MIMO systems consist of multiple antennas at both the transmitter and receiver. By adding
multiple antennas, a greater degree of freedom
(in addition to time and frequency dimensions)
in wireless channels can be offered to accommodate
more information data. Hence, a significant
performance improvement can be
obtained in terms of reliability, spectral efficiency,
and energy efficiency. In massive MIMO
systems, the transmitter and/or receiver are
equipped with a large number of antenna elements
(typically tens or even hundreds). Note
that the transmit antennas can be co-located or
distributed (i.e., a DAS system) in different
applications. Also, the enormous number of
receive antennas can be possessed by one device
or distributed to many devices. Besides inheriting
the benefits of conventional MIMO systems,
a massive MIMO system can also
significantly enhance both spectral efficiency
and energy efficiency [9]. Furthermore, in massive
MIMO systems, the effects of noise and
fast fading vanish, and intracell interference
can be mitigated using simple linear precoding
and detection methods. By properly using multiuser
MIMO (MU-MIMO) in massive MIMO
systems, the medium access control (MAC)
layer design can be simplified by avoiding complicated scheduling algorithms [14]. With MUMIMO,
the BS can send separate signals to
individual users using the same time-frequency
resource, as first pro. Consequently, these main
advantages enable the massive MIMO system
to be a promising candidate for 5G wireless
communication networks.