  Unmanned aerial vehicles (UAVs) have found numerous applications and are
expected to bring fertile business opportunities in the next decade. Among
various enabling technologies for UAVs, wireless communication is essential and
has drawn significantly growing attention in recent years. Compared to the
conventional terrestrial communications, UAVs' communications face new
challenges due to their high altitude above the ground and great flexibility of
movement in the three-dimensional (3D) space. Several critical issues arise,
including the line-of-sight (LoS) dominant UAV-ground channels and resultant
strong aerial-terrestrial network interference, the distinct communication
quality of service (QoS) requirements for UAV control messages versus payload
data, the stringent constraints imposed by the size, weight and power (SWAP)
limitations of UAVs, as well as the exploitation of the new design degree of
freedom (DoF) brought by the highly controllable 3D UAV mobility. In this
paper, we give a tutorial overview of the recent advances in UAV communications
to address the above issues, with an emphasis on how to integrate UAVs into the
forthcoming fifth-generation (5G) and future cellular networks. In particular,
we partition our discussions into two promising research and application
frameworks of UAV communications, namely UAV-assisted wireless communications
and cellular-connected UAVs,where UAVs serve as aerial communication platforms
and users, respectively. Furthermore, we point out promising directions for
future research and investigation.
