  Extremely large antenna array (ELAA) is a common feature of several key
candidate technologies for sixth-generation mobile networks (6G), such as
ultra-massive multiple-input-multiple-output (UM-MIMO), cell-free massive MIMO,
reconfigurable intelligent surface (RIS), and terahertz communications. Since
the number of antennas is very large for ELAA, the electromagnetic radiation
field needs to be modeled by near-field spherical waves, which is opposed to
the conventional planar-wave-based radiation model of 5G massive MIMO. As a
result, near-field communications will become essential in 6G wireless
networks. In this article, we systematically investigate the emerging
near-field communication techniques. Firstly, we present the fundamentals of
near-field communications and the metric to determine the near-field ranges in
typical communication scenarios. Then, we investigate recent studies specific
to near-field communications by classifying them into two categories, i.e.,
techniques addressing the challenges and those exploiting the potentials in
near-field regions. Their principles, recent progress, pros and cons are
discussed. More importantly, several open problems and future research
directions for near-field communications are pointed out. We believe that this
article would inspire more innovations for this important research topic of
near-field communications for 6G.
