Integrating the unmanned aerial vehicles (UAVs) into the cellular network is envisioned to be a promising technology to significantly enhance the communication performance of both UAVs and existing terrestrial users. In this paper, we first provide an overview on the two main paradigms in cellular UAV communications, namely, cellular-enabled UAV communication with UAVs as new aerial users served by the ground base stations (GBSs), and UAV-assisted cellular communication with UAVs as new aerial communication platforms serving the terrestrial users. Then, we focus on the former paradigm and study a new UAV trajectory design problem subject to practical communication connectivity constraints with the GBSs. Specifically, we consider a cellular-connected UAV in the mission of flying from an initial location to a final location that are given, during which it needs to maintain reliable communication with the cellular network by associating with one of the available GBSs at each time instant that has the best line-of-sight (LoS) channel (or shortest distance) with it. We aim to minimize the UAV's mission completion time by optimizing its trajectory, subject to a quality-of-connectivity constraint of the GBS-UAV link specified by a minimum receive signal-to-noise ratio (SNR) target, which needs to be satisfied throughout its mission. To tackle this challenging non-convex optimization problem, we first propose an efficient method to verify its feasibility via checking the connectivity between two given vertices on an equivalent graph. Next, by examining the GBS-UAV association sequence over time, we obtain useful structural results on the optimal UAV trajectory, based on which two efficient methods are proposed This work will be presented in part at stations (GBSs). On the other hand, thanks to the continuous improvement in UAV payload weight and communication device miniaturization, it becomes more feasible for UAVs (such as drones, helikites, balloons) to carry communication equipments in the sky, to provide or enhance the communication services for the terrestrial users in the cellular networks (see e.g.[8], [9] for initial trials made by Facebook and Google). Generally speaking, the interplay between UAVs and the cellular network can be fully exploited to improve the communication performance of both UAVs and existing cellular users on the ground, which calls for a new research paradigm to investigate their joint design and performance optimization.In the rest of this section, we first give an overview on the promising technologies for integrating UAVs into the cellular network. Next, we provide a review of related prior work, and discuss a new and critical communication-aware trajectory design problem in cellular-enabled UAV communication, which motivates the main research in this work. Last, we summarize the main results and contributions of this paper for solving this problem.
A. UAV Meets Cellular Network: An OverviewAs briefly introduced above, there are two main paradigms for integrating UAVs into the cellular n...