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