Signal Processing in RFID · Introduction •RFID – Radio Frequency Identification •Wireless...
Transcript of Signal Processing in RFID · Introduction •RFID – Radio Frequency Identification •Wireless...
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
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Outline
• Introduction
• Tag Collision Model
• Simple Collision Recovery Techniques
• Transmission Model
• Advanced Recovery Techniques
• Leakage Compensation Techniques
• Summary and Conclusions
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Introduction
• RFID – Radio Frequency Identification
• Wireless identification technology
• Allows non line-of-sight identification
• Multiple goods can be inventoried almost simultaneously
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How it Works
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• 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
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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
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Outline
• Introduction
• Tag Collision Model
• Simple Collision Recovery Techniques
• Transmission Model
• Advanced Recovery Techniques
• Leakage Compensation Techniques
• Summary and Conclusions
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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,
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
RN [0, F-1]:
4
2
2
0
3
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
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
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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
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Throughput calculation
• Results in throughput (= #slots with r=1):
• Given N and F what is the maximum throughput?
• Answer: maximum for F/N=1T=0.368
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11
11
1
N
FF
NT
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Throughput calculation
• 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
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Throughput calculation
• 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:
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rFFr
NT
rNM
r
rt
11
1
1
t
rNM
Mr
rrNM
r
r
MFFr
Nr
FFr
NT
r
t
t
11
111
1
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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
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NF /
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M Fopt/N T
1 1 0.368
2 0.707 0.587
4 0.452 0.817
8 0.265 0.962
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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
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Outline
• Introduction
• Tag Collision Model
• Simple Collision Recovery Techniques
• Transmission Model
• Advanced Recovery Techniques
• Leakage Compensation Techniques
• Summary and Conclusions
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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.
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Captured samples of a two tag collision
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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
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Recovering from collisions of two tags:
Optimal single antenna receivers
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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~
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Challenges
•For recovering from collisions of more than
two tags, we need:
• More antennas
• Channel impulse responses for each tag!
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Outline
• Introduction
• Tag Collision Model
• Simple Collision Recovery Techniques
• Transmission Model
• Advanced Recovery Techniques
• Leakage Compensation Techniques
• Summary and Conclusions
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Transmission Model
Double Rayleigh Pinhole Channel
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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
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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:
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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
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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
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whk
hK
Kh
1
1
1
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• 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
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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
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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
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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
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Performance Simulations - Bit Error Ratio
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Performance Simulations - Bit Error Ratio
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Expected throughput
• The receiver can only decode one of the colliding packets
• The expected throughput:
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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
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Performance Simulations – Expected throughput
Mr=8
Mr=4
Mr=2
Mr=1
× 2.6
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Tag Signal
• Tag respond to Query command:
• How to separate signals at reception and to estimate channel ?
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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.
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Tags with “colours”
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Tag population N
Partitioned population C×N/C
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Channel Estimation
• This provides sufficiently good
quality for channel estimation
• Here, we used a simple
LS channel estimation
• continuous: known channel
• dashed: estimated channel
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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
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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
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Collision scenarios with random
orthogonal "postpreambles"
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Resolvable
Likely resolvable?
Maybe not resolvable ?
Not fully resolvable
Not resolvable
Not fully resolvable
Not resolvable
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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:
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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
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Collision scenarios with random orthogonal
"postpreambles“
– just unique(=all different) ones are taken into account
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50% drop
100% gain
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Collision Recovery
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• Received signal:
• New channel estimation technique:
• successive interference cancelation
• projection of the constellation into the orthogonal subspace of the interference
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Successive Interference Cancelation
• LS estimator:
• We extract modulation signal with MMSE receiver
• Signal for the next iteration :
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Performance Analysis - throughput
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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.
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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.
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Crosstalk and Active Carrier Compensation
0 d
Bm
-80 dBm
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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
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Measurement Setup – Active Carrier Compensation
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BLF: 320 kHz
PTX: 33 dBm
30 dBm
27 dBm
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Antenna - Setup
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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
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Measured SNR
without CCU with CCU
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SNR increases with decreased transmit power No SNR increase due to decreased transmit power
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Measured Signal-to-Self-Interference Ratio
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without CCU with CCU
SSIR increases with decreased transmit power
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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.
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THANKS FOR YOUR ATTENTION !
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