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Overcoming UWB Implementation Challenges in Wireless Sensor Networks

This document discusses the implementation challenges associated with Ultra-Wideband (UWB) technology in wireless sensor networks (WSNs). It covers market and application requirements, regulation and standardization issues, and the technical limitations that hinder global wireless sensor adoption. The paper outlines the specific requirements for sensor motes, including link budgets, synchronization, and forward error correction (FEC) strategies. Insights into creating reliable, energy-efficient, and small devices capable of meeting these demands are provided. The conclusions emphasize the need for innovation and standard compliance to facilitate market readiness.

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Overcoming UWB Implementation Challenges in Wireless 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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