can we get wi-fi connectivity for 15 µw? bryce kellogg
TRANSCRIPT
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
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1
1
0
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• 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?
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
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 it affect Wi-Fi Performance?
1 2 3 40
0.5
1
1.5
2
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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
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0
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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