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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

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

content
Content
  • Introduction
  • Market/Application requirements
  • Regulation
  • Standardization
  • Wireless Sensor Mote
    • Link budget
    • MAC
    • Synchronization
    • FEC
  • Conclusions

UWB4SN – November 2005

a problem under constraints

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

major limitations to global wireless sensor adoption
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

application requirements expressed at ieee 802 15 4a

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

regulation
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

standardization 1
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

standardization 2
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

slide9

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

stmicroelectronics ast areas of work in wsn
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

mote mac
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

example of a ldr budget link

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

synchronization 1
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

synchronization 2
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

super orthogonal vs convolutional codes
Super Orthogonal vs convolutional codes
  • Gap between SOC and CC decreases in dense multipath environment.
  • SOC performances for non coherent Rx ?

UWB4SN – November 2005

fec general requirements
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

ranging

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

symmetric double sided two way ranging

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

toa estimation error

 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

conclusion
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