Massive
MIMO - Why Massive MIMO?
We use
very high frequency (mm Wave) signal in 5G. High frequency mean that the size
of single antenna will be very small and the aperture (the area for receiving
energy) will be very small. To overcome this small aperture on receiver side at
high frequency, we need to use a large number of transmission antenna.
It would
be the main reason to adopt the Massive MIMO technology, we can enjoy some
other advantages coming from using a large array antenna.
It would
be the main reason to adopt the Massive MIMO technology, we can enjoy some
other advantages coming from using a large array antenna.
The
received power is decreased in proportion to the square of the distance from
the transmission antenna. For example, if the distance gets two times farther
away, the received power gets decreased by 4 times.
Here we
can see there is no any parameter about frequency or the gain of the receiver’s
antenna. But we know in real scenario the received signal power IS affected by
the frequency (wave length) and receiver antenna gain.
If we
improve the mathematical model to include the frequency (wave length) and
receiver antenna gain, the model can be described as shown below.
As per
below equation, the received power is in proportion to the square of the
wavelength.
As
Example, if we assume the antenna gain does not changes, the frequency gets
increased by 2 times (this mean that the wavelength gets shorten by 2 times),
the received power gets decreased by 4 times.
As
Example, if we use 3 GHz frequency in current communication and we will use 30
GHz frequency in 5G, the wavelength in 30 GHz is 10 times shorter than the
wavelength of 3 GHz. It means the received power at 30 GHz will be 100 times lower
than the received power at 3 GHz.
In
reality, the situation gets even more complicated because not only the receiver
antenna gain but also transmission antenna gain plays role as well. If we add
the transmission antenna gain into the equation, it would become as shown
below.
Now
question is 'how we can make Prx larger?
·
Massive
MIMO can increase the capacity 10 times or more and simultaneously, improve the
radiated energy-efficiency in the order of 100 times.
·
Massive
MIMO simplifies the multiple-access layer
·
Massive
MIMO enables a significant reduction of latency on the air interface (due to
robustness against fading)
·
If we
assume that we are using a fixed antenna size relative to the wavelength (e.g,
size of 1/4 wavelength, 1/2 wavelength etc), as the carrier frequency goes
higher, the path loss increases.
·
This means
the absolute physical size of the antenna gets smaller as carrier frequency
goes higher. It means we can put more antenna in the same area in higher
carrier frequencies. Based on this facts, we may compensate the high path loss
in high carrier frequencies by putting more antenna without increasing the
total size of the antenna array.
Simple Formula
for Network Throughput [bit/s/km2]:
Throughput
(bit/s/km2/) =
Cell density
(Cell/km2) * Available spectrum (Hz) * Spectral efficiency (bit/s/Hz/Cell)
Cell density => Cell density refers to the number of cells per unit
volume. Expensive to deploy more base
stations
Available spectrum=> Plenty of mm-wave spectrum, but only for short-range scenarios
Spectral efficiency=> Can we improve it? Improve technology, upgrade base stations
Maximum amount of data can be
transmitted with the fewest transmission error
In a cellular
telephone network, spectrum efficiency equates to the maximum number of users
per cell that can be provided while maintaining an acceptable quality of
service (QoS).
There is one thing that is automatically acquired by Massive
MIMO. It is the fact that most of energy transmitted from the antenna array
focus on very narrow area. It means the beam width get narrower as you use more
antenna. Following plot would give you an example for the effect of beam width
narrowing with the increased number of antenna.
More Antennas → More Directivity
You would notice how narrower the beam width become as the
number of antenna in the array get larger.
This effect would cause both advantage and disadvantage at
the sometime. Advantage would be that there will be less interference between
beams for different users since each of the beam would be focused in very small
area and the disadvantage would be that you have to implement very
sophisticated algorithm to find exact location of the user and directing the
beam to the user with high accuracy.
Massive MIMO:
Taking Multi-User MIMO to the Next Level
Main Characteristics
·
Many BS antennas; e.g., M=200 antennas, K=40 single-antenna users
·
Many more antennas than users: M>K
·
Very directive signals
·
Little interference leakage
Spectral efficiency
grows with number of users.
http://www.commsys.isy.liu.se/~ebjornson/bjornson_popularscience_2017.pdf
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