# Massive MIMO Explained: It Work? MM

```Massive MIMO Explained:
Part 1: What is Massive MIMO and How Does
It Work?
March 2014
MM
Div. of Communication Systems
Dept. of Electrical Engineering (ISY)
oping University
oping, Sweden
www.commsys.isy.liu.se
YS
Massive MIMO
k=1
M=
x100
antennas!
K
terminals
k=K
◮
Massive MIMO is massive multiuser MIMO:
◮
◮
M ≫ K ≫ 1 (think M = ×00, K = ×0)
coherent, but simple, processing
◮
Spatial multiplexing gain O(K), in favorable propagation
◮
√
Array gain (radiated energy efficiency) gain O(M ), or O( M )
1/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
Massive MIMO Deployment
◮
◮
Testbeds:
Argos (USA), 96× ∼10
FP7-MAMMOET, 100×?
Massive MIMO is not mmWave MIMO!
2/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
λ
4v
c
∆
Tc =
Bc =
frequency
Coherence Interval
τc , Bc Tc symbols
time
At 2 GHz carrier frequency:
pedestrian
v = 1.5 m/s
indoors
∆ ∼ 30 meters
Bc = 10 MHz
Tc = 25 ms
τc = 250000
vehicular
v = 30 m/s
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
outdoors
∆ ∼ 1000 meters
Bc = 300 kHz
Tc = 25 ms
τc = 7500
Bc = 300 kHz
Tc = 1.25 ms
τc = 375
Communication Systems
oping University
3/12
τc for a Coherence Bandwidth of Bc = 200 kHz
fc = 1GHz
fc = 2GHz
fc = 6GHz
fc = 60GHz
τc = 15000
τc = 7500
τc = 2500
τc = 250
10 km/h
τc = 5400
τc = 2700
τc = 900
τc = 90
50 km/h
τc = 1080
τc = 540
τc = 180
τc = 18
100 km/h
τc = 540
τc = 270
τc = 90
τc = 9
350 km/h
τc = 154
τc = 77
τc = 26
τc = 3
pedestrian
1 m/s
(with the convention that Tc =
λ
)
4v
4/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
Massive MIMO Operation
◮
Main challenge in MU-MIMO: getting CSI at the base station.
◮
FDD training: BS transmits pilots, terminals report CSI.
◮
◮
◮
◮
◮
◮
Downlink training: each antenna transmits sequence ψm , of length
τp , in each coherence interval!
Optimal training on the downlink requires
(
const 6= 0, m = m′
H
ψm
ψm′ =
0,
m 6= m′
Require M ≤ τp ≤ τc !
FDD operation is not scalable, and probably infeasible in most cases
TDD training: terminals transmit pilots, BS estimates channel
◮
◮
◮
◮
The K terminals send pilots of length τp in each coherence interval
Require K ≤ τp ≤ τc !
Pilot overhead independent of M — TDD entirely scalable w.r.t. M
5/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
Massive MIMO Operation
◮
◮
◮
◮
◮
acquire CSI from uplink pilots and/or blindly from data
detect symbols
M ≫ K ⇒ linear processing (MRC, ZF, MMSE) nearly optimal
◮
◮
◮
◮
use CSI obtained on the uplink
make necessary adjustments based on reciprocity calibration
apply multiuser MIMO precoding
simple precoders desirable (and very good!): MRT, ZF, R-ZF, ...
6/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
◮
M antennas
◮
K terminals (single-antenna)
 1

1
g1 · · · gK

.. 
..
G ,  ...
.
. 
◮
g1M
◮
M
· · · gK
√
Uplink: y = Pu Gx + w,
◮
√
Downlink: y = Pd GT x + w.
◮
gkm =
terminal 1
1
terminal K
2
m
gkm
M
base station array
terminal k
√
βk h m
k
7/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
◮
MRC/MRT:
◮
◮
ˆ=
GH
2
kGk

·y

∗
G

◮ MRT precoder (downlink): x =  q
·q
E kGk2
X
1
◮ Channel hardening:
βk , M ≫ 1
kGk2 ≈
M
k
◮ intracell interference treated as noise
◮ typical operating point is 1 bps/Hz/terminal; K bps/Hz total
◮ distributed implementation
ZF:
ˆ = (GH G)−1 GH · y
◮
◮
◮

G∗ (GT G∗ )−1


ZF precoder (downlink): x =  q ·q
E trace{(GT G∗ )−1 }
can cancel out (some) interference
computationally somewhat more demanding
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
8/12
Communication Systems
oping University
Bc
Nsmooth subcarriers
Nslot OFDM symbols
coherence interval
frequency
OFDM with TDD Operation
Nsmooth
Nslot
Ts
Tslot
time
τc = Nslot Nsmooth symbols
τu
τp
τd
UL data
UL pilots
DL data
τc symbols
9/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
Typical OFDM Parameters
coherence bandwidth
subcarrier spacing
number of subcarriers within coherence bandwidth
slot duration
OFDM symbol duration
number of OFDM symbols within one slot
Bc
Bs
Nsmooth
Tslot
Ts
Nslot
210 kHz
15 kHz
14
1 ms
71 µs
14
10/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
Literature for this Lecture
◮
T. L. Marzetta, “Noncooperative MU-MIMO with unlimited
numbers of base station antennas,” IEEE Trans. WC 2010.
◮
E. G. Larsson, F. Tufvesson, O. Edfors and T. L. Marzetta, “Massive
MIMO for Next Generation Wireless Systems,” IEEE Comm.
Magazine, Feb. 2014.
◮
F. Rusek, D. Persson, B. K. Lau, E. G. Larsson, T. L. Marzetta, O.
Edfors, and F. Tufvesson, “Scaling up MIMO: Opportunities and
Challenges with Large Arrays”, IEEE SP Magazine, Jan. 2013
◮
C. Shepard et al., “Argos: practical many-antenna base stations”, in
Proc. MobiCom 2012.
11/12
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
Communication Systems
oping University
Massive MIMO Explained
is Based on Joint Work and Discussion with My Colleagues:
◦
◦
◦
◦
◦
◦
◦
Hien Q. Ngo (LiU, Sweden)
Christopher Moll´en (LiU)
Marcus Karlsson (LiU)
Antonios Pitarokoilis (LiU)
Hei Victor Cheng (LiU)
Emil Bj¨
ornson (LiU)
◦ Fredrik Tufvesson (LU, Sweden)
◦ Ove Edfors (LU)
◦ Fredrik Rusek (LU)
◦ Thomas L. Marzetta (Bell Labs/Alcatel-Lucent, USA)
◦ Saif K. Mohammed (IIT, Dehli)
Massive MIMO Explained: Part 1: What is Massive MIMO and How Does It Work?
12/12
Communication Systems
oping University
```