MIMO As a First-Class Citizen in 802.11
Kate C.-J. Lin
Academia Sinica
Shyamnath Gollakota and Dina Katabi
MIT
Wireless nodes increasingly haveheterogeneous numbers of antennas
1-antenna devices 2-antenna devices3-antenna devices
802.11 Was Designed for 1-Antenna Nodes
When a single-antenna node transmits,multi-antenna nodes refrain from transmitting
Alice
Bob
Chris
But, MIMO Nodes Can Receive Multiple Concurrent Streams
Alice
Bob
Chris
Alice
It’s Not That Simple
But, how do we transmit concurrentlywithout interfering with ongoing transmissions?
Interference!!
Interference!!Bob
Chris
Enable concurrent transmissions
without harming ongoing transmissions
Goal
802.11n+
802.11n+
• Allows MIMO nodes to join ongoing transmissions without interfering with them
• Maintains 802.11 random access
• Implemented and shown to significantly improve the throughput
1. How to transmit without interfering with ongoing transmissions? Interference nulling Interference alignment
2. How do we achieve it in a random access manner? Multi-dimensional carrier sense
1. How to transmit without interfering with ongoing transmissions? Interference nulling Interference alignment
2. How do we achieve it in a random access manner? Multi-dimensional carrier sense
Interference Nulling
Alice
Bob
nulling
• Signals cancel each other at Alice’s receiver
• Signals don’t cancel each other at Bob’s receiver
Because channels are different
€
y
€
h1αy+h2βy=0
Interference Nulling
• Signals cancel each other at Alice’s receiver
• Signals don’t cancel each other at Bob’s receiver
Because channels are different
Alice
€
βy
€
αy
€
h1
€
h2
€
≠0Bob
€
⇒ Nulling: h1α =−h2β
Interference Nulling
Interference Nulling
Q: How to transmit without interfering with ongoing
transmissions?
A: Nulling
€
h1αy+h2βy=0Alice
€
βy
€
αy
€
h1
€
h2
€
≠0Bob
€
⇒ Nulling: h1α =−h2β
Alice
Bob
Chris
Is Nulling Alone Enough? NO!!
Alice
Bob
Chris
NO!
Is Nulling Alone Enough? NO!!
Alice
Bob
Chris
nulli
ng
NO!
nulling
N-antenna transmitter cannot null
at N receive antennas
Chris needs to null at three antennas
But, he can’t!
€
(h11α +h21β+h31γ)z = 0
Is Nulling Alone Enough? NO!!
Alice
Bob
€
βz
€
αz
€
γzChris
€
(h12α +h22β+h32γ)z = 0
€
(h13α +h23β+h33γ)z = 0
€
Only solutionα =β=γ=0
Transmit Nothing!!!
NO!null
€
z
Do we really need to null at all antennas?
No, we can use interference alignment
MIMO Basics
1. N-antenna node receives in N-dimensional space
antenna 1
antenna 2
antenna 1
antenna 1
antenna 2
antenna 3
MIMO Basics
1. N-antenna node receives in N-dimensional space
2. Transmitter can rotate the received signal
To rotate received signal y to y’ = Ry, transmitter multiplies its transmitted signal by the same rotation matrix R
y’y2-antenna receiver
= Ry
Interference Alignment
N-antenna node can only decode N signals
wanted signalI1I2
If I1 and I2 are aligned,
appear as one interferer
2-antenna receiver can decode the wanted signal
2-antenna receiver
Interference Alignment
If I1 and I2 are aligned,
appear as one interferer
2-antenna receiver can decode the wanted signal
N-antenna node can only decode N signals
2-antenna receiver I1 + I2
wanted signal
alig
ning
Use Nulling and Alignment
nulli
ng Alice
(unwanted)Bob
Chris
Alice
Bob
Chris
Null as before
alig
ning
Use Nulling and Alignment
Alice
Bob
Chris
nulli
ng
Alice + Chris
(unwanted)Bob
Can decode Bob’s Signal and (Alice + Chris)
All senders transmit, but the throughput is as high as if only
the 3-antenna node is transmitting all the
time
General Protocol
• Each sender in a distributed way computes where and how to null where and how to align
• Analytically proved: # concurrent streams = # max antenna per
sender
1. How to transmit without interfering with ongoing transmissions? Interference nulling Interference alignment
2. How do we achieve it in a random access manner? Multi-dimensional carrier sense
1. How to transmit without interfering with ongoing transmissions? Interference nulling Interference alignment
2. How do we achieve it in a random access manner? Multi-dimensional carrier sense
Alice
Bob
Chris
Centralized controller
But, lost the benefit of 802.11 random access
Alice
Bob
Chris
Bob, Chris, both you can transmit a packet concurrently
Centralized controller
n+ maintains random access!
In 802.11, contend using carrier sense
Multi-Dimensional Carrier Sense
But, how to contend despite ongoing transmissions?
Alice
Bob
Alice
Alice
one signal
AliceBob
two signals
Say that Ben is performing carrier senseBen
Distinguishable using simple linear algebra
Multi-Dimensional Carrier SenseAlice
Bob
Contend
BenAlice
Contend
Alice
Multi-Dimensional Carrier SenseAlice
Bob
Project
BenAlice
Bob and Ben project orthogonal to Alice’s signal
Alice
Project
Multi-Dimensional Carrier SenseAlice
Bob
Project
orthogonal to Aliceno signal from Alice!!
Alice
orthogonal to Aliceno signal from Alice!!
BenAlice
Project
Bob and Ben project orthogonal to Alice’s signal
Multi-Dimensional Carrier SenseAlice
Bob
ProjectAlice
Apply 802.11 contention after projection
BenAlice
Project
orthogonal to Aliceno signal from Alice!!
orthogonal to Aliceno signal from Alice!!
Alice
Alice
Bob
Bob
Detect energy after projection
Multi-Dimensional Carrier Sense
Win
Lose
Ben
Works for arbitrary number of antennas
1. How to transmit without interfering with ongoing transmissions? Interference nulling Interference alignment
2. How do we achieve it in a random access manner? Multi-dimensional carrier sense
Performance
Implementation
• Implemented in USRP2
• OFDM with 802.11-style modulations and convolutional codes
Testbed
Randomly assign the nodes to the marked locations
1. How to transmit without interfering with ongoing transmissions? Interference nulling Interference alignment
2. How do we achieve it in a random access manner? Multi-dimensional carrier sense
1. How to transmit without interfering with ongoing transmissions? Interference nulling Interference alignment
2. How do we achieve it in a random access manner? Multi-dimensional carrier sense
Nulling Experiment
wanted signal
unwanted signal
Can Bob null his signal at Alice’s receiver?
BobAlice
010 15 20 25 27.5-32.5
0
0.5
1
1.5
2
SNR of unwanted signal [dB]
Resi
dual
inte
rfer
ence
[dB]
Nulling Experiment
802.11 SNR range
Nulling Experiment
Residual interference from Bob can reduce the SNR of wanted signal by at most ~1dB
010 15 20 25 27.5-32.5
0
0.5
1
1.5
2
SNR of unwanted signal [dB]
Resi
dual
inte
rfer
ence
[dB]
Alignment and Nulling Experiment
010 15 20 25 27.5-32.5
0
0.5
1
1.5
2
SNR of unwanted signal [dB]
Resi
dual
inte
rfer
ence
[dB]
Alignment and Nulling Experiment
Though alignment is harder, residual interference is still small
~1.5dB
10 15 20 25 27.5-32.50
0.5
1
1.5
2nullingalignment
SNR of unwanted signal [dB]
Resi
dual
inte
rfer
ence
[dB]
0
1. How to transmit without interfering with ongoing transmissions? Interference nulling Interference alignment
2. How do we achieve it in a random access manner? Multi-dimensional carrier sense
Carrier Sense Experiment
tx1 tx1 + tx2 tx1 tx1 + tx2Traditional CS CS after projection
0
0.1
0.2
0.3
Time
Pow
er (m
Watt
)
0
0.005
0.01
0.015
0.02
0.025
Time
Pow
er a
fter
pro
jecti
on
0
0.1
0.2
0.3
Time
Pow
er (m
Watt
)
Carrier Sense Experiment
tx1 tx1 + tx2 tx1 tx1 + tx2
Can’t identify
0
0.005
0.01
0.015
0.02
0.025
Time
Pow
er a
fter
pro
jecti
on
Hard to distinguish
Traditional CS CS after projection
0
0.1
0.2
0.3
Time
Pow
er (m
Watt
)
Carrier Sense Experiment
0
0.005
0.01
0.015
0.02
0.025
Time
Pow
er a
fter
pro
jecti
on
Hard to distinguish
9dB jump
tx1 tx1 + tx2 tx1 tx1 + tx2
Can identifyCan’t identify
Traditional CS CS after projection
Throughput Experiment
Compare 802.11n with 802.11n+
BobAlice Chris
Throughput Experiment
0 10 20 30 40 50 60 70 800
0.2
0.4
0.6
0.8
1
Total throughput [Mb/s]
CDFs
0 10 20 30 40 50 60 70 800
0.2
0.4
0.6
0.8
1
802.11n+ 802.11n
Total throughput [Mb/s]
CDFs
Throughput Experiment
0 10 20 30 40 50 60 70 800
0.2
0.4
0.6
0.8
1
802.11n+ 802.11n
Total throughput [Mb/s]
CDFs
Throughput Experiment
~2x
n+ delivers significant throughput gain in practice
Related Works
• Information theory [CJ08], [MMK08], [JS08], …
• MIMO systems Beamforming [AASK10], SAM [TLFWZC09],
and IAC [GPK09]
First MIMO concurrent transmissions
without any central coordination
Conclusion
• In today’s 802.11, MIMO is an add-on
• In 802.11n+, MIMO is a first-class citizen Higher concurrency With random access
• Shown practical via implementation and testbed evaluation
Top Related