CS434/534: Topics in Networked (Networking) Systems Mobile Networking System: Making Connections: Backscatter Geng Li Computer Science Department Yale University 208A Watson Email: yry@cs.yale.edu http://zoo.cs.yale.edu/classes/cs434/

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)

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

Recap: Near Field RFID <= 100 Mhz

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

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

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

RFID Backscatter

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

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

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

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

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

Leverage Existing Wireless Signals TV Cellular Wi-Fi Available at almost any time and place, rain or shine

Communication between two battery-free devices

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

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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.

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

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 500 - 700 mW

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

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

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

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

Contact lenses that measure blood sugar Google- Business Computing World

Contact lenses that measure blood sugar

Inter-Technology Backscatter Source Receiver Contact lens

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

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

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

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

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

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

How to create WiFi packets? First-Order Solution: Use Passive WiFi (NSDI ‘16) 0 1 0

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

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

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

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