1. CS434/534: Topics in Networked (Networking) Systems Mobile Networking System: Making Connections: Backscatter Geng Li Computer Science Department Yale University 208A Watson

2. Outline Admin Android Platform overview Basic concepts
Inter-thread: execution model with multiple threads
Inter-process: component composition
Inter-machine: network-wise composition
Service discovery
Make connections
Bluetooth
WiFi Direct
Cellular
Mobile cloud
RFID – Backscatter (New wireless technology)

3. Recap: RFID Tag
Tag is a device used to transmit information such as a serial number to the reader in a contact less manner
Classified as
Passive : energy from reader
Active : energy from battery
Semi-passive : energy from battery and energy from reader

4. Recap: Near Field RFID
<= 100 Mhz

5. Near Field RFID Non-radiative techniques
Either inductive or magnetic resonant coupling
Varying magnetic flux induces current
Transfer power over short distances N S
Receiver
Source

6. Far Field RFID Radiative techniques
Electric field of the electromagnetic waves
Radio frequency (RF) waves as an energy delivery medium
Transfer power over longer distances
Provision of energy to many receivers simultaneously
broadcast nature
Charge low-power embedded devices
RFID tags
chip
antenna

7. Far Field RFID
Active
Battery-free communication!
Semi-passive

8. RFID Backscatter

9. Backscatter – Key Principle
When a wave encounters a boundary between two media （impedances/densities）, the wave is reflected back
Creating an additional impedance boundary between the antenna and the backscatter circuit

10. Backscatter in SIGCOMM
2013: Best paper
2014: 2/8 in Wireless Session
2015: 2/4
2016: 3/5, including the Best Paper

11. Three Types of Novel Backscatter
Ambient Backscatter
Wi-Fi Backscatter
Inter-Technology Backscatter

12. Three Types of Novel Backscatter
Ambient Backscatter
Wi-Fi Backscatter
Inter-Technology Backscatter

13. Ambient Backscatter Ambient RF signal source and energy source? TV
Radio
Cellular transmissions
WiFi
Bluetooth

14. Leverage Existing Wireless SignalsTV
Cellular
Wi-Fi
Available at almost any time and place, rain or shine

15. Communication between two battery-free devices

16. Ambient Backscatter- ON-OFF keying modulation
TV Tower
Works with only ~5% of the harvested power!
Reception
Unaffected
Additional
Multipath
Bob
Alice
‘0’ bit – Absorb TV Signals
‘1’ bit – Reflect TV signals

17. Block diagram
It consists of a transmitter, a receiver and a harvester that all use the same ambient RF signals

18. RFID Backscatter Ambient Backscatter Reader Tag
Reader sends constant wave
Receive chain: 100s of mW
Reader centrally coordinates

19. RFID Backscatter Ambient Backscatter Reader Tag Tag Tag
Reader sends constant wave
Receive chain: 100s of mW
Reader centrally coordinates
Uses uncontrollable signals
Receive chain: 0.5 μW
Need distributed MAC

20. Challenges
Extracting backscattered signals from ambient signals we don’t control
Decoding on a battery-free device
Designing distributed MAC for battery-free devices

21. Alice’s reflections change the average amplitude
Send 0 and 1
TV Tower
Send 0: Alice absorbs
At Bob: TV signal
Send 1: Alice reflects
+ Weak Reflection
Alice
Bob
Alice’s reflections change the average amplitude

22. How Do We Extract The Backscattered Signals?
Alice Sends 1010…
Alice Inactive
Moving Window Average

23. Use RC Circuits to Average
Capacitor slowly charges/discharges when voltage is applied/removed
Provides a cheap, analog,
exponential moving average
By picking the right RC values,
we can selectively filter out the high TV frequencies

24. Now that we can decode bits…
Link Layer
Distributed MAC?
Physical Layer

25. Carrier Sense Preamble correlation Energy detection
Detect the beginning of other packet transmissions
Energy detection
Detect energy in the middle of a packet transmission

26. Packet Format Detecting the Beginning of a Packet Transmission
Preamble
Continuous correlation
Comparator hardware takes very little power
A few bits to wake up the receiver’s hardware

27. Recall: CSMA
CSMA/CD: Collision Detection
CSMA/CA: Collision Avoidance

28. Evaluation The distance between the transmitter and receiver
The distance from TV tower
Outdoor and indoor
can achieve 1 kbps, at distances of up to 2.5 feet in outdoor and 1.5 feet indoor.

29. Three Types of Novel Backscatter
Ambient Backscatter
Wi-Fi Backscatter
Inter-Technology Backscatter

30. Wi-Fi Backscatter
Wi-Fi Backscatter: Internet Connectivity for RF-Powered Devices
SIGCOMM 2014
Passive Wi-Fi: Bringing Low Power to Wi-Fi Transmissions
NSDI 16
Best Paper

31. Wi-Fi Backscatter Why? Stronger power source
Connect commodity Wi-Fi devices to the Internet

32. Wi-Fi Backscatter - Signal Source
Wi-Fi Backscatter: Internet Connectivity for RF-Powered Devices
SIGCOMM 2014
Passive Wi-Fi: Bringing Low Power to Wi-Fi Transmissions
NSDI 16
Best Paper
WiFi AP
Special device transmitting a single-frequency tone.

33. Changing reflection consumes 0.65 µW of power
Wi-Fi Backscatter
Key Idea: it conveys a ‘1’ and a ‘0’ bit by either reflecting or absorbing the Wi-Fi packets
Wi-Fi AP
Changing reflection consumes 0.65 µW of power
RSSI 3 5 6 4 1 2 9 8 7
Wi-Fi Device
Wi-Fi Backscatter Tag 1

34. Wi-Fi AP Consumes Energy
Power scaled with Moore’s law O(10 uW)
No such scaling O(100 mW)
Digital
(Baseband)
Analog
(RF)
Wi-Fi transmitter consumes mW

35. Generate Wi-Fi packets using reflections
Passive Wi-Fi
Generate Wi-Fi packets using reflections
sin (2𝜋𝑓𝑡)
Plugged-in Device 9
Passive Device
Wi-Fi Receiver

36. Three Types of Novel Backscatter
Ambient Backscatter
Wi-Fi Backscatter
Inter-Technology Backscatter

37. Inter-Technology Backscatter
SIGCOMM 16 Best Paper
Towards Internet Connectivity for Implanted Devices

38. Brain implants for reanimation of limbs
Need wireless connectivity to make this practical and safe
Brown University - Guardian News & Media Ltd

39. Contact lenses that measure blood sugar
Google- Business Computing World

40. Contact lenses that measure blood sugar

41. Inter-Technology Backscatter
Source
Receiver
Contact lens

42. Problem: Bluetooth and WiFi are different protocols
Challenge 1: Different modulation
Frequency Shift Keying
Phase Shift Keying
Challenge 2: Different frequencies and bandwidth

43. Inter-Technology Backscatter
Step 1: Transform Bluetooth transmissions into single tone signals
Step 2: Reflect the single tone signal to create Wi-Fi packets

44. How to create a single tone from Bluetooth?
Bluetooth uses frequency shift keying
flow = 0
fhigh = 1

45. How to create a single tone from Bluetooth?
Bluetooth uses frequency shift keying 1 1
Transmitting all ‘0’ or ‘1’ bits creates a single tone

46. Inter-Technology Backscatter
Step 1: Transform Bluetooth transmissions into single tone signals
Step 2: Reflect the single tone signal to create Wi-Fi packets

47. How to create WiFi packets?
First-Order Solution: Use Passive WiFi (NSDI ‘16)
Problem: Bluetooth and WiFi use different frequencies
BLE 38
Smart contact lens

48. How to create WiFi packets?
First-Order Solution: Use Passive WiFi (NSDI ‘16)

49. Problem: Passive WiFi creates mirror copy outside ISM band
2.4 GHz ISM Band
BLE 38
Mirror Copy
Wi-Fi CH11

50. Problem: Passive WiFi creates mirror copy outside ISM band
Create the complex signal
I + Q*j
Real impedance
Imaginary impedance

51. Key Idea: Cancel mirror copy using two switches
Single sideband backscatter
Key Idea: Cancel mirror copy using two switches
Mirror Copy + +

52. Conclusion : Backscatters
Can achieve battery-free communication
Can utilize various available signal sources
Only adopt ON-OFF keying amplitude modulation
Have limited range and data rates
Can be potential technology fo IoT