Wireless Sensors - ZHAW Zürcher Hochschule für ...kunr/publikationen/wireless_sensor2_hout.pdf ·...

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Wireless Sensors

A Modern Approachto Eliminate Cabling

29.10.2002© Elektrobit AG, <wireless sensor.ppt> 2

What Wireless can do

• The fundamental formula:

• Pr = received power• Pt = transmitted power• λ = wavelength• d = distance• n = propagation exponent• L = system loss factor

Ld4GrGtPtPr n2

2

•••••

=)( π

λ

2



Check health of a eagle anywhere in the world using LEO satellitesf = 402 MHz (wavelength λ = c / f= 75 cm),Pt = 100 mW, Pr > 10-16 W ( assumes B=1 kHz, NF = 4 dB, SNR=10 dB)Gr = 1 (Piece of Wire), Gt = 10 (Satellite Antenna), L = 2Freespace Propagation n = 2

Energy:120mA every 10 min. for 1 s from 2000 Ah Li-Cell:10’000 hour lifetime (>1 year)

km42002LPr4

GrGtPtd 2

2

=•••••

=)( π

λIt can be done: ARGOS 850 km



Monitor the temperature of machinery tools anywhere in the plantf = 868 MHz (wavelength λ = c/f= 35 cm),Pt = 10 mW, Pr > 10-15 W ( assumes B=10 kHz, NF = 4 dB, SNR=10 dB)Gr = 4 (directional antenna), Gt = 1 (printed loop),L = 100 (fading margin for moving objects), Non line of sight propagation n = 4....6

Energy:20mA every second. for 100 ms from 2000 Ah Li-Cell:1’000 hour lifetime (>1 month)

m1334LPr4

GrGtPtd 2

2

=•••••

=)( π

λ m266LPr4

GrGtPtd 2

2

=•••••

=)( π

λ

It may be a hard job: optimized solutions

3


Frequency Bands

Different countries-

Different regulations

e.g. CEPT Short Range Devices SRD


Frequency Bands

4


Frequency Bands

The Big Three:

• 434 MHz B = 1.75 MHz Pt = 10 mW Duty Cycle Restrictions Car door openers, RFID, Telemetry

• 868 MHz B = 2 MHz (24 MHz), Pt = 10 ... 500 mW (1 W) (915 MHz USA) Telemetry, Social Alarm, Alarm, (WLAN)• 2.45 GHz B = 83 MHz, Pt = 10 mW...100 mW (1 W)

WLAN, Bluetooth, RFID, Telemetry, Wireless Video...

And the upcoming:

• 5.2 GHz B = 100 MHz, Pt = 10 mW... 1W• 5.8 GHz B = 150 MHz, Pt = 25 mW...1 W

WLAN, ITS (LSVA), Wireless Multimedia, Telemetry


Propagation Effects

TxRxLOS

ScatteringReflection

Diffraction

Main Impact on System:• Signal is also available behind obstacles• Propagation index n increases in non line of sight situations (non-LOS)• Multiple copies of the transmitter signal arrive at receiver (Multipath)

5


Propagation Effects

Environment Propagation exponent n

Free Space 2Open Field (far) 4Urban area 2.5 - 4Shadowed urban 5 - 6In building LOS 1.8 - 2In building obstructed 4 - 6

Max useful path loss is typically 120 dB


Propagation Effects

• Multipath signals can add or subtract → Deep fade notches can occur, L increases• Multipath changes when objects are moving

0 dB

-30 dB

840 MHz 880 MHzfrequency

SpectraCHx

20log [S(f, t1)]

20log [S(f, t2)]

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Propagation Effects

Reliability in % Fading margin (L) in dB

low reliable link90 1099 2099.9 3099.99 40

high reliable link

• 1 Channel curve is valid for low cost system without diversity• Diversity of 2 can bring 10... 20 dB reduction in margin !!


Transmitter / Receiver

Low cost architecture

Synthesizer

PLL

VCO

Slicer

Tx/RxSwitch

IF

RF

Data

Antenna

Microcontroller

Gt / Gr

B

NF

λ S/N

Pt

Pr

7



LoopAntenna

Low cost architecture

• single channel• fixed output power • low Rx sensitivity • simple filtering

• external µC• simple detector• no error correction• no networking


Receiver sensitivity

S/N

BER

Parameters influencing receiver sensitivity• Bit Error Rate (BER): related to link reliability• Modulation: Amplitude/Frequency/Phase: PSK, FSK, OOK... best for sensor application• Noise figure (NF) of receiver front-end: technology, price• Bandwidth B: proportional to data rate

1030Pr

mindBm

dBm

10Pr

S/Nlog(B)10NF dBm -174PrRkB

−

=

+•++=•=

PSK = phase shift keyingFSK = frequency shift keyingOOK = on-off (amplitude) keying

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1030)dBmPr(

min

10Pr

S/Nlog(B)10NF dBm -174Pr(dBm)RkB

−

=

+•++=•=

Receiver sensitivity

Example: Data rate R = 10 kbit/s, NF = 4 dB BER < 0.001 FSK, k = 1.0

S/N

BERB = 10 kHzPr(dBm) = -174 + 4 + 40 + 11 = -110 dBm

→ Pr = 10-14 W


Antenna Design

• The antenna is the interface between transmitter or receiver and the propagation medium• It is therefore a deciding factor in the performance of a radio system• Principal properties are the same for transmitting and receiving antennas

Key parameters:BandwidthGain, Directivity, AreaImpedance matching

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Most important type of antennas

Monopole / Dipole

+ cheap+ wide bandwidth+ fair gain, omnidirectional- size is λ/4... λ- place dipole away from metall/ground- monopole needs ground plane

Antenna gain:

•λ /2 Dipole: Gr = 1.6• λ/4 Monopole Gr = 3.2• 5 λ/8 Monopole Gr = 6.4

λ /2

λ /2λ /4



Directional Patch

+ wide bandwidth+ high gain: Gr = 4...10+ easy placing on metall+ easy forming of arrays: Gr >10- size is λ/2 • λ/2- higher costs + - high directivity

OmnidirectionalDirectional

air printed

10



Small Printed LoopExample for 434 MHz:

+ small and cheap- narrow bandwidth- low gain when small- place away from metall/ground yresistivit

skindeepthwidthlenghtR

NA320R

l

4

22

r

••

=

••=

λπ )(

lr

r

RRR100Efficiency+

•=(%)


New antenna technologies

• High frequency ceramic solutions (LTCC technology)

• Research on fractal antennas for reduced size of loop and dipoles

Application size for full gain at 2.4 GHz, Gr = 1

868 MHz: ∅ only 55 mm for Gr = 1

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Power source

If there is no wired power supply:• Energy is the most valuable resource• All components must support low power modes• What software can do to conserve energy: save power by turning radio off, when nothing is to hear

Xmit:

Recv:

preamble messagesleepb

Activesleep Activesleep

µs time scale

ms time scale


Power source

But what does this mean for Pt = 1 mW link?

– Lithium Battery runs for 28 hours at peak load and years atminimum load!3 orders of magnitude difference

– A one byte transmission uses same energy as approx 10’000 cyclesof computation!Use software to increase performance

CR2354 560 mAh

003 mAEE-Prom

7 mA00

5 mA (RX)2 mA

Idle

10 uA20 mA01 mA04 mASensor Part

5 µA7 mA (TX)Radio5 µA5 mACPU

SleepActive

Wake-upSum

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Diversity

Time Diversity• Use 2 or more different time slots for transmission of a message

• Separate slots more than Tc, the coherence time

• Use coding and interleaving of dataNeeds dynamic environment and time

Space Diversity•Use 2 or more antennas•Separate antennas 0.5λ....0.8 λ

•433 MHz 35....56 cm !• Use RSSI for processing

Needs additional space at transmitter and/or receiver


Diversity, FHSS

Frequency Diversity• Use 2 or more channels / frequencies for transmission of a message• Separate the channels far enough → needs programmable synthesizer

Spread Spectrum• Frequency Hopping in K channels• Coherence bandwidth Bc depends on multipath delay spread στ → needs large bandwidth and fast synthesizer

f

Sf1 f2

Bc ≈ ( 5στ )-1 Range of 100 kHz .... 4 MHz

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Undetected error Pb

Error correction

• Interleave data from different blocks

• Use forward error correction instead of simple parity check

• Golay (23.12) can correct 3 errors, detect 4 and more errors (B.Sklar)

Forward Error Correction (FEC)• Channel may show fading• Channel may be disturbed by jamming signal


Error correction

Designing reliability in wireless link:

16e

7eundet )p(1)p(253P −••≈

•pe = uncoded error probability (@ same message duration)• For Eb/Noinformation bit = 7 dB → pe ≈ 10-2 → Pundet = 2·10-12

• If data rate is 10 kbit/s continuously: undetected error every 530 days !

Sophisticated error correction and data link layer protocol needed

• Golay (23,12) undetected error probability:

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Connectivity

Simplex: needs high degree of FEC (sensor is transmitter only)Half-duplex: offers chance to implement Automatic Repeat Request (ARQ) at cost of having transmitter and receiver at both endsFull-duplex: offers more throughput with ARQ at more costly hardware

Half-duplex

Full-duplex

3 5

4 7

53

4 7

Error Error

Error Error



High end architecture

Direct Digital

Synthesizer

PLL

RSSI

ADC DACVCO

TunableDiscriminator

FEC Processor

Slicer

ADC

Tx/RxSwitch

IF

RF

Data

Antenna

Microcontroller

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Data Link Layer

Adapted Data Frames: simple but powerfull

• Learning sequence for adjusting discriminator/slicer for offsets • Barker sequence for bit and byte synchronization • Data blocks followed by forward error correction (FEC)• FH mode could send one frame per hop

Example for 50 kbit/s and 500 hops/s:

Hopping Time 2 ms

1 Byte 160 µs

FEC FEC FECAdress



High end architecture

• Multi channel, FH• High Rx sensitivity• Onboard processor• Error correction• Networking support• Adaptive features

Tx/RxSwitch

LNAPA

Transceiver Host Interface

Sensor Codec

Generic TRX

Power Mgmt

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Wireless networks

Many topologies: Point to Point, Multi-point, Ad-hoc, Avalanche, Centralized.....


Wireless networks

Application LayerPresentation Layer

Session LayerTransport LayerNetwork Layer

Physical LayerData Link Layer

Additional protocol layers need to be optimized for wireless

Example for Bluetooth:

Minimum needed for sensor

ISI / OSI

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Multiple Access

Systems with > 1 sensor may need an access scheme:• Reduction of probability of a collision• Higher throughput• Faster response time


And Bluetooth ?

Small chip solution, potential for low cost: • Pt =1 mW, 10 mW, (100 mW)• Prmin = 10-11 W (NF > 20 dB)• f = 2.40....2.48 GHz, Frequency Hopping• Data rate < 723 kbit/s, Half- ... Full-duplex• B = 1 MHz, GFSK Modulation• Error correction with 1/3 or 2/3 FEC & ARQ• d typically 10 m (non LOS, 1 mW)• Current consumption > 50 mA (@ 1mW)

Points to be solved for sensor applications: • antenna size dilemma• unintentional interference versus reliability• current consumption versus distance• extensive protocol know-how needed

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Conclusions

Wireless Sensors - Means not just cutting the cables !

• Study propagation issues• Check regulatory options• Map system application to

•wireless architecture•modulation & diversity•data reliability & protocol•network aspects

• Design wireless circuits & antenna• Solve the power puzzle


Conclusions

Wireless communications solutions• BLUETOOTHTM HW Integration• Elektrobit BLUETOOTHTM Stack• DECT / GSM / GPRS / HSCSD / WLAN• Radio Control / Telemetry System Design

Robust Data Communication• Spread Spectrum Technology (DSSS, FHSS)• High data rate

Integrated wireless systems• Low power / low cost radio (network) solutions• IC and RFIC Design • Embedded SW, TCP/IP, WEB

Elektrobit has the keys to your successful wireless application

Wireless Sensors - ZHAW Zürcher Hochschule für ...kunr/publikationen/wireless_sensor2_hout.pdf ·...

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