RFID Technologies Coding and Modulation

Dipartimento di Ingegneria dell’Informazione

e Scienze Matematiche

RFID TechnologiesCoding and Modulation

Alberto Toccafondi

Alberto Toccafondi

Coding and modulation (I)Ø Signal and data flow in a digital communication system

Ø Signal coding• Bits need to be converted into sequences of pulses (a waveform)• Mapping of bits (0,1) to amplitudes and pulse shaping.• Optimize with respect to the channel characteristics• Add some degree of protection (interference, collisions)• Base band operation

Ø Modulation• Altering amplitude, frequency of phase of a HF carrier in relation to a baseband

signal

Signalcoding

Modulation Channel DemodulationSignal

decoding

Reader Transponder (tag)Noise

Data stream

Data stream

Alberto Toccafondi

Coding and modulation (II)Ø Signal and data flow in a digital communication system

Ø Transmission channel• Introduce noise• Uses a transmission media (inductive coupling or microwaves)

Ø Demodulation• Signal processing to extract baseband signal from HF carrier

Ø Decoding• Reconstruct message• Fix possible transmission errors

Signalcoding

Modulation Channel DemodulationSignal

decoding

Reader Transponder (tag)Noise

Data stream

Data stream

Alberto Toccafondi

Coding in RFID systemØ Non Return to Zero (NRZ)

• 1->High 0->Low• Typically used in FSK (Frequency) or PSK (phase)

Ø Unipolar RZ code• 1->negative transition in the first half-bit period 0->remain low• Typically used for avoid long high state

τ pulse

Tbit

Alberto Toccafondi

Coding in RFID systemØ Manchester code

• 1->negative transition 0->positive transition (in the half bit period)• Typically used in tag->reader communication with subcarrier modulation

Ø Differential biphase (DBP-FM0)• 0->transition of any type in the half bit period 1->lack of transition• Level is inverted at the start of every bit period• there is at least one transition per bit

Alberto Toccafondi

Coding in RFID systemØ Miller code

• 1->transition of any type in the half bit period• 0->hold the previous 1 level, invert the previous 0 level

Ø Modified Miller code• High level. Each transition in the Miller code is replaced by a short negative pulse;• Ensures a continuous power supply in RFID system

Alberto Toccafondi

Coding in RFID systemØ Differential coding

• 1->change in the signal level at the start of every bit• 0->signal level remains unchanged

Ø Pulse-Interval code• 0->pause of duration t; 1->pause of duration 2t;• Typically used for Reader->Transponder data transfer;• Ensures a continuous power supply in RFID system during data transfer.PIE EncodingPIE Encoding

http://rfidsecurity.uark.edu 15

0 0 01 1

Alberto Toccafondi

Coding selectionØ Signal spectrum after modulation

Ø Susceptibility to transmission error

Ø Passive RFID Reader->Tag

• power supply during signal transmission

• easy demodulation at the tag

Ø Passive RFID Tag->Reader

• power consumption

• easy carrier suppression

Alberto Toccafondi

ModulationØ Coded signal is converted from baseband to RF-Band

Ø Parameters of a high frequency carrier

amplitudephase (or instantaneous frequency)

Ø Possible modulation schemes• amplitude• frequency• phase

Ø Digital modulations• amplitude shift keying (ASK)• frequency shift keying (FSK)• phase shift keying (PSK)

umod(t) = a(t)cos[2π f

ct + φ(t)]

Alberto Toccafondi

ASK modulationØ Amplitude of the carrier frequency switched between two states, u0

and u1 by the binary coded signal

6.2 DIGITAL MODULATION PROCEDURES 187

Carrier

Sideband

P

f

Figure 6.5 Each modulation of a sinusoidal signal — the carrier — generates so-called (mod-ulation) sidebands

To find the duty factor m we calculate the arithmetic mean of the keyed and unkeyedamplitude of the carrier signal:

um = u0 + u1

2(6.1)

The duty factor is now calculated from the ratio of amplitude change u0 − um tothe mean value um:

m = !um

um= u0 − um

um= u0 − u1

u0 + u1(6.2)

In 100% ASK the amplitude of the carrier oscillation is switched between the carrieramplitude values 2um and 0 (On-Off keying ; Figure 6.6). In amplitude modulationusing an analogue signal (sinusoidal oscillation) this would also correspond with amodulation factor of m = 1 (or 100%) (Mausl, 1985).

The procedure described for calculating the duty factor is thus the same as thatfor the calculation of the modulation factor for amplitude modulation using analogue

∆û m

û mû 1

û 0

t

m = 0.5; (ASK 50%)

Figure 6.6 In ASK modulation the amplitude of the carrier is switched between two states bya binary code signal

1 0

Ø Duty factor m

um=u0+ u

1

2m =

u0− u

1

um

Alberto Toccafondi

ASK modulation

Ø simple to demodulate with envelope detector

School of Engineering

0

2A

Modulationssignal: a(t)= A [1 + s(t)]

1 0 1 1 0 1 0 0 1 1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

-2A

0

2A

ASK-Signal: y(t) = a(t)�sin(2Sf0t)

t

MSE, Rumc, mod, 2

ASK: Amplitude Shift Keying

1 0

2A 2 21 (0 4 / 2)

2S A A �

on/off Keying simple to demodulate with envelope-detector

signal space diagram

signal power


0

2A

Modulationssignal: a(t)= A [1 + s(t)]

1 0 1 1 0 1 0 0 1 1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

-2A

0

2A

ASK-Signal: y(t) = a(t)�sin(2Sf0t)

t

MSE, Rumc, mod, 2

ASK: Amplitude Shift Keying

1 0

2A 2 21 (0 4 / 2)

2S A A �

on/off Keying simple to demodulate with envelope-detector


signal power

Ø 100% ASK the amplitude is switched between u0=2um and u1=0

Alberto Toccafondi

ASK modulationØ ASK modulation is achieved by multiplying this code signal ucode(t) by

the carrier oscillation uCr(t).

188 6 CODING AND MODULATION

signals (sinusoidal oscillation). However, there is one significant difference betweenkeying and analogue modulation. In keying, a carrier takes on the amplitude u0 inthe unmodulated state, whereas in analogue modulation the carrier signal takes on theamplitude um in the unmodulated state.

In the literature the duty factor is sometimes referred to as the percentage carrierreduction m′ during keying:

m′ = 1 − u1

u0(6.3)

For the example in Figure 6.7 the duty factor would be m′ = 0.66 (= 66%). In thecase of duty factors <15% and duty factors >85% the differences between the twocalculation methods can be disregarded.

The binary code signal consists of a sequence of 1 and 0 states, with a periodduration T and a bit duration τ . From a mathematical point of view, ASK modulationis achieved by multiplying this code signal ucode(t) by the carrier oscillation uCr(t).For duty factors m < 1 we introduce an additional constant (1 − m), so for this casewe can still multiply uHF(t) by 1 in the unkeyed state:

UASK(t) = (m · ucode(t) + 1 − m) · uHF(t) (6.4)

The spectrum of ASK signals is therefore found by the convolution of the codesignal spectrum with the carrier frequency fCr or by multiplication of the Fourierexpansion of the code signal by the carrier oscillation. It contains the spectrum of thecode signal in the upper and lower sideband, symmetric to the carrier (Mausl, 1985).

A regular, pulse-shaped signal of period duration T and bit duration τ yields thespectrum of Table 6.1 (see also Figure 6.8).

HFGen

0 t Time

Amplitude

HF amplitudeASK modulator

Digitalsignal

HFsignal

T

Figure 6.7 The generation of 100% ASK modulation by the keying of the sinusoidal carriersignal from a HF generator into an ASK modulator using a binary code signal

uASK (t) = (m ⋅ucode(t)+1−m)cos(2π fct)

Ø 100% ASK pulse-shaped signal with τ <<T

period duration

bit duration

Alberto Toccafondi

ASK modulationØ Spectrum of the ASK modulated signal is therefore found by

multiplication of the Fourier expansion of the code signal by the carrieroscillation.

Ø Es. Unipolar NRZ with 100% ASKPSD of unipolar NRZ

SJSU RFID Systems: Spectral shaping techniques 25

-6 -4 -2 0 2 4 6

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

Normalized frequency, fTb

S s(f)

PSD of Unipolar NRZ, A=1, Tb=1

7Modulation of Digital Data: ASK (cont.)

ASKASK--Modulated Signal: Frequency SpectrumModulated Signal: Frequency Spectrum

t)cos(Ȧt)fcos(2(t)v ccc ʌ

»¼º

«¬ª �� ...tcos5Ȧ

5ʌ2tcos3Ȧ

3ʌ2tcosȦ

ʌ2

21A(t)v 000d

Carrier signal: , where 2Sfc=Zc

Digital signal:(unipolar!!!)

Zc ZZd_max

Modulated signal:

� � � �> @

� � � �> @ ...t3ȦȦcost3ȦȦcos3ʌ1

-tȦȦcostȦȦcosʌ1tcosȦ

21

...tcos3ȦtcosȦ3ʌ2-tcosȦtcosȦ

ʌ2tcosȦ

21

...tcos5Ȧ5ʌ2tcos3Ȧ

3ʌ2tcosȦ

ʌ2

21tcosȦ

(t)v(t)v(t)v

0c0c

0c0cc

0c0cc

000c

dcASK

��

��

��

»¼º

«¬ª ��

�

� �B)cos(AB)-cos(A21cosBcosA ��

Zc ZZc+Zd_maxZc-Zd_max

Alberto Toccafondi

FSK modulationØ The frequency of a carrier oscillation is switched between two

frequencies f1 and f2 by a binary code signal.

Ø Carrier frequency fc=f1+ f2

2

Ø Frequency deviation Δfc=f2− f1

2

u1(t) = Acos[2π(f

c− Δf

c)t ]

u2(t) = Acos[2π(f

c+ Δf

c)t ]

f2= f

c+ Δf

c f1 = fc − Δfc

Alberto Toccafondi

FSK modulationØ A binary 2FSK signal can be considered as the composition of two

amplitude shift keyed signals of frequencies f1 and f2.


Table 6.1 Spectral lines for a pulse-shaped modulated carrieroscillation

Designation Frequency Amplitude

Carrier oscillation fCR uHF · (1 − m) · (T − τ )/T1st spectral line fCR ± 1/T uHF · m · sin(π · τ/T )2nd spectral line fCR ± 2/T uHF · m · sin(2π · τ/T )3rd spectral line fCR ± 3/T uHF · m · sin(3π · τ/T )nth spectral line fCR ± n/T uHF · m · sin(nπ · τ/T )

0

T

t Time

Amplitude

Figure 6.8 Representation of the period duration T and the bit duration τ of a binarycode signal

0 t Time

Amplitude

HF amplitude

Digitalsignal

HFsignal

2FSK modulator

f2

f1

T

Figure 6.9 The generation of 2 FSK modulation by switching between two frequencies f1 andf2 in time with a binary code signal

6.2.2 2 FSK

In 2 frequency shift keying the frequency of a carrier oscillation is switched betweentwo frequencies f1 and f2 by a binary code signal (Figure 6.9).

The carrier frequency fCR is defined as the arithmetic mean of the two charac-teristic frequencies f1 and f2. The difference between the carrier frequency and the

Alberto Toccafondi

FSK modulationØ The spectrum of a 2 FSK signal is therefore obtained by superimposing

the spectra of the two amplitude shift keyed oscillations


characteristic frequencies is termed the frequency deviation !fCR:

fCR = f1 + f2

2!fCR = |f1 + f2|

2(6.5)

From the point of view of the time function, the 2 FSK signal can be consideredas the composition of two amplitude shift keyed signals of frequencies f1 and f2.The spectrum of a 2 FSK signal is therefore obtained by superimposing the spectra ofthe two amplitude shift keyed oscillations (Figure 6.10). The baseband coding used inRFID systems produces an asymmetric frequency shift keying:

τ = T

2(6.6)

In these cases there is also an asymmetric distribution of spectra in relation to themid-frequency !fCR (Mausl, 1985).

6.2.3 2 PSK

In phase shift keying the binary states ‘0’ and ‘1’ of a code signal are converted intocorresponding phase states of the carrier oscillation, in relation to a reference phase.In 2 PSK the signal is switched between the phase states 0◦ and 180◦.

Mathematically speaking, the shift keying of the phase position between 0◦ and180◦ corresponds with the multiplication of the carrier oscillation by 1 and − 1.

The power spectrum of a 2 PSK can be calculated as follows for a mark-space ratioτ /T of 50% (Mansukhani, 1996):

P(f ) =(

P · Ts

2

)· [sin c2π(f − f0)Ts + sin c2π(f + f0)Ts] (6.7 )

where P is transmitter power, Ts is bit duration (= τ ), f0 is centre frequency, andsin c(x) = (sin(x)/x).

Sidebands

P

f

f2f1

fCR

Figure 6.10 The spectrum of a 2 FSK modulation is obtained by the addition of the individualspectra of two amplitude shift keyed oscillations of frequencies f1 and f2

Alberto Toccafondi

PSK modulationØ Binary states “1” and “0” are converted in different phase states of the

carrier in relation to a reference phase. In 2PSK the phase of the carrieris switched between 0� and 180�


-A

0

A

Modulationssignal: a(t)= A�s(t)

1 0 1 1 0 1 0 0 1 1

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

-A

0

A

PSK-Signal: y(t) = a(t)�sin(2Sf0t) = A�sin(2Sf0t+I(t))

t

MSE, Rumc, mod, 3

BPSK: Binary Phase Shift Keying

1 0 A

2 / 2S A A

same BER as ASK with 3 dB less signal power


180° phase jumps

Alberto Toccafondi

PSK modulationØ Mathematically, its equivalent to the multiplication of carrier oscillation by

+1 and -1


The envelope of the two sidebands around the carrier frequency f0 follows thefunction (sin(x)/x)2. This yields zero positions at the frequencies f0 ± 1/Ts, f0 ±2/TS, f0 ± n/TS. In the frequency range f0 ± 1/TS, 90% of the transmitter power istransmitted. See Figure 6.11.

6.2.4 Modulation procedures with subcarrier

The use of a modulated subcarrier is widespread in radio technology. In VHF broad-casting, a stereo subcarrier with a frequency of 38 kHz is transmitted along with thebaseband tone channel. The baseband contains only the monotone signal. The differ-ential ‘L–R’ signal required to obtain the ‘L’ and ‘R’ tone channels can be transmitted‘silently’ by the modulation of the stereo subcarrier. The use of a subcarrier thereforerepresents a multilevel modulation. Thus, in our example, the subcarrier is first modu-lated with the differential signal, in order to finally modulate the VHF transmitter onceagain with the modulated subcarrier signal (Figure 6.12).

In RFID systems, modulation procedures using a subcarrier are primarily usedin inductively coupled systems in the frequency ranges 6.78 MHz, 13.56 MHz or27.125 MHz and in load modulation for data transfer from the transponder to thereader. The load modulation of an inductively coupled RFID system has a similareffect to ASK modulation of HF voltage at the antenna of the reader. Instead ofswitching the load resistance on and off in time with a baseband coded signal, alow frequency subcarrier is first modulated by the baseband coded data signal. ASK,FSK or PSK modulation may be selected as the modulation procedure for the sub-carrier. The subcarrier frequency itself is normally obtained by the binary division ofthe operating frequency. For 13.56 MHz systems, the subcarrier frequencies 847 kHz(13.56 MHz ÷ 16), 424 kHz (13.56 Mhz ÷ 32) or 212 kHz (13.56 MHz ÷ 64) are usu-ally used. The modulated subcarrier signal is now used to switch the load resistor onand off.

The great advantage of using a subcarrier only becomes clear when we consider thefrequency spectrum generated. Load modulation with a subcarrier initially generates

× 1, −1Ttime

Amplitude

HF amplitude

Digitalsignal

HF signal

2 PSK modulator

0f1

t

Figure 6.11 Generation of the 2 PSK modulation by the inversion of a sinusoidal carrier signalin time with a binary code signal

Alberto Toccafondi

PSK modulationØ Mathematically, its equivalent to the multiplication of carrier oscillation by

+1 and -1


The envelope of the two sidebands around the carrier frequency f0 follows thefunction (sin(x)/x)2. This yields zero positions at the frequencies f0 ± 1/Ts, f0 ±2/TS, f0 ± n/TS. In the frequency range f0 ± 1/TS, 90% of the transmitter power istransmitted. See Figure 6.11.

6.2.4 Modulation procedures with subcarrier

The use of a modulated subcarrier is widespread in radio technology. In VHF broad-casting, a stereo subcarrier with a frequency of 38 kHz is transmitted along with thebaseband tone channel. The baseband contains only the monotone signal. The differ-ential ‘L–R’ signal required to obtain the ‘L’ and ‘R’ tone channels can be transmitted‘silently’ by the modulation of the stereo subcarrier. The use of a subcarrier thereforerepresents a multilevel modulation. Thus, in our example, the subcarrier is first modu-lated with the differential signal, in order to finally modulate the VHF transmitter onceagain with the modulated subcarrier signal (Figure 6.12).

In RFID systems, modulation procedures using a subcarrier are primarily usedin inductively coupled systems in the frequency ranges 6.78 MHz, 13.56 MHz or27.125 MHz and in load modulation for data transfer from the transponder to thereader. The load modulation of an inductively coupled RFID system has a similareffect to ASK modulation of HF voltage at the antenna of the reader. Instead ofswitching the load resistance on and off in time with a baseband coded signal, alow frequency subcarrier is first modulated by the baseband coded data signal. ASK,FSK or PSK modulation may be selected as the modulation procedure for the sub-carrier. The subcarrier frequency itself is normally obtained by the binary division ofthe operating frequency. For 13.56 MHz systems, the subcarrier frequencies 847 kHz(13.56 MHz ÷ 16), 424 kHz (13.56 Mhz ÷ 32) or 212 kHz (13.56 MHz ÷ 64) are usu-ally used. The modulated subcarrier signal is now used to switch the load resistor onand off.

The great advantage of using a subcarrier only becomes clear when we consider thefrequency spectrum generated. Load modulation with a subcarrier initially generates

× 1, −1Ttime

Amplitude

HF amplitude

Digitalsignal

HF signal

2 PSK modulator

0f1

t

Figure 6.11 Generation of the 2 PSK modulation by the inversion of a sinusoidal carrier signalin time with a binary code signal

Alberto Toccafondi

PSK modulationØ Spectrum is similar to ASK

Alberto Toccafondi

Tag power supply and demodulationSchool of Engineering Reader => Tag: ASK-Modulation

ASK modulation depth 10-100%

from [2] MSE, HF-RFID, 7

Alberto Toccafondi

HF-RFID-Standard ISO-15693Ø 13.56 MHz, range < 1 m (vicinity coupling). VCD: vicinity coupling

device („reader“). VICC: vicinity card („tag“).

Ø VCD-to-VICC Communication

School of Engineering VCD-to-VICC Communication

ASK-Modulation

„The VICC shall decode both. The VCD determines which index is used.“

Source: ISO-15693-standard


ASK-modulation with 10% and 100% modulation depth

MSE, HF-RFID, 15

School of Engineering VCD-to-VICC Communication

ASK-Modulation

„The VICC shall decode both. The VCD determines which index is used.“



ASK-modulation with 10% and 100% modulation depth

MSE, HF-RFID, 15

Ø VCD determines which index is used. VICC shall demodulate both

Alberto Toccafondi

Tag -> Reader communicationØ Inductively coupled systems (frequency ranges 6.78MHz, 13.56MHz or

27.125MHz and in load modulation) uses modulation subcarrierprocedures.

Ø Instead of switching the load resistance on and off in time with abaseband coded signal, a low frequency subcarrier is first modulatedby the baseband coded data signal. (ASK, FSK or PSK)


Subcarrier 212 kHz

Data stream − baseband coded

Carrier signal 13.56 MHz

Modulated subcarrier

ASK-Modulation 2= Load modulation

ASK-Modulation 1

Load modulated signal with subcarrier

Figure 6.12 Step-by-step generation of a multiple modulation, by load modulation with ASKmodulated subcarrier

two spectral lines at a distance ± the subcarrier frequency fH around the operatingfrequency (Figure 6.12). The actual information is now transmitted in the sidebandsof the two subcarrier lines, depending upon the modulation of the subcarrier with thebaseband coded data stream. If load modulation in the baseband were used, on the otherhand, the sidebands of the data stream would lie directly next to the carrier signal atthe operating frequency.

f

Sig

nal

0 dB

−80 dB

f T = 13.560 MHz

f H = 212

Carrier signal of the reader,measured at the antenna coil

Modulation products by loadmodulation with a subcarrier

13.772 MHz13.348 MHz

Figure 6.13 Modulation products using load modulation with a subcarrier


Subcarrier 212 kHz





ASK-Modulation 1




f

Sig

nal

0 dB

−80 dB

f T = 13.560 MHz

f H = 212



13.772 MHz13.348 MHz


ASK

Ø Subcarrier frequency is normally obtained by the binary division of theoperating frequency.

Alberto Toccafondi

Modulation with subcarrierØ For 13.56 MHz systems, the subcarrier frequencies 847kHz (13.56

MHz� 16), 424 kHz (13.56 Mhz� 32) or 212 kHz (13.56 MHz� 64)may be used.

Ø The modulated subcarrier signal is now used to switch the loadresistor on and off.


Subcarrier 212 kHz





ASK-Modulation 1




f

Sig

nal

0 dB

−80 dB

f T = 13.560 MHz

f H = 212



13.772 MHz13.348 MHz


ASK


Subcarrier 212 kHz





ASK-Modulation 1




f

Sig

nal

0 dB

−80 dB

f T = 13.560 MHz

f H = 212



13.772 MHz13.348 MHz


Ø A carrier signal ismodulated by amodulated subcarrier.

Alberto Toccafondi

Modulation with subcarrierØ Spectrum of the modulated subcarrier


Tag-response in baseband

Tag-response subcarrier-modulated

fsub -fsub

Tag-response @ reader

f

Tag => Reader: Load Modulation MSE, HF-RFID, 9

t Tb

f t Tb

Tb t

envelope

fc+fsub f fc-fsub fc

0

0

Alberto Toccafondi

Modulation with subcarrierØ Demodulation and influence of the Q factor

School of Engineering Tag => Reader: Load Modulation

f

IY(f)I

fc+fsubcarrier fc-fsubcarrier fc

huge dynamic range (e.g. 80 dB)

filtering sideband (with info)

f fc + 423.75 kHz fc

13.56 MHz

antenna BP-characteristic (Q = fc / B3dB < 20

rec. for ISO 15693)

fc - 423.75 kHz

- 3 dB

Q too large

Spectrum of the Rx-signal

IY(f)I

Q-factor of inductive reader- and tag-antenna compromise between long range (high voltage peak) and good carrier-subcarrier separation (facilitates demodulation)

MSE, HF-RFID, 10

Alberto Toccafondi

Some LF- and HF-RFID-StandardsØ Readers and tags from different suppliers are interoperable. Typically

Tx-procedure is standardized, but not the Rx.

Ø LF• ISO/IEC 11784/5 and extension 14223 (identification of animals)

Ø HF• ISO/IEC 14443 Identification Cards

ü Proximity Cards (2001) range up to 10 cm. Data rate 106 kb/sd. Mifare-product variant

• ISO/IEC 15693 Identification Cardsü Vicinity Cards (2001) range up to 1 m. Data rates up to 26 kb/s. Also ISO/IEC

18000-3 mode 1• ISO/IEC 18000-3 mode 3

ü Item management standard. HF-version of EPC UHF Gen2. High data andhigh reading rates

Alberto Toccafondi

UHF: EPC Gen 2Ø EPCGlobal: non-profit organization set up to achieve worldwide

adoption and standardization of Electronic Product Code

Ø EPC Generation-2 StandardØ Class 1 and 2: Passive-backscatter Tags

Ø Most recent standard, to be on all goods Works up to a couple ofmeters

Ø Very sophisticated inventorying, session management, outliersingulation, etc.

Ø Multiple physical standards supported

Alberto Toccafondi

Gen-2 Reader-to-Tag CommunicationØ Modulation

ü Double sideband amplitude shift keying (DSB-ASK)

ü Single-sideband ASK (SSB-ASK)ü Phase reversal ASK (PR-ASK)

Ø Encoding - Pulse interval encoding (PIE)

Ø Data rate based on Tari

Tari (#$%&)Bit Rate (Kbps)

Max Average

25 40 27

12.5 80 53

6.25 160 107

Alberto Toccafondi

Gen-2 Tag-to-Reader CommunicationØ Modulation

ü Amplitude shift keying (ASK)ü Phase shift keying (PSK)

ü Tag manufacturer selects the modulation format. Interrogators shall demodulate both modulation types

Ø Encodingü FM0 baseband ü Miller modulation of a subcarrier (M=2, 4 or 8)

Ø Data rates are variableü FM0 [single reader mode]–40 Kbps up to 640 Kbpsü Miller (M=2) [multi-reader mode]– 20 Kbps up to 320 Kbps

ü Miller (M=4) [dense reader mode]– 10 Kbps up to 160 Kbpsü Miller (M=8) [dense reader mode]– 5 Kbps up to 80 Kbps

ü Typical rates in the lab vary between 60-70 Kbps using Miller (M=4)

Alberto Toccafondi

Miller modulated subcarrier (I)

Ø Rulesü Inverts its phase between two Data-0s

in sequence

ü Phase inversion in the middle of a Data-1 symbol

ü The transmitted waveform is the baseband waveform multiplied by a square wave at M times the symbol rate for M = 2, 4, 8

ü M specified by the reader in Query command

ü Miller encoding has memory. Choice of sequences depends on prior transmissions.

ü Subcarrier frequency: 40 kHz to 640 kHz

Miller M=2 ExampleMiller M 2 Example

http://rfidsecurity.uark.edu 21

Alberto Toccafondi

Miller modulated subcarrier (II)

Ø More transitions per bit make detection easier but reduces thedata rate

Ø Miller modulated subcarrier works better in the presence of noise

Ø Another advantage of more transitions per second is that theresponse from the tag is farther from the carrier frequency

Ø Trade-off of interference rejection vs. data rate

24JAG. Sept 2005Texas Instruments Proprietary Information

Principles of Operation

Tag !!!! Reader Modulation

ASK Modulation(Amplitude Shift Keyed)

PSK Modulation(Phase Shift Keyed)

" The Tag uses either ASK or PSK modulation to return its data:(Miller encoding shown in example)

Miller Bits (2 Sub carrier cycles)

1 0 1 1 0 1

Alberto Toccafondi

European regulations

Copyright Elektrobit Corporation 2007, Amsterdam RFID LIVE Conference 2007 Elektrobit.com, 10/8/2008, Slide 4

European Regulatory Requirements

Cha

nnel

4

Cha

nnel

10

Cha

nnel

13

Cha

nnel

7

Lower adjacentsub-band

Upper adjacentsub-band

Selectedsub-band

fcfc – 400 kHz fc – 200 kHz fc + 200 kHz fc + 400 kHz

60 kHz

0 dBc

200 kHz 200 kHz 200 kHz

Interrogatorchannel

Tagresponsechannels

60 kHz

Documents

RFID Technologies Coding and Modulation