FLIGHT: Clock Calibration Using Fluorescent Lighting Zhenjiang Li*, - PowerPoint PPT Presentation

FLIGHT: Clock Calibration Using Fluorescent Lighting Zhenjiang Li*, Wenwei Chen, Cheng Li, Mo Li, Xiang-Yang Li, Yunhao Liu Nanyang Technological University, Singapore Hong Kong Universiy of Science & Technology, Hong Kong Tsinghua University, China Illinois Institute of Technology, USA

Time Synchronization ! A variety of applications ! Phone-to-phone gaming Virtual Table Tennis [Sensys’11] Precise 3D localization Virtual Sword Fight [MobiSys’12] 2/35

Time Synchronization ! A variety of applications E v e n t & o ! Body-area networks r d d e e t r e i n c t g i o n Fall Down Detection [SenSys’08-Demo] 3/35

Time Synchronization ! A variety of applications ! MAC-layer protocol design Time slot alignment C D B A Active State Dormant State Opportunistic flooding [MobiCom’09] 4/35

Time Synchronization ! Design challenges (1/2) ! Initial clock offset Embedded Devices Initial offset: seconds or even more ! Smart phones, Tablets,... 5/35

Time Synchronization ! Design challenges (2/2) ! Clock uncertainty CMOS oscillator ✓ Inherent clock uncertainty 5-100 ppm Temperature ✓ Ambient environments 4 x 10 Humidity 3.2768 3.2768 32767.10 Vcc = 3.0 V ... 32767.00 3.2768 32766.90 Vcc = 2.1V 32767.10 Clock Frequency (Hz) 3.2767 32767.00 Supply voltage 32766.4 3.2767 32766.90 1820 1840 1860 3.2767 32766.3 ✓ Internal factors Pressure Vcc = 2.4 V 3.2767 32768.0 32766.2 3.2767 32767.9 ... 32766.1 3620 3640 3660 3.2766 32767.80 3.2766 32767.70 0 50 6/35 3.2766 0 1000 2000 3000 4000 5000 6000 Time Elapsed (s)

Time Synchronization ! Calibration vs. synchronization ! Clock Calibration ✓ Similar speeds ✓ Various apps. Time synchronization = Initial offset cancelation + Clock calibration 7/35

Time Synchronization ! The state of the art ! Communication-based solutions ! RBS[OSDI’02], TPSN[SenSys’03], and FTSP [SenSys’04] ✓ Easy to implement ✓ High power consumption & overhead Internet Sink Node Wireless Sensor Networks 8/35

Time Synchronization ! The state of the art ! External signal source based solutions GPS module ✓ Less communication overhead ✓ Hardware support Electromagnetic radiation [Sensys’09] FM station [MobiSys’11] 9/35

Idea Overview ! Key observations (1/3) ! Fluorescent lighting Alternating l a c i d o Current (AC) i r e P 50 or 60 Hz ! l a n g i S Light Intensity (100 or 120 Hz) 10/35

Idea Overview ! Key observations (2/3) Energy efficient 30~50W ! Availability of fluorescent light 24hr < 0. 1 USD Airport Data center Home Indoor Environments 11/35 Hospital Supermarket Office

Idea Overview ! Key observations (3/3) ! Mobile devices ! Sensor motes, smart phones, tablets, notebooks,... Light sensor / Camera 12/35

Idea Overview ! FLIGHT Oscilloscope Sampling light sensor 13/35

Idea Overview ! FLIGHT Periodical Pattern ! Phase differences g n i l p m a r S o s n e s t h g i l 14/35

Design Overview ! System architecture of FLIGHT Logic time Logic Time Control (Get / Set) interface α (t) correction Logic Time Maintenance Logic time updating Clock calibration Sampling Window Config. Period Extraction Filtering Error Estimation Interval Calibration Light Sensor Interval Calculation adaptation Interval Prediction Interval Updating 15/35

Design Overview ! Period extraction ! Where is the delimiter? Facing the lamp Rotation Opposite to the lamp 16/35

Design Overview ! Period extraction 100 m ! Roaming 230 m Bright place 480 m Dark place 17/35

Design Overview ! Period extraction ! Frequency domain Self-correlation, folding,...? 4 2 x 10 ✓ Long computation delay 1 ✓ Buffer occupancy 0 ✓ Energy consumption − 1 FFT − 2 − 3 Feasible cutoff frequency region − 4 − 5 50 100 150 200 250 300 350 Frequence (Hz) 18/35

Design Overview ! Period extraction ! Filtering solution First-order: Second-order: order=1 order=2 ... 195 Light Intensity (mV) Light Intensity (mV) 190 190 185 185 Advantages of 180 180 175 filtering 170 175 20 40 60 80 100 20 40 60 80 100 Sampling Index Sampling Index ✓ Less computation order=4 order=6 187 Light Intensity (mV) 188 Light Intensity (mV) ✓ Energy efficient 186 186 185 184 ✓ Less buffer occupancy 184 182 183 180 182 178 20 40 60 80 100 20 40 60 80 100 19/35 Sampling Index Sampling Index

Design Overview ! Logic time maintenance L Light periods: t Clock ticks: t I M - I 1 = I Define frequency ratio 20/35

Design Overview ! Logic time maintenance n . . . Light periods: t 1 / ( α · f g ) m Logic time: . . . t Clock ticks: . . . t ✓ Two clocks ✓ Granularity 21/35

Design Overview ! Calibration interval ! Energy concern c l ( τ 1 ) + t / ( α ( τ 1 ) · f g ) Logic ˆ ˆ ˆ α ( τ 1 ) 1 / ( α ( τ 2 ) · f g ) α ( τ 2 ) time Calibration calibration 1 calibration 2 ˆ Interval . . . τ 1 τ 1 + t τ 2 Native clock Time synchronization: a common starting point + Clock calibration 22/35

Design Overview ! Calibration interval 0 1 2 L-1 L ✓ Long sampling window . . . Light periods ✓ Robust to sampling jitters t ✓ Better accuracy of freq. ratio I 1 I M . . . Clock ticks ✓ However... ✓ Buffer concern t ✓ Uncertain delay of computation τ Parallelize sampling & filtering & delimiter searching operations 23/35

Design Overview ! Calibration interval ! Filter order vs. sampling rate 24/35

Experimental Evaluation ! Experiment setup TinyOS 2.1x TelosB mote (32.768KHz) ✓ Kernel: NesC Beacon node 25/35

Experimental Evaluation ! Clock offset ! Initial offset = 0 1000 Max logic time error Avg logic time error 800 600 Logic time error ( µ s) 400 200 0 − 200 − 400 − 600 0 200 400 600 800 1000 1200 1400 Time (min) 26/35

Experimental Evaluation ! Distance to lamps The rear of the laboratory 27/35

Experimental Evaluation ! Distance to lamps 1000 Error − 10min 800 Error − 20min Error − 30min 100 600 Light Intensity µ s) ✓ Each lamp can coope- Light Intensity (mV) 400 Logic Time Error ( rate a large number of 200 nodes 0 0 ✓ Lamps can be sparsely − 200 used − 400 − 600 − 800 − 1000 4 4 5 5 6 6 7 7 Different Distances (m) 28/35

Experimental Evaluation ! Mixed with other types of light 1 0.9 0.8 ✓ Sunlight, Filament light, LED light 0.7 0.6 ✓ Direct signals Sunlight CDF 0.5 Filament light LED light 0.4 ✓ Adding an additional 0.3 direct component (e.g., 800mV + 90mV) 0.2 0.1 0 0 200 400 600 800 1000 Logic Time Errors ( µ s) 29/35

Experimental Evaluation ! Clock offset with roaming 1000 800 Period 2 600 Periods 1 & 2: Logic Time Error ( µ s) 400 roaming inside the 200 academic building 0 Period 3: roaming in − 200 the laboratory − 400 − 600 Period 3 Period 1 − 800 0 200 400 600 800 1000 1200 1400 Time (min) 30/35

Experimental Evaluation ! Clock offset in two different buildings Two nodes are separated by more than 100m Within [350,550] min, two nodes are locally roaming 31/35

Experimental Evaluation ! Energy consumption ✓ ROCS [MobiSys’11] ✓ WizSync [RTSS’11] ✓ FTSP [SenSys’04] 5 350 4.5 300 4 Average Power ( µ W) Average Power ( µ W) 250 3.5 3 200 2.5 150 2 1.5 100 1 50 0.5 0 0 10 20 30 10 20 30 10 20 30 30 60 100 10 15 20 25 30 35 FLIGHT(min) ROCS(min) WizSync(min) FTSP(sec) Calibration Interval (min) 32/35

Discussion & Future Work ! Main features ✓ No extra hardware support ✓ Energy efficient ✓ Robust to network disconnection 33/35

Discussion & Future Work ! Limitation ! Indoor environments ! Future works ! Fingerprint of indoor ! Other mobile / embedded environments platforms ✓ One strong indication ✓ Applicability ✓ Automatically control ✓ Advanced techniques LBS, GPS, ... 34/35

Q & A Thank you !