ETSI TS 102 902 V1.2.1 (2013-04)

Technical Specification
Electromagnetic compatibility
and Radio spectrum matters (ERM);
Methods, parameters and test procedures for cognitive
interference mitigation towards ER-GSM for use by UHF RFID
using Detect-And-Avoid (DAA) or other similar techniques

2 ETSI TS 102 902 V1.2.1 (2013-04)

Reference
RTS/ERM-TG34-21
Keywords
radio, RFID, SRD, UHF
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ETSI
3 ETSI TS 102 902 V1.2.1 (2013-04)
Contents
Intellectual Property Rights . 5
Foreword . 5
Introduction . 6
1 Scope . 7
2 References . 7
2.1 Normative references . 7
2.2 Informative references . 7
3 Definitions, symbols and abbreviations . 8
3.1 Definitions . 8
3.2 Symbols . 9
3.3 Abbreviations . 9
4 UHF RFID deployment scenario . 10
4.1 Introduction . 10
4.2 Frequency plan . 10
4.3 Basic operational principal of RFID technology . 12
4.3.1 Introduction. 12
4.3.2 Characteristics of RFID at UHF . 12
4.3.2.1 Antennas . 13
4.3.2.2 Data Rates . 13
4.3.2.3 Intermodulation Products . 14
4.3.2.4 De-tuning and absorption . 14
4.3.2.5 Shielding . 15
4.3.2.6 Transparent materials . 15
4.4 Static deployment scenario . 16
4.4.1 Overview . 16
4.4.2 RFID equipped portal system . 16
4.4.3 Packing table . 17
4.4.4 Point of sales system (PoS) . 18
4.4.5 Industrial applications . 20
4.5 Dynamic deployment scenario . 21
4.5.1 Introduction. 21
4.5.2 Hand held readers . 21
4.5.3 Vehicle mounted readers . 22
5 ER-GSM 900 system deployment scenario and protection criteria . 22
5.1 Introduction . 22
5.2 Frequency plan . 22
5.3 Basic operational principle . 23
5.4 Signal Formats . 23
5.5 Information contained in the BCCH . 24
5.6 Deployment scenarios . 25
5.7 Summary . 27
6 Mitigation techniques . 27
6.1 Introduction . 27
6.2 Site licensing . 29
6.3 Service area detection (DL detection) . 29
6.4 Victim terminal detection (UL Detection) . 31
6.4.1 Considerations . 31
6.4.2 Realization . 32
7 Summary and Conclusion . 32
Annex A (informative): Future requirements for RFID . 34
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4 ETSI TS 102 902 V1.2.1 (2013-04)
A.1 General . 34
A.2 RFID applications . 34
A.3 Technical Radio Spectrum requirements and justification . 34
A.3.1 Current regulations for RFID . 34
Annex B (informative): Detailed market information - Market size, Applications and
requirements. 36
Annex C (informative): Technical information . 37
C.1 RFID . 37
C.1.1 Performance requirements from leading RFID manufacturers and users . 37
C.1.2 Power . 37
C.1.3 Bandwidth . 37
Annex D (informative): Avoidance options . 39
D.1 Introduction . 39
D.2 Transmit power management . 39
D.3 Band relocation . 39
D.4 LDC . . 39
D.5 Antenna techniques . 40
D.6 Combinations . 40
D.7 Avoidance parameters . 40
D.7.1 Minimum avoidance bandwidth . 40
D.7.2 Maximum avoidance power level . 40
Annex E (informative): Mitigation zone definition . 41
E.1 Introduction . 41
E.2 Required Isolation . 41
E.3 Protection distances . 42
E.3.1 Non specific RFID systems (Portal Systems) . 42
E.3.2 Indoor RFID systems (Handheld Systems) . 42
E.3.3 Indoor low power RFID systems (PoS, Packaging Table) . 43
Annex E (informative): Bibliography . 44
History . 45

ETSI
5 ETSI TS 102 902 V1.2.1 (2013-04)
Intellectual Property Rights
IPRs essential or potentially essential to the present document may have been declared to ETSI. The information
pertaining to these essential IPRs, if any, is publicly available for ETSI members and non-members, and can be found
in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to ETSI in
respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the ETSI Web
server (http://ipr.etsi.org).
Pursuant to the ETSI IPR Policy, no investigation, including IPR searches, has been carried out by ETSI. No guarantee
can be given as to the existence of other IPRs not referenced in ETSI SR 000 314 (or the updates on the ETSI Web
server) which are, or may be, or may become, essential to the present document.
Foreword
This Technical Specification (TS) has been produced by ETSI Technical Committee Electromagnetic compatibility and
Radio spectrum Matters (ERM).
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6 ETSI TS 102 902 V1.2.1 (2013-04)
Introduction
In order to accommodate the spectrum needs for the increasing number of RFID devices and systems, an extension band
for high power RFID systems in the frequency range between 915 MHz and 921 MHz is under discussion. This band is
already used by RFID in several countries worldwide and its availability in Europe would simplify the global movement
of tagged goods. In Europe, part of this new frequency band will be shared between the primary user ER-GSM and
RFID. In order to guarantee an interference-free coexistence between the two systems, mechanisms have to be
implemented on the RFID side to reduce the probability of interference to a minimum. These mechanisms can be
regulatory, technical mechanisms or operational restrictions.
The present document proposes a set of these mechanisms which, together with the necessary parameters, will avoid
interference with the ER-GSM system in the frequency band 918 MHz to 921 MHz. In addition to the main goal of
reducing the potential for interference with ER-GSM, the overall achievable performance of RFID systems should be
optimized. The main strategy adopted in the present document has been to split the task into phases with three different
time horizons:
• Short term solution for the next 1 to 3 years;
• Mid term solutions for the coming 3 to 6 years; and
• Long term solutions for a stable deployment of RFID systems in the shared bands.
This strategy will allow for the fast implementation of initial systems in the band without having to wait for the full
implementation of the final solution. Some of the proposed techniques will be based on software upgrades of existing
systems and only the mechanisms for the long term solution will require additional hardware and the implementation of
new reader systems. On the other hand the long term solution will use all available mechanisms and procedures and
thus will guarantee the best performance.
The values used in the present document are working assumptions and therefore these values have to be verified in
practical measurements and adapted to the state of the art technology.
The present document will be complemented by TS 102 903 [i.11], which will include the results of an initial practical
evaluation of the proposed mechanisms, and a description of the test procedures necessary for verifying the compliance
of RFID devices and systems.
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7 ETSI TS 102 902 V1.2.1 (2013-04)
1 Scope
The present document provides the technical specifications for mitigation techniques and procedures for the coexistence
of ER-GSM 900 terminals and RFID systems in the frequency range of 918 MHz to 921 MHz.
2 References
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
reference document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found at
http://docbox.etsi.org/Reference.
NOTE: While any hyperlinks included in this clause were valid at the time of publication ETSI cannot guarantee
their long term validity.
2.1 Normative references
The following referenced documents are necessary for the application of the present document.
[1] ETSI EN 302 208 (all parts) (V1.4.1): "Electromagnetic compatibility and Radio spectrum Matters
(ERM); Radio Frequency Identification Equipment operating in the band 865 MHz to 868 MHz
with power levels up to 2 W".
2.2 Informative references
The following referenced documents are not necessary for the application of the present document but they assist the
user with regard to a particular subject area.
[i.1] TEDDI database.
NOTE: See http://webapp.etsi.org/Teddi/.
[i.2] ISO/IEC 18000-6: "Information technology - Radio frequency identification for item
management - Part 6: Parameters for air interface communications at 860 MHz to 960 MHz".
[i.3] ETSI TR 102 649-2: "Electromagnetic compatibility and Radio spectrum Matters (ERM);
Technical characteristics of Short Range Devices (SRD) and RFID in the UHF Band; System
Reference Document for Radio Frequency Identification (RFID) and SRD equipment;
Part 2: Additional spectrum requirements for UHF RFID, non-specific SRDs and specific SRDs".
[i.4] J. D. Jackson: "Classical Electrodynamics", John Wiley, 1975.
[i.5] T. Rappaport: "Wireless Communications", Prentice Hall, 1996.
[i.6] ETSI ERM TG34: "Report: Kolberg Measurements", June 2009 and June 2010.
[i.7] ETSI ERM TG34: ERM-TG34#15-04r1: "ETSI tests at a Distribution Centre", September 2006.
[i.8] CEPT Report 14 (July 2006): "Develop a strategy to improve the effectiveness and flexibility of
spectrum availability for Short Range Devices (SRDs) in response to the EU Commission
mandate".
[i.9] Study on legal, economic & technical aspects of "Collective Use of Spectrum" in the European
Community (November 2006) by order of EU Commission.
[i.10] ERC Recommendation 70-03: "(Tromso 1997 and subsequent amendments) relating to the use of
Short Range Devices".
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8 ETSI TS 102 902 V1.2.1 (2013-04)
[i.11] ETSI TS 102 903: "Electromagnetic compatibility and radio spectrum matters (ERM); Compliance
tests for cognitive interference mitigation for use by UHF RFID using Detect-And-Avoid (DAA)
or other similar techniques".
[i.12] Void.
[i.13] ETSI TS 144 018:"Digital cellular telecommunications system (Phase 2+); Mobile radio interface
layer 3 specification; Radio Resource Control (RRC) protocol (3GPP TS 44.018)".
[i.14] ETSI TS 145 002: "Digital cellular telecommunications system (Phase 2+); Multiplexing and
multiple access on the radio path (3GPP TS 45.002)".
[i.15] EIRENE System Requirements Specification Version 15.1.
[i.16] Decision No 804/2004/EC of the European Parliament and of the Council of 21 April 2004
establishing a Community action programme to promote activities in the field of the protection of
the Community's financial interests (Hercule programme).
3 Definitions, symbols and abbreviations
3.1 Definitions
For the purposes of the present document, the following terms and definitions apply:
NOTE: Further/other definitions can be found in Terms and Definitions Interactive Database (TEDDI) [i.1].
Cognitive Radio System (CRS): Radio system (optionally including multiple entities and network elements), which
has the following capabilities:
• to obtain the knowledge of radio operational environment and established policies and to monitor usage
patterns and users' needs;
• to dynamically, autonomously and whenever possible adjust its operational parameters and protocols
according to this knowledge in order to achieve predefined objectives, e.g. minimize a loss in performance or
increase spectrum efficiency;
and to learn from the results of its actions in order to further improve its performance.
Detect And Avoid (DAA): technology used to protect radio communication services by avoiding co-channel operation
NOTE: Before transmitting, a system senses the channel within its operative bandwidth in order to detect the
possible presence of other systems. If another system is detected, the first system should avoid
transmission until the detected system disappears.
Downlink (DL): direction from a hierarchic higher network element to the one below, in the case of a typical RFID
system direction from the interrogator to tag or from the (E)R-GSM Base Transceiver Station to the terminal
Dynamic Frequency Allocation (DFA): protocol that allows for changing transmit frequency during operation
Dynamic Power Control (DPC): capability that enables the transmitter output power of a device to be adjusted during
operation in accordance with its link budget requirements or other conditions
fixed: physically fixed, non- moving device; includes temporary event installations as well
link adaptation: result of applying all of the control mechanisms used in Radio Resource Management to optimize the
performance of the radio link
Listen before Talk (LBT): spectrum access protocol requiring a cognitive radio to perform spectrum sensing before
transmitting
location awareness: capability that allows a device to determine its location to a defined level of precision
master: controls the radio resource changing actions
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9 ETSI TS 102 902 V1.2.1 (2013-04)
mobile: physically moving device
radio environment map: integrated multi-domain database that characterizes the radio environment in which a
cognitive radio system finds itself
NOTE: It may contain geographical information, available radio communication services, spectral regulations and
policies, and the positions and activities of co-located radios.
Service Level Agreement (SLA): defined level of service agreed between the contractor and the service provider
slave: performs the commanded actions by the Master
Uplink (UL): direction from Slave to Master
white space: label indicating a part of the spectrum, which is available for a radio communication application at a given
time in a given geographical area on a non-interfering / non-protected basis with regard to other services with a higher
priority on a national basis
3.2 Symbols
For the purposes of the present document, the following symbols apply:
α Pathloss Exponent in the Friis Equation
dB decibel
d distance
f frequency measured under normal test conditions
fc centre frequency of carrier transmitted by interrogator
λ wavelength
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
AM Amplitude
ARFCN Absolute Radio Frequency Channel Number
ASCI Advanced Speech Call Items
BCCH Broadcast Control Channel
BCH Broadcast Channel
BSC Base Station Controller
BTS Base Transceiver Station
CA cell allocation
CBCH Call Broadcast CHannel
CRS Cognitive Radio System
DAA Detect and Avoid
DFA Dynamic Frequency Allocation
DL DownLink
DPC Dynamic Power Control
e.r.p. effective radiated power
EECC European EPC Competence Center
EM ElectroMagnetic
ER-GSM Extended Railways GSM
FM Frequency
FRS Functional Requirement Specification
GSM Global System for Mobile Communication
GSM-R GSM for Railways
HF High Frequency
LBT Listen before Talk
LDC Low Duty Cycle
LoS Line-of-Sight
LTE Long term evolution
MA mobile allocation
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10 ETSI TS 102 902 V1.2.1 (2013-04)
MS Mobile System
NCC Network Control Centre
NCH Notification Channel
NLoS Non-Line-of-Sight
PM Phase
PoS Point of sales system
RACH Random Access Channel
RF Radio Frequency
RFID Radio Frequency Identification
R-GSM Railway GSM
RSCOM Radio Spectrum Committee
RSL Radio Signalling Link
RX Receiver
SI System Information
SLA Service Level Agreement
SRD Short Range Device
SRS System Requirement Specification
TCH Traffic Channel
TDMA Time Division Multiple Access
TX Transmitter
UHF Ultra High Frequency
UIC Union Internationale des Chemins de fer
UL UpLink
VBS Voice Broadcast Service
VGCS Voice Group Call Service
4 UHF RFID deployment scenario
4.1 Introduction
RFID systems are used in item management, logistics and in a wide range of other applications.
Many of these applications require reading ranges of at least 2 meters, and in certain logistics applications extended
ranges from 5 meters to 10 meters. These extended ranges cannot be achieved by alternative technologies and in the
existing designated UHF band due to regulatory constraints.
In this clause different deployment scenarios for RFID systems will be described and the need for additional mitigation
factors will be considered. After an initial introduction to the proposed frequency plan in clause 4.2 and a general
description of the operational principle of an RFID system, the RFID systems will be split into two main categories:
• Static systems, which are fixed RFID read-only systems; and
• Dynamic systems, like portable interrogator systems and RFID interrogators integrated into consumer devices
(e.g. Mobile phones, Cameras, etc.).
The two categories should be treated in different ways in order to guarantee the right level of protection towards the
potential victim system (ER-GSM).
4.2 Frequency plan
It is proposed to designate spectrum for UHF RFID high performance interrogators as shown in table 1 within the
frequency range from 915 MHz to 921 MHz.
This proposal is in accordance with the recommendations of the EC (including RSCOM) [i.9] and of CEPT [i.8], as
proposed in TR 102 649-2 [i.3] and the Kolberg Measurements [i.6]. These recommendations promote the
co-existence of multiple types of equipment within bands by the use of common technical characteristics.
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11 ETSI TS 102 902 V1.2.1 (2013-04)
Table 1: Proposal for high performance RFID interrogators
Maximum
Frequency bands Power Duty cycle Channel Notes
bandwidth
Interrogators: No mandatory limit for Interrogators
≤ 4 W e.r.p. on a single
915 MHz to 921 MHz interrogator channel for transmitter on-time. may operate
However interrogators in any of the
each individual
Interrogator centre will not be allowed to four high
interrogator
frequencies f transmit longer than is power
c
916,3 MHz necessary to perform f ± 200 kHz channels
c
917,5 MHz the intended operation
918,7 MHz
919,9 MHz
Tags: < -10 dBm e.r.p. per tag f ± 1 000 kHz for
c
Between 915 MHz
tag response
to 921 MHz
NOTE: f are the carrier frequencies of the interrogators.
c
NOTE: Some member states (France, Germany, Italy and The Nederland's) have allocated the band 915 MHz to
918 MHz exclusively for governmental and military use.
Figure 1 shows the current draft proposal for high performance RFID interrogators as in TR 102 649-2 [i.3].
Note that the situation for SRDs will be very different since they occupy the low power channels and their power is
limited to 25 mW.
Figure 1: Revised proposal for high performance RFID applications
The Kolberg measurements [i.7] concluded that the following should be considered in the channel plan:
• A 100 kHz carrier offset between RFID and GSM/ER-GSM/R-GSM channels will result in an additional
mitigation factor of 9 dB. As GSM/ER-GSM/GSM-R use even multiplies of 100 kHz (e.g. 914,8 MHz, 918,2
MHz and 921,2 MHz), RFID should use odd multiples of 100 kHz (e.g. 916,3 MHz).
• A 300 kHz carrier offset between GSM/ER-GSM/GSM-R channels will result in an additional mitigation
factor of 55 dB.
• The minimum frequency separation between an ER-GSM/R-GSM carrier and an RFID carrier should be
700 kHz under worst case conditions.
In summary a channel plan should be considered with the first high power channel at a centre frequency of 916,3 MHz
and the remaining high power channels spaced at 1,2 MHz. This is depicted in figure 1.
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12 ETSI TS 102 902 V1.2.1 (2013-04)
4.3 Basic operational principal of RFID technology
4.3.1 Introduction
A basic RFID system comprises an interrogator with its associated antennas and a collection of tags. The antennas are
arranged to transmit their signal within an interrogation zone. Tags are attached to either animate or inanimate objects
that are to be identified. When a tag enters an interrogation zone, it is activated by the transmitted signal from the
interrogator. Typically the tag will respond by sending its identity and possibly some associated data. The identity and
data from the tag is validated by the receiver in the interrogator and passed to its host system. A block diagram of the
principle is shown in figure 2.
To Host
To Host
Tag Interrogator
Tag Interrogator
System
System
Figure 2: Principle of RFID
A sophisticated protocol is used to handle the transfer of data between the interrogator and tags. This ensures the
integrity of data transfer and may include error checking and correction techniques. In addition the protocol handles the
process for writing data to the tag and controls the procedure for reading multiple tags that may be present
simultaneously within the same interrogation zone.
Across the whole of the radio spectrum three different forms of communication are used for the transfer of information
between interrogators and tags. These are:
• Electrostatic.
• Inductive.
• Electromagnetic waves.
The present document confines itself solely to electromagnetic waves and near field techniques since they are the only
forms of communication that are relevant for RFID at UHF.
To transfer information between an interrogator and a tag it is necessary to superimpose the data on a carrier wave. This
technique is known as modulation. Various schemes are available to perform this function. They each depend on
changing one of the primary features of an alternating sinusoidal source in accordance with the transmitted data. The
most frequent choices of modulation are Amplitude (AM), Frequency (FM) and Phase (PM).
Tags exist in a range of shapes and sizes to satisfy the particular needs of their intended application. Many tags are
passive and derive the power for their operation from the field generated by the interrogator. However some tags are
fitted with batteries, which may provide additional features (e.g. sensors) and may enable them to operate at
significantly greater ranges.
4.3.2 Characteristics of RFID at UHF
UHF transmission takes place by means of electromagnetic (EM) waves. At these frequencies EM waves have
properties that have many similarities to light. Transmissions travel in a straight line and the power of the received
signal is a function of the inverse square of the distance from its source. For example if the distance from a transmit
antenna is doubled the received power drops to one quarter. This property means that it is possible with UHF systems to
achieve significant reading ranges. Operation in the UHF band also makes it possible to transfer information at high
data rates. Both of these characteristics make UHF systems well suited for use in applications where tags are moving at
speed or in which there are multiple tags present in an interrogation zone.
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13 ETSI TS 102 902 V1.2.1 (2013-04)
UHF can present the installer with a number of challenges. Electromagnetic transmissions at UHF are readily reflected
from many surfaces. The reflections can cause the activation of unwanted tags and can also give rise to an effect known
as standing wave nulls. These can produce points within the interrogation zone where there are very low levels of
signal. UHF signals also experience significant levels of attenuation in the presence of water. In applications where
water may be present, system integrators must therefore make suitable provision for a reduction in reading range during
the design and configuration of the installation.
Operation is also possible using near field coupling between an interrogator and tags. This technique is useful in
situations where there are many tags in a confined area and it is necessary to control the transmitted field. Near field
systems generate magnetic fields that attenuate in accordance with the inverse cube of distance. Their properties
therefore make them useful for reading tags at close range while avoiding activation of tags outside the area of interest.
Near field techniques require the use of special antennas that are configured in the shape of a loop. Some tags have
antennas that are capable of operating with both EM transmissions and near field coupling.
4.3.2.1 Antennas
At UHF the shape of the interrogation field generated by the EM antennas of an interrogator will typically be in the
form of a cone. The angle subtended between the half power (or 3 dB) points of this cone is known as the beamwidth.
Often beamwidth is specified in both horizontal and vertical values, which need not necessarily be the same. In many
installations the long reading ranges possible at UHF mean that tags outside the wanted interrogation zone are
inadvertently activated. The use of antennas with a narrow beamwidth provides one means by which it is possible to
limit the area where tags may be read.
The most common type of antenna used at UHF is the patch antenna. This typically has a beamwidth of the order of
70 degrees. The patch antenna is fully satisfactory for many short to medium range applications where there are no
other interrogators and unwanted tags in the immediate vicinity. In applications where longer reading ranges are
required it may be necessary to control the extent of the interrogation zone more precisely. A first order of improvement
may be achieved by using a variant of the standard patch antenna that is physically larger. This makes it possible to
produce antennas with a horizontal beamwidth down to 30 degrees. Other types of antenna exist with narrower
beamwidths. One of these is the helical antenna, which can have a beamwidth of as little as 10 degrees. This narrow
beamwidth makes it possible to generate an interrogation zone that is very directional.
As the beamwidth of an antenna is reduced the transmitted power is compressed into a smaller volume, which produces
increased field intensity. This effect is quantified by the term "antenna gain". Since the radio regulations limit the
maximum field level that is permitted, it is necessary to reduce the level of power generated by the interrogator to
compensate for the increased gain of the antenna. Where the use of different antennas is allowed by the manufacturer,
details of how this adjustment should be carried out should be included within the product manual for the interrogator.
Generally transmissions from the antenna of the interrogator will be circularly polarized. This eliminates differences in
the reading range of tags caused by their orientation in the x and y planes (but not the z plane, which is the direction of
travel of the radio wave).
4.3.2.2 Data Rates
The maximum data rate of the communication link from the interrogator to the tag (sometimes called the downlink) is
determined by the size of the permitted channel of operation of the interrogator. The size of the channel is specified in
ERC Recommendation 70-03 [i.10] and is effectively a fixed parameter. For channels of 200 kHz channel spacing as
defined in annex 11 of ERC Recommendation 70-03 [i.10] the maximum possible data rate is of the order of 40 kbits
per second. However the protocol used for transferring the information includes error checking and other features,
which reduce the effective speed of information transfer. Details of the agreed standard data rates are included in
ISO/IEC 18000-6 [i.2].
In most situations the response from the tag (sometimes called the uplink) will lie in the same, or adjacent channels as
the downlink. This will place a practical limit on the achievable data rate. Where interrogators operate in accordance
with the 4 channel plan as specified in EN 302 208 [1], the tag may be set to operate at link frequencies of
approximately 300 kHz. In such circumstances data rates of 75 kbits per second are achievable.
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14 ETSI TS 102 902 V1.2.1 (2013-04)
However EN 302 208 [1] also permits the wanted signal from the tag to occupy the entire designated band from
865 MHz to 868 MHz provided that the levels specified in the spectrum mask are met. For some applications this
provides scope for manufacturers to create systems with substantially faster uplinks, which could provide significant
benefits. Where this technique is used, system designers must ensure that any transmissions from other nearby
interrogators do not block the response from the tag. This implies the need for some form of system planning to manage
either the timing of transmissions or the permissible sub-bands of operation.
4.3.2.3 Intermodulation Products
Where two or more devices are sited close to each other and are transmitting at closely spaced frequencies, they may
interfere with each other. This can arise through the generation of intermodulation products. These are unwanted
emissions that occur at frequencies that are at multiples of the sums and differences of the transmitting frequencies.
Intermodulation products can adversely affect the performance of both interrogators and tags.
The effect of intermodulation products may be reduced to an acceptable level by reducing the power received from
adjacent transmitters. This may be achieved either by the introduction of shielding or by increasing the physical
separation between transmitters. As a general guide for acceptable operation the power received by an interrogator or
tag from an adjacent transmitter should be at least 20 dB less than the power received from the wanted transmission.
An alternative mitigation technique is to arrange for adjacent transmitters to operate on different channels. The
frequencies must be sufficiently spaced apart that any intermodulation products do not degrade the performance of the
device. From practical tests and measurements it has been determined that for adjacent interrogators and their tags to
operate satisfactorily, the frequency separation between them should be at least 1 MHz.
4.3.2.4 De-tuning and absorption
The proximity of certain materials to UHF tags may cause a significant reduction in their reading range. This effect is
due predominantly to de-tuning of the resonant frequency of the tag. Spacing the tag a small distance away from the
material can significantly reduce this effect. However the application may impose a restriction on the extent to which
spacing is acceptable. Alternatively where the material to be tagged is known in advance, it may be possible to adjust
the tuning of the tag to compensate. Nevertheless recovery of the full free space reading range is unlikely to be
achieved. This difference is due to power absorption by the material.
In situations where an electromagnetic wave meets a boundary between two dissimilar materials, some of the energy is
reflected at the surface and some of the energy passes into the material. The proportion of the energy that passes into the
material is a function of its physical properties (known as its dielectric constant). This process is repeated at each
boundary between two dissimilar materials.
Where a tag is read through an object the consequent reduction in the level of signal reaching the tag will reduce its
reading range. Some indication of the scale of reduction in reading range caused by different materials is given in
table 1. The figures in the table are based on some informal tests and are illustrative only.
Table 2: Typical effect of materials on performance
Scenario Reference Distance (cm) Range (cm) (R/Rref)**2 Loss dB
Air 200 200 1,00 0,00
Tag on front of plastic case 200 180 1,23 0,90
Tag on front of plywood sheet 200 131 2,33 3,68
Tag on front of wood block 2,5 cm deep 200 120 2,78 4,44
Tag on front of paper 3 cm thick 200 108 3,43 5,35
Tag on front of empty plastic jug 200 149 1,80 2,56
Tag on rear of empty plastic jug 200 138 2,10 3,22
Tag on front of plastic jug filled with tap water 200 46 18,90 12,77
Tag on rear of plastic jug filled with tap water 200 31 41,62 16,19
Tag behind metal mesh 10 x 10 cm 200 28 51,02 17,08
Tag behind metal mesh 1 x 1 m 200 10 400,00 26,02
NOTE: For the purpose of making these measurements the transmit level from the interrogator was set to a
constant value.
ETSI
15 ETSI TS 102 902 V1.2.1 (2013-04)
An associated effect, which can also reduce the reading range of a tag, is its proximity and orientation with respect to
other adjacent tags. The effect is greatest where tags are parallel with each other since this produces the highest level of
mistuning and absorption. A similar situation arises where a second tag is positioned a short distance behind the first
one and in line with the transmission path from an interrogator. The tag nearest to the interrogator creates a "shadow",
which reduces the field available to power the tag that is further away.
It is important for end-users to understand and assess the impact of all of the above effects on their application.
In applications in which near field techniques
...