combating cross-technology interference

Combating Cross-Technology Interference

Shyamnath Gollakota

Fadel AdibDina Katabi

Srinivasan Seshan

ISM Band Is Increasingly Crowded

• Most problems are from cross-technology high-power interferers

• Responsible for more than 50% of the customer complaints

• Lead to complete loss of connectivity

Microwave Ovens Cordless PhonesBaby Monitors

Multiple independent studies [Cisco, Ofcom, Jupiter, Farpoint]

Experimental Setup

• Two Netgear 802.11n devices

• Baby monitors, cordless phones and microwave ovens

• WiFi devices about 20 feet away from each other

• Move interferer 1-90 feet away from WiFi receiver

WiFi tx

WiFi rx

20 feet

Effect of High-Power Interferers on WiFiW

iFi T

hrou

ghpu

t (i

n M

bps)

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80

1 foot 90 feet

Line of sight Non- Line of sight

Interferer Location #

0 1 2 3 4 5 6 7 8 9 100

20

40

60

80Without Interferers

With Microwave

With baby MonitorWith Cordless Phone

Effect of High-Power Interferers on WiFi



WiF

i Thr

ough

put

(in

Mbp

s)

1 foot 90 feet

Traditional Solutions to Cross Technology Interference Don’t Work

• Avoid interferer frequencies Much wider bandwidth than WiFi Interferer can occupy multiple WiFi channels

Traditional Solutions to Cross Technology Interference Don’t Work

• Avoid interferer frequencies Much wider bandwidth than WiFi Interferer can occupy multiple WiFi channels

• Treat interferer as noise and use lower rate High power interferers (e.g., 8-100X WiFi power) Can’t get even lowest WiFi rate

How can we deal with such high-power interference?How can we deal with such high-power interference?

Technology Independent Multiple Output (TIMO)

• First WiFi receiver that decodes in presence of high-power

cross-technology interferers

• Is agnostic to the interferer’s technology

• Implemented and evaluated with baby monitors, microwave

ovens and cordless phones Convert no-connectivity scenarios to operational networks

Idea: Try to leverage MIMO

APClient

Today, streams are of the same technology


APClient

If MIMO can work across diverse technologies


APClient

Challenge: Current MIMO doesn’t work with diverse technologies

If MIMO can work across diverse technologies

MIMO Primer

APClient1h

2h

4h

3h

111Shy

If channels are known, AP can solve equations to decode the two streams, S1 and S2

23Sh

122Shy

24Sh

1S

2S

1y

2y

How do current APs estimate the channels?• Client sends a known preamble on the two antennas• AP correlates with known preamble to estimate channels• Doesn’t work across technologies

How do current APs estimate the channels?• Client sends a known preamble on the two antennas• AP correlates with known preamble to estimate channels• Doesn’t work across technologies

Ih4

Shy22

Say, Interferer is One of the Streams

APClient1h

2h

But, AP doesn’t know interferer technology / preamble Can’t compute interferer channels, h3 and h4

Shy11

Ih3

I

S

3h

4h

Scenario 2

c

IchShy

311

c

IchShy

422

InterferenceChannel

Scenario 1

InterferenceChannel

IhShy311

IhShy422

Fundamental Limitation of Channel Estimation

Can’t distinguish between the two scenario Impossible to exactly estimate interferer channels

How Does TIMO Work?

AP is not interested in decoding baby monitor

I APClient

IhShy311

I

S 1h

2h

3h

4h

IhShy422

• Reduce the number of unknowns to three

How Does TIMO Work?

I APClient

IhShy311

I

S 1h

2h

3h

4h

IhShy422

Ih

h

3

4


• Reduce the number of unknowns to three

How Does TIMO Work?

APClient

IShy 11

I

S 1h

2h

3h

4h I

h

hShy

3

4

223

4

h

h


• Reduce the number of unknowns to three• β is the interferer channel ratio

How Does TIMO Work?

APClient

IShy 11

I

S 1h

2h

3h

4h

IShy 22

3

4

h

h


• Reduce the number of unknowns to three• β is the interferer channel ratio • Focus on channel ratio instead of channels

Getting Around the Fundamental Limitation

Unlike channels, the channel ratio is not ambiguous

Scenario 2

c

IchShy

311

c

IchShy

422

InterferenceChannel

Scenario 1

InterferenceChannel

IhShy311

IhShy422

3

4

h

h

ch

ch

3

4The scaling factor, c, introduces ambiguity into channels

If β Can be Computed, AP Can Decode WiFi Client

APClient

IShy 11

I

S 1h

2h

3h

4h

IShy 22

3

4

h

h

AP can solve the two equations to decode the WiFi client

Question: How do we compute β?

Shy11

I

Shy22

I

Answer: Send known symbol

• WiFi client sends known symbol at beginning of its packet



Known

Known

• WiFi client sends known symbol at beginning of its packet

• Solve equations to get β

• Once β is known, it can be used to decode subsequent

symbols

I Shy11

Shy22

I

Use β to decode subsequent symbols

But, what if interferer is concentrated in time

Time

Known symbol



Known symbol

Time

Known symbol



But, what if interferer is concentrated in timeWe have a solution to compute β without known symbolsWe have a solution to compute β without known symbols

Intuition: Exploit the WiFi Symbol Structure

Real

Imaginary

• BPSK – ‘1’ bit sent as +1 and ‘0’ bit sent as -1

+1-1


Real

Imaginary

• BPSK – ‘1’ bit sent as +1 and ‘0’ bit sent as -1• If no interference, received symbols are close to expected symbols

+1-1


Real

Imaginary

+1-1

• BPSK – ‘1’ bit sent as +1 and ‘0’ bit sent as -1• If no interference, received symbols are close to expected symbols• If interference, received symbols are far from expected symbols

Correct estimate Average error is small

Error

correct


Real

Imaginary

+1-1


Bad estimate Average error is big

Error

correct

guess1


Real

Imaginary

+1-1


Better Estimate Average error reduce

Error

• Design gradient descent style algorithm to iteratively converge to actual channel ratio

• Paper described algorithm that works across modulations

• Design gradient descent style algorithm to iteratively converge to actual channel ratio

• Paper described algorithm that works across modulations

correct

guess1guess2

Performance

• Implement using USRP2s

• WiFi modulations and coding rates

• OFDM over 10 MHz

• Bits rates between 3-27 Mbps

• No carrier sense

Implementation

Testbed

• Place USRP prototype for 802.11 at blue locations

• Change the location of interferer over red locations

RxTx

Throughput Performance with Baby Monitor


802.

11 T

hrou

ghpu

t (i

n M

bps)

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25


WiFi

1 foot 90 feet

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25



60 feet away

802.

11 T

hrou

ghpu

t (i

n M

bps)


USRP WiFi

WiFi

1 foot 90 feetDespite disabling carrier sense, complete loss of connectivity in more than half the location

Despite disabling carrier sense, complete loss of connectivity in more than half the location

USRP WiFi with TIMO

Interferer Location #1 foot 90 feet


802.

11 T

hrou

ghpu

t (i

n M

bps)


Without interference

WiFi

USRP WiFi

1 2 3 4 5 6 7 8 9 1005

10152025

Throughput Performance

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25

802.

11 T

hrou

ghpu

t (in

Mbp

s)


Cordless Phones

w/o TIMO

with TIMO

802.

11 T

hrou

ghpu

t (in

Mbp

s)

1 2 3 4 5 6 7 8 9 100

5

10

15

20

25


w/o TIMO

with TIMO

Microwave Ovens

TIMO transforms scenarios with a complete loss of connectivity to operational networks

TIMO transforms scenarios with a complete loss of connectivity to operational networks

• Decoding Interference [IC, SAM, Beamforming, …]

• Cognitive Communication [Samplewidth, Jello, Swift, …]

Related Work

- Don’t work with cross-technology interference

- Don’t operate on the same frequency

First system to decode in the presence of cross-technology interference on same band

Conclusions

• First WiFi receiver that decodes in presence of high-power

cross-technology interferers

• Enable MIMO to work across technologies

• Implemented and evaluated with baby monitors, microwave

ovens and cordless phones Convert no-connectivity scenarios to operational networks

combating cross-technology interference

Documents

wifi interference

wifi transmitter

wifi receiver wifi txwifi

wifi experiences interference

wifi doesnt ge

traditional interferers

nonline of sight location

digital cordless phones