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10 Things You Should Know About Massive MIMO
1
2
Massive MIMO is multi-user MIMO where the number of base station antennas is much larger than the number of users.
a Uplink
Base station
Single-User MIMO
Multi-User MIMO
Multiple spatial channels for higher data rates
Precoding data for multiple users
h00
S0
h01
TX
h10
S1
h11
^S 0
r0 ... r1 ...
RX
H-1 RX
^S 1
Base station
User terminal
S0
h01
W TX S1
h10
X1
h11
^S r0 ... RX 0 ^S r1 ... RX 1
r0
h00 h01
S0
X0
W00 W01 S0
^S 0
h00 h01
X0
r1
h10 h11
S1
X1
W10 W11 S1
^S 1
h10 h11
X1
In single-user MIMO, all of the processing of calculating channel state information and decoding of the downlink data is performed in the user terminal based on the known pilot or preamble sequence.
b
User terminal
3
In multi-user MIMO, all of the processing of calculating channel state information and precoding of the downlink data is performed in the base station. The user terminal in a multi-user MIMO system has no additional complexity.
Multi-user MIMO uses beamforming to improve signal to interference ratio for a user by forcing the desired signal from each antenna to add in phase and the signals for all other users to add out of phase.
a2 a3
h12
a4
h13
a1 X
b1 Y
k1.x + ~0.y
h14
b2
h21
b3
h22
b4
h23
Precoding A number of precoding techniques may be used with additional processing to mange the peak/average of the signal.
By convention the receiver location is placed first in the h notation.
~0.x + k2.y
h24
200 Element Linear Array
50 Element Linear Array
8-Port Butler Matrix
a4x
h00
X0
Coefficients a1-4 and b1-4 are calculated with knowledge of h1n, h2n to maximize x and y at the two locations. h11
UE1
a3x
Downlink a2x a1x b3y b1y
b2y
C
M
Y
CM
MY
CY
K
4a
Frequency
7
Users that are too close together for spatial multiplexing can be assigned to different time frequency blocks.
b4y
1-of-8 fixed beam selection plus combinations
b
Simple precoding beam steering will not work for massive MIMO as it only provides approximate beamforming and will not provide spatial multiplexing.
5
Massive MIMO provides spatial multiplexing since each user only receives their own signal, allowing all users to share the same time/frequency resources.
The Butler matrix is a type of beamforming network. Depending on which N input is accessed, the antenna beam is steered in a specific direction in one plane.
... Time
Frame structure Uplink data
Uplink pilots
Downlink data
Bc
8
In a TDD system, the channel state information is calculated in the base station by having all of the users transmit a sequence of orthogonal pilot signals at the same time. The channel state information is valid over a specific amount of time (coherence time) and over a specific amount of frequency (coherence bandwidth).
9
Tc
W1906EP 5G Baseband Verification Library
M8190A 12 GSa/s Arbitrary Waveform Generator
6
The performance of massive MIMO improves with adding more antennas, without limit. More antennas allow the energy to be increasingly focused at a specific physical antenna location, reducing overall transmitted power by reducing power transmitted in non-useful directions.
Time to Move ¼ Wavelength
...
CMY
The massive MIMO base station needs to know the channel state from each antenna to each user before transmitting data to allow the signals to add in the correct phase for each user. To control the shape of the beam, you need to control both the amplitude and phase of the antenna feeds.
1R 4L 3R 2L 2R 3L 4R 1L
M8195A 65 GSa/s Arbitrary Waveform Generator with M8197A Synchronization Module
Speed
2 GHz
28 GHz
60 GHz
3 km/h
45 ms
3.2 ms
1.5 ms
30 km/h
4.5 ms
320 µs
150 µs
120 km/h
1.125 ms
80 µs
37 µs
500 km/h
27 µs
19 µs
9 µs
The channel state information is only valid for the duration of the channel coherence time. Massive MIMO will work best in low mobility scenarios and performance will decrease as mobility rates increase.
M9703A AXIe 12-bit High-Speed Digitizer/Wideband Digital Receiver
HARDWARE + SOFTWARE + PEOPLE = 5G INSIGHTS
256 Element (64x4) Antenna Array Patch antenna
10
Planar array size
Frequency
Width
Height
Height
Width
Area
2 GHz
23 mm
40 mm
225 mm
4730 mm
1063 mm2
28 GHz
3.56 mm
2.44 mm
16 mm
338 mm
5.42 mm2
60 GHz
1.66 mm
0.87 mm
7.5 mm
158 mm
1.18 mm2
Massive MIMO is well suited for millimeter wave frequencies due to the small antenna size but is also practical at existing mobile frequency bands as significant performance gains can be realized with 10s to 100s of antenna elements.
Infiniium S-Series High-Definition Oscilloscope with 10-bit ADC
