Unmanned Aerial Vehicles (UAV) play a critical role in the low-altitude economy; however, they face significant challenges in network coverage during transit. This paper investigates the characteristics and challenges of communication networks for UAV. First, we delineate typical scenarios, traffic characteristics, and the dual-layer heterogeneous network topology necessary for three-dimensional continuous coverage and network coexistence. Additionally, the paper addresses the channel characteristics and specific challenges faced by UAV Integrated Sensing and Communication (ISAC) networks, including the limitations of traditional APAA-based networks in terms of cost, complexity, and site selection. Subsequently, we present Reconfigurable Intelligent Surface (RIS)-assisted networks as a viable solution for enhancing UAV signal coverage. Key technical characteristics of RIS are discussed, along with design principles, antenna tilt design, and the application of new beam types. We also explore the implications of high-frequency bands and their absorption peaks on signal attenuation, as well as network architecture designs aimed at improving UAV signal coverage and facilitating network coexistence. Furthermore, the importance of standardization in RIS-based UAV networks is emphasized. We also present field trial results evaluating the effectiveness of RIS in improving UAV coverage. Finally, we outline future technology trends, highlighting potential advancements that could further optimize UAV communication systems.
