Joint Location and Beamforming Design for Cooperative UAVs With Limited Storage Capacity

In this paper, we investigate downlink transmissions in a wireless communication system enabled by a swarm of unmanned aerial vehicles (UAVs) which are spatially dispatched to cooperatively deliver requested contents to ground users. First, we propose a communication scheme that exploits the flexible deployment of UAVs as well as their cooperative transmissions to improve in-network user admission. Unlike previous literature, a practical operational constraint of limited storage capacity for UAVs is considered. Then, from the knowledge that cooperation among UAVs depends on the availability of the contents in their limited storage space, we propose a novel joint optimization problem to determine the content placement, location planning, user admission decision and transmit beamforming to maximize the number of users experiencing a minimum required rate, so-called admitted users. Since the formulated problem is a mixed-integer non-linear program which is generally non-deterministic polynomial-time hard, we proposed a framework that is developed on the basis of difference-of-convex (DC) programming to transform the original problem into a series of approximate convex problems which can be iteratively solved until convergence. Our extensive simulation results reveal that the proposed scheme outperforms other schemes that have been introduced in previous work and reflect a notable trend that deploying more cooperative UAVs with fewer resources (power and storage capacity) is more efficient than deploying fewer UAVs with more resources. In particular, in one of our collected results, the total communication power can be reduced by roughly 40 dB when doubling the number of cooperative UAVs.

I. Introduction

Future wireless communication technologies are under development to adapt to different application scenarios of 5G and beyond (5G/B5G). Specifically, Enhanced Mobile Broadband (eMBB), Massive Machine-Type Communication (mMTC) and Ultra-Reliable Low-Latency Communication (URLLC), also called mission-critical communications, are widely considered the key services in 5G [1]. In other words, 5G networks are expected to accommodate a huge number of mobile devices with heterogeneous and stringent requirements in terms of data rate, latency, and reliability. To meet the heightened expectations for 5G, numerous wireless technologies have been developed including mmWave communications [2], massive MIMO [3], or ultra-densification [4]. However, capacity-enhancing solutions such as installing more cell sites (ultra-densification) or using additional spectrum (mmWave) are costly and may sometimes lead to underutilization of resources due to the time-varying network traffic. Thus, an alternative is to integrate a highly flexible communication platform that can be swiftly deployed to adapt to the dynamic traffic demand, yet ensure adequate quality of service (QoS).