With the explosive growth of data communications, existing infrastructure networks are under ever-increasing pressure. Due to the advantages of fully controllable mobility, rapid deployment, and low cost, the unmanned aerial vehicles (UAVs) have attracted much attentions from both industry and academia in recent years, and it has become an inevitable trend to employ UAVs to enhance the network performance in different environments.As an important paradigm of UAV-assisted communications, UAV relaying communications has been regarded as a promising solution in enhancing connectivity and improving transmission rate. This paper for the first time comprehensively summarizes UAV relaying communications and its application scenarios, including single UAV relaying networks, multi-user UAV relaying networks, multi-hop UAV relaying networks, as well as Internet of UAVs, and deeply analyzes the key technologies and challenges to be solved under this topic. Furthermore, the state-of-the-art researches and opportunities of UAV relaying communications are discussed in detail.
INTRODUCTIONWith the dramatic development of wireless communications as well as the rapid increase in the scale and variety of mobile communication services, the amount of data to be transmitted is constantly exploding. And thus mobile data transmissions with high capacity, high speed, and high coverage has become the focus of attention in the industry, which is also an urgent problem to be solved. In this regard, the existing wireless communication system seems to be insufficient. Fortunately, the application of UAV-assisted communications provides opportunities to solve this problem, which can be employed as a supplement and extension of current communication systems to improve the performance [1,2]. In recent decades, with the continuous maturity of technologies such as flight control, navigation, power, and communications, the safety and reliability of UAVs have also been constantly improved, so the applicationThis is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.