UWB4SN – November 2005 Implementation challenges of UWB for sensor networks Laurent Chalard, Didier Hélal, Gian-Mario Maggio, Yinqwei Qiu, Lucille Verbaere-Rouault,

UWB4SN – November 2005

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


Content• Introduction• Market/Application requirements• Regulation• Standardization• Wireless Sensor Mote

– Link budget– MAC– Synchronization– FEC

• Conclusions


A problem under constraints

Innovative WSN

solutions

Ready for market

Market understanding

Completemote solution

ST’s technologycompatibility

Standard compliancy

Competitiveadvantage


Major Limitations to Global Wireless Sensor Adoption

Source ON-World 2004

Ease/install

Reliability

Interference

Battery

Cost

Interoperability

Security

Bit rate

No need

Size


Application requirementsexpressed at IEEE 802.15.4a

Applications' requirements

1

10

100

1000

1 10 100 1000 10000

Range (meter)

Da

ta r

ate

(k

bp

s)

+ ranging with accuracy inside 5% of range

Low data

rate does

not mean

simple !


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 hour1 sec

• Minimum instantaneous burst repetition period over 1 second30 to 200ms

– emission level limitation-41.3dBm/MHz or -45dBm/MHz


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]


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)


• 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

ADC

SensorDIO

Actuator

DAC

Calibration

TEDS

-controllerBB + RF

transceiver

Power management

BatteryCapacitors

Energy scavenging

Sensor

Mote: Complete Solution


STMicroelectronics – ASTareas of work in WSN

• 802.15.4 / ZigBee (PHY, MAC and networking protocol)

• UWB Physical Layer

• Localization enabled networking

Target is convergence !


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)


Example of a LDR budget link

Throughput Rb (Mb/s) 1Distance (m) 100.0Average TX power Pt (dBm) -8.00Tx antenna gain Gt (dBi) 0.0Fc (Hz) 4.0E+09Path loss 1 meter L1 (dB) 44.5Path loss at d meter L2 (dB) 40.0Rx antenna gain Gr (dBi) 0.0Rx power Pr (dBm) -92.5N = -174 + 10*LOG10(Rb) (dBm) -114.0Noise Figure (dB) 7.0Average noise power per bit Pn (dBm) -107.0Eb/No min (dB) 10.0Implementation Loss (dB) 3.0Link Margin (dB) 1.5Proposed Min Rx sensitivity Level (dBm) -94.0

TX Power

RX Power

Eb/No min

Link Margin

System Noise

Noise Figure

Path Loss

Regulation

Thermal Noise

Data throughput

Noise per bit

Temperature

ImplementationLoss


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.


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?


Super Orthogonal vs convolutional codes

• Gap between SOC and CC decreases in dense multipath environment.

• SOC performances for non coherent Rx ?


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.

K

Coherent Rx

AWGN EbN0 @ 1%PER

Non-coherent Rx

AWGN EbN0

@1%PER

Un-coded

9.0 dB 12.25 dB

3 5.2 dB 9.8 dB

4 4.5 dB 9.2 dB

5 4.0 dB 8.8 dB

Eb/No requirements with convolutional codes, coding

rate=1/2.


Terminal B

Request

Prescribed Protocol

Delay and/or Processing

Time

To

TReply

T1

Terminal A TX/RX

Terminal B RX/TX

TOF TOF

cTd

TTTT

AOFAB

AOF

.~~2

1~Reply01

TOF EstimationTerminal

A

RANGING

Courtesy: LETI

TOA error + clock drift @ A

TOA error + clock drift @ B

TOA & Two-Way Ranging

Ranging error < 1m TOAerror < 3.3 ns


Symmetric Double Sided-Two Way Ranging

(IEEE 802.15.4a: Nanotron)

Device A

Device B

Time of flight

reply time

4

tttt replyBroundBreplyAroundATOF

Device B

TOF

TOF

TOF

TOF

Two big numbers measured with the same time-base (clock A)

Two big numbers measured with the same time-base (clock B)

TReplyB >> TOF

TReplyA TReplyBTRoundB TReplyB

TRoundAThe condition TReplayA TReplayB

limits the MAC protocol !

BUT:

(SDS -TWR)


TOA estimation error

+ AWGN + Relative clock drift between Terminal A and B

matched filter output (of coherent bins)

80 ppm

• LOS• NLOS• Delay spread


Conclusion• Only a complete wireless sensor network solution will

enable the emergence of a mass market• This can only be achieved through cross optimization

Innovative WSN

solutions

Market understanding

Completemote solution

ST’s technologycompatibility

Standard compliancy

Competitiveadvantage

Documents

UWB4SN – November 2005 Implementation challenges of UWB for sensor networks Laurent Chalard, Didier Hélal, Gian-Mario Maggio, Yinqwei Qiu, Lucille Verbaere-Rouault,