Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms

Project goal Outline Acquiring Transmitting Receiving Conclusions

Evaluation of Bluetooth Hands-Free Profile forSensors 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 SymposiumUniversity 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


Preamble: modern smartphones

AccelerometersGyroscope

GPS

Internal sensors

CameraMicrophone

Proximity.....

WiFi

Bluetooth

Zigbee, Ant+, ...Ext.

hardware

External sensors

GSM/UMTS/4G

World

I Wide range of internal sensors.

I Wide range of connectivity capabilities.




Objective

Create a common interface between an external sensor (e.g.biometric) and a generic smartphone platform.

Common interface

SmartphoneBiometric data

I Real time streaming.

I OS/Platform independent.




Selecting the right interface

Availability on Smartphones

BluetoothWiFi

GSM/UMTS

commoncommoncommon

Peak energy consumption

during transmission

lowmedium

high

Data transmission

cost

NONOYES

External HW limited ? NO

Additionalcosts

NONONOYES




Advantages of Bluetooth

Available on almost every smartphone on the market.Many profiles for different uses (not every one is always available).




Bluetooth Hands-Free Profile HFP

Profile for connection with an external hands-free device.

I Bi-directional audio channel.

I Available on almost every smartphone on the market.

I Already in use or ready for other applications (telephony,navigation, ..).

I Biometric signal can be multiplexed to preexistingtransmissions.




Bluetooth HFP: signal requirements

Signal received for different inputamplitude values, versusfrequency.

SNR [dB] for received signal fordifferent input amplitude values,versus frequency.

The best band to transmit is between ∼ 100 Hz and ∼ 600 Hz.




Working example: heart monitor

To test the use of Bluetooth HFP to transmit sensor data, we builta simple heart signal monitor. The system is composed of:

I An optical sensor.

I A filtering/amplifying/modulation stage.

I A commercial bluetooth headset.

I Custom applications for both iOS and Android platforms toshow the signal.




Heart monitor system outline

Sensor

FilterAmplifier

Channelmodulation

Transmission

Reception

Demodulation

Visualization

Recording

Analysis




Heart monitor system:

Acquiring

Transmitting

Receiving



Outline

Acquiring

Transmitting

Receiving


Photoplethysmography: principles

An optical non-invasive way to measure the blood flow through abody part.

Body part

Vout

Photodiode Phototransistor

Blood flow

The output voltage is proportional to the opacity of the body part,hence to the blood flow through the capillaries.




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.




Treated signal

The filtered and amplified signal appears as shown in figure.

Time [seconds]

Arbi

trary

uni

t

Frequency [Hz]

Arbi

trary

uni

tFigure: Signal from sensor and its spectrum

The heart beat can be clearly seen.



Outline

Acquiring

Transmitting

Receiving


Signal modulation

The PPG signal is below the lower cut-off frequency of theBluetooth module, so it needs to be modulated to translate itsspectrum in the band pass.

Square wave366 Hz

Filtered PPGsignal

Schmitt trigger

oscillator



Outline

Acquiring

Transmitting

Receiving


FilteringThe demodulation is based on a software moving average filter,which selects partially-overimposed windows, eliminates peaksamples (lowest and highest) and averages the rest.

Modulated Signal

Recovered Signal

t

Windowing Ordering Selecting Averaging

Sample implementation of a moving average digital filter




Displaying

Android software. iOS (iPod Touch) software.




Conclusions

Following our research, we found that the Bluetooth HFP profile,normally used to transmit a vocal signal to a smartphone, can bealso used to transport biometric data from an external sensor,provided that the biometric signal can be modulated in the100-600 Hz band.

Advantages:

I Wide availability: all smartphones OSs (iOS, Android, ...), butalso PCs and embedded systems.

I Enough bandwidth to transmit (more than one) slowly varyingbiometric signals.

I Low cost, easy to use.



Technology

Evaluation of Bluetooth Hands-Free Profile for Sensors Applications in Smartphone Platforms