Enabling On-Body Transmissions with Commodity Devices Mehrdad Hessar, Vikram Iyer, Shyam Gollakota Can we send signals confined to the body with mobile devices? Enable secure on body communication Unlock doors with a phone Share secret keys How do we achieve this on a phone? Look beyond radios for wireless communication
System Requirements 1. Signals dont propagate through the air Cant use radios like Bluetooth or Wi-Fi 2. Direct contact with the body Explore using input modalities 3. Produce electromagnetic signals Should emit low frequency signals Our Contributions First transmission throughout the body using commodity devices Fingerprint sensors and touchpads Evaluate effects of posture, motion, and body type Data transmissions at up to 50 bps What signals do these devices generate?
Fingerprint Sensor Spectrum 0 cm 20 cm dBm -10 -30 -50 -70 -90 0 20 40
60 80 100 Frequency (MHz) 120 140 What signals do these devices generate? Touchpad Spectrum 0 cm 20 cm dBm -20
Touchpads and fingerprint sensors produce secure signals How well do signals propagate on body? Transmitting phone placed in hand How well do signals propagate on body? SNR at Different Locations on the Body Hands R Hand R Leg R Hand L Leg
L Hand R Leg L Hand L Leg R Hand Chest L Hand Chest SNR dB 22 18 14
10 iPhone 6s Fingerprint iPhone 5s Fingerprint Adafruit Trackpad Lenovo Trackpad Mobile devices send signals throughout the body Understanding fingerprint sensors
Capacitive coupling through body Modeling the human body VTX Fingerprint Sensor Receiver VRX What Capacitive about coupling lack of common throughground? body
Modeling the human body VTX Fingerprint Sensor Receiver VRX Earth GND 1. Weak coupling to ground Inaccurate voltage at receiver 2. But fingerprint sensors generate strong signals Can still receive reliably
Earth GND Transmitting signals through the body Amplitude (V) Fingerprint Sensor Signal 3.5 2.5 1.5 0.5 0.5 -0.5 0
0.5 1 1.5 2 Time (s)s) 2.5 3 3.5 4 Fingerprint sensors produce high amplitude signals
Designing a transmitter Modulate data by turning on and off the signal 1 0 1 1 2 0 3 1
4 5 Time (s) 0 1 6 Fingerprint Sensor- Verifi P5100 Start and stop fingerprint scan Touchpad- Adafruit Capacitive Touchpad Power cycle the touchpad 0 7
8 Receiving and decoding data RX iPhone 6s Fingerprint Spectrogram Receiving and decoding data iPhone 6s Fingerprint Spectrogram RX Amplitud e
Decoded Data 0.8 0.4 0 1 2 3 4 5 Time (s) 6
7 8 Evaluation How does SNR vary in different scenarios? Body type Posture Motion What data rates can we achieve? Effect of Height and Weight Total Participants: 10 Weight Range: 118 195lbs Height Range 54 63 R Hand,
L Hand R Hand, L Leg L Hand, L Leg iPhone 6s SNR dB 20 15 10 1
2 3 4 5 6 7 Participants 8
9 10 Effect of Different Postures iPhone 6s SNR Measurements Standing SNR dB 22 Sitting Sleeping 18 14 10
Left Arm, Chest Right Arm, Chest Left Arm, Left Leg Right Arm, Left Leg Difference of 1-2 dB across postures Effect of Motion
SNR dB 20 15 10 Still Hands Moving Hands Walking SNR >10 dB across different motion scenarios Bit Rate Evaluation Bit Rate vs. SNR for Fingerprint Sensor and Touchpad Fingerprint Sensor Touchpad
SNR dB 20 15 10 0 5 10 15 20 25 30 35 40 45 50 55 Bit Rate (bps) Bit rates are sufficient for our target applications
Conclusion Commodity devices can communicate throughout the body Evaluate effects of posture, motion, and body type, data rate Enable new applications on body communication onbody.cs.washington.edu
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