RFID Modulation

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Contents

1. Introduction 22. RFID types 3

2.1 Tags 3

2.2 Operating frequencies 4

3. General Working principles 64. Data encoding 8

4.1 NRZ encoding 8

4.2 PIE encoding 8

4.3 Manchester encoding 84.4 Miller encoding 9

4.5 FM0 encoding 9

4.6 PPM encoding 9

4.7 MFM encoding 10

5. Modulation techniques 115.1 PR-ASK modulation 11

5.2 PJM modulation 11

5.3 GMSK modulation 12

6. RFID standards 136.1 ISO/IEC 18000-2 13

6.2 ISO/IEC 18000-3 14

6.3 ISO/IEC 18000-4 15

6.4 ISO/IEC 18000-6 16

6.5 ISO/IEC 18000-7 17

References 18


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1. Introduction

Radio Frequency Identification (RFID) is a wireless system that uses radio-

frequency electromagnetic fields to transfer information, and information in this

case is identification number. In the simplest sense, it is like a digital barcode reader

as it helps to identify particular products. But in this case Barcodes are

transponders (tags) which have identification code that is read by RFID readers when

tag is in the range of reader. Ranges differ by technology and used device. And

comparing with barcode reading technology, tag does not need to be within the line

of sight because it uses magnetic fields. However RFID technology has more

advantages over barcodes for example tags have read and also write capabilities and

with the ability to store, change and transmit much more data. RFID also is more

complex as generally it means that it provides identification using radio frequency so

there are hundreds of possible uses. And for different applications there need to be

different approaches for example RFID technology used for door opening cannot

be used for aircraft identification. That is why RFID technology includes different

versions which use different kind of tags, readers, frequencies, standards, etc.

Historically predecessor of this technology was developed in year 1948 when the

idea of identifying friendly aircrafts was introduced. However just in year 1978

passive radio transponder with memory was introduced. That was the first true

ancestor of RFID [1].

RFID is used for different appliances such as tracking, identifying, sorting,

locating, even for paying and toll collection. There are lots of possible appliances. In

the future, it is even expected that there will be a system for shopping in

supermarkets to attach a tag to every product and after crossing the purchasing zone

all products will be counted in system without barcode reading to each product

separately [1].

However as RFID technology goes towards more and more serious appliances,

security needs to be considered and security and privacy threats should be avoided.

These risks include possibilities to access data in tag without the owners allowance.

If someone could access protocols used in some application, this person could cheat

for example by changing information in shops price tags. And even there could be

viruses in tags which could create security threats in the system.


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2. RFID types

As identification using electromagnetic waves offers lots of usage possibilities there

are lots of RFID versions which include different protocols, frequencies, modulation

techniques, data speeds, etc., as well as different kinds of tags.

2.1Tags:From power source viewpoint there are 3 kinds of tags: active, semi-passive and

passive. Active tags have their inner power source such as battery which provides

tags with power. These tags usually periodically send out their signal even if the

readers signals are not present. Passive tags doesnt contain inner power source and

is fully dependent on interrogators sent energy. When interrogator radiates out

radio waves, antennas on the tag receive energy and it is accumulated in chip in

order for the Integrated Circuit (IC) to work and send out signal. Semi-passive tag is

something in between as it has its own power source however the power source is

not used as in active mode to periodically send out signal but provides tag with

energy just if the interrogators signals are present. So it is like passive tag but with

exception that it takes energy not from interrogator but from its inner power source.

The most often used type of tags is passive as it is much cheaper to produce tags

without energy element. [2]

Another way to classify tags, is by considering the storage type. There are basically 3

such types [3].

1) Read-only memory: This is the simplest type of tags, because their ID number iswritten in factory and later it is just possible to read it. For a lot of applications, it

is acceptable solution as ID is in the database and after reading tag, computer

compares the obtained ID with the ones which are in the database.

2) Write once, read many (WORM): it is clear from its title that this type is similar tothe previous type but with a possibility for the system administrator to write the

ID on his own. After that the tag can be just read.

3) Read-write tag: Tag can be read and written as many times as it is needed.However there is also the 4

thtype, which is like pseudo-type since it is a tag without

any memory. These are the tags which are attached to for example clothes. As there

is no memory, the only thing that these tags can do is to signal its existence. For

prevention from stealing it is an acceptable solution, since after buying the item, the

tag is removed.


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2.2 Operating frequencies:

Different RFID systems operate in different frequency bands because each frequency

range offers particular possibilities over others including operating range, powerrequirements and performance, also the size of the tag.

There are frequency bands [2]:

Low Frequency (LF) range: 120150 kHz

High Frequency (HF) range: 13.56 MHz

Ultra High Frequency range: 433 MHz,

In Europe 865.6 867.6 MHz (regulated

by ETSI), USA 902 928 MHz, other

countries particular range in between860960 MHz

Microwave: 2450 MHz, 5.8 GHz (standardization

discontinued)

Ultra Wide Band (UWB) 3.110 GHz

Low frequency range advantages include being able to operate in proximity of

liquids, metal or dirt. Commonly they are passively powered and have short range

around 10 cm. Disadvantage could be low data rates.

13.56 MHz frequency in high frequency range offers better data rate than LF but

doesnt perform so good in proximity of liquids and metals. As 13.56 MHz frequency

is in highly regulated band where in nearby frequencies some sensitive electronics

like medical equipment works, it makes them undesirable in places such as hospitals.

In this frequency also passive tags are used so it is good choice for short range

identification. In addition these tags are quite cheap, for example as in 2006, one tag

could be bought for less than 0.50 US Dollars.

UHF 860960 MHz advantages include wider read range and the tags are cheaper

to manufacture. In 2006, the price per tag was approximately 0.15 US dollars and

nowadays it is around just 5 US cents. However the tag cannot operate in proximity

of liquids and metals because of interference. So applications like metal container

tracking, animal tracking and access control are not feasible with UHF systems.

In 433 MHz frequency active tags are used because of low power allowance of

10mW. Two major advantages are maximum communication range and propagation

within crowded environments. However in 433 MHz system only active tags are used

which means increased price, size and weight of the tag comparing with passive tag

and battery changing.


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3. General working principles

As RFID is a wireless technology, it uses radio waves for data transferring. RFID

systems differ very much, but in figure 1 traditional working principle with passive

tag is shown.

Figure 1. Typical RFID system overview

This kind of system with passive tag and reader connected to database could be

used for example as door unlocking system using the tag instead of the key. All

initiation comes from the interrogator (reader) unit which typically sends out

periodical RF signals in order to power up tag if it is present [5]. Tag then

accumulates energy obtained from reader and rectifies and filters it in order to get

direct current which powers up tags IC with memory. From memory data are sent to

modulator which encodes and modulates the ID code. The signal then goes to the

antenna which transmits the signal in space. The interrogator needs to be sensitive

enough to receive signal from the tag. Tag signal is very weak comparing with

interrogators signal, that is why the working distance with passive tags is limited. The

code from the reader usually goes to a computer where it is compared with another

IDs in database and can for example permit an access. Figure 2 illustrates a typical

communication system which is used also in RFID.

Figure 2. RFID System from Communications System viewpoint [6, figure 4]


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In order for the reader to communicate with tags, it needs to send out radio

signals which can be either just a ping for powering up tag - or could be multi

round communication signals. If there are many tags present reader could perform

anti-collision protocol. Tag usually consists of:

An antenna, which in active tags case just transmits and receives radiosignal, in passive tags also collects energy,

Integrated circuit with memory which performs communicationalgorithms such as encoding,

Tags inner clock, which generates frequency to transfer data frommemory in particular data rate,

Rectifier, filter and regulator help to provide tag with direct current, In rewritable tags there could be also memory writing circuit.


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4. Data encoding

4.1 Non-return to zero Coding(NRZ):

Figure 4. NRZ coding[4, figure 6.8]

This is the simplest coding scheme, because there actually is no encoding. It is used

rarely because of many disadvantages for example produce high DC level.

4.2 Pulse interval encoding (PIE):

Figure 5. PIE coding [3, Figure 4-3]

In this type of encoding pulse interval is modulated, where for 1 there is longer

pulse than for 0. For pulse ending signalizes constant zero pulse.

4.3 Manchester encoding:

Figure 6. Manchester encoding[4, figure 6.8]


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Manchester encoding is quite popular in RFID systems. It is simple and easy to

implement, just by checking phase to clock signal. With this kind of encoding

problem of transmitting long dc values is eliminated. It is also good from

synchronization and error control viewpoint.

4.4 Miller encoding:

Figure 7. Miller encoding [4, figure 6.8]

From the first look it is not so easy to understand, but there is a transition in the

middle of a bit period if it is a 1 bit. There is a transition at the start of the bit period

if the 0 bit is followed by a 0 bit. For a 0 followed by a 1 or a 1 followed by a 0, no

transition occurs at the symbol interval. This code is very effective in terms of used

bandwidth.

4.5 FM0 encoding:

Figure 8. FM0 encoding[4, figure 6.8]

This also is often used type of encoding, where 1 consists of constant pulse duringbit interval and 0 consists of 2 different level pulses. But there need to be transition

between 2 bits. So duty cycle is between 45% and 55%.

4.6 Pulse position modulation data coding:

In 18000-3 standard there is used data coding mode 1 of 4 and 1 of 256. In this kind

of data coding in constant symbol time interval pulse can be in different places. In 1

of 4 mode, pulse can be in one of 4 places pulse position determines two bits at a


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time. With mode 1 of 256 is the same principle but one pulse position determines 8

bits.

Figure 9. PPM 1 of 4 data encoding[9, ISO 18000-3, Figure G.5]

4.7 Modified frequency modulation

MFM advantage is the lowest bandwidth occupancy of the binary encoding methods.

Simple principle is: A bit 1 is defined by a state change at the middle of a bit interval,

A bit 0 is defined by a state change at the beginning of a bit interval and where a bit

0 immediately follows a bit 1 there is no state change.

Figure 10. MFM encoding.[9, ISO 18000-3, Figure 41]


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5. Modulation Techniques

Typically ASK, PSK, FSK and their forms are used in RFID systems. Mostly ASK is

used and in many cases in form of OOK. For ISO 18000-6c standard there is used

different forms of ASK - SSB ASK, DSB ASK and PR ASK.

5.1 Phase reversal ASK:

PR-ASK changes phase 180 each time a symbol is sent , it allows narrow band while

maximizes power transport to tag. It creates AM index 100%.

Figure 11. Traditional ASK modulation compared with PR-ASK modulation

[10, Figure 4]

5.2 Phase Jitter Modulation:

Figure 12. PJM example[9, Figure A.1]


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This is the variation of PSK where phase is changed just by 1 - 2. The advantage of

using this kind of modulation is that sideband levels can be set to any arbitrary level

without affecting the data rate.

5.3 Gaussian Minimum Shift Keying (GMSK):

It is variation of FSK, but it uses spectrum more efficiently. It doesnt use amplitude

modulation that is why it is more resilient to noise. Gaussian minimum shift keying is

originated from Minimum Shift Keying (MSK) but after this modulation is digital data

is shaped with Gaussian filter.

Figure 12. MSK signal waveforms.[11]

With MSK there are just 4 different types of bit signal. 1 is with some particular

frequency and 0 is with frequency that is 1,5 times bigger. Also for each frequency

there are 2 phase angle states for smooth 0 to 1 or opposite transition.


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assuming equally distributed 0 and 1 bits the average data rate is 5.1 kbps.

Channel bandwidth is 4 kHz.

For tag to interrogator communication: inductive coupling is the way to

transfer signal. 4 kbps Manchester encoding is used. To improve tags collision

detection, during the inventory process 2 kbps Dual Pattern (DP) data coding is

used. In Figure 3 Manchester encoding with DP coding is compared. Channel

bandwidth is 10 kHz.

Start Of Frame (SOF) pattern is code used to start communication. In this

standard it is Manchester coded bit sequence of 110 and no End Of Frame

(EOF) pattern is used. End of frame pattern signals to tag that communication

ends.

Figure 3. Manchester coding (left) inventory command coding (right). [9,

figure 5]

HDX tags:

For interrogator to tag communication: ASK modulation with PIE is used. Data

rates for slow communication is 1 kbps and for fast data rate 2.3 kbps. Slow rate and

fast rate differ on pulse width.

For communication tag to interrogator: NRZ coding with FSK modulation is

used. Low bit frequency is the same carrier frequency 134.2 kHz and High bit

frequency is in range 123,7 4,2 kHz. Data rate is improved in such a way and

average data rate is 8 kbit/s. SOF and EOF patterns consist each of 6 bits.

Interrogator to tag bandwidth is 8 kHz, whereas for tag to interrogator it is 15 kHz.

6.2 ISO/IEC 18000-3 [9]: Parameters for Air Interface Communications at 13.56

MHz

In ISO 18000-3 standard there are 2 operating modes which can work

without interfering with each other.


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MODE 1: When communication is initiated, there are 2 tag response formats - with

precursor and without it. Precursor is first part of communication, it helps to

perform anti-collision algorithms in early stages. During the precursor

communication, frequency that is 32 times smaller than carrier frequency and

Differential Binary Phase Shift Keying (DBPSK) is used for modulation.

Interrogator to tag communication is in bandwidth 13,56 MHz +/- 7 kHz. 2 kinds of

amplitude modulation are used with 100% or 10% modulation index. Tags shall

decode both. Data coding technique is Pulse Position Modulation (PPM). 2 data

coding modes shall be supported by tag1 of 256 (with data rate 1.65 kbit/s) and

1 of 4(with data rate 26.48 kbps).

For tag to interrogator communication 1 or 2 subcarrier frequencies are used: either

just 423.75 kHz or also 484.28 kHz. With 2 subcarriers occupied channel bandwidth is

13,56 MHz (484,28 kHz 40 kHz). Manchester coding is used for data encoding and

CRC 16 code for error correction.

MODE 2:

Interrogator to tag communication: 13,56 MHz +/- 7 kHz bandwidth is occupied.

Data are modulated in Phase Jitter Modulation (PJM) with min level +/- 1,0 and

max. level +/- 2,0 . PJM is PM variation where signal phase differ by just small angle

in this case 1 - 2. For data coding is used Modified Frequency Modulation (MFM).

In this way data rate is 423.75 kbps.

Tag to interrogator communication uses 13,56 MHz 3,013 MHz frequency

range. Tag can use one of 8 subcarrier frequencies in this range (969; 1233; 1507;

1808; 2086; 2465; 2712; 3013 kHz) where each subcarrier frequency has 106 kHz

bandwidth. BPSK is used for modulation in this case with MFM data coding. MFM is

used because it has the lowest bandwidth occupancy comparing with other binary

encoding methods. From tag to reader data rate is 105,9375 kbps. This is full-duplex

system. CRC codes are used also in this standard: 16 bit CRC for interrogator to tag

and 32 bit - tag to interrogator.

When many tags are present, Time and Frequency Division Multiple Access

(FTDMA) principle is used. Tags then each randomly choose one of 8 subcarrier

channels and after valid command, transmit the reply, after next command

choose another channel and sends again. Tags which are identified are muted so

they dont respond to commands anymore.

6.3 ISO/IEC 18000-4 [9]: Parameters for Air Interface Communications at 2.45 GHz

In this standard there also are 2 modes.

MODE 1 is a passive backscatter RFID system.


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Interrogator to tag Tag to interrogator

Encoding Manchester encoding FM0

Error control coding CRC 16

Operating frequency

range2400 to 2483.5 MHz

Max occupied channel

bandwidth0.5 MHz

Modulation ASK with modulation

index of 99%ASK

Data rate 3040 kbps

MODE 2: Long range high data rate RFID system it is appropriate for long

range with active tags.

Interrogator

to tag

Tag to interrogator

R/O-tag R/W-tag

(notification)

R/W-tag

(communication)

Encoding No encoding Miller Miller Manchester

Error control

codingDifferent types of CRCs for detection are used

Operating

frequency

range

2400 to 2483.5 MHz

Max

occupied

channel

bandwidth

1 MHz

ModulationGMSK

DBPSK or

OOKDBPSK DBPSK

Data rate 384 kbps 76,8 kbit/s 76,8 kbit/s 384 kbit/s

The communication between interrogator and tag is based on Time DivisionDuplexing/Time Division Multiplexing (TDD/TDM).

6.4 ISO/IEC 18000-6 [9]: Parameters for Air Interface Communications at 860

to 960 MHz

This is half-duplex system. In this group there are 3 types of RFID systems.

Type A:


Data coding PIE FM0


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Modulation ASK

Modulation index 27% to 100%

Data rate 33 kbps (mean) 40 or 160 kbps

Error control coding 5 bit CRC; 16 bit CRC 16 bit CRC

Type B characteristics:


Data coding Manchester FM0

Modulation ASK

Modulation index 18% or 100%

Data rate 10 or 40 kbit/s (according to local regulations)

Error control coding 16 bit CRC

EPC Global had class 1 and class 2 standards which were not compatible with ISO

18000 standards. As EPC Global grew in popularity their new standard - Class1

Generation 2 (Gen2) - was included in ISO 18000-6 part as type C in year 2006.

Type C characteristics:


Encoding PIE FM0 or Miller

Modulation DSB-ASK, SSB-ASK or

PRASK (Phase-reversalamplitude shift keying)

PRASK

Modulation depth not less than 80%

Data rates up to 128 kbps up to 320 kbps

Error correction codes 16 bit CRC

6.5 ISO/IEC 18000-7 [9]: Parameters for Air Interface Communications at

433.92 MHz

Interrogator to tag Tag to interrogatorEncoding Manchester

Modulation FSK with frequency deviation of +/- 50 kHz

Data rate 27,7 kbps

Error correction codes 16 bit CRC

Channel bandwidth 500 kHz 200 kHz

ISO/IEC 18000-5 was standard for microwave communications at 5.8 GHz, but

is withdrawn.


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References

[1] Wikipedia web page. Last edited 13. april 2013. Available:

http://en.wikipedia.org/wiki/Radio-frequency_identification

[2] Stephen A. Weis, RFID (Radio Frequency Identification): Principles and Applications, MIT

CSAIL, pp. Available: http://www.eecs.harvard.edu/cs199r/readings/rfid-article.pdf

[3] Jerry Banks et al., RFID applied, Wiley & Sons inc., 2007, pp. 61 123.

[4] Harvey Lehpamer, RFID design principles, Artech House inc., 2008, pp. 103132, 223-

227.

[5] Louis E. Frenzel Jr, McGraw Hill, Principles of Electronic Communication Systems

3rdedition, 2008, pp. 840844.

[6] Guang Yang, Coding for passive RFID commuication, Ph. D. dissertation, The

selmer center, department of informatics, University of Bergen, Norway, March

2012, Available:

https://bora.uib.no/bitstream/handle/1956/6208/44996%20Yang%20main_thesis.pdf?sequ

ence=1

[7] rfid.net webpage, last updated: 13 March 2012, Available:

http://rfid.net/basics/196-rfid-standards-101-

[8] Bob Violino, A Summary of RFID Standards, 16 January 2005, Available:

http://www.rfidjournal.com/articles/view?1335/

[9] ISO/IEC 18000 standards: Information technology -- Radio frequency identification for

item management. (ISO/IEC 18000-2:2009, ISO/IEC 18000-3: 2010, ISO/IEC 18000-4: 2008,

ISO/IEC 18000-6: 2013, ISO/IEC 18000-7: 2009)

[10] Darren McCarthy, RFID Technology and Testing 3 February, 2009, Available:

http://www.eetimes.com/design/microwave-rf-design/4019025/RFID-Technology-and-

Testing

[11] Radio-electronics.com webpage, What is GMSK Modulation - Gaussian Minimum Shift

Keying, Available: http://www.radio-electronics.com/info/rf-technology-design/pm-phase-

modulation/what-is-gmsk-gaussian-minimum-shift-keying-tutorial.php

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RFID Modulation