Can we get Wi-Fi connectivity for 15 µW?

Bryce Kellogg

Prototype consumes 14 µW of power

Why is this hard?

Wi-Fi consumes 100 mW of power 10,000x too much

RSSI

Wi-Fi Backscatter

Wi-Fi Device

Wi-Fi Backscatter Tag

Wi-Fi AP2

2

4

4

Changing reflections consumes 0.65 µW of power

Key Idea: Piggyback on Wi-Fi Packets

1

1

1

0

3

3

5

5

6

6

7

7

8

8

9

9

• How to work in a network of Wi-Fi devices?

• How does the Wi-Fi device send data to the tag?

• How do we design a network with multiple tags?

• How to work in a network of Wi-Fi devices?

• How does the Wi-Fi device send data to the tag?

• How do we design a network with multiple tags?

Challenge: Cannot Decode Packet Headers

Wi-Fi Device

Wi-Fi Backscatter Tag

???

Solution: Be Agnostic to Wi-Fi Traffic

0 0 11 0

10:3

4:12

.126

5710

:34:

12.1

2663

10:3

4:12

.126

7810

:34:

12.1

2698

10:3

4:12

.127

19

10:3

4:12

.127

34

10:3

4:12

.127

48

10:3

4:12

.127

57

10:3

4:12

.127

7010

:34:

12.1

2773

10:3

4:12

.127

7510

:34:

12.1

2777

10:3

4:12

.127

79

10:3

4:12

.127

8610

:34:

12.1

2789

Bin backscatter bits into time slots

Use Wi-Fi timestamp to reconstruct bits

Time Slot

tAdjust timeslots based on network traffic

• How to work in a network of Wi-Fi devices?

• How does the Wi-Fi device send data to the tag?

• How do we design a network with multiple tags?

Encode data as presence/absence of Wi-Fi Packet

Wi-Fi Backscatter Tag

CTS-to-Self

Wi-Fi Device

0 0 0001 1 11

OFDM: High peak-to-avg power ratio

Low Power Wi-Fi Packet DetectionLeverage structure of Wi-Fi signal

Can detect packets as short as 50 µsConsumes µWs of power

Antenna EnvelopeDetector Peak finder Set threshold Comparator

Low Power Wi-Fi Packet Detection

• How to work in a network of Wi-Fi devices?

• How does the Wi-Fi device send data to the tag?

• How do we design a network with multiple tags?

Design Request/Response Protocol

Wi-Fi Device

CTS 1 1 1 11

0 00

CTS 1 1 1 1 10 0 0

CTS 1 1 1 1 10 0

0

None of the tags transmit concurrently

Wi-Fi device act as a central coordinator

Our Prototype

Intel 5300 Wi-Fi Cards

Wi-Fi BackscatterPrototype

• 2.4 GHz Wi-Fi channels• Use both RSSI and CSI values

Power Consumption

MSP430: 5.0 µW

RF Switch: 0.65 µW

Peak Finder: 9.0 µW

Total: 14.65 µW

What is the maximum rate and range?

2.2 meters, 1 kbps

3.0 meters, 20 kbpsWi-Fi Backscatter

Tag

Wi-Fi Device

Does AP Location Affect Backscatter Decoding?

33

3

3

3

94%99%99%

98%

8.5 m

12.2 m

Probability of receiving a backscatter packet correctly

AP can be far away or even in a different room

Does Backscatter Affect Wi-Fi Decoding?

33

3

3

3

8.5 m

12.2 m

Does it affect Wi-Fi Performance?

1 2 3 40

0.5

1

1.5

2

2.5

3

3.5

No Backscatter 100 bps

AP Location

Dat

a Ra

te (M

B/s)

Wi-Fi is robust to multipath environment

How does it work in a busy network?

Setup in UW Networks Lab: 8 – 15 People

Used University Wi-Fi as AP

Observed maximum achievable bit rate over period of 8 hours

How does it work in a busy network?

12:00 PM 2:00 PM 4:00 PM 6:00 PM 8:00 PM0

200

400

600

800

1000

1200

0

50

100

150

200

250

Time of Day

Pack

ets

Per S

econ

d

Bit R

ate

(bps

)

More network traffic → faster Backscatter rates

Related Work

• RFID• Requires dedicated reader and new

hardware installation• Wi-Fi Backscatter transforms Wi-Fi devices into

RFID readers

• Ambient Backscatter• Device to device communication• Wi-Fi Backscatter connects devices to existing

Wi-Fi networks

Conclusion

Can enable RF-Powered Internet of Things, connecting next billion devices

Feasibility of Wi-Fi connectivity at < 15 µW

iotWiFi.cs.washington.edu