A holistic survey of UAV-assisted wireless communications in the transition from 5G to 6G: State-of-the-art intertwined innovations, challenges, and opportunities

Due to the rapid progress in communication technologies, unmanned aerial vehicles (UAVs) have become increasingly capable of providing reliable and cost-effective wireless communication from aerial vantage points. Unlike conventional stationary infrastructure, UAVs exhibit attractive features such as high scalability and improved line-of-sight (LoS) connectivity. Consequently, UAV-assisted wireless communications have become a promising paradigm to enhance coverage and connectivity in terrestrial and non-terrestrial networks. Nevertheless, the efficient deployment of UAVs in continuously evolving wireless network scenarios has remained to be a challenging task. These challenges have attracted a large body of research literature and subsequently several survey papers on UAV-assisted wireless communications. One of the distinctive features of research in UAV-assisted wireless networks is its broad array of experimental and analytical tools and techniques. A thorough review of these methodologies can swiftly familiarize researchers with the most recent efforts within this expansive field. However, most of the existing review/survey papers in this domain lack a comprehensive discussion about the advanced technologies used in UAV-assisted wireless networks, such as rate-splitting multiple access (RSMA), simultaneous wireless information and power transfer (SWIPT), digital twin (DT), cognitive radio (CR), space-air-ground integrated network (SAGIN), cell-free massive multiple-input multiple-output (CF mMIMO), integrated sensing and communication (ISAC), quantum technology, holographic MIMO (HMIMO). Motivated by this limitation and considering the novel UAV-assisted communication scenarios that can benefit from the adoption of such technologies; this work provides a thorough analysis of state-of-the-art intertwined technologies relative to UAV-assisted communications along with a discussion of their effectiveness and limitations. Furthermore, this study provides a brief overview of the comprehensive challenges of UAV-assisted networks, along with their security challenges, and opens future direction in this domain. This work finally explores the unique challenges in each of the existing technologies developed for UAV-assisted wireless networks that have been limitedly explored in prior literature aimed at providing a set of directions for future works.