Wi-Fi Backscatter: Internet Connectivity for RF-Powered Devices

Transcription

1 Wi-Fi Backscatter: Internet Connectivity for RF-Powered Devices Bryce Kellogg, Aaron Parks, Shyamnath Gollakota, Joshua R. Smith, and David Wetherall University of Washington Presenter: Xiaolong Zheng

2 Internet of Things All things are connected to the Internet 2

3 Internet of Things Internet is ready: IPv6: Every grain of sand on the earth has millions of IP addresses Are things ready? 3

4 Internet of Things Some things are ready: Smartphones, vehicles and household appliances. Powerful hardware to run Wi-Fi protocols 4

5 Internet of Things RF-powered devices are not ready: RFID tags Internet Control Center Handheld RFID Reader RFID Reader 5

6 Goals RF-powered devices that reuse existing infrastructure to directly connects to the Internet Wi-Fi infrastructure Can RF-powered devices directly communicate with commodity Wi-Fi devices? 6

7 Challenges RF-powered tags cannot speak Wi-Fi Energy consumption Hardware limitations It is not clear how Wi-Fi devices decode signals from RF-powered devices Wi-Fi Backscatter A novel communication system that solves above challenges, making RF-powered things directly communicate with Wi-Fi devices. 7

8 Roadmap Motivation Challenges Design of Wi-Fi Backscatter Uplink Design Downlink Design Evaluation Summary 8

9 Overview of Wi-Fi Backscatter Communicate with Wi-Fi devices Uplink Downlink 9

10 Overview of Wi-Fi Backscatter Communicate with Wi-Fi devices Uplink Downlink 10

11 Roadmap Motivation Challenges Design of Wi-Fi Backscatter Uplink Design: From tags to the Internet Downlink Design: From the Internet to tags Evaluation Summary 11

12 CSI Amplitude Uplink design Modulating Wi-Fi channel at the tag Wi-Fi channel has RSSI/CSI, as the channel measurements of the channel quality. Wi-Fi Helper Wi-Fi Reader Packet Number 12

13 Uplink design Modulate 0/1 bits on CSI of Wi-Fi channel Bit 1: Reflects Wi-Fi helper packets to influence CSI Bit 0: No reflection Wi-Fi Helper Wi-Fi Reader Wi-Fi Backscatter Tag 13

14 Uplink design CSI at the Wi-Fi reader A clear binary modulation on top of CSI can be found 14

15 Uplink design Decoding at the Wi-Fi reader Preprocessing to improve the signal quality Decoding bits 15

16 Preprocessing Exploiting Wi-Fi frequency/spatial diversity Reflections from tags show different effects on different sub-channels due to frequency diversity and multipath. Probability Density Function (PDF) 17

17 Preprocessing Exploiting Wi-Fi frequency/spatial diversity Reflections from tags show different effects on different sub-channels due to frequency diversity and multipath. Probability Density Function (PDF) 18

18 Exploiting diversity Step 1: Identify good sub-channels If good ones are always good, identification will be much easier. But No Wi-Fi sub-channel is consistently good. Wi-Fi sub-channels with BER < 10-2 at various distances 19

19 Exploiting diversity Step 1: Identify good sub-channels No permanently good channels Wi-Fi reader should identify good sub-channels for each uplink transmission Adding preamble before the message Pick out the good sub-channels based on the correlation method. Key insight: The better a sub-channel correlates with the preamble, the better the signal from Wi-Fi Backscatter tag is. 20

20 Exploiting diversity Step 2: Combine information across good subchannels to improve qualities Different sub-channels have different noise variance Using a weighted average : noise variance in the i-th sub-channel. σ i 2 Higher weight for lower variance 21

21 Decoding the bits Decoding by threshold mechanism Bit 1: Bit 0: Practical issues CSI weighted > threshold CSI weighted < threshold Issue: CSI is noisy due to hardware Solution: Configure one Wi-Fi Backscatter tag bit affects multiple Wi-Fi packets, then collect multiple channel measurement and decode by majority voting. 22

22 Roadmap Motivation Challenges Design of Wi-Fi Backscatter Uplink Design: From tags to the Internet Downlink Design: From the Internet to tags Evaluation Summary 23

23 Encoding at the Wi-Fi reader Presence/absence of packets to encode downlink data CTS_to_SELF: block other Wi-Fi devices Preamble: Help tags to detect the packets 24

24 Wi-Fi backscatter tag (Receiver) Detect packets => bit 1; Otherwise, bit 0. Cannot decoding Wi-Fi packets Energy Detection Chips consumes high power Self-designed detector Key insight: Wi-Fi transmissions have high Peak to Average Power Ratio (PAPR). Receiver circuit at Wi-Fi Backscatter tag 25

25 A whole picture Request-response model, similar to RFID Uplink Downlink 26

26 Roadmap Motivation Challenges Design of Wi-Fi Backscatter Evaluation Summary 27

27 Uplink Evaluation BER (Bit Error rate) vs. Distance BER increase with distance BER significant reduces with #of packets per bit increases. Up to 65cm communication range 28

28 Uplink Evaluation Bit rate vs. Helper s transmission rate Tag modulates information on helper s transmission. The smaller the helper s transmission rate is, the lower data rate of uplink is. 29

29 Uplink Evaluation Bit rate in the office network 30

30 Summary A novel communication system makes things directly communicate with commodity Wi-Fi devices to get online. Bit rate: Up to: 1kpbs Office environment: >100bps Communication range: Up to: 65cm Using coding to extend: 2.1m Bit error rate: Easy to keep: <

31 Thank You!

32 Receiver Circuit Design Leverage Envelope Detector 33

33 Receiver Circuit Design Envelope Detector Voltage drop=c1*(r1+r2), control the rate at which the signal s envelope is tracked. 34

34 Prototypes Wi-Fi Backscatter prototype antenna 40.6mm * 30.9mm 35

35 Uplink Evaluation BER vs. Distance BER increase with distance CSI is better than RSSI BER significant reduce with #of packets per bit. Up to 65cm communication range 36

36 Uplink Evaluation BER vs. Frequency diversity Significantly reduce BER by leveraging good sub-channels 37

37 Downlink Evaluation False positive due to failures of preamble 38

38 Downlink Evaluation BER vs. Distance Can identify Wi-Fi packets as small as 50us (20kbps) Communication range: 2.13m for BER<

39 Effects on Wi-Fi communications Location 1: Wi-Fi reader and Wi-Fi Backscatter tag Location 2, 3, 4 and 5: Wi-Fi helpers with line-of-sight and non line-of-sight 40

40 Effects on Wi-Fi communications Location 1: Wi-Fi reader and Wi-Fi Backscatter tag Location 2, 3, 4 and 5: Wi-Fi helpers with line-of-sight and non line-of-sight 41