Harvesting a Clock from a GSM Signal for the Wake-Up of a Wireless Sensor Network

Transcription

1 Harvesting a Clock from a GSM Signal for the Wake-Up of a Wireless Sensor Network Jonathan K. Brown and David D. Wentzloff University of Michigan Ann Arbor, MI, USA ISCAS 2010 Acknowledgment: This material is based upon work supported by the National Science Foundation under Grant No

2 Wireless Sensor Networks (WSNs) Network of energy-constrained nodes Node functionality Sense information n 1 n 4 Communicate wirelessly Potential applications Environmental sensing Biomedical implants Industrial monitoring Major design challenges Small volume, low cost Long lifetime n 2 n 3 n 5 n 6 n k Low-power circuits 2

3 Motivation for Synchronization Relative power consumption of circuit components on a node 1 mw Comm Radio 1 μw Sensor Processor 1 nw 1 pw Duty-cycled communication Not synchronized? X n 1 n 2 Synchronized n 1 n 2 3

4 Motivation for Synchronization Radio Synchronization strategies High-accuracy (high-power) timer Wake-up radio Low-accuracy (low-power) timer + synchronization radio Relative power for each synchronization strategy 1 mw 1 μw Comm Radio Sensor Processor 1 mw 1 μw HA Timer WU Radio Sync Radio 1 nw 1 nw 1 pw 1 pw LA Timer 4

5 Motivation for Clock Harvesting Timing with a wake-up radio Unknown time for wake-up signal Wake-up radio on continuously Beacon strategy Node Beacon Power Pwr Generate within network Requires network power to generate it Requires custom infrastructure Sleep (WU radio) n 1 Sense Wake-up Sense & comm Time n k n 2 5

6 Motivation for Clock Harvesting Timing with a synchronization radio Known time for sync signal Sync radio on intermittently Beacon strategy Node Beacon Power Pwr Generate within network Harvest existing signal Doesn t require network power to generate it Sleep (Sync radio) Sleep (LA timer) GSM Wi-Fi n 1 Sync Sense & comm Sense Time n k n 2 6

7 Motivation for GSM-Based Clock GSM Coverage [Image Courtesy of GSM Association] Provides worldwide coverage Broadcasts high-power signals Contains an embedded clock Low-frequency Simple to extract (i.e. low-power) 7

8 Characteristics of GSM Standard 4 major frequency bands worldwide 850 MHz, 900 MHz, 1800 MHz, 1900 MHz 200-kHz bandwidth physical channels Basic services (BCCH carrier) Includes Frequency correction (FCCH) Broadcast control (BCCH) Exist on all GSM, GPRS, and EGDE networks Channel properties Gaussian minimum shift keying (GMSK) spectrum Constant envelope 8

9 Proposed GSM-Based Clock Frequency correction burst (FB) Generates tone 67.7 khz above center freq of BCCH carrier Repeats at rate of approx 21 Hz Add filter here PSD (dbm/mhz) Simulated Measured Frequency (MHz) Power (dbm) Measured Time (ms) Looks like a clock 9

10 Proposed Receiver Architecture Measures power of BCCH carrier and at FB offset freq Uses periodicity of FB as a clock for synchronization LNA BCCH Path FB Path Ref In Sync Clk Goal is to harvest a clock, not extract GMSK-modulated data 10

11 Proposed Receiver Architecture Measures power of BCCH carrier and at FB offset freq Uses FB periodicity as a clock for synchronization Matlab Simulations V(t) Time V(t) V(t) V(t) V(t) LNA Time Time BCCH Path FB Path Ref In Time Sync Clk V(t) Time Time Desire signal at output of FB filter only during freq bursts; otherwise, AWGN 11

12 Proposed Receiver Architecture Measures power of BCCH carrier and at FB offset freq Uses periodicity of FB as a clock for synchronization LNA BCCH Path FB Path Ref In Sync Clk Expected power similar to wake-up radios previously reported 12

13 Proposed Receiver Architecture Measures power of BCCH carrier and at FB offset freq Uses periodicity of FB as a clock for synchronization LNA BCCH Path FB Path Ref In Sync Clk Requires some design, but relatively little power at IF 13

14 Proposed Receiver Architecture Measures power of BCCH carrier and at FB offset freq Uses periodicity of FB as a clock for synchronization LNA BCCH Path FB Path Ref In Sync Clk Potentially high power because potentially high-q, especially narrowband FB filter 14

15 Characterization of the FB Filter Generated GMSK-modulated FBs and PR bursts Set BCCH filter to 200 khz bandwidth 2 nd -order bandpass Swept FB filter bandwidth and order Generated 10 4 FB intervals (>10 8 GMSK symbols) Counted number of synchronization errors Probability of Synchronization Error Order = 2 Order = 4 Order = 8 Order = FB Filter Bandwidth (khz) Set FB filter to 7 khz bandwidth 4 th -order bandpass 15

16 Prototype Receiver for Proof-of-Concept Set IF to 275 khz Digitized real GSM data Used measured data as input to simulated baseband Antenna Filter LNA Signal Generator Discrete Prototype Mixer Amplifier Ampifier Filter Attenuator ADC Digitizer Card Matlab Simulations BCCH Path FB Path Ref In Sync Clk 16

17 Extraction of Real GSM-Based Clock Counted number of synchronization errors Compared simulated and measured results Probability of Synchronization Error Sim, O = 2 Meas, O = 2 Sim, O = 20 Meas, O = FB Filter Bandwidth (khz) Similar trends with discrepancies 17

18 Extraction of Real GSM-Based Clock ADC 1 Ref In 2 Sync Clk 3 Amplitude (V) BCCH Path (Ref) Amplitude (V) Time (s) 2 FB Path (In) Clock Output Time (s) Time (s) 18 Amplitude (V)

19 Summary / Conclusions Introduced technique of clock harvesting Synchronizes network with existing signal Conserves energy in sensor network Identified embedded clock in GSM standard Proposed radio architecture for synchronization Amenable to low-power design Characterized probability of synchronization error Verified functionality with prototype Harvested clock from real GSM signal 19