  Next-generation mobile networks promise to support high throughput, massive
connectivity, and improved energy efficiency. To achieve these ambitious goals,
extremely large-scale antenna arrays (ELAAs) and terahertz communications
constitute a pair of promising technologies. This will result in future
wireless communications occurring in the near-field regions. To accurately
portray the channel characteristics of near-field wireless propagation,
spherical wavefront-based models are required and present both opportunities as
well as challenges. Following the basics of near-field communications (NFC), we
contrast it to conventional far-field communications. Moreover, we cover the
key challenges of NFC, including its channel modeling and estimation,
near-field beamfocusing, as well as hardware design. Our numerical results
demonstrate the potential of NFC in improving the spatial multiplexing gain and
positioning accuracy. Finally, a suite of open issues are identified for
motivating future research.
