Microsoft Research Asia BeepBeep: A High Accuracy Acoustic Ranging System using COTS Mobile Devices Chunyi Peng Guobin(Jacky) Shen, Yongguang Zhang, Yanlin Li, Kun Tan Microsoft Research Asia

Ranging, basic to localization Many excellent systems Cricket (Mobicom’00) RIPS (Sensys’05) ENSBox (Sensys’06) … Hardware and algorithmic innovation

Our motivation Proximity detection between devices Among portable devices MobiUS (Mobisys’07) Between portable and non-portable devices Phone to PC, Xbox, Printers, Projectors …

The requirement A widely applicable solution High accuracy! Work on COTS devices No additional hardware (e.g., ultrasound) Pure user space software (no change to OS/driver) Not dependent on infrastructure Applicable in spontaneous, ad hoc situations Minimum set of sensors High accuracy!

A matter of time measurement Mostly, based on time-of-flight measurement Distance = speed x time Sound often chosen Slower speed => less demanding on time accuracy Still, a challenging task 1 ms error in time = 34 cm error in distance 1 cm ranging accuracy requires 30us timing accuracy Extremely challenging on COTS/software

The root cause of inaccuracy – three uncertainties Clock synchronization uncertainty Sending uncertainty Receiving uncertainty software issuing command software aware of arrival ... t0 = wall_clock(); write(sound_dev, signal); ... read(sound_dev, signal); t1 = wall_clock(); unknown delays (software, system, driver, hardware, …) ? unknown delays (hardware, interrupt, driver, scheduling, …) ? sound leaves speaker sound reaches mic time

Effects of the sending and receiving uncertainties Example measurement of the lower bound on COTS mobile devices (HP iPAQ rw6828) Highly fluctuating, appears unpredictable Easily adds up to 1-2 ms (=> a few feet error) CPU idle CPU heavily loaded

Our approach – BeepBeep A simple and effective solution Each device just needs to emit a sound signal and record them simultaneously Only require a speaker, a mic, and some way of communicating with the other device Achieving 1cm accuracy while satisfying all the requirements

Beepbeep’s basic procedure Device A Device B Device A emits a beep while both recording Device B emits another beep while both continue recording Both devices detect TOA of the two beeps and obtain respective ETOAs Exchange ETOAs and calculate the distance A’s recording B’s recording ETOAA ETOAB DAB=|ETOAA-ETOAB|/2

Mathematical derivation 1st Beep tB1 tA0 tA1 dB,A-dA,A = c·(tB1-tA1) ETOAA dA,B-dB,B = c·(tA3-tB3) ETOAB tA3 2nd Beep tB3 tB2 dB,A+dA,B= c·[(tA3-tA1)-(tB3-tB1)] +dA,A+dB,B = c·(ETOAA-ETOAB)+dA,A +dB,B

Key techniques, effects and rationale (I) Self-recording Record signals from both the other party and itself Establish the starting reference point of the whole ranging process Duplex audio channel Two-way sensing Avoid clock synchronization uncertainty To capture the ending reference point of the whole ranging process not attempt to capture any system time info

Key techniques, effects and rationale (II) Sample counting Avoid referring to system clocks for timing info Dedicated A/D converter, w/ fixed sampling rate Achievable precision is determined by the sampling frequency: 0.8cm at 44.1kHz sampling rate Putting together: Bypass all the three uncertainties by making time measurement irrelevant to system clocks

Engineering Challenges (I) Signal Design Good signal design helps detection Easily detectable in digital recording Robust against ambient noise Robust against acoustic distortion Low-fidelity speaker & mic in COTS mobile device Within hardware capability Most COTS devices have limited voice frequency range Our empirical design (“chirp” sound) 50ms long, shifting frequency from 2 to 6 kHz

Engineering Challenges (II) Signal Detection Algorithm Design Efficient and fast signal detection algorithm Quickly locate possible signal regions Robust against low SNR Utilize noise floor to boost SNR Combat multipath effect Multipath: big issue indoor environment We derived special algorithm to detect first “sharp peak” signal correlation

Engineering Challenges (III) Protocol design Coordinate two (or more) devices in entire ranging process Minimize ranging time duration Device form factor Speaker/mic’s placement affect ranging results Vary from one device model to another Need calibration to adjust ranging calculation

System Implementation Platform: Windows Mobile 5.0 Sound: “wave” API Communication: “WinSock” (WiFi or Bluetooth) Software architecture User-mode dynamic linkable library As service for other applications Test devices Dopod838 HP iPAQ 6828

Evaluation Case-A: Indoor, quiet Case-B: Indoor, noisy Case-C: Outdoor, car park entrance Case-D: Outdoor, subway station 50 runs each setting Expr Setting Operation Range Conf. Level AA(|Err|) cm MA(|Err|) A(Std) M(Std) Case-A 4.0m 94% 0.9 1.4 1.2 1.9 Case-B 1.1 1.7 1.0 1.3 Case-C 12m 98% 2.7 3.8 2.1 Case-D 10m 92% 2.2 1.6

Summary Identified three uncertainties and mitigated them with three key technologies Two-way sensing Self-recording Sample counting BeepBeep provides a simple ranging solution Achieves 1cm accuracy On very basic hardware set Purely in software (user-space)

Thanks! Demo session. Welcome to try BeepBeep! Software downloads: http://research.microsoft.com/wn/beepbeep.aspx

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Lower bound of sending and receiving uncertainties