The document evaluates using the Bluetooth Hands-Free Profile (HFP) to transmit biometric sensor data to smartphones. It describes building a heart monitor that uses an optical sensor, signal processing, Bluetooth transmission to a headset, and apps to receive the data. Testing showed the HFP works for this use by modulating the biometric signal into the supported 100-600Hz range. The authors conclude the HFP is suitable due to its wide availability across devices and ability to transmit slowly varying biometric signals with low cost.

1. Project goal Outline Acquiring Transmitting Receiving Conclusions
Evaluation of Bluetooth Hands-Free Profile for
Sensors Applications in Smartphone Platforms
Dept. of Information Engineering, University of Brescia
C. M. De Dominicis, D. Mazzotti,
M. Piccinelli, S. Rinaldi, A. Vezzoli
IEEE 2012 Sensors Applications Symposium
University of Brescia, Italy
February 7-8-9
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

2. Project goal Outline Acquiring Transmitting Receiving Conclusions
Preamble: modern smartphones
External sensors
Internal sensors WiFi
Accelerometers
Gyroscope
GPS Bluetooth
Camera
Microphone Zigbee, Ant+, ...
Proximity Ext.
..... hardware
GSM/UMTS/4G
World
Wide range of internal sensors.
Wide range of connectivity capabilities.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

3. Project goal Outline Acquiring Transmitting Receiving Conclusions
Objective
Create a common interface between an external sensor (e.g.
biometric) and a generic smartphone platform.
Biometric data Smartphone
Common interface
Real time streaming.
OS/Platform independent.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

4. Project goal Outline Acquiring Transmitting Receiving Conclusions
Selecting the right interface
Peak energy
Data
Availability on consumption Additional
transmission
Smartphones during costs
cost
transmission
Bluetooth common low NO NO
WiFi common medium NO NO
GSM/UMTS common high YES NO
External HW limited ? NO YES
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

5. Project goal Outline Acquiring Transmitting Receiving Conclusions
Advantages of Bluetooth
Available on almost every smartphone on the market.
Many profiles for different uses (not every one is always available).
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

6. Project goal Outline Acquiring Transmitting Receiving Conclusions
Bluetooth Hands-Free Profile HFP
Profile for connection with an external hands-free device.
Bi-directional audio channel.
Available on almost every smartphone on the market.
Already in use or ready for other applications (telephony,
navigation, ..).
Biometric signal can be multiplexed to preexisting
transmissions.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

7. Project goal Outline Acquiring Transmitting Receiving Conclusions
Bluetooth HFP: signal requirements
Signal received for different input SNR [dB] for received signal for
amplitude values, versus different input amplitude values,
frequency. versus frequency.
The best band to transmit is between ∼ 100 Hz and ∼ 600 Hz.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

8. Project goal Outline Acquiring Transmitting Receiving Conclusions
Working example: heart monitor
To test the use of Bluetooth HFP to transmit sensor data, we built
a simple heart signal monitor. The system is composed of:
An optical sensor.
A filtering/amplifying/modulation stage.
A commercial bluetooth headset.
Custom applications for both iOS and Android platforms to
show the signal.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

9. Project goal Outline Acquiring Transmitting Receiving Conclusions
Heart monitor system outline
Sensor Reception
Filter Demodulation
Amplifier
Visualization
Channel
modulation Recording
Analysis
Transmission
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

10. Project goal Outline Acquiring Transmitting Receiving Conclusions
Heart monitor system:
Acquiring
Transmitting
Receiving
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

11. Outline
Acquiring
Transmitting
Receiving

12. Project goal Outline Acquiring Transmitting Receiving Conclusions
Photoplethysmography: principles
An optical non-invasive way to measure the blood flow through a
body part.
Blood flow
Vout
Body
part
Photodiode Phototransistor
The output voltage is proportional to the opacity of the body part,
hence to the blood flow through the capillaries.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

13. Project goal Outline Acquiring Transmitting Receiving Conclusions
Raw signal from sensor
The raw signal from the sensor is low (∼ 4-6 mV) and noisy.
It must be low pass filtered (∼ 6 Hz) and amplified.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

14. Project goal Outline Acquiring Transmitting Receiving Conclusions
Treated signal
The filtered and amplified signal appears as shown in figure.
Arbitrary unit
Arbitrary unit
Time [seconds] Frequency [Hz]
Figure: Signal from sensor and its spectrum
The heart beat can be clearly seen.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

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Acquiring
Transmitting
Receiving

16. Project goal Outline Acquiring Transmitting Receiving Conclusions
Signal modulation
The PPG signal is below the lower cut-off frequency of the
Bluetooth module, so it needs to be modulated to translate its
spectrum in the band pass.
Filtered PPG
signal
Square wave
366 Hz
Schmitt
trigger
oscillator
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

17. Outline
Acquiring
Transmitting
Receiving

18. Project goal Outline Acquiring Transmitting Receiving Conclusions
Filtering
The demodulation is based on a software moving average filter,
which selects partially-overimposed windows, eliminates peak
samples (lowest and highest) and averages the rest.
Sample implementation of a moving average digital filter
Windowing Ordering Selecting Averaging
Recovered Signal
t
Modulated Signal
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

19. Project goal Outline Acquiring Transmitting Receiving Conclusions
Displaying
Android software. iOS (iPod Touch) software.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

20. Project goal Outline Acquiring Transmitting Receiving Conclusions
Conclusions
Following our research, we found that the Bluetooth HFP profile,
normally used to transmit a vocal signal to a smartphone, can be
also used to transport biometric data from an external sensor,
provided that the biometric signal can be modulated in the
100-600 Hz band.
Advantages:
Wide availability: all smartphones OSs (iOS, Android, ...), but
also PCs and embedded systems.
Enough bandwidth to transmit (more than one) slowly varying
biometric signals.
Low cost, easy to use.
C. M. De Dominicis, D. Mazzotti, M. Piccinelli, S. Rinaldi, A. Vezzoli
Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms