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ISM Band Is Increasingly Crowded

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

- Most problems are from cross-technology high-power interferers
- Responsible for more than 50% of the customer complaints
- Lead to complete loss of connectivity

Cordless Phones

Microwave Ovens

Baby Monitors

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

20 feet

WiFirx

Effect of High-Power Interferers on WiFi

WiFi Throughput

(in Mbps)

Interferer Location #

1 foot

90 feet

Line of sight

Non- Line of sight

Effect of High-Power Interferers on WiFi

Without Interferers

WiFi Throughput

(in Mbps)

With Microwave

With baby Monitor

With Cordless Phone

Interferer Location #

1 foot

90 feet

Line of sight

Non- Line of sight

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?

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

Client

AP

If MIMO can work across diverse technologies

Challenge: Current MIMO doesn’t work with diverse technologies

MIMO Primer

AP

Client

- 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

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

Say, Interferer is One of the Streams

AP

Client

But, AP doesn’t know interferer technology / preamble

Can’t compute interferer channels, h3 and h4

Fundamental Limitation of Channel Estimation

Scenario 1

Scenario 2

Interference

Channel

Interference

Channel

Can’t distinguish between the two scenario

Impossible to exactly estimate interferer channels

How Does TIMO Work?

AP

Client

AP is not interested in decoding baby monitor

- Reduce the number of unknowns to three

How Does TIMO Work?

AP

Client

AP is not interested in decoding baby monitor

- Reduce the number of unknowns to three

How Does TIMO Work?

AP

Client

AP is not interested in decoding baby monitor

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

How Does TIMO Work?

AP

Client

AP is not interested in decoding baby monitor

- 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

Scenario 1

Scenario 2

Interference

Channel

Interference

Channel

The scaling factor, c, introduces ambiguity into channels

Unlike channels, the channel ratio is not ambiguous

If β Can be Computed, AP Can Decode WiFi Client

AP

Client

AP can solve the two equations to decode the WiFi client

Question: How do we compute β?

Answer: Send known symbol

- WiFi client sends known symbol at beginning of its packet

Question: How do we compute β?

Answer: Send known symbol

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

Question: How do we compute β?

Answer: Send known symbol

Time

Known symbol

But, what if interferer is concentrated in time

Use β to decode subsequent symbols

Question: How do we compute β?

Answer: Send known symbol

Time

Known symbol

Known symbol

- We have a solution to compute β without known symbols

But, what if interferer is concentrated in time

Intuition: Exploit the WiFi Symbol Structure

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

Imaginary

+1

-1

Real

Intuition: Exploit the WiFi Symbol Structure

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

Imaginary

+1

-1

Real

Intuition: Exploit the WiFi Symbol Structure

- 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

Imaginary

correct

Correct estimate

Average error is small

Error

+1

-1

Real

Intuition: Exploit the WiFi Symbol Structure

- 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

Imaginary

guess1

correct

Bad estimate

Average error is big

+1

-1

Real

Error

Intuition: Exploit the WiFi Symbol Structure

- 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

Imaginary

guess2

guess1

correct

Better Estimate

Average error reduce

Error

+1

-1

Real

- Design gradient descent style algorithm to iteratively converge to actual channel ratio
- Paper described algorithm that works across modulations

- Implement using USRP2s
- WiFi modulations and coding rates
- OFDM over 10 MHz
- Bits rates between 3-27 Mbps
- No carrier sense

- Place USRP prototype for 802.11 at blue locations
- Change the location of interferer over red locations

Tx

Rx

Throughput Performance with Baby Monitor

802.11 Throughput

(in Mbps)

WiFi

Interferer Location #

Line of sight

Non- Line of sight

1 foot

90 feet

Throughput Performance with Baby Monitor

802.11 Throughput

(in Mbps)

USRP WiFi

60 feet away

WiFi

Interferer Location #

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

Line of sight

Non- Line of sight

1 foot

90 feet

Throughput Performance with Baby Monitor

Without interference

USRP WiFi with TIMO

802.11 Throughput

(in Mbps)

USRP WiFi

WiFi

Interferer Location #

Line of sight

Non- Line of sight

1 foot

90 feet

Cordless Phones

Microwave Ovens

with TIMO

with TIMO

802.11 Throughput(in Mbps)

802.11 Throughput(in Mbps)

w/o TIMO

w/o TIMO

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

Interferer Location #

Interferer Location #

- Decoding Interference [IC, SAM, Beamforming, …]
- Cognitive Communication [Samplewidth, Jello, Swift, …]

- 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

- 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

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