1. Chunyi Peng, Guobin Shen, Yongguang Zhang, Yanlin Li, Kun Tan
BeepBeep: A High Accuracy Acoustic Ranging System using COTS Mobile Devices
Chunyi Peng, Guobin Shen, Yongguang Zhang, Yanlin Li, Kun Tan
Presented by
Prithvi Raj

2. Overview Traditional ranging systems BeepBeep ranging system
System Architecture
System Design
Experiment results
Conclusion

3. Traditional TOA-based ranging system
 𝐷=𝑐 . ∆𝑡
Distance between sender and receiver is product of time of flight and propagation speed of signal.
Choice of signal: radio, light, acoustic (error of 30cm per 1ms error)
Clock synchronization uncertainty ( 𝜇 𝑐 )
reason: clock skews and drifting between two clocks
GPS for clock synchronization
round trip
Sending uncertainty ( 𝜇 𝑠 )
arbitrary delay after output command is issued till sound signal comes out from speaker
reasons: software delay, system load, speaker architecture
Receiving uncertainty ( 𝜇 𝑟 )
arbitrary delay of sound signal arrival being recognized
reasons: real-time control, software delay, interrupt handling delay, system load

4. ( 𝜇 𝑠 + 𝜇 𝑟 )
Experiment to find out a lower bound for ( 𝜇 𝑠 + 𝜇 𝑟 ), if a TOA measurement is done in software.
COTS mobile phone(HP iPAQ rw6828)
Time of flight negligible by placing microphone and earphone together

5. ( 𝜇 𝑠 + 𝜇 𝑟 )

6. Ranging Protocol
Each device has a WiFi radio
Initiation: A ranging process is started by an initiating device, which calculates and disseminates a schedule in an initiation message to all the devices that participate in the ranging process
Sensing: Each device calculates a delay according to the schedule and sets a timer. Upon the timer expires, it emits a sound signal.
ETOA exchanging: After the last device has emitted the sound signal, each device processes the recorded signals and determine ETOA between its own signal and signals from all other devices. These ETOA values are packed into one packet broadcast to other devices. Upon receiving EOTA information from all other devices one device can calculate the distance to all other devices.

7. BeepBeep Ranging Mechanism
Assumption: Both devices A and B are in recording state.
Two way sensing: First device A, emits sound signal which is recorded by recorders of both A and B. Then after some time, B does same thing.
Both devices examine their recorded data and locate the sample points when previously emitted two signals arrived. And calculate ETOA(elapsed time between two signals)
A and B, exchange locally computed ETOAs. And based on this two ETOAs, distance between two devices is calculated

10. ETOA Determination
Received sound signal is always sampled at a fixed sampling frequency ( 𝑓 𝑠 )
To calculate ETOA, instead of system clock to tell the timestamps of arrival of both the signal, we rely on samples by recording module.
EOTA=(number of samples between first sample points of each of two signals )/ 𝑓 𝑠

11. Signal Detection
Signal is detected by correlating with the reference signal.
Cross correlation is a measure of similarity of two waveforms as a function of a time lag applied to one of them.
Maximum “peak” is located and this is considered as a point when the signal arrived by the property of autocorrelation.
𝐿 2 (𝑆) , 𝐿 2 (𝑁), 𝑤 0 =100, 𝑇𝐻 𝑆𝐷 =3db=2
𝐿 2 𝑆 𝐿 2 𝑁 > 𝑇𝐻 𝑆𝐷

12. Multipath Effect
Sharpness: level of peak regarding to it’s surrounding side-lobes
Cross correlation values from difference path s should have same sharpness value
First peak which has comparable sharpness is the maximum “peak”
𝛾 𝑝 > 𝛾 𝑚𝑎𝑥 . 𝑇𝐻 𝑀𝑃 𝑇𝐻 𝑀𝑃 =85%, w1=100

13. BeepBeep Advantages
Clock synchronization is eliminated by using two way sensing
No local clock of end system or time-stamping is required
Sending uncertainty is removed as we don’t use instance when sound is sent
Receiving uncertainty by measuring ETOA using samples and as 𝑓 𝑠 is constant
No instantaneous signal detection required
Signal detection can be done offline and hence complex signal processing techniques can be applied to detect the signal
Granularity proportional to c/ 𝑓 𝑠 , in typical setting granularity = 0.77cm

14. Sources of error In c (speed of signal): Temperature and humidity
Drifting of sampling frequency
Signal to noise ratio(SNR): communication channel, environment noise, energy
Multipath effects: reverberation, indoor settings
Signal distortion: limited spectrum band(around 3kHz), attenuation outside this band.

15. System Architecture

16. Signal design Good autocorrelation property
Good Frequency response near human voice range(2-8kHz)
Length: 50cm : tradeoff between good SNR and multipath effect

17. Performance Matrices
Accuracy: difference between ranging results and real distances
minimum / maximum / medium error / standard deviation
absolute value /percentage to real distance
α-Confidence: percentage of times accuracy level α is achieved
Operational range: it is maximum range that ranging system can still achieve accuracy level α with confidence β.

18. Test case design Quiet indoor: room(5mx11m) ( 𝑇𝑒𝑚𝑝=25 ° c)
Noisy Indoor:(noise of AC, music, chatting)( 𝑇𝑒𝑚𝑝=25 ° c)
Outdoor, car park( 𝑇𝑒𝑚𝑝=10 ° c)
Outdoor, subway station( 𝑇𝑒𝑚𝑝=15 ° c)
𝑐 𝑎𝑖𝑟 = ∗𝜃
Failed experiments (accuracy>20cm or 𝑇𝐻 𝑆𝐷 <3db) not included
K=3cm and 8cm respectively
COTS mobile device used are
50 times for each setting
LOS ensured

19. Experiment Results
CASE A: Quiet indoor

20. Experiment Results
CASE B: Noisy Indoor

21. Experiment Results
CASE C: Outdoor car park

22. Experiment Results
CASE D: Outdoor subway station

23. Experiment Results
Cumulative distribution function of the α-confidence

24. Experiment Results Using earphones instead of speaker
Earphones have less signal power but larger operational range than speaker
So we compare effect of SNR VS effect of signal distortion(CASE A)

25. Experiment Results
Earlier experiment with speakers for CASE A

26. Experiment Results
Finding out effectiveness of multipath effect mitigation effort

27. Conclusion
BeepBeep eliminates uncertainties related to clock synchronization, sending and receiving
No local clock timestamp required
No dedicated hardware required
Implemented totally in software and in user space
Using COTS devices