primary users should get. To preempt existing spectrum usage is not allowed. We study the case in which the conflict graph of secondary users is a complete graph, which occurs in the urban area where the distribution of the secondary users is very dense. In this case, we design strategyproof (truthful) mechanisms for both the primary users and secondary users. To the best of our knowledge, we are the first to design truthful online double auction mechanisms for spectrum allocation. Our simulation results show that the expected social efficiency ratio of our mechanism is always above 80% compared with the off-line VCG mechanism and the spectrum utilization ratio is around 70% when the system is highly loaded.

Wireless communication enabled by unmanned aerial vehicles (UAVs) has emerged as an appealing technology for many application scenarios in future wireless systems. However, the limited endurance of UAVs greatly hinders the practical implementation of UAV-enabled communications. To overcome this issue, this paper proposes a novel scheme for UAV-enabled communications by utilizing the promising technique of proactive caching at the users. Specifically, we focus on content-centric communication systems, where a UAV is dispatched to serve a group of ground nodes (GNs) with random and asynchronous requests for files drawn from a given set. With the proposed scheme, at the beginning of each operation period, the UAV pro-actively transmits the files to a subset of selected GNs that cooperatively cache all the files in the set. As a result, when requested, a file can be retrieved by each GN either directly from its local cache or from its nearest neighbor that has cached the file via device-to-device (D2D) communications.It is revealed that there exists a fundamental trade-off between the file caching cost, which is the total time required for the UAV to transmit the files to their designated caching GNs, and the file retrieval cost, which is the average time required for serving one file request. To characterize this trade-off, we formulate an optimization problem to minimize the weighted sum of the two costs, via jointly designing the file caching policy, the UAV trajectory and communication scheduling. As the formulated problem is NP-hard in general, we propose efficient algorithms to find high-quality approximate solutions for it.Numerical results are provided to corroborate our study and show the great potential of proactive caching for overcoming the limited endurance issue in UAV-enabled communications. enabled wireless communication systems, where a UAV is dispatched to serve a group of GNs with random and asynchronous file requests, i.e., the same content may be requested by different GNs at different time. Note that "content-centric" communications have many practical applications nowadays, such as for video on-demand (VoD) streaming and software download [21]. As shown in Fig. 1, the proposed scheme operates in a periodic manner, with each period consisting of two phases, namely the file caching phase and the file retrieval phase. In the file caching phase, the UAV pro-actively transmits each of the files from a given set of interest to a subset of selected GNs that cooperatively cache all the files. Next, in the file retrieval phase, each GN that has a file request can retrieve the file either directly from its own local cache or from its nearest neighbor that has cached the file via device-to-device (D2D) communications [22]. For the proposed scheme, the UAV is only involved in the file caching phase.Thus, the required UAV operation time for each period only depends on how fast it can transmit the files to the selected caching GNs, instead of the random file request pattern of the GNs. This thus offers a pro...

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