mimo as a first-class citizen in 802.11
DESCRIPTION
MIMO As a First-Class Citizen in 802.11. Kate C.-J. Lin Academia Sinica Shyamnath Gollakota and Dina Katabi MIT. 1-antenna devices. 2-antenna devices. 3-antenna devices. Wireless nodes increasingly have heterogeneous numbers of antennas. 802.11 Was Designed for 1-Antenna Nodes. Alice. - PowerPoint PPT PresentationTRANSCRIPT
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MIMO As a First-Class Citizen in 802.11
Kate C.-J. LinAcademia Sinica
Shyamnath Gollakota and Dina KatabiMIT
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Wireless nodes increasingly haveheterogeneous numbers of antennas
1-antenna devices 2-antenna devices3-antenna devices
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802.11 Was Designed for 1-Antenna Nodes
When a single-antenna node transmits,multi-antenna nodes refrain from transmitting
Alice
Bob
Chris
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But, MIMO Nodes Can Receive Multiple Concurrent Streams
Alice
Bob
Chris
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Alice
It’s Not That Simple
But, how do we transmit concurrentlywithout interfering with ongoing transmissions?
Interference!!
Interference!!Bob
Chris
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Enable concurrent transmissionswithout harming ongoing transmissions
Goal
802.11n+
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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
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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
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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
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Interference Nulling
Alice
Bob nullin
g
• Signals cancel each other at Alice’s receiver• Signals don’t cancel each other at Bob’s
receiver Because channels are different
€
y
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€
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
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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
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Alice
Bob
Chris
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Is Nulling Alone Enough? NO!!
Alice
Bob
Chris
NO!
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Is Nulling Alone Enough? NO!!
Alice
Bob
Chris
nullin
g
NO!
nullin
g
N-antenna transmitter cannot null
at N receive antennas
Chris needs to null at three antennas But, he can’t!
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€
(h11α +h21β+h31γ)z = 0
Is Nulling Alone Enough? NO!!
Alice
Bob
€
βz
€
αz
€
gzChris
€
(h12α +h22β+h32γ)z = 0
€
(h13α +h23β+h33γ)z = 0
€
Only solution α =β=g=0
Transmit Nothing!!!
NO!null
€
z
Do we really need to null at all antennas?
No, we can use interference alignment
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MIMO Basics1. N-antenna node receives in N-dimensional
space
antenna 1
antenna 2
antenna 1
antenna 1
antenna 2
antenna 3
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MIMO Basics1. N-antenna node receives in N-dimensional
space2. 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
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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
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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 + I2wanted signal
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align
ing
Use Nulling and Alignment
nullin
g Alice(unwanted)Bob
Chris
Alice
Bob
Chris
Null as before
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align
ing
Use Nulling and Alignment
Alice
Bob
Chris
nullin
g
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
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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
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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
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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
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Alice
Bob
Chris
Centralized controller
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But, lost the benefit of 802.11 random access
Alice
Bob
ChrisBob, Chris, both you can transmit a packet concurrently
Centralized controller
n+ maintains random access!
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In 802.11, contend using carrier sense
Multi-Dimensional Carrier Sense
But, how to contend despite ongoing transmissions?
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Alice
Bob
Alice
Alice
one signal
AliceBob
two signals
Say that Ben is performing carrier sense Ben
Distinguishable using simple linear algebra
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Multi-Dimensional Carrier SenseAlice
BobContend
BenAlice
Contend
Alice
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Multi-Dimensional Carrier SenseAlice
BobProject
BenAlice
Bob and Ben project orthogonal to Alice’s signal
Alice
Project
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Multi-Dimensional Carrier SenseAlice
BobProject
orthogonal to Aliceno signal from Alice!!
Alice
orthogonal to Aliceno signal from Alice!!
BenAlice
Project
Bob and Ben project orthogonal to Alice’s signal
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Multi-Dimensional Carrier SenseAlice
BobProject
Alice
Apply 802.11 contention after projection
BenAlice
Project
orthogonal to Aliceno signal from Alice!!
orthogonal to Aliceno signal from Alice!!
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Alice
Alice
Bob
Bob
Detect energy after projection
Multi-Dimensional Carrier Sense
Win
LoseBen
Works for arbitrary number of antennas
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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
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Performance
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Implementation
• Implemented in USRP2
• OFDM with 802.11-style modulations and convolutional codes
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TestbedRandomly assign the nodes to the marked locations
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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
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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
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Nulling Experiment
wanted signal
unwanted signal
Can Bob null his signal at Alice’s receiver?
BobAlice
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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
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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]
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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]
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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
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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
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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 p
roje
ction
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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 p
roje
ction
Hard to distinguish
Traditional CS CS after projection
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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 p
roje
ction
Hard to distinguish9dB jump
tx1 tx1 + tx2 tx1 tx1 + tx2
Can identifyCan’t identify
Traditional CS CS after projection
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Throughput Experiment
Compare 802.11n with 802.11n+
BobAlice Chris
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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
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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
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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
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Related Works• Information theory
[CJ08], [MMK08], [JS08], …
• MIMO systems Beamforming [AASK10], SAM [TLFWZC09],
and IAC [GPK09]
First MIMO concurrent transmissions
without any central coordination
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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