Advanced MIMO
One promising technology for meeting the future
demands is massive MIMO transmission/reception
[21]. When used with multi-user precoding schemes
such as Maximum Ratio Transmission (MRT) precoding,
also known as channel conjugate precoding,
massive MIMO systems experience small interuser
and inter-cell interferences, and consequently
achieve significantly higher throughput than the
state-of-the-art MIMO systems.
One of the main challenges to build massive MIMO
systems in practice is the limitation in the number of
antennas that can be equipped at a BS, caused by
the BS form factors and operating carrier frequencies.
For example, to horizontally install a large number
of antenna elements (e.g., > 8) at the top of a BS
tower operating with the lowest 4G system frequency
bands of 700 MHz, eight antenna elements with
0.5λ spacing require up to 1.7 m width, where λ
is the carrier wavelength. For the typical 4G system
frequency bands of 2.5 GHz, fitting 32 antenna elements
with 0.5λ spacing require up to 1.9 m width,
which is still not feasible in many BSs that have only
limited room on the tower. This practical limitation
has motivated Full-Dimension MIMO (FD-MIMO)
cellular communication systems, which place a
large number of active antenna elements in a two
dimensional grid at the BSs.
A typical FD-MIMO deployment scenario is illustrated
in Figure 20, for a macro BS with 3 sectors
equipped with 2D Active Antenna Array (AAA) panels