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Phase-Synchronicity in Active Load Modulation
for NFC and Proximity
Michael Stark & Michael Gebhart
5th International Workshop on Near Field Communication (NFC)
ETH Zurich, Switzerland, Feb. 5th 2013
Content

Motivation
-

NFC combines several Contactless Communication Protocols
Smallest Antenna to achieve Side Band Amplitude requirement with PLM
How Active Load Modulation can overcome this limit for NFC and PxD
Impact of Phase-Drift on Contactless Communication
PICC TX maximum allowable Phase-Drift (PD)
Phase-Drift Measurement Setup
- Cross-Talk in ISO/IEC10373-6 Test Assembly
- PICC Signal Pre-Processing Approach
- How Signal-to-Interference (SIR) degrades PD measurement

Signal Processing with Homodyne and Hilbert Demodulator Algorithm
- Schematics and simulated Signal Processing

Measurement Results at different H-field

Conclusions
5th International Workshop on Near Field Communication, ETH Zurich
2
February 7, 2013
NFC: A chance for world-wide interoperability
in contactless HF communication
NFC combines all relevant Contactless Communication (protocol) Standards
•
•
ECMA 340: NFCIP-1
ISO/IEC14443 Type A (106 kbit/s)
+ ECMA 356: RF Test Methods
FeliCa (212 / 424 kbit/s)
= ISO/IEC 18092
„Active mode“ (0,847 … 6,78 Mbit/s ASK)
ECMA 352: NFCIP-2
+ ECMA 356: RF Test Methods
„NFC mode“ = NFCIP-1
+ Reader ISO/IEC14443 A+B (106 … 847 kbit/s)
+ Reader ISO/IEC15693
•
NFC Forum
NFC-A
= ISO/IEC14443 A (106 kbit/s)
Analogue Specification
NFC-B
= ISO/IEC14443 B (106 kbit/s)
NFC-F
= FeliCa (212 / 424 kbit/s)
ISO/IEC15693
5th International Workshop on Near Field Communication, ETH Zurich
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NFC operating modes
NFC Card (Emulation) Mode
Mobile payment, Ticketing,
Access control, Transit,
Top-ups, Toll-Gate
NFC Reader Mode
TX
RX
Target Mode
RX
TX
NFC Device
Contactless Reader
TX
RX
Initiator Mode
Content distribution,
Information access,
Smart advertising
Peer to peer Mode
Data transfer:
Fast, easy & convenient
device association, setup &
configuration
RX
TX
Contactless Card
NFC Device
TX
RX
RX
NFC Device
TX
NFC Device
5th International Workshop on Near Field Communication, ETH Zurich
4
Smallest antenna size to achieve SBA for PLM
P3SR008: Measured data
Minimum transponder antenna size to comply ISO/IEC 14443-2 LMA
4000
Outer transponder antenna area in mm 2
P3SR009: Measured data
3000
Passive 14443
transponders
class 1
class 3
2000
class 2
1500
class 4
1000
Battery powered
NFC devices
800
600
class 5
Impossible with passive
antenna concept
400
N=3
N=4
N=5
N=6
200
0.5
1
class 6
2
3
5
10
Transponder quality factor QT
15
Postulated lower
border for
antenna size
20 25 30 35
Amongst other constraints: H=1.5 A/m(rms); only PCD 1 and class 1-3 LMA limits
5th International Workshop on Near Field Communication, ETH Zurich
5
Active Load Modulation
 Active Load Modulation (ALM) can break the limitation of a minimum Antenna Size to
achieve Side Band Amplitudes with Passive Load Modulation (PLM).
 The Reader defines the time reference in Contactless Communication.
 The Card has to respond synchronously to the Reader alternating H-field.
 This is given for PLM, but is a challenge for ALM.
 To guarantee interoperability, a specific method for Phase Drift Measurement must be
developed.
Possible ALM realization: Zero-Coupling Antenna
k
U RX LRX
U TX LTX
5th International Workshop on Near Field Communication, ETH Zurich
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Card to Reader Communication using ALM
Original CW Reader Carrier
:24
:24
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5th International Workshop on Near Field Communication, ETH Zurich
7
Time domain
Amplitude in Volts (peak)
Analysis,
(forward) DFT
+1.5 a
+a
time
Frequency domain
+a lower
side
band
SC
SC
frequency
-a
frequency
Time series of discrete amplitudes
Frequency spectrum of discrete amplitudes
N samples
Index i
0
N

i  0...N 1
Real part
(cos wave amplitudes)
N-1
 2 k i 
 Re X  cos  N 
2
k
k 0
N

Phasors, the polar view
N/2 + 1 samples
Imaginary part
(sine wave amplitudes)
xi
Phasor view
upper
side band
amplitude
Real part
Synthesis,
inverse DFT,
Imaginary part
Amplitude in V(p)
Side Band Amplitude Analysis algorithm
 2 k i 
 Im X  sin  N 
2
k
k 0
e.g. 128 pt time-domain

Rotation speed
0 Index k N/2
N/2 + 1 samples
   2  fC
Phasor radius  amplitude a
0 Index k N/2
N 1
 2 k i 
ReX k     xi  cos 

 N 
k 0
N 1
 2 k i 
ImX k     xi  sin 

 N 
k 0
65 pt frequency-domain x 2
2 pt contain no information: Im { X0 } = 0 and Im { XN/2 } = 0
Real part contains the DC component
5th International Workshop on Near Field Communication, ETH Zurich
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Phase drift impact on bit representation & coding:
Type A – 106 kbit/s
Example of a synthetic signal with significant phase drift
–
–
–
Reference (black) is the intended signal amplitude in base band without drift
I-channel (blue) is the in-phase channel of the demodulated HF signal with phase drift
Q-channel (red) is the 90 ° shifted quadrature phase of the demodulated HF signal with phase drift
1
I-channel
Q-channel
reference
Significant
subcarrier
energy at ½ etu
amplitude
0.5
0
-0.5
0
10
20
30
40
50
60
70
time in s
Conclusion: Even for high phase drift the demodulator may correctly receive the
information (For Type A 106 kbit/s the presence, not the polarity of the subcarrier is
relevant. Type A 106 kbit/s is very sensitive to bit grid violation.)
5th International Workshop on Near Field Communication, ETH Zurich
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Phase drift impact on bit representation & coding
Example for Type A (212 – 848 kbit/s) and Type B
phase (degree)
180
phase
reference
reference line
135
Phase drift observations:
90
45
0
0
5
10
15
20
time in s
25
30
35
40
10
15
20
time in s
25
30
35
40
1
amplitude
0.5
0
I-channel
Q-channel
reference
-0.5
-1
0°:
Signal energy in I-channel only
45°:
Signal energy is equally distributed
between I- and Q-channel
90°:
a) Signal energy in Q-channel only;
b) I-channel signal inversion
compared to reference
135°:
Signal energy is equally distributed
between I- and Q-channel, signal
energy in I-channel inverted
0
5
Conclusion: Due to absolute reference in channel coding there is a break down point for information recovery in case of
signal inversion. Dependency on demodulator initial phase!
•
Best case break down for 135° phase shift,
•
Worst case break down for 45° phase shift  criterion for bit coding violation may be 30° for one frame (some
margin for non ideal signal transmission & detection assumed).
5th International Workshop on Near Field Communication, ETH Zurich
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PICC TX maximum phase drift
Bit coding violation:
– Valid for Type A bit rates > 106 kbit/s and all Type B bit rates
– Maximum allowable phase drift for bit coding violation depends on the
initial demodulator phase of the PCD
• Best case max phase drift over one frame: 135°
• Worst case max phase drift over one frame: 45°
– The criterion for bit coding violation may be a maximum phase drift of
30° over one frame.
Bit grid violation:
– Valid for all Type A + Type B PICC-PCD bit rates
– Type A 106 kbit/s:
• Manchester coding: The presence of the subcarrier during a half etu carries the
information, not the phase of the subcarrier
– The criterion for bit grid violation may be 0.1 etu over one frame.
5th International Workshop on Near Field Communication, ETH Zurich
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Phase drift measurement setup

Use conventional ISO/IEC 10373-6 test assembly

Capture calibration coil (CalCoil) signal – can it be used as reference?

Capture Helmholtz bridge signal (HHB) – can it be used for PICC
emission measurement?
Arbitrary Waveform Generator
13.56 MHz sinewave carrier
PICC signal + phase deviation
Class 1 Ref PICC
HHB
CalCoil Signal
5th International Workshop on Near Field Communication, ETH Zurich
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Cross-talk quantification by measurement
 Alternating H-field sine-wave @ 13,56 MHz emitted by PCD antenna (1,5 A/m)
 2nd frequency sine-wave (15 MHz) emitted by Ref PICC Class 1 antenna (varied voltage)
 Signals captured at CalCoil and HHB  2-tone ratio evaluated (cross-talk)
CalCoil: Signal mainly consists of PCD
30
20
CW carrier  can be used as reference
Level [dB]
10
HHB: PICC signal is min. 20 dB (SIR) lower
0
than PCD reference sine-wave carrier
-10
o PCD HHB was compensated without DUT
before measurement
-20
-30
-40
-50
o 20 dB ratio is independent of side band
amplitude (SBA)  blue trace
Helmholtz Bridge 2-Tone ratio
CalCoil 2-Tone ratio
Difference of Helmholtz-Cal coil ratio
0
0.5
1
1.5
2
2.5
Voltage (PICC driver) [V]
3
3.5
4

SIR  20 log10  S
I

 in dB

Conclusions:
– HHB signal does not represent well the PICC burst emission signal
Cause: PICC antenna circuit emits a secondary H-field to compensate PCD CW carrier
– It may help to remove this PCD CW carrier signal from the Helmholtz Bridge signal in
a HF signal pre-processing step, or to compensate the bridge (Df, DG) in Software
5th International Workshop on Near Field Communication, ETH Zurich
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U RMS 
PICC Signal pre-processing approach
CW Carrier
106A Modulation

1 n 2
1 2
2
2
 ui  n u1  u2  ...  un
n i 1
Max. PhaseDrift, DFT Algorithm
Original
Measurement
data
HelmholtzBridge
1 signal
captured
URMS = 0,592 V(rms)
Intended (correct)
result
evaluated
result
2 Sense
Coils –
2 signals
captured
SW
Compensation
Df, DG
URMS = 0,109 mV(rms)
+ BP Filter on
SenseCoil
signals
2 Sense
Coils –
2 signals
captured
URMS = 0.019mV(rms)
Conclusions:
SW Compensation + Filtering gives 30 dB better noise suppression than bridge
DFT algorithm is piecewise able to calculate Phase Drift over time not sufficient
5th International Workshop on Near Field Communication, ETH Zurich
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
How Signal-to-Interference (SIR) degrades PD
The plot illustrates the relation between SIR and evaluated phase drift (degradation
of the true value) in a simulation.
o X-Axis: HHB signal (no pre-processing) at various SIR values used as input.
o Y-Axis: Ratio between the ideal phase drift (e.g. 40 °) of PICC burst emission and the
evaluated phase drift (e.g. 28 °) of the HHB signal mixture, vs. PCD CW reference.
1
0.9
0dB SIR  at best 70% of the
true phase drift is measured
Phase ratio: target-measured
0.8
0.7
0.6
U RMS
0.5
0.4
S
SIR  20 log10 
I
0.3
90 deg-848kbps
5 deg-848kbps
90 deg-106kbps
5 deg-106kbps
0.2
0.1
-30
-20
-10
0
10
20
SIR [dB]
30
40
50
1 n
2


 
u

u
 i MEAN
n i 1
60



S... Wanted Signal (e.g. ALM burst carrier)
I... Interference Signal (e.g. PCD CW carrier)
Both components will be in the HHB signal
5th International Workshop on Near Field Communication, ETH Zurich
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Signal processing of proposed Algorithms
Helmholtz
bridge
signal
BP filter
CalCoil
signal
hilbert
BP filter
arg
hilbert
x
 HHB
CC
arg
-
ˆ
LP
Helmholtz
bridge
signal
In-phase
ˆ
CalCoil
signal
90°
phase
shift
x
DC block
arg
Q-phase
LP
DC block
Hilbert Demodulator Algorithm
Homodyne Demodulator Algorithm
Phase: PD Analysis; SIR: -30dB 50degree; bit rate: 848kbps
Hilbert demodulator method:PD Analysis; SIR: -30dB 50degree; bit rate: 848kbps
50
200
phase trace
target
measured upper PD run
measured lower PD run
180
40
140
35
120
30
angle (degree)
angle (degree)
160
45
100
80
25
20
15
60
10
40
5
20
0
0
0
5
10
15
20
number of etu
25
30
35
0
5
10
15
20
number of etu
25
30
35
Same signals evaluated by both algorithms  Homodyne Demod. result is correct
Note: Both diagrams SIR of HHB signal: - 30 dB (ALM vs. PCD)
5th International Workshop on Near Field Communication, ETH Zurich
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Measurement Results at H-field: 1.5A/m
Helmholtz
bridge
signal
BP filter
CalCoil
signal
hilbert
BP filter
hilbert
x
 HHB
arg
CC
arg
-
ˆ
Helmholtz
bridge
signal
x
-110
-110
-120
-120
-130
-130
-140
-150
-160
-180
-190
-190
120
DC block
-160
-180
80
100
number of etu
Q-phase
LP
-150
-170
60
arg
-140
-170
40
ˆ
Hilbert demodulator method:40 degrees phase drift
-100
angle (degree)
angle (degree)
Phase: 40 degrees phase drift
20
In-phase
Hilbert Demodulator Algorithm
-100
0
DC block
CalCoil
signal
90°
phase
shift
Homodyne Demodulator Algorithm
-200
LP
140
160
-200
0
20
40
60
80
100
number of etu
120
140
160
PICC TX phase drift: 40°over frame
5th International Workshop on Near Field Communication, ETH Zurich
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Measurement Results at H-field: 2.5A/m
Helmholtz
bridge
signal
BP filter
CalCoil
signal
hilbert
BP filter
hilbert
x
 HHB
arg
CC
arg
-
ˆ
Helmholtz
bridge
signal
x
-110
-110
-120
-120
-130
-130
-140
-150
-160
-180
-190
-190
120
DC block
-160
-180
80
100
number of etu
Q-phase
LP
-150
-170
60
arg
-140
-170
40
ˆ
Hilbert demodulator method:40 degrees phase drift
-100
angle (degree)
angle (degree)
Phase: 40 degrees phase drift
20
In-phase
Hilbert Demodulator Algorithm
-100
0
DC block
CalCoil
signal
90°
phase
shift
Homodyne Demodulator Algorithm
-200
LP
140
160
-200
0
20
40
60
80
100
number of etu
120
140
160
PICC TX phase drift: 40°over frame
5th International Workshop on Near Field Communication, ETH Zurich
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Measurement Results at H-field: 3.5A/m
Helmholtz
bridge
signal
BP filter
CalCoil
signal
hilbert
BP filter
hilbert
x
 HHB
arg
CC
arg
-
ˆ
Helmholtz
bridge
signal
x
-110
-110
-120
-120
-130
-130
-140
-150
-160
-180
-190
-190
120
DC block
-160
-180
80
100
number of etu
Q-phase
LP
-150
-170
60
arg
-140
-170
40
ˆ
Hilbert demodulator method:40 degrees phase drift
-100
angle (degree)
angle (degree)
Phase: 40 degrees phase drift
20
In-phase
Hilbert Demodulator Algorithm
-100
0
DC block
CalCoil
signal
90°
phase
shift
Homodyne Demodulator Algorithm
-200
LP
140
160
-200
0
20
40
60
80
100
number of etu
120
140
160
PICC TX phase drift: 40°over frame
5th International Workshop on Near Field Communication, ETH Zurich
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Measurement Results at H-field: 4.5A/m
Helmholtz
bridge
signal
BP filter
CalCoil
signal
hilbert
BP filter
hilbert
x
 HHB
arg
CC
arg
-
ˆ
Helmholtz
bridge
signal
x
-110
-110
-120
-120
-130
-130
-140
-150
-160
-180
-190
-190
120
DC block
-160
-180
80
100
number of etu
Q-phase
LP
-150
-170
60
arg
-140
-170
40
ˆ
Hilbert demodulator method:40 degrees phase drift
-100
angle (degree)
angle (degree)
Phase: 40 degrees phase drift
20
In-phase
Hilbert Demodulator Algorithm
-100
0
DC block
CalCoil
signal
90°
phase
shift
Homodyne Demodulator Algorithm
-200
LP
140
160
-200
0
20
40
60
80
100
number of etu
120
140
160
PICC TX phase drift: 40°over frame
5th International Workshop on Near Field Communication, ETH Zurich
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Measurement Results at H-field: 5.5A/m
Helmholtz
bridge
signal
BP filter
CalCoil
signal
hilbert
BP filter
hilbert
x
 HHB
arg
CC
arg
-
ˆ
Helmholtz
bridge
signal
x
DC block
In-phase
ˆ
CalCoil
signal
90°
phase
shift
arg
Q-phase
LP
DC block
Hilbert Demodulator Algorithm
Homodyne Demodulator Algorithm
Phase: 40 degrees phase drift
Hilbert demodulator method:40 degrees phase drift
-100
-100
-110
-110
-120
-120
-130
-130
-140
angle (degree)
angle (degree)
LP
-150
-160
-170
-140
-150
-160
-170
-180
-180
-190
-190
-200
0
20
40
60
80
100
number of etu
120
140
160
-200
0
20
40
60
80
100
number of etu
120
140
160
PICC TX phase drift: 40°over frame
5th International Workshop on Near Field Communication, ETH Zurich
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Conclusions

ALM can overcome the challenge of PLM regarding SBA

Small Loop antennas can be used for NFC Card Mode or active PICC and
replace large Class 1 antennas of ID-1 Cards

Technically ALM is more than just a Wireless Transmission –
Synchronicity to the Reader is the main challenge

To accurately measure the Phase Deviation of a low signal in the presence
of a strong signal at the same frequency is tricky

A Homodyne Demodulation Algorithm + removal of DC component can
solve this problem in the Baseband

Explanation: Homodyne demodulation of the CW carrier component in HHB
with the CalCoil CW carrier produces DC voltage  can be removed

So it is possible to use the existing ISO/IEC10373-6 test bench, and to
define an additional RF compliance test, to guarantee interoperability of
ALM devices with existing infrastructure!
5th International Workshop on Near Field Communication, ETH Zurich
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Thank you for your
Audience!
Please feel free to ask questions...
Contact:
Michael.Stark @nxp.com,
or gebhart@ieee.org
5th International Workshop on Near Field Communication, ETH Zurich
5th International Workshop on Near Field Communication, ETH Zurich
24
ALM test signal generation with phase-shift
Original 13.56 MHz sine wave carrier
artificial phase drift
Short-term carrier phase drift (freq. offset)
gated with subcarrier
Half-sub-carrier: 8 carrier periods
gated with cannel coding
Channel coding: 14443 Type A 106 kbit/s
1 etu = 128 carrier periods
5th International Workshop on Near Field Communication, ETH Zurich
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Homodyne Algorithm measurement results
Phase drift: 50°
Bit Rate: 848kbps, NFC-A
Phase drift: 50°
Bit Rate: 106kbps, NFC-A
Phase: PD Analysis; 50degree; bit rate: 848kbps
140
120
120
100
100
80
80
angle (degree)
angle (degree)
Phase: PD Analysis; 50degree; bit rate: 106kbps
140
60
40
20
60
40
20
0
0
phase trace
target
measured upper PD run
measured lower PD run
-20
-40
0
20
40
60
80
number of etu
phase trace
target
measured upper PD run
measured lower PD run
-20
-40
100
H-field: 1.5A/m
RefPICC: max loading 6VDC
Modulation: unipolar
120
140
0
20
40
60
80
number of etu
100
120
140
Lower SBA: 12.639mV(p)
Upper SBA: 14.051mV(p)
Carrier abs: 739.672mV(p)
5th International Workshop on Near Field Communication, ETH Zurich
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Example for communication with ALM
End of Reader command
Modified Miller channel coding
Induced voltage received by RX loop
Alternating H-field emitted by the opposite Reader
gives the 13.56 MHz frequency reference
Reader
Frame Delay Time (Reader to Card)
bit duration
Voltage measured at loop 1 (TX to GND)
Bursts of 13.56 MHz carrier cycles buid a 847.5 kHz
subcarrier, modulated by Manchester channel coding
NFC Device
ALM
8 carrier cycles
per half-bit
must be synchronous to opposite Reader carrier frequency
Time
5th International Workshop on Near Field Communication, ETH Zurich
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