Signal Processing in RFID · Introduction •RFID – Radio Frequency Identification •Wireless identification technology •Allows non line-of-sight identification •Multiple goods

Signal Processing in RFID

Markus Rupp, Jelena Kaitović, Robert Langwieser

Institute of Telecommunications, Vienna University of Technology

Duisburg

July 5, 2012

Christian Doppler Laboratory for Wireless

Technologies for Sustainable Mobility

Outline

• Introduction

• Tag Collision Model

• Simple Collision Recovery Techniques

• Transmission Model

• Advanced Recovery Techniques

• Leakage Compensation Techniques

• Summary and Conclusions

2



Introduction

• RFID – Radio Frequency Identification

• Wireless identification technology

• Allows non line-of-sight identification

• Multiple goods can be inventoried almost simultaneously

3


Technologies for Sustainable Mobility 4

How it Works

4

• Tag is powered by the reader passive tag

• Tag reflects a fraction of the reader signal backscattering

• Tag changes its scattering behavior backscatter modulation

Energy

DataDatareader or interrogator transponder or tag



Challenges and Motivation

• Several RFID tags operate in reader range

• Multiple tags respond simultaneously

collision occurs

information is discarded

throughput decreases

• Motivation:

• use information from colliding tags

• throughput increase

5



Outline

• Introduction







6



Framed Slotted Aloha

• Multiple tags are scheduled by Framed Slotted Aloha

Query,

Frame Size F Qr Qr Qr Qr Qr

Slot 0 Slot 1 Slot 2 Slot 3 Slot 4 Slot F-1

Reader

Tag 0

Tag N-1

Tag 3

Tag 2

Tag 1

t

Query,



Reader

Tag 0

Tag N-1

Tag 3

Tag 2

Tag 1

t

RN [0, F-1]:

4

2

2

0

3

Query,



Reader

Tag 0

Tag N-1

Tag 3

Tag 2

Tag 1

RN [0, F-1]:

4

2

2

0

3

t

RN16

RN16

RN16

RN16

RN16

• F: selected framesize

• N: tag population size

• RN: Random Number

• Qr: Query repeat

• RN16 16 bit Random . number packet



Throughput calculation

• Framed slotted Aloha • #slots: F

• #tags: N

• # Mr: recover from collisions of up to Mr tags

• # Mt: acknowledge up to Mt tags

• “fill level” in one slot: r

• Expected #slots with r tags:

• Standard: recover if Mr=1 and acknowledge Mt=1

• Results in throughput (= #slots with r=1):

rNr

FFr

NF

11

1

8

11

11

1

N

FF

NT




• Results in throughput (= #slots with r=1):

• Given N and F what is the maximum throughput?

• Answer: maximum for F/N=1T=0.368

9

11

11

1

N

FF

NT




• How does the reader/interrogator know the number of tags N?

• Answer: set some value F and measure number of slots for which: r=0, r=1 and r>1

• Use ML estimator to derive N and thus optimal value of F

• B.Knerr, M.Holzer, C.Angerer, M.Rupp, ''Slot-wise maximum likelihood

estimation of the tag population size in FSA protocols,'' IEEE Transactions on Communications, vol. 58, no. 2, February 2010. http://dx.doi.org/10.1109/TCOMM.2010.02.080571

10




• How large is the throughput if we can recover from Mt tags?

• Unfortunately, the standard allows only to acknowledge a single tag at a time. Let us assume we can acknowledge Mt tags, although we can recover from Mr >Mt collisions:

11

rFFr

NT

rNM

r

rt

11

1

1

t

rNM

Mr

rrNM

r

r

MFFr

Nr

FFr

NT

r

t

t

11

111

1

11



Higher Throughput by Collision Recovery

• r<=Mr … recover from collision – r tags received

• r<=Mt … acknowledge r tags

• Expected throughput (r<=Mr):

• Calculate optimum framesize F

• Throughput plot for Mt=1; M=Mr=1…8:

t

rNM

Mr

rrNM

r

r

MFFr

Nr

FFr

NT

r

t

t

11

111

1

11

12

NF /



M Fopt/N T

1 1 0.368

2 0.707 0.587

4 0.452 0.817

8 0.265 0.962

13

Framed Slotted Aloha with Collision Recovery

M Fopt/N T

1 1 0.368

2 0.707 0.587

4 0.452 0.817

8 0.265 0.962

× 2.6

F denotes frame size

N denotes tag population size

M=Mr denotes collision recovery factor

T maximal theoretical throughput



Outline

• Introduction







14



Challenges

• We want to find transmission methods,

such that many tags can be identified

even under collision scenrios (r>1)

• We want to employ several antennas at the

reader/interrogator to avoid changes in the existing

standards as much as possible.

15



Captured samples of a two tag collision

16


Technologies for Sustainable Mobility 17 17

Inphase

Quadrature

Tag 1

modulation h1a1(t)

L

φ1

φleak

h1

S(a,a)

S(r,a)

Inphase

Quadrature

L

Tag 2

modulation h2a2(t)

φ2

φleak

S(a,a)

h2

Inphase

Quadrature

Tag 1

modulation h1a1(t)

L

Tag 2

modulation h2a2(t)

φ1 φ2

φleak

h1

S(a,a)

S(a,r)

S(r,r)

S(r,a)

h2Reader

TX

RX

Energy / Data

DataEnergy / D

ata

Data

Tag 1

Tag 2

...

NR

ss

cro

talk

Inphase

Quadrature

L

φleak

S(a,a)

Reader

TX

RX

Energy / Data

Energy / Data

Tag 1

Tag 2

...

NR

ss

cro

talk

Reader

TX

RX

Energy / Data

DataEnergy / D

ata

Tag 1

Tag 2

...

NR

ss

cro

talk

Reader

TX

RX

Energy / Data

Energy / Data

Data

Tag 1

Tag 2

...

NR

ss

cro

talk

Recovering from collisions of two tags

• Baseband signal at reader receive path i:

• Receive signals:

tntahtahLts iiiii 22,11,

ttt nIHas

C. Angerer , R.Langwieser, M.Rupp: „RFID Reader Receivers for Physical Layer Collision Recovery“,

IEEE Transactions on Communications, 58 (2010), 12, p. 3526 - 3537



Recovering from collisions of two tags:

Optimal single antenna receivers

18

Single Antenna Zero Forcing Receiver (SAZF): Project signal constellation into subspace orthogonal to interference (force

interference to zero) Independent of synchronisation between tags Gain in RX signal power prop. to sin(φ1-φ2) Slice each projection separately

Successive Cancellation Receiver:

Decode stronger component Remodulate and substact Slice 2nd component Prevents SNR loss due to projection on second component

ML-Receiver:

as modulation signals in general are not synchronous, not feasible

Inphase

Quadrature

Sa,a

Sr,a

tag 1 modulat ion

tag 2 modulat ion

Sa,r

Sr,r

φ1 φ2

φ1-φ2

s2~

s1~ S2

S1

S1

S2

S2,a~

S2,r~

S1,r~

S1,a~



Challenges

•For recovering from collisions of more than

two tags, we need:

• More antennas

• Channel impulse responses for each tag!

19



Outline

• Introduction







20



Transmission Model

Double Rayleigh Pinhole Channel

21

Energy Data

Data

2,4

2,3

2,2

2,1

1,4

1,3

1,2

1,1

h

h

h

h

h

h

h

h

H

b

ji

f

jji hhh ,,

index i represents receive antenna i

index j denotes tag j



Transmission Model

Channel

• hjf follows a Rayleigh fading

• hi,jb follows a Rayleigh fading

• Channel matrix follows a double Rayleigh fading

• Channel matrix can also have a form of:

22

index i represents antenna i

index j denotes tag j

NR is number of receiving antennas

R is number of tags transmitting in the same slot

b

ji

f

jji hhh ,,

RN

R

ji

N RRh

h

h

h

h

H

,

,1

.

1,

1,1

...

...

...

...

H

HHRI



Channel Model

• Ricean model: Channel with a line of sight component E{h}= and a

fading componnent hw

• K denotes Ricean factor of the channel

• K=0 is pure Rayleigh fading

• is non fading link

23

whk

hK

Kh

1

1

1



• Once an estimate of the individual tag channels is given, we can apply

linear receivers

• Zero Forcing (ZF) receiver:

• Minimum Mean Square Error (MMSE) receiver:

Multiple Antenna Collision Recovery Receivers

denotes the Hermitian transpose of the estimated channel matrix

a(t) modulation vector

σ2 noise power

IR R × R identity matrix

aa laHsHHHr tEtttt HH

ZF

,ˆˆˆˆ1

laHsHIHHr

ttt H

R

H

MMSEˆˆˆˆ

12



Multiple Antenna Collision Recovery Receivers

• ZF and MMSE receivers - capable to recover from collision with R tags as long

as R≤ 2NRA

• Further improvement:

• Due to the fact that modulation signal a(t) is real-valued:

• In the equations for the ZF and the MMSE receivers the channel matrix and

the received signal have the form of:

number of equations is doubled

allows the separation of R≤ 2NR tags

tn

tn

I

Ita

H

H

ts

ts

H

HHRI

t

tt

s

ss



Performance Simulations

Simulation parameters Parameters value

Number of receiving antennas NRA ={1,2,3,4}

Number of responding tags r={1,2,4,8}

Channel Double Rayleigh fading channel

Perfect knowledge assumed

26



Performance Simulations - Bit Error Ratio

Confidence interval (95%)

J. Kaitovic et al.: „RFID Reader with Multi Antenna Physical Layer Collision Recovery Receivers “, IEEE International

Conference in RFID-Technologies and Applications, Sitges, Spain, 2011




28




29



Expected throughput

• The receiver can only decode one of the colliding packets

• The expected throughput:

30

rN

opt

r

opt

N

r

N

optopt FFr

N

FF

NT

RA

11

111

1

1

2

2

1

Mr=2=2NRA

M Fopt/N T

1 1 0.368

2 0.707 0.587

4 0.452 0.817

8 0.265 0.962

M Fopt/N T

1 1 0.368

2 0.707 0.587

4 0.452 0.817

8 0.265 0.962



Performance Simulations – Expected throughput

Mr=8

Mr=4

Mr=2

Mr=1

× 2.6



Tag Signal

• Tag respond to Query command:

• How to separate signals at reception and to estimate channel ?

32

J.Kaitovic, M.Simko, R.Langwieser, M.Rupp, ''RFID Reader Receivers with Multiple Antennas

for Physical Layer Collision Recovery, '' 6th annual IEEE International Conference on RFID

(RFID'2012), Orlando, Florida, April, 2012.

32



Tags with “colours”

33

Tag population N

Partitioned population C×N/C



Channel Estimation

• This provides sufficiently good

quality for channel estimation

• Here, we used a simple

LS channel estimation

• continuous: known channel

• dashed: estimated channel

34



tM

r

rC

Nr

rFFr

C

N

CT1

11

1

Throughput in collision scenarios with random orthogonal "postpreambles“

• Theoretical maximum:

•C=8 with 8 orthogonal “postpreambles”

•J=2 project signal constellation into

subspace orthogonal to interference

• number of tags per slot with identical color

35

F .. frame size

C .. number of used “postpreambles”=colors

N .. tag population size

RC : number of tags per slot with identical color

T .. maximal theoretical throughput

Mt=J: number of acknowledgements



Collision scenarios with random

orthogonal "postpreambles"

37

Resolvable

Likely resolvable?

Maybe not resolvable ?

Not fully resolvable

Not resolvable

Not fully resolvable

Not resolvable



Collision scenarios with random

orthogonal "postpreambles"

• This causes a reduction of the throughput from

• to being constraint on Scenario 1 and 2:

• To further include the receiver capability:

38

r

rrp

r

rrp

FFr

Nr

rprpFFr

Nr

FFr

Nr

sol

ss

sol

ss

rN

opt

r

opt

N

r

ss

rN

opt

r

opt

N

r

rN

opt

r

opt

N

r

RA

RA

RA

22

11

2

1

21

2

1

2

1

)()(1

11

)()(1

11

11

1



Collision scenarios with random orthogonal

"postpreambles“

– just unique(=all different) ones are taken into account

39

50% drop

100% gain



Collision Recovery

40

• Received signal:

• New channel estimation technique:

• successive interference cancelation

• projection of the constellation into the orthogonal subspace of the interference



Successive Interference Cancelation

• LS estimator:

• We extract modulation signal with MMSE receiver

• Signal for the next iteration :

41



Performance Analysis - throughput

42

almost perfect

for 4 tags in 2

antennas,

good

improvement

for 8 tags in 4

antennas

J.Kaitovic, R.Langwieser, M.Rupp, '' Advanced Collision Recovery Receiver for RFID, ''

4th EURASIP RFID Workshop, Turino, Italy, 26-27.Sep. 2012.



Leakage

• What about the leakage?

• We have always assume to get perfectly rid of it.

• But was this assumption realistic?

• Means: active carrier compensation • R.Langwieser, G.Lasser, A.L.Scholtz, M.Rupp, ''Comparison of Multi-Antenna

Configurations of an RFID Reader with and without Active Carrier

Compensation,'' Proc. of IEEE RFID-TA 2011, Sites, Spain, Sept. 2011.

43



Crosstalk and Active Carrier Compensation

0 d

Bm

-80 dBm

44

Tag is powered by the reader Crosstalk from transmitter to receiver at the reader Tag response is interfered by the crosstalk Energy transfer from reader to tag during the whole communication Crosstalk depends on the reader-antenna configuration

RFID Reader

TX

RX

(semi) passive Tag

33 dBm



Measurement Setup – Active Carrier Compensation

45

BLF: 320 kHz

PTX: 33 dBm

30 dBm

27 dBm



Antenna - Setup

46

TX

RX1 RX2

Nylon cord

0 1 2 2.5 [m]

2.1m

1.7m

Reader

RX/TX antennas: off-the-shelf patch antennas right hand circularly polarized TX: 9 dBi RX: 7 dBi positions for best RX/TX decoupling: ~45 dB

Tag

off-the-shelf - (DogBone)

EPCglobal UHF Class 1 Gen2

25 measurements all 10 cm

static scenario



Measured SNR

without CCU with CCU

47

SNR increases with decreased transmit power No SNR increase due to decreased transmit power



Measured Signal-to-Self-Interference Ratio

48

without CCU with CCU

SSIR increases with decreased transmit power



Summary and Conclusion

• It is possible to recover from collisions that have a number of tags two

times higher than the number of receiving antennas

• Multi antenna receivers significantly improve performances of RFID

readers

• Additional means for channel estimation required.

• Carrier leakage is usually the limiting factor

• Physical layer collision recovery: 2.6 times throughput increase, when

acknowledging a single tag (standard compliant)

• Substantially higher throughput possible with changes in standard.



THANKS FOR YOUR ATTENTION !

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Documents

Signal Processing in RFID · Introduction •RFID – Radio Frequency Identification •Wireless identification technology •Allows non line-of-sight identification •Multiple goods