1) Massive MIMO: It is an important 5G technology.
In single user case, the number of antennas per device is
limited by antennas separation constraint while for multi-user
MIMO, multi-antennas corresponds to many users, allowing a
space diversity gain. Notions such as coordinated multipoint
(CoMP) can turn interference in the system into advantage as
already used in LTE but which should be extended to a dense
network where the BSs have a very large number of antennas
[36] to support a larger number of users per time-frequency
signaling resource. This would enhance the number of users
even without much densification of the network in a simple
way but still requires a fine channel estimation, in addition to
the possible large overhead. This also requires a totally new
architecture to realize the Full-Dimension MIMO (FD-MIMO)
[37]. Note that extensive field measurements are needed for
channel modeling, and some research is already conducted for
mmwave characterization is several environments [34].