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Implementation challenges of UWB for sensor networks

Implementation challenges of UWB for sensor networks . Laurent Chalard, Didier H é lal , Gian-Mario Maggio, Yinqwei Qiu, Lucille Verbaere-Rouault, Armin Wellig, Julien Zory STMicroelectronics, Geneva, Switzerland. Content. Introduction Market/Application requirements Regulation

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Implementation challenges of UWB for sensor networks

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  1. Implementation challenges of UWB for sensor networks Laurent Chalard, Didier Hélal, Gian-Mario Maggio, Yinqwei Qiu, Lucille Verbaere-Rouault, Armin Wellig, Julien Zory STMicroelectronics, Geneva, Switzerland UWB4SN – November 2005

  2. Content • Introduction • Market/Application requirements • Regulation • Standardization • Wireless Sensor Mote • Link budget • MAC • Synchronization • FEC • Conclusions UWB4SN – November 2005

  3. Ready for market A problem under constraints Market understanding Complete mote solution Competitive advantage Innovative WSN solutions Standard compliancy ST’s technology compatibility UWB4SN – November 2005

  4. Major Limitations to Global Wireless Sensor Adoption Ease/install Reliability Interference Battery Cost Interoperability Security Bit rate No need Size Source ON-World 2004 UWB4SN – November 2005

  5. Applications' requirements 1000 100 Data rate (kbps) 10 1 1 10 100 1000 10000 Range (meter) Application requirementsexpressed at IEEE 802.15.4a Low data rate does not mean simple ! +ranging with accuracy inside 5% of range UWB4SN – November 2005

  6. Regulation • No harmonization done by ITU-R so far… • EC final decision in March 2006 • Low data rate is still under discussions • duty cycle • Minimum average burst repetition period over an hour 1 sec • Minimum instantaneous burst repetition period over 1 second 30 to 200ms • emission level limitation -41.3dBm/MHz or -45dBm/MHz UWB4SN – November 2005

  7. Standardization (1) • Standards has exhibited limitations up to know for wireless sensor network applications • 802.15.4: poor reliability • Zigbee: too complex • WiFi: too expensive • BT: limited in number of nodes • Now appear 3 different alternate PHY options in IEEE 802.15.4a • Low-band UWB [DC-960MHz] • Chirp Spread Spectrum [2.4GHz ISM band] • High band UWB [3.1-10.6GHz] UWB4SN – November 2005

  8. Standardization (2) IEEE 802.15.4a status • Band plan defined • PRF will be a multiple of 7.21875MHz • Perfect Balanced Ternary Sequences (PBTS) of length 31 and 127 have been agreed. • All systems should support a mandatory non-coherent mode • Still 6 Forward Error Correction proposals (Super Orthogonal Codes, Convolutional codes) UWB4SN – November 2005

  9. Mote: Complete Solution Energy scavenging Battery Capacitors A D C Sensor Power management D I O Sensor Calibration BB + RF transceiver -controller D A C TEDS Actuator • Fully integrated wireless sensor devices • Small: < 1cm3 (System-in-Package ) • Cheap: <1$ (low cost electronics) • Low power: <10mW peak • Operate from energy scavenging: <100uW average UWB4SN – November 2005

  10. STMicroelectronics – ASTareas of work in WSN • 802.15.4 / ZigBee (PHY, MAC and networking protocol) • UWB Physical Layer • Localization enabled networking Target is convergence ! UWB4SN – November 2005

  11. Mote: MAC • Support for ranging procedures (including mobility) • Backward compatibility (w.r.t. 802.15.4 MAC) • Cross-layer (PHY-MAC) optimization • Medium access: • “Carrier Sensing” type mechanisms for UWB (to enable CSMA) • Random access schemes (e.g. ALOHA) • Interference mitigation: • LDC operation • DAA (Detect and Avoid) UWB4SN – November 2005

  12. Regulation TX Power Path Loss RX Power Link Margin Implementation Loss Eb/No min System Noise Noise Figure Noise per bit Data throughput Thermal Noise Temperature Example of a LDR budget link UWB4SN – November 2005

  13. Synchronization (1) • Context • Inaccurate reference clocks (typ. >> 1ppm) • Multi-user, asynchronous & random communications • Low SNR => Need for pulse energy accumulation (CI and/or MF, etc.) • Short pulses => down-convert to limit processing speed • Synchronization shall overcome… • Jitter (reference clock & PLL) • Drift between motes’ clocks (frequency offset) • Noise • Interferences • multi-user • Mobility • etc. UWB4SN – November 2005

  14. Synchronization (2) • A few illustrative numbers… • Coherency time of a 500MHz pulse is in the order of 100ps • Preamble duration is between 1us and 33us • Possible drift due to oscillator's accuracy • Over 1us, 10ppm to ±10ps, 40ppm to ±40ps • Over 33us, 10ppm to ±330ps, 40ppm to ±1.32ns • Hence a few design challenges • Acquisition/detection • How to coherently accumulate energy? • How to estimate frequency drift, so as to relax tracking requirements? • Tracking • How to do it on a non-continuous signal? • How to do it with minimum complexity? UWB4SN – November 2005

  15. Super Orthogonal vs convolutional codes • Gap between SOC and CC decreases in dense multipath environment. • SOC performances for non coherent Rx ? UWB4SN – November 2005

  16. FEC general requirements • Unique solution for coherent AND non coherent mode: puncturing ? • Trade-off Complexity vs performance • L constraint length • Nb operation per bit=2L • hard decoding for min complexity (soft decoding -> ~2dB improvment) • (De)Interleaver mandatory to obtain good decoding performances. Eb/No requirements with convolutional codes, coding rate=1/2. UWB4SN – November 2005

  17. TOA error + clock drift @ A TOF Estimation Request TOA error + clock drift @ B RANGING TOA & Two-Way Ranging T1 To Terminal A TX/RX Terminal B RX/TX TOF TOF TReply Terminal A Prescribed Protocol Delay and/or Processing Time Terminal B Ranging error < 1m TOAerror < 3.3 ns Courtesy: LETI UWB4SN – November 2005

  18. BUT: The condition TReplayA  TReplayB limits the MAC protocol ! Two big numbers measured with the same time-base (clock B) Two big numbers measured with the same time-base (clock A) Symmetric Double Sided-Two Way Ranging (SDS -TWR) Device A Device B Device B Time of flight TOF TReplyB >> TOF TRoundA reply time TOF TOF TRoundB TReplyB TReplyA TReplyB TOF (IEEE 802.15.4a: Nanotron) UWB4SN – November 2005

  19.  matched filter output (of coherent bins) 80 ppm TOA estimation error • LOS • NLOS • Delay spread + AWGN + Relative clock drift between Terminal A and B UWB4SN – November 2005

  20. Conclusion • Only a complete wireless sensor network solution will enable the emergence of a mass market • This can only be achieved through cross optimization Market understanding Complete mote solution Competitive advantage Innovative WSN solutions Standard compliancy ST’s technology compatibility UWB4SN – November 2005

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