Page 252 - Maxwell House

232 ANTENNA BASICS

antenna effective aperture (in general, roughly equals to occupied by antenna and associated
electronics geometrical area) is proportional to . It
2
means, an antenna of the same directivity / gain at 5.7
GHz grabs 23 times more space than at 27.5 GHz and
155 (!) times more than at 71 GHz. Consequently,
every BS tower might host up to 230 - 1550 antennas
(instead of typical 10) thereby forming a so-called
Figure 5.3.7 MIMO set of beams massive MU-MIMO (Multi-User MIMO) with
enormous number (in the range of hundreds and even
thousands) of beams of diverse shapes. Accordingly, the Base Station (BS) acquires the ability
simultaneously communicate with vast spatially separated user terminals changing the
direction, number of beams and their shape according to the position and speed of the mobile
terminal. Particularly, the beams of enhanced directivity can be formed almost in directions to
every user with pattern minimums in directions of other users or undesirable intrusion sources
19
to mitigate thereby interference. Figure 5.3.7 illustrates schematically BS station forming
individual beam of different directivity depending on the distance to the user. As a result, the
user 5G portable terminals might be relatively simple and cheap devices.

The massive MIMO or Large-Scale Antenna Systems (LSAS) requires cutting-edge system
design and signal processing as soon as the number of antennas at each BS increases up to
hundreds. However, accompanying system complicacy pays over and above as soon as several
additional options are exploited [39 - 42]:

1. LSAS enables BS to use advanced signal processing techniques to target in unison multiple
users while reusing the same time and frequency resource. Thereby, the capacity of a
wireless network may increase more than tenfold in extra reliable and dense network
acquiring the ability to handle the never-ending growth in network capacity.
2. In general, the several copies of the same signal from any terminal approach BS from
different directions due to the phenomenon of multipath propagation . It means that two
20
or more spatially separated antennas might be tuned up independently in a timely manner
to peak up the energy of these copies (spatial diversity reception). As soon as these antennas
are several wavelengths apart (corresponds to one or few centimeters in mm-wave band),
the signal portions become almost uncorrelated and their energy might be summarized
21
thereby suppressing to some extend so-called fading effect . Consequently, the
instantaneous signal-to-noise level and communication stability greatly improves.
3. The required transmitted energy per bit vanishes as the number of antennas in a massive
MIMO cell grows to infinity [42].
Several challenging issues of concern regarding antenna arrays and beamformers should be
solved:

19 Public Domain Image, source: https://5g.co.uk/guides/what-is-massive-mimo-technology/
20 Multipath is the propagation phenomenon that results in signals reaching the receiving antenna by two
or more paths as a result of atmospheric ducting, reflection from water surface, ground, buildings, etc.
21 The signal strength at the receive point is the vector sum of the direct space wave and the waves
randomly reflected from ground or water surface and ground objects on their path to the receiver. As a
result, the received signal might fluctuate and fade widely depending on number of waves and their
relative magnitude and phase.