Anh Huong Cua He Thong TTDD

Engineering and Physical Sciences ResearchCouncil

Department of Trade and Industry

LINKPersonal Communications Programme

COLLABORATIVE RESEARCH

Electromagnetic CompatibilityAspects of Radio-based MobileTelecommunications Systems

FINAL REPORT

Produced in 1999 for the LINK Personal CommunicationsProgramme by ERA Technology Ltd, Cleeve Road, Leatherhead,

Surrey, KT22 7SA, England

Industrial sponsors:

Technical Partners:

of 93

List of Contents

LIST OF CONTENTS......................................................................................................................3

LIST OF FIGURES ..........................................................................................................................5

LIST OF TABLES ...........................................................................................................................7

EXECUTIVE SUMMARY ..............................................................................................................9

ABBREVIATIONS ........................................................................................................................11

1. BACKGROUND ....................................................................................................................13

1.1 INTRODUCTION.....................................................................................................................131.1.1 Electromagnetic Compatibility....................................................................................131.1.2 Trends in the Mobile Telecommunications Market.......................................................14

1.2 OVERVIEW OF CURRENT AND FUTURE SYSTEMS .....................................................................151.3 OBJECTIVES..........................................................................................................................191.4 APPROACH...........................................................................................................................20

2. RESULTS...............................................................................................................................23

2.1 DETERMINING THE THREAT ENVIRONMENT ...........................................................................232.1.1 Defining the Threat Parameters..................................................................................242.1.2 Modulation Schemes...................................................................................................272.1.3 Amplitude Modulation Effects.....................................................................................292.1.4 Models........................................................................................................................302.1.5 Signal Statistics...........................................................................................................312.1.6 Measurements.............................................................................................................32

2.2 INTERACTION BETWEEN THE EM ENVIRONMENT AND EQUIPMENT .........................................342.2.1 Direct Coupling to Stand-alone Equipment.................................................................35

2.2.1.1 Aperture Coupling............................................................................................................362.2.1.2 Interaction with Internal Electronics..................................................................................38

2.2.2 Coupling to Wired Networks........................................................................................382.2.2.1 Basic Concepts.................................................................................................................382.2.2.2 Common Mode Effects.....................................................................................................402.2.2.3 Transfer Functions............................................................................................................422.2.2.4 Twisted and Shielded Cables............................................................................................442.2.2.5 Statistical Measurements on Ethernet Cables....................................................................44

2.3 EQUIPMENT IMMUNITY .........................................................................................................472.3.1 Equipment Level Radiated Susceptibility Testing.........................................................472.3.2 Component Level Susceptibility...................................................................................50

2.4 REPRESENTATIVE SCENARIOS................................................................................................552.4.1 Hospital......................................................................................................................552.4.2 Railway.......................................................................................................................572.4.3 Office IT Installation...................................................................................................58

of 93

3. ANALYSIS .............................................................................................................................61

3.1 ANALYSIS OF RESULTS..........................................................................................................613.1.1 The Hospital Scenario.................................................................................................613.1.2 The IT Scenario...........................................................................................................663.1.3 The Railway Scenario..................................................................................................69

3.2 THREAT PROBABILITY ..........................................................................................................723.2.1 Method and Parameters..............................................................................................723.2.2 Relation to Modeling and Measurements.....................................................................743.2.3 Statistical Determination of the Threat........................................................................743.2.4 Relation to Existing EMC Standards and Test Methods...............................................743.2.5 Threat Determination for the Representative Scenarios...............................................75

3.2.5.1 The Hospital Scenario.......................................................................................................753.2.5.2 The IT Scenario................................................................................................................763.2.5.3 The Railway Scenario.......................................................................................................76

3.2.6 Summary of Threat Probability....................................................................................773.3 MITIGATION TECHNIQUES.....................................................................................................78

3.3.1 Additional EM Immunity Tests....................................................................................783.3.1.1 Proposed Specific Free Field Tests Levels........................................................................793.3.1.2 Proposed Generic Free Field Tests....................................................................................813.3.1.3 Conducted Immunity Tests................................................................................................83

3.3.2 Limit/Control Use of Mobile Phones...........................................................................833.4 IMPLICATIONS FOR FUTURE SYSTEMS.....................................................................................863.5 CONCLUSIONS......................................................................................................................88

3.5.1 General.......................................................................................................................883.5.2 For Equipment Designers and Manufacturers.............................................................883.5.3 For Mobile Telecommunication Systems Operators.....................................................893.5.4 For Standards Organisations.......................................................................................893.5.5 For Administrators and Managers...............................................................................89

REFERENCES ...............................................................................................................................91

LIST OF APPENDICES ................................................................................................................93

of 93

List of Figures

Figure 1: Project Plan..........................................................................................................21

Figure 2: Radiation from an Isotropic Source.......................................................................23

Figure 3: An Element of a Threat Scenario (Threat Element)................................................24

Figure 4: Simulated Spectrum of a GMSK Signal.................................................................27

Figure 5: GMSK Modulator, based on a Quadrature, Baseband Architecture........................30

Figure 6: Typical Model with Ground Plane.........................................................................31

Figure 7: Average Distribution for the 2MHz Bandwidth Measurements...............................33

Figure 8: Coupling Paths for RF Energy into a Generic Piece of Equipment..........................34

Figure 9: SE of an Enclosure with an Aperture for different Aperture Lengths.......................37

Figure 10: SE of an Enclosure with a 100mm Aperture for different Distances behind theAperture............................................................................................................37

Figure 11: Differential and Common Mode Currents on a Twin Wire Cable..........................39

Figure 12: Differential and Common Mode Currents on a 20m Twin Wire Cable..................39

Figure 13: Image Theory Model of Common Mode Currents.................................................40

Figure 14: Schematic of Mode Conversion Mechanism at a Cable Termination.....................41

Figure 15: Effect of the Environment and Termination Practice on Mode Conversion............42

Figure 16: Transfer Function of a Category 5 Ethernet Cable and Wall Socket......................43

Figure 17: Block Diagram of Peak Detection System............................................................45

Figure 18: Statistics of the Base-band Signal Incident on the Envelope Detector with NineGSM900 Phones Operational Near the Cable.....................................................45

Figure 19: Time Series of Samples Showing GSM Frames...................................................46

Figure 20: Radiated Immunity Test Configuration for Single Mobile Source.........................47

Figure 21: Test Configuration for Ensemble Signal from Three Sources................................48

Figure 22: Simulated Signals for GSM and TETRA Tests....................................................49

Figure 23: Transfer Function of a Typical Logic Inverter......................................................51

Figure 24: AC Noise Margin for 4000B CMOS with Different Supply Voltages...................52

Figure 25: Variation in Switching Time with a 30MHz RFI Voltage.....................................53

Figure 26: Envelope of Signal from 3 GSM Phones Operating on Channels 1, 47 & 124.......63

Figure 27: Envelope of Signal from 3 GSM Phones Operating on Channels 1 3 & 5..............63

Figure 28: TF Between a Source and a Cable, Differential and Common Mode.....................65

of 93

Figure 29: Envelope of Signal from 5 GSM Phones Operating on Channels 1, 13, 59, 101 &124....................................................................................................................67

Figure 30: Envelope of Signal from 5 GSM Phones Operating on Channels 1, 3, 5, 7 & 9.....67

Figure 31: Distributed Pickup of EMI from Multiple Sources along a Length of Cable..........68

Figure 32: Concept of Variability Applied to EMC...............................................................72

Figure 33: Extension of Variability Concept to Measurements and Modelling........................73

Figure 34: Pulse Modulation Waveforms for Generic Immunity Test....................................82

Figure 35: Generic Test Configuration..................................................................................82

of 93

List of Tables

Table 1: Summary of Current and Emerging Mobile Telecommunications Systems...............17

Table 2: Characteristics of Fields and Conductors at Mobile Radio Frequencies....................36

Table 3: Immunity Levels and Safe Distances for Equipment to GSM and TETRA...............49

Table 4: Worst Case DC Noise Margin for Common Logic Families (*5V supply)................52

Table 5: Peak E Field Strength for One and Three Sources in Overlapping Time-Slots..........62

Table 6: Peak Load Voltage for a 3m Un-shielded Cable, Against a Single Source................66

Table 7: Peak E Field for One and Five Sources in Overlapping Time-slots...........................66

Table 8: Summary of EMC Radiated Immunity Standards....................................................75

Table 9: Test Parameters for Mobile Phone Systems.............................................................79

Table 10: Severity Level Definitions and Field Multiplier.....................................................80

Table 11: Peak Fields for Specific Systems in each Severity Level........................................80

Table 12: Change in Total, Average, Power in CDMA Based Systems..................................87

of 93

This page is intentionally left blank

of 93

Executive Summary

Electromagnetic Compatibility Aspects of Radio-based Mobile Telecommunications Systems isa research study conducted under the Link Personal Communications Programme (PCP). Theaim of this study was to quantify the risk associated with unwanted EM interactions betweenmobile communications devices, such as mobile phones, and surrounding electrical/electronicequipment. In addition to the current situation, emphasis was placed on considering theperceived future position, to do this mobile communications technology and market trends wereconsidered along with predicted developments in general electronics technology.

The general approach to this study was to define the EM threat produced by single and multiplemobile communications devices and then determine how this would interact/couple with standalone and distributed electrical/electronic systems. The predicted threat generated at/within thevictim equipment was then compared with current EMC immunity test techniques and levels todetermine if they adequately covered the threat posed by mobile communication devices.Susceptibility experiments were conducted on a selection of medical equipment using simulatedmobile communications EM environments. To understand and demonstrate typical/realisticsituations where mobile communications devices and electronic systems coexist, three differentscenarios were developed.

The EM threat posed by mobile communications devices has been defined in terms of the mainparameters that constitute it and these include; frequency, power, range(s), number of devicesand modulation/multiplexing scheme. The threat itself will obviously have characteristics andthese will be related to the parameters that constitute it. The threat characteristics include;bandwidth, repetition frequency, amplitude and probability.

Coupling models have been developed and validated for the interaction between single andmultiple sources and wired/distributed systems. These models have shown that at mobilephone frequencies considerable common mode current can be induced into interconnectingwiring and this can lead to potential interference signals in the cable loads due to modeconversion.

It has been determined that the EM threat generated by mobile communication devices will, inmany realistic situations, exceed the immunity test limits currently defined in most EMCstandards. Limited susceptibility experiments were conducted and it was found that somemedical equipment failed when exposed to the EM signals that can be generated by mobilecommunications devices. Although this highlights potential risks this study has shown that theprobability of this situation occurring is relatively small and this is supported by the lownumber of reported incidences.

When assessing the potential future risk both electronic technology within victim equipmentand anticipated future mobile communications techniques need to be addressed.

of 93

All the information suggests that the speed of electronics will continue to increase and thepower consumption will continue to reduce as will operating voltages. This advancement inelectronics has always been considered as detriment to the EMI performance. This is becausehigher speed is associated with greater bandwidth, thus more interference energy (noise) will beseen by the electronics, and this coincides with lower signal power and voltage levels and boththese effects will reduce the signal to noise ratio. On a more positive note modern electronics isbecoming more sophisticated and is likely to be better at handling and/or recovering fromtransient interference effects.

It is believed that the EM threat produced by future mobile communications systems will beless than that associated with current systems. This is based on the assumption that futuresystems will rely more on base-band signal processing techniques and thus use lowertransmission power. This is true for CDMA, which is widely accepted as being the thirdgeneration access scheme and will be used in UMTS. Compared to TDMA systems, CDMAwill further reduce the EM threat as CDMA transmissions are continuous rather than burst.

This report suggests mitigation techniques that could be employed to reduce the potential riskassociated with unwanted EM interactions between mobile communication devices andelectronic systems. Also advice and recommendations have been made for equipmentdesigners, network operators, users and standardisations groups.

of 93

Abbreviations

AM Amplitude Modulation

BPSK Binary Phase Shift Keying

CDMA Code Division Multiple Access

CT Cordless Telephone

DART Digital Advanced Radio for Trains

DECT Digital European Cordless Telecommunications

DCS Digital Cellular System

DQPSK Differential Quadrature Phase Shift Keying

DTI Department of Trade and Industry

DTX Discontinuous Transmission

EM Electromagnetic

EMC Electromagnetic Compatibility

EMI Electromagnetic Interference

EPSRC Engineering and Physical Sciences Research Council

ETSI European Telecommunications Standards Institute

FDD Frequency Division Duplex

FDMA Frequency Division Multiple Access

FM Frequency Modulation

GFSK Gaussian Frequency Shift Keying

GMSK Gaussian Minimum Shift Keying

GSM Global System for Mobile communications

IT Information Technology

LAN Local Area Network

PCN Personal Communications Network

PCP Personal Communications Programme

PMR Private Mobile Radio

PCS Personal Communications System

QPSK Quadrature Phase Shift Keying

RF Radio Frequency

RFI Radio Frequency Interference

TACS Total Access Communications System

TDD Time Division Duplex

TDMA Time Division Multiple Access

TETRA Trans-European Trunked Radio Architecture

UMTS Universal Mobile Telecommunication System

of 93


of 93

1. Background

1.1 Introduction

The Link Personal Communications Programme (PCP) was established to investigate theimplications and impact of an expanding mobile communications market. This reportaddresses Electromagnetic Compatibility Aspects of Radio-based Mobile TelecommunicationsSystems, which was one of the areas investigated under this programme.

EMC Aspects of Radio-based Mobile Telecommunications Systems originally gained approvalin March 1995 and work commenced in April 1996. The project was funded under the LinkPCP with the DTI and EPSRC providing half of the funding and the industrial partnersproviding the other half. The total funding was approximately 370,000 over the three and ahalf year project.

The industrial partners were BT Cellnet, Nokia, Orange and Vodefone who in addition toproviding funding also provided invaluable information about existing and emergingtelecommunication technologies and standards.

The technical partners were ERA Technology Ltd, University of Hull and University of York.ERA Technology Ltd was responsible for the project management.

1.1.1 Electromagnetic Compatibility

The ability of electrical systems to operate in an electromagnetic (EM) environment withoutadverse effects is known as electromagnetic compatibility (EMC) and this is an establishedphenomenon in electronics engineering. The reality is that all electrical systems can bedisturbed by EM energy if sufficient power is available. For this reason EMC is achieved bylimiting/controlling EM emissions in addition to ensuring that electrical systems are sufficientlyimmune to EM interactions.

Mobile communications devices are intentional EM emitters and as such their radiationcharacteristics (frequency, power, etc) are tightly regulated, by organisations such as theEuropean Telecommunications Standards Institute (ETSI). However, by their very nature theoperating location will be uncontrolled and thus the distance between a mobile communicationsdevice and a potential victim electrical system is undefined. The power density generated by anEM emitter in free space is proportional to 1/d2, where d is the distance from the emitter. Thismeans that as d is reduced the power density increase to very high levels.

Although the emissions characteristics of each mobile device is regulated, the number ofdevices operated in a given area is not and is only limited by the capacity that the serviceproviders offer. EM emissions from separate emitters can combine together, either in freespace or within an electrical system, to produce a more severe EM threat.

of 93

The number of mobile communications devices in use is rapidly increasing and serviceproviders are constantly investigating methods to increase network capacity to meet thedemand. Given the known electromagnetic interference (EMI) problems associated withelectrical systems it is apparent that the uncontrolled use of mobile communications devicescould present potential EMC problems.

1.1.2 Trends in the Mobile Telecommunications Market

In the past 15 years the mobile telecommunications market has expanded from almost nothinginto a multi billion pound industry. In the UK the current market penetration is quoted as beingabout 20%, that is one in five people owns a mobile communications device of somedescription, most likely a mobile phone. To put it another way there are approximately 10million mobile phones in use in the UK at the present time. This may seem to be an incrediblelarge market, however the UKs use of mobile phones looks quite limited when compared withother European countries, where in some cases the market penetration has reached 90 %.

Given the level of market penetration in other countries and the current growth rate it isconsidered that the use of mobile phones within the UK will increase significantly over the nextfew years. From section 1.1.1 it may be concluded that this increase in mobile phones wouldbe detrimental to the EMC of other electrical systems. Although this is true to an extent, it maybe mitigated by the anticipated lower transmission power, in future mobile communicationsdevices.

of 93

1.2 Overview of Current and Future Systems

There are many types of mobile radio systems in use today, each designed to meet therequirements of a diversity of users and therefore having widely varying technical parameters.In this study we will confine our interest to systems which use transmitting elements (mainlyhand-held portables) which are capable of being brought into close proximity to electronicsystems in normal usage, producing a variable and unpredictable EM threat. Abnormal usageof systems, or special interactions such as that between a base-station and its immediateenvironment are not considered, since these interactions are likely to be the subject ofinstallation guide lines and normal EMC requirements. The effects of base-stations mayhowever need to be considered in future systems where cell sizes may be reduced to increasesystem capacity, thus increasing the density of base-stations.

This overview is therefore limited to systems in common, current use and to future systemswhose parameters are either known or may be sensibly estimated. One of the main aims of thisbrief overview is to identify the general system parameters that need inclusion for a meaningfulEMC analysis.

The major defining parameters of a radio system are the frequency band of operation, radiatedpower level, type of modulation, multi-user access technique and duplex technique. The EMthreat posed by the simultaneous use of several such systems is dependent upon the nature ofthe time-varying ensemble received signal as observed in a particular (non-radio) bandwidth aswell as the susceptibility characteristics of the victim system. It is therefore necessary toevaluate the variation of this ensemble signal on a range of time-scales, including thosecorresponding to carrier cycles (individual carrier frequencies), phase interactions (total systemoperating band), system time frame (time division multiple access, time division duplex, packettransmission etc.), signal fading (automatic power level control) and transmitted messagecharacteristics (discontinuous transmission etc).

Three "generations" of mobile radio communications systems are commonly defined: firstgeneration analogue systems are still in widespread use but are increasingly being replaced bysecond generation digital systems. Third generation digital systems, offering major advances inservice provision, including e-mail and broadband data, are now in the final phase ofstandardisation.

Cellular systems divide the coverage area into a large number of cells, whose size ranges fromtypically a few hundred metres to a few kilometres, in order to increase radio spectrumutilisation by reusing frequencies in non-adjacent cells. The radio spectrum resource (usuallyin the form of a number of carrier frequency channels) allocated to each cell may be sharedallowing multiple-user access, in a number of ways:

Frequency Division Multiple Access (FDMA): Each communications channel occupies adifferent carrier frequency and hence the number of available carriers must be at least equal tothe number of simultaneous users.

of 93

Time Division Multiple Access (TDMA): Each communications channel is allocated a carrierfrequency for only part of the time. Usually the available time is shared into a number of equaltime-slots. The system has to remain synchronised throughout. Inclusion of synchronisationand control data leads to the definition of higher-level timing structures known as frames.Generally if N channels share one carrier, the required bandwidth of the carrier is in excess ofN times that of a single communication channel.

Code Division Multiple Access (CDMA): The available spectrum is divided into a smallnumber (possibly even one, but usually around ten) of high bandwidth channels. Many usersare allocated the same channel simultaneously, each being able to decipher their owncommunications by the use of a large number of non-interacting (orthogonal) digital codes. Anessential requirement for successful operation of CDMA is that all the signals forcommunication with a particular base station must arrive at that base station withapproximately equal power levels.

Some systems use a combination of the above methods to improve capacity. Simultaneous twoway communication is achieved by one of the following techniques:

Time Division Duplex (TDD): The mobile station and base station each transmit in differenttime intervals.

Frequency Division Duplex (FDD): The mobile station and base station each transmit ondifferent carrier frequencies.

The main technical parameters of a number of systems both in use and proposed aresummarised in Table 1.

of 93

UK TACS PMR TrunkedDigitalPMR

GSM900 CT-2 IS95

USCDMA

CT3DCT900

DECT

DCS1800 TETRA UMTS

Generation 1 1 2 2 2 2 2 2 + PCS 2 + 3

Primary Use AnalogueCellular

AnalogueTelephony

Digital Data DigitalCellular

DigitalCordless

DigitalCellular

DigitalCordless

DigitalCellular

PCS

DigitalCellular

DigitalCellular

Handset Freq (MHz) 890 05 425463 933935 890 915 864868 824849 862866

18001900

17101785380-400 1920-1980

Peak Power (mW) 600 5000 2000 10 10 80

250

1000 3000 112.5

Mobile Power Control Yes No Yes (Slow)2dB steps

Yes Yes No

No

Yes (Slow)2dB steps

Yes (Fast)0.25/1.5dB

steps

Multi-Access FDMA FDMA FDMA4TDMA

TDMAFDMA

FDMA CDMA(DS)FDMA

TDMAFDMA

TDMAFDMA

TDMAFDMA

W-CDMAFDMA

Duplexing FDD FDD FDD FDD TDD FDD TDD FDD FDD/TDD

Modulation (Up-Link) FM FM QPSK GMSK GFSK BPSKQPSK

GFSK GMSK DQPSK Dual ChanQPSK

Frame Duration (ms) N/A N/A N/A 4.615 2 20 16

10

4.615 59 1016 625ms

Channel Spacing (kHz) 25 12.5 25 200 100 1250 1000

1728

200 25 5000

Table 1: Summary of Current and Emerging Mobile Telecommunications Systems

of 93

Table 1 summarises the main systems in use in the UK at the present time. The main threats tobe assessed from high-volume usage of these systems are expected to arise from:

1. Carrier cycle phase addition effects due to the wide operating band of the systems. Forexample, in the GSM900 system, mobile channels may be spaced by up to 25MHz givingrise to coherent signal additions of duration down to 40ns.

2. TDMA frame-rate effects.

3. Effects due to power ramping and power control, although these are limited by thespecified maximum power ramping rate and the maximum power level change betweentime-slots.

GSM, DCS and TETRA employ power management to conserve battery power. For the GSMsystems the mobile transmitted power is adjustable in steps of 2dB between each time-slot, witha dynamic range of 30dB. Note that for a system using 1:N TDMA the average mobiletransmitted power is 1/N. For example GSM handsets with a peak power of 2W have anaverage power, measured over several tens of ms, of 0.25W as N = 8. In most cases however,the timescale of interest is within one time-slot and we must therefore consider the peak powerlevel.

Also summarised in Table 1 are later, second generation systems, some of which are oftentermed generation two-and-a-half systems, these offer enhanced personal communicationservices. Details of the third generation UMTS have not yet been finalised although it is knownthat it will be based on wide-band CDMA, other details have been determined from [1] and [2].Because of the low transmission power associated with each mobile handset, it is anticipatedthat the ensemble signal received by a victim circuit would be noise-like, lacking in any of thecoherent effects, which may be observed in narrow-band systems.

In UMTS there will, however, be a received power variation associated with the time-slotduration and the fast closed-loop power control. The power control maintains a constantreceived signal/noise ratio at the base-station. However at other locations the received signalwill consist of the power transmitted by the mobile handset (controlled by fading on the mobileto base-station path) multiplied by the (independent) fading over the handset-victim path(assuming a non-stationary handset).

In summary, various current and future mobile radio communication systems have beenreviewed with emphasis on the parameters which, when the systems are used in a congestedenvironment, are likely to contribute towards an EM threat.

of 93

1.3 Objectives

The objective of this study was to determine and quantify the potential for unwanted EMinteractions from mobile communication devices to surrounding electrical systems and suggestpractical mitigation techniques where appropriate. The study was also to consider theprojected future usage of mobile communications and technology such that the future threatcould be assessed.

Only mobile communication devices were to be considered in this study as fixed installationsshould be controlled and regulated in accordance with existing guide lines. Also onlyinteractions with electrical systems were to be considered, personal safety issues were onlyconsidered in the context of the consequence of interference with an electrical system.

The unwanted interaction of EM signals with electrical systems can be considered as threeseparate steps:

1. The characteristics of the incident EM signal.

2. How the incident signal is coupled into the victim electrical system.

3. The characteristics of the EM energy needed to interfere with the electrical system.

The characteristics of an EM signal will include many different parameters, i.e. peak level,average level, total energy, frequency, modulation, etc.

The EM emissions from a single communications device are well regulated and thus can beaccurately characterised. However if more than one device is operated in close proximity theindividual emissions will interact to generate a combined EM environment. This combinedenvironment is unlikely to be a simple addition of the individual sources it represents as relativedistances, signal frequency and phasing will all determine the resultant environment. Beingable to define the EM environment when more than one mobile communications device is inoperation is considered to be a major component of this work programme.

of 93

1.4 Approach

Originally the project was divided into five work packages which could initially be investigatedseparately, these were:

1. Coupling Mechanism and EM Modelling

2. Radio Signal Sources and Emission Characteristics

3. Non-Radio Systems Emissions and Susceptibility Characteristics

4. Radio/Non-Radio Interactions

5. Testing and Verification

Although there were regular technical meetings where ideas and information was exchanged forthe first two years of the project these packages were essentially investigated separately. Atthis stage the project was reviewed and subsequently restructured to ensure that the variouswork packages would converge, thus allowing the programme objectives to be fulfilled. Themodified structure was intended to act as a framework for the final report as this would provideclear direction and allow shortcomings to be identified early. The outline project planimplemented during the final 18 months of the project is shown in Figure 1, and from this it canbe seen that there are three initial topics, as listed below:

1. Determine the Threat Environment

2. Coupling Channels/Mechanisms & Levels

3. Consider Representative Scenarios

It was always recognised that immunity testing of a representative selection of electronicequipment against the anticipated threat environment would be an important part of the project.When a reasonable understanding of the threat environment and how it interacts with potentialvictim systems had been established, equipment immunity tests were conducted. The threeinitial topics listed above and the equipment immunity tests therefore form the main resultssection of the final report.

Using the information and results obtained from the four areas highlighted above, it waspossible to analyse different situations to determine if there were any potential risks. Also byunderstanding how the threat is generated and coupled into systems it is possible to suggestmitigation techniques to reduce any potential threats.

of 93

Project Plan

Determine the threat environmentFrequency Band(s)Worst case ERPModulation characteristicsMin distance from victim equipment IdentifyConstructive / destructive interference Relevant coupling channels / mechanismsConsider future systems Dominant channel for each scenario/equip type

Coupling channels / mechanisms (& Levels) Immunity/Susceptibility assessmentConsider frequency band(s) Use modeling to reduce scope / amount of testingSource - Free field radiation Test representative selection of equipment

Direct coupling to PCB / devicesCoupling to interconnecting cablesCoupling to local cablesCoupling directly to antennas (out / in band) Conduct a risk assessment for typical scenarios

Consequence of a failureHospital - (loss of life)

Consider a representative selection of scenarios Railway - (loss of life / economic)Hospital IT Network - (economic)Railway (track, station, concourse) Likelihood of a failureIT Network Reported incidences in known environment

Type / amount of interconnecting cables Immunity/susceptibility tests on various equipmentType / amount of electronic equipment Consider future technologyImportance of equipmentTypical layoutConsider worst case scenario + probability

Identify/Quantify risk areasMake recommendations on risk reductionIdentify predominant interference parameterProvide guidance on test techniques

Figure 1: Project Plan

of 93


of 93

2. Results

2.1 Determining the Threat Environment

In this section the parameters of the RF environment which constitute an EMC threat to non-radio equipment and systems are defined and related to measured results where appropriate.To place this work into its correct context it should be borne in mind that here we are definingindividual features of the RF threat The summation of individual threat components in realisticsituations is discussed in section 2.4 whilst the probability that these threats will add up to anactual EMC problem is the subject of section 3.2.

The concept of an elementary isotropic source of radio waves is used throughout this reportand is therefore defined here. An isotropic source is one which emits radiowaves equally in alldirections. It may be shown, by consideration of the vector nature of electromagnetic waves,that an isotropic source cannot actually exist, but the concept is nevertheless useful andprovides approximate estimates of field strength for radiating elements which are shortcompared with the wavelength of radiated waves. Radiation from an isotropic source may bevisualised as follows:

Figure 2: Radiation from an Isotropic Source

For an isotropic source in free space the radiated power spreads out equally in all directionsand therefore the power density, Pd W/m

2 at a distance d m from the source is given as:

PP

dd

T=4

2

p

Equation 1

Where PT = total radiated power in Watts

of 93

For free field impedance the corresponding electric field strength (E) is given by:

d

PmVE T

=

30)/(

Equation 2

A typical "threat element" may be envisaged as shown in Figure 3.

SOURCE

VICTIM

z

y

x

LOAD

Figure 3: An Element of a Threat Scenario (Threat Element)

In section 2.1.1 the parameters of the above threat element are summarised and in section 2.4 anumber of typical scenarios are constructed using ensembles of these elements. A generaldescription of the features of a modulated RF carrier which may lead to EMC problems inwired systems is followed by discussion of modulation schemes in common usage as well asthose planned for future systems. The methods used to model and simulate these modulationschemes are presented along with a discussion of likely signal fading and statistics of a singleRF emitter, such as a mobile radio handset. A method of assessing the probability associatedwith individual and combined risk parameters is presented in section 3.2. Finally, a series ofmeasurements is described and related to the modelling. It is concluded that the models usedare capable of simulating the ensemble signal due to a realistic disposition of mobile radiohandsets, provided that suitable input parameters are used.

2.1.1 Defining the Threat Parameters

In Section 2.2 it will be shown in detail how energy from EM waves can couple into conductorsto produce EMC problems. A simplified appreciation of the main characteristics of thesecoupling methods is required in order that modulated RF carriers may be assessed in terms oftheir threat parameters. Without elaborating on the details of coupling mechanisms at thispoint, it may be shown that, in general, the coupling between a RF field and a simple wireconductor depends upon two parameters:

1. The length of the conductor, in terms of the wavelength of the incident field.

2. The strength, type and polarisation of the field incident upon the conductor.

of 93

Each of these parameters is itself dependent upon a large number of other factors. The firstobviously depends upon the length of the conductor but is also dependent upon the detailedspectrum of the radiated field which is in turn dependent upon signal modulation and otheramplitude and phase effects. The incident field strength is dependent upon all of the factorsrelated to transmitted power, amplitude modulation effects, distance between the conductor andthe RF source and the relative geometry of the field and conductor. Many of these parameterscan only be accurately described in a statistical manner. Equation 3 illustrates the relationshipbetween the main parameters that contribute to the threat.

=

=

N

n nZonennn dlcpPkt

1 .

..

Equation 3

Where: t = threatk = constant of proportionalityN = number of emitters (handsets)P = transmission power of handset np = propagation loss, including statisticsc = coupling from field to load, including statisticsZone = length over which a significant interaction may occur

The following paragraphs describe each threat parameter in more detail and relate each toobservable characteristics of real radio systems.

Frequency band, channel and system bandwidth: The overall spectrum allocated to thesystem governs the bandwidth of possible phase interference effects since the maximum beatfrequency arises from phase addition of signals at opposite extremes of the allowable range.The duration of pulse-type events arising from specific modulation states will be inverselyproportional to this bandwidth. In GSM, for example, this bandwidth is 25MHz, giving thepossibility of pulse-like events of 40ns duration. The distributed nature of coupling into fixedconductors and of propagation along those conductors may further compress this minimumfeasible pulse duration, see appendix G.

Distance of RF source from victim conductor: For communication between two isotropicantennas in free space the received power is inversely proportional to the square of the distancebetween the antennas. If both antennas are situated above a simple infinite reflecting groundthe received power varies in a more complex way due to interference between the direct andreflected wave and at large distances become inversely proportional to the fourth power of thedistance. In the scenarios considered in this study (see section 2.4) the following conditionsgenerally apply:

1. The receiving antenna is a victim conductor that is distributed in extent.

2. For significant coupling the transmitter is generally close (less than 2m) from some part ofthe victim conductor.

of 93

The models used in this study consider the radiation field and include reflections, close rangeinduction effects have not been considered, see appendix B.

Transmitted Power: Since the coupling techniques considered here are linear, the threatmagnitude will be related to the transmitted power for an individual emitter. In determining thefeasibility of a particular EMC problem, peak, maximum transmitter powers should be used,typically 2W for GSM or 3W for TETRA. Base-station transmitters are not specificallyconsidered in this study since although they have much higher power levels, they are fixed inposition and their installation is controlled by established EMC and radio regulatory legislation.

Modulation scheme: Each type of modulation has its own characteristic radiated spectrum andthose used in modern cellular systems are chosen for their excellent radio spectrum utilisationcharacteristic. In this study we are not concerned with amplitude modulation schemes whosespectrum may contain data-specific components. Modulation schemes considered includeGMSK, GFSK and DPSK, which are all constant-envelope schemes. Appendix D contains adetailed analysis of GMSK modulation with emphasis on the phase addition for signals from anumber of emitters. The potential threat is will be related to the carrier frequency and theoverall system bandwidth; higher carrier frequencies and bandwidths give rise to greater ratesof change of induced voltage and interference pulses of shorter duration.

Multiple-access technique: The three multiple-access techniques considered are FDMA,TDMA and CDMA. Each of these contributes to an overall EMC threat in a different way. Ina simple FDMA system the overall system bandwidth has to be large enough to accommodatethe level of traffic offered whilst the individual channels are of relatively low bandwidth,accommodating only a single traffic channel. In an N-channel TDMA system radio channelshave in excess of N times the bandwidth of one message channel but there are less radiochannels available. More importantly, a receiver in the vicinity of a mobile station willtypically only receive power from one or two time-slots within the TDMA frame, causing anamplitude modulation effect.

More complex systems may use a combined CDMA/TDMA approach, again resulting inamplitude modulation of the signal received at a non-intended location. Fast power control willalso result in amplitude modulation when the receiver is not coincident with the base-station.The bandwidth of such power control is likely to be low when compared with the modulationbandwidth. For example in UMTS, transmitted power increments may be between 0.25dB and1.5dB depending upon cell size. Control signals are sent at a rate of 800 per second, resultingin a maximum rate of change of transmitted power of 1200dB per second. More realistically,fast fading could cause 6dB power excursions with an amplitude modulation frequency of100Hz in the most extreme case. This frequency also coincides with the frame rate in UMTS.

Duplex Technique: GSM and UMTS both employ FDD with separate band allocations for theup-link and down-link channels. The use of TDD (in CT2 and CT3 for example) generallyincreases the required traffic channel bandwidth by a factor of two with consequent EMCimplications as described above.

of 93

There are a range of other modulation-related effects that have implications for the EMC threatposed by a congested RF environment including DTX, power management and the rate ofpower ramping. These effects, often causing amplitude modulation of the received signal, arediscussed in more detail in Section 2.1.3.

2.1.2 Modulation Schemes

Throughout the history of radio communications, modulation schemes have been continuouslydeveloped in order to improve the performance and spectral utilisation. In recent years smallerscales of circuit integration and faster DSP devices and techniques have meant that verycomplex modulation schemes can now be used within compact handsets.

In simple modulation schemes such as AM, all changes in the carrier frequency are due to themodulating signal. In more complex systems the introduction of separate channels forsignalling and multiple access techniques, means that in practice the behaviour of the RFcarrier cannot be described using simple, theoretical types of modulation. In order to examinethe EMC implications of current and future mobile radio systems a, perhaps artificial,distinction is made between effects due to "pure" modulation (such as GMSK), which arediscussed in this section, and other sources of amplitude modulation which may arise due tosystem considerations, which are discussed in the following section.

Constant envelope techniques such as PSK and FSK have been refined by stages to reducespectral spreading due to abrupt instantaneous phase or frequency changes, leading tominimum shift keying (MSK) and gaussian minimum shift keying (GMSK). In GMSK, thebandwidth of the gaussian filter (B), relative to the data rate (1/T), can be chosen to reach adesired compromise between bit error rate and out of band interference. In the GSM system a

value of a third is chosen for the product BT, with a symbol duration (T) of 3.69ms and a filterbandwidth of 90.3kHz. A typical spectrum of a GMSK signal (derived from simulation) isshown in Figure 4.

Frequency (kHz)

Figure 4: Simulated Spectrum of a GMSK Signal

of 93

In simple terms the spectrum of Figure 4 can be considered to repeat every 200kHz across thespectrum allocated to GSM. It will be noted that this spectrum differs from the smooth curvesoften shown in text books, which are normally a theoretical average over all possiblecombinations of input data. It may be seen that low frequency structure may be introduced intothe signal spectrum by repetition of data sequences but this effect does not involve significantenvelope amplitude changes and is therefore not considered.

In the case where each carrier may be considered to be a narrow band signal, amplitudechanges would be expected due to phase effects between multiple carriers when the receiver (inthis case a broadband victim circuit) receives more than one carrier signal. In the simple case,if there are N carriers equally spaced K kHz apart, then there are (N-m) possible interactionswhich give rise to a beat frequency mK kHz where m ranges from 1 to (N-1). For example,with five carriers with spacing of 20kHz there are three combinations of carriers which producea 40kHz beat frequency (N=5, K=20kHz, m=2, N-m=3). In general there would be no well-defined phase relationship between the carriers and the simultaneous addition of all carriers inphase becomes progressively less likely, although of greater effect, as N increases, seeappendix K.

In practice, the signals are not narrow band and each is spread over approximately 200kHz bythe (independent) modulating signals. Multi path propagation and motion of mobile handsetsfurther decreases the likelihood of significant carrier enhancements and, more importantly,physical factors limit the number of handsets that fall within a defined zone of interaction for avictim conductor. These practicalities are discussed when defining realistic scenarios (section2.4) and presenting results for each scenario (section 3.1). Measurements and realisticsimulations are described in appendices E and F respectively.

Amplitude modulation due to system TDMA and power control is described in section 2.1.3.

Other forms of constant envelope modulation used in modern mobile radio systems, such asGFSK, 4-QPSK and DQPSK have spectra similar to that of Figure 4 with the bandwidthdepended upon the transmitted symbol rate.

A further class of systems deserving particular consideration is that including planned third-generation high-speed digital services, to be used in various forms world-wide. Specifically,the European UMTS is planned to use wideband CDMA with parameters summarised in Table1. The implications of higher bit (chip) rate and lower power are as follows:

1. Compared with conventional systems, the signal bit energy in a W-CDMA system isspread over a wide bandwidth and the desired channel signal-to-noise ratio is achieved as aconsequence of coherent, matched, detection. The spectral density of signal energy due to asingle channel is therefore much less than in a narrow band system (In simple terms, theenergy is reduced by the processing gain, which in UMTS is 128, i.e. 21dB). Whilst, forcomparable fully occupied narrow band and CDMA systems the signal power spectraldensity may be comparable, for a lightly loaded CDMA system the signal power spectral

of 93

density will be much less, possibly comparable with natural or man-made radio noiselevels.

2. The probability of significant phase coherent addition of signals from multiple users is alsomuch reduced compared with the conventional case and the coupled energy associated withany such additions is much smaller due to the higher chip rate. CDMA systems arediscussed further in section 3.4 and the TDMA aspects of UMTS are discussed in section2.1.3.

2.1.3 Amplitude Modulation Effects

Radio systems utilising constant envelope modulation schemes can produce amplitudemodulation of the RF carrier in a number of ways, most of which are consequences of systemoperational protocols.

As previously stated there are two main effects arising from changes in a signal amplitude,which may cause EMC problems in wired systems of the type considered within this study:

1. Rate of change of voltage (dV/dt) coupled into conductors, via cross-talk type mechanisms,gives rise to voltage spikes at the semiconductor load.

2. Repetitive amplitude modulation may contain frequency components that fall within theoperational frequency range of the victim electronic equipment.

In the cellular mobile radio systems considered here, amplitude modulation may be produceddue to the TDMA framing structure and due to adaptive power control. The TDMA structuremay produce amplitude modulation with a fundamental frequency equal to the frame rate, dueto differing amounts of power received from each time-slot in general. In a fully utilisedsystem, at the base station (only) the power control will strive to maintain equal received powerin each slot, this is particularly critical in CDMA systems. However, if some slots are notused, or if, as is the general case, the unintentional receiver is not co-incident with the basestation, amplitude changes of a repetitive nature will occur, this effect has been verified byexperiment, see section 2.2.2.5. The frame rate associated with commonly used systems istherefore of importance and in this study it has been shown that the frame rate used in TETRA(17 frames per second) may have particular implications for a certain class of medicalequipment. In general there will always be the possibility of specific interactions betweenindividual frame rates and equipment although, due to the noise like nature of CDMA, it islikely that this threat will diminish in future systems such as UMTS.

The other source of amplitude changes is the power control itself. If a system controls thepower transmitted from a mobile to maintain constant power at the base station, a receiverclose to the mobile will observe the resulting amplitude changes in their entirety. It is thereforenecessary to examine the maximum rate at which such power control can operate.

The most severe case is likely to be in UMTS where fast power control can result in 800changes in transmitted power per second. If these power steps are all in the same direction andare of maximum allowed size, the signal amplitude could be changing at 1200dB per second.

of 93

Again in a worst case, if a mobile was moving through a faded field at 60mph it wouldexperience 60 fades per second (it may be shown that fading at the 10dB level occurs at a ratewhich is numerically equal to the vehicle speed in mph) with an amplitude variation of 10dB,which would produce amplitude modulation comparable to that arising from the TDMAstructure. However, in this study we are concerned predominantly with close range effects andit has been shown that the zone of interaction, which may produce EMC effects of the typeconsidered, is limited to a few metres from the victim conductor. Whilst the effects of theabove fading and consequent power control may be experienced by vehicle-borne systems, it isnot physically possible to move at the speed described above and still remain within the zone ofinteraction of a fixed conductor for any significant time. It is therefore concluded that powercontrol as a direct effect is of secondary importance, and that amplitude modulation, in thecontext of this report, will arise predominantly from the TDMA structure and associated slowpower control.

Other effects, such as DTX may also be considered using the same argument since theoperation of DTX will be different in different time-slots.

Randomisation of the usage of time-slots by frequency hopping will also tend to suppress theeffect of the fundamental frame rate.

2.1.4 Models

The study involves the modeling of individual and ensemble signals transmitted from mobilehandsets, with a view to characterising the features which pose a threat to otherelectronic/electrical devices operating in the surrounding environment. With this in mind aGMSK modulation simulator was implemented. Since this would be representative of themajority of current second-generation digital systems using the GSM standard. At the time thestudy was undertaken it seemed likely that the third generation systems would also adopt aGSM type protocol but the decision has since been made in favour of W-CDMA. Theimplications of CDMA have been discussed above. The GMSK simulator was based on aquadrature, baseband architecture and a representation of this is shown in Figure 5.

Figure 5: GMSK Modulator, based on a Quadrature, Baseband Architecture

of 93

The simulator, which produces a phase and amplitude representation of the cycle-by-cycle RFwaveform in response to a given pattern of input data bits, was used to generate the field due toa random disposition of sources in the vicinity of a victim conductor. Account was also takenof polarisation of the dipole antennas representing the sources and their orientation relative tothe victim conductor. Initially a very simple model was set up as in Figure 6 where line ofsight propagation and reflection from ground planes is taken into account.

B = (0, -2, 1)

y

x

z

Ground Plane at z 0

A = (0, 2, 1)

C = (-1, 2, 1)

S = (1.5, 0, 1)

Figure 6: Typical Model with Ground Plane

Using the ensemble signal model, the time varying signal at any point along the cable can becalculated, and these results have been used in conjunction with the coupling models tocalculate the potential system level EM threat.

2.1.5 Signal Statistics

The models described in Section 2.1.4 produce a time series of signal samples that relate to aspecified disposition of emitters, each transmitting a specified bit stream and each having aspecified carrier phase and TDMA synchronisation. In a practical situation there are a numberof variations in individual signal components that are statistical in nature and which can onlybe modelled by statistical means. In this study a distinction has been made between statisticalvariations of the signal (or threat) due to variations in the signal from one emitter or specifiedgroup of emitters and statistical variations in the composition of the group of emitters. Thelatter statistical variations are governed by system usage patterns and are discussed in thecontext of threat probability in section 3.2.

Statistical variations of the signal received from a single emitter can be related to two factors,propagation and transmitted signal.

1. Variations in signal propagation arise from changes in the multi-path structure of thereceived signal due to motion of the transmitter, receiver or features in the environment. Inthis study we are concerned predominantly with signals received by electronic equipmentwhich is fixed in position. Received signal fluctuations are therefore due to motion of thetransmitter or motion of reflecting or scattering surfaces. In systems employing power

of 93

control, the power control acts in such a way that the signal at the intended receiver is keptconstant within certain limits. However, at other positions the statistical independence ofthe propagation paths to the intended and non-intended receivers may result in an enhancedrange of signal variations at the non-intended (victim) receiver. The time-scale of suchpower control depends on the system design. In GSM for example, the primary function ofpower control is to preserve battery power and the power changes are relatively slow. InUMTS the power control is much faster and is fundamental to the operation of the system.

2. Variations in the received signal arise from the pattern of transmitted bits in the messageitself, as well as from framing etc, imposed by the communication protocol. The effect oftransmissions from individual emitters on an ensemble 'received' signal depends upon manysystem-dependent factors including the modulation used. In most of the analysis performedin this study GMSK modulation has been assumed which is fundamentally constant-envelope for a single emitter. Under these conditions the ensemble signal due to a numberof emitters is determined by phase addition of individual components.

In the modelling described in this section the following assumptions are made:

1. Propagation: Typical conditions apply to each path.

2. Multi-path: Frequency-dependent multi-path is automatically taken into account by thebroadband nature of the modelling. Motion of the emitters is not specifically modelled.

3. Message: It is assumed that all of the emitters are transmitting simultaneously. For aTDMA system such as GSM this is equivalent to all emitters being allocated to the sametime-slot. Statistics of the signals on a bit-by-bit basis are automatically taken into accountin the modelling.

2.1.6 Measurements

The computer-based methods for modeling the ensemble EM emissions from a number ofmobile sources needed to be verified and there were two perceived ways of doing this. Onemethod was to set up controlled experiments in the laboratory and the other was to measure areal environment. In an attempt to reduce uncertainties in establishing the real environment thesecond option was adopted.

All the measurements were performed on the main concourse at Waterloo Station in London.The spectrum occupancy of GSM900 and DCS1800 (PCN) mobile channels was monitoredusing an antenna and a receiver. Three different intermediate frequency (IF) bandwidths wereused, 15kHz, 100kHz and 2MHz. A complete description of the measurement system used andall the results is given in appendix E.

Figure 7 shows the amplitude distribution of the measured data with a 2MHz IF bandwidth.

of 93

Relative level (dB)

Perc

enta

ge (

%)

0

5

10

15

20

0 20 40 60 80 100

min

max

Figure 7: Average Distribution for the 2MHz Bandwidth Measurements

For the modelling a number of randomly distributed sources, each with its power controlled bythe proximity of the base-station with maximum power being reached at the cell boundary,were placed inside a cell. A test position was selected at random within the cell and thedistribution of the anticipated levels was calculated. Using this simple model a reasonablematch to the measured data was achieved. Different test positions were considered and it wasfound that the best results were obtained when the test point was at the cell boundary, i.e. as faraway from the base-station as possible. Additional simulations were conducted, includingsignals propagating from mobile stations in neighbouring cells.

of 93

2.2 Interaction Between the EM Environment and Equipment

The interaction of electrical equipment with the EM environment at high frequencies is acomplex process. The wavelength of many mobile telecommunication systems is comparableto, or smaller than, the dimensions of potential victim equipment. Thus, in general, a fullelectromagnetic analysis is required to quantify all the possible coupling paths for external EMIinto the electronics of the equipment. Accurate prediction of the immunity of bulk equipmentbased on its component parts is a very difficult task and leads to highly specific results. Forthis reason this study has concentrated on identifying typical coupling routes and immunitylevel(s) of equipment against the perceived mobile telecommunications systems EM threat.Typical equipment immunity levels were determined by testing in a quantitative assessmentprogram and this is the subject of section 2.3.1.

THREAT Field

Network Auxilarly Cable

Grill

Power Cable

Apertures

Shielded Chassis

PCB

CB

PGrid

Power

Figure 8: Coupling Paths for RF Energy into a Generic Piece of Equipment

Figure 8 shows a schematic of a generic piece of electronic equipment, i.e. a personal computersystem. The electronics are mounted inside an enclosure that may or may not be metallic. It isextremely rare for the enclosure to be completely closed as there are usually openings to allowexternal cables to be attached (power leads and other auxiliary cables), ventilation grills, andother apertures for access or functionality of the equipment. Obviously there are many pathswhich external EMI can couple to the electronics inside this enclosure:

1. Direct penetration of the RF energy through the enclosure.

2. Coupling of RF energy through apertures in the enclosure.

3. Coupling of RF energy onto the mains power grid and/or the equipment power cable, whichis then conducted into the equipment.

4. Coupling of RF energy onto network cables or auxiliary leads, which is then conducted intothe equipment.

The dominant coupling path(s) for a specific case depends on many factors including thefrequency of the threat field, the geometry of the external cables and the layout of the internalcircuits.

of 93

If significant EMI energy is able to penetrate into the enclosure then interference currents willbe induced on internal cables and circuits. If the enclosure is relatively complete then resonantmodes of the cavity may be excited at certain frequencies, increasing the energy in the vicinityof susceptible circuits. Even inside the equipment the path taken by EMI into susceptiblecircuits may not be direct: cross talk with other sub-circuits, re-radiation, absorption andconduction can all contribute to failure of the equipment.

The coupling of EMI onto external cables is considered to be a particularly importantmechanism at high frequencies, especially for extended wired networks. This is discussed indetail in section 2.2.2. First we consider other direct coupling paths into a piece of equipmentvia shields and apertures.

2.2.1 Direct Coupling to Stand-alone Equipment

The immunity of equipment, which is predominantly enclosed in a conductive shield, can berelated to the effectiveness of the shield. The electric field shielding effectiveness, SE, of alayer of conductor can be defined by Equation 4.

=

||

||log20)(

dtransmitte

incident10 E

EdBSE

Equation 4

Where E incident is the electric field incident field on the outside of the shield anddtransmitteE is the

field transmitted through to the inside of the shield. A good shield has a large shieldingeffectiveness, typically many tens of decibels.

There are three contributions to the shielding effectiveness of a completely enclosing shield.

1. A proportion of the incident electric field is reflected at the outer surface of the conductor,as determined by the reflection coefficient of the conductor in air. This reflection loss isgreatest at low frequencies and decreases at 10dB/decade with frequency, reaching around70dB at 1GHz,[5].

2. Absorption loss due to attenuation in the conductor. For mobile telecommunicationsfrequencies the skin depth in a good conductor is a few micrometers or less (see Table 2).The absorption loss is therefore very high in all but the thinnest of conducting shields. Theabsorption loss dominants the shielding effectiveness above 100MHz, contributingtypically hundreds of decibels to the shielding effectiveness.

3. Multiple reflections inside the conductor will also contribute to the shielding effectivenessof a conductor, however for good conductors the contribution made is small compared tothe other two mechanisms and can thus be neglected.

In summary, the penetration of EMI energy directly through shielding conductors can beignored at mobile telecommunications frequencies.

of 93

Frequency(MHz)

Wavelength(cm)

Skin Depth(mmm)

ResonantSize (cm)

Notes

10 3000 21 1500

100 300 6.6 150

400 75 3.3 37.5 TETRA

900 33.3 2.2 16.7 GSM

1000 30 2.1 15.0

1800 16.7 1.6 8.3 PCN

2000 15.0 1.5 7.5 UMTS

Table 2: Characteristics of Fields and Conductors at Mobile Radio Frequencies

2.2.1.1 Aperture Coupling

The amount of RF energy that penetrates through an aperture is determined by the electricalsize of the aperture (the circumference divided by the wavelength). For large apertures, with acharacteristic size much greater than a wavelength, an external electromagnetic field incidenton the aperture will penetrate directly into the enclosure. For electrically small apertures theenergy transmitted through the aperture can be determined by considering the aperture as anantenna excited by the incident electromagnetic field. The theory of small antennas thenindicates that the coupled far fields increase at 20dB/decade as long as the aperture remainselectrically small.

If the enclosure is relative complete then resonant modes of the cavity can be excited by anelectromagnetic field coupling through the aperture, the fields inside the enclosure are thenstrongly dependent on the modes of the cavity and therefore strong functions of frequency andposition inside the cavity. For these reasons it is difficult to make general quantitativestatements about the level of EMI fields inside enclosures, particularly at high frequencies.However, estimates were made for a number of simple structures based on a transmission linemodel of an aperture and cavity. A simple illustrative example is given below, [6].

Figure 9 shows the electric field shielding effectiveness at the centre of a 300mm by 150mm by300mm metal enclosure of thickness 1.5mm with a 5mm wide slot of varying lengths centredone of the rectangular faces. The shielding effectiveness can be quite small, even forelectrically small apertures, near the resonant frequencies of the cavity. For the 100mm longslot the resonance of the slot itself, at 1.5GHz, also increases the electric fields inside theenclosure, enhancing the effect of cavity resonance at a slightly higher frequency. Figure 10shows how the electric field varies with distance from the slot, for a slot length of 100mm, dueto the resonant modes of the cavity.

In general we can say that for frequencies above 500MHz the characteristic size of typicalequipment enclosures, and the apertures in them, will lead to windows of low shieldingeffectiveness at resonant frequencies. The likelihood for such susceptibility windows in a piece

of 93

of equipment increases with increasing frequency. Table 2 shows the characteristic resonantsize for a cavity/aperture over a range of frequencies. Systems less than ten centimetres in sizehave the potential to resonate when exposed to frequencies in the UMTS band.

-20

0

20

40

60

80

100

0 500 1000 1500 2000 2500

Frequency (MHz)

Shie

ldin

g E

ffect

iveness

(dB

)

25mm

50mm

100mm

Figure 9: SE of an Enclosure with an Aperture for different Aperture Lengths

-20

0

20

40

60

80

100

0 500 1000 1500 2000 2500

Frequency (MHz)

Shie

ldin

g E

ffect

iveness

(dB

)

50mm

100mm

150mm

Figure 10: SE of an Enclosure with a 100mm Aperture for different Distances behind theAperture

of 93

2.2.1.2 Interaction with Internal Electronics

Once EMI has penetrated into an enclosure (or if the equipment does not have a conductiveshield) it couples onto PCB tracks, internal cables, IC pins, etc. This coupling is highlydependent on the details of each specific piece of equipment. However the trend is similar tothat of coupling through apertures in enclosures; internal tracks and cables are more efficientantennas at higher frequencies, with particularly strong coupling at resonant frequencies.

For short cables and tracks a simple estimate of the external EMI coupled into the loads can bemade from excited transmission line theory, appendix G. Consider a track of length (L) and a

height (h) above a ground, place in a space with an effective relative permittivity of er. Theratio of the voltage (VL) induced in a matched load to an incident plane wave of field strength(EEMI) can be calculated using the expression in Equation 5.

rrEMI

L 22 ep

el

p Lfhc

Lh

E

V==

Equation 5

Providing the track is electrically short the coupled EMI is proportional to the length of thetrack, the height of the PCB and the frequency of the EMI. The coupled EMI is seen toincrease at 20dB/decade with frequency until the track is no longer electrically short. For the

maximum track length for which the model is valid, L/l = 0.2, a PCB height of 1mm and aneffective permittivity of 2 we obtain |VL|/|EEMI| = 1.810

-3. A 10V/m incident field willtherefore give a load voltage of 18mV. In reality EMI inside a piece of equipment will not be aplane wave so this model is only suitable for predicting orders of magnitude. Also the modelonly predicts the direct differential mode coupling to a PCB track. In practice common modecoupling may also be important and this needs to be considered on a case by case basis.

2.2.2 Coupling to Wired Networks

The project identified at an early stage the potential for wired networks to guide energy frommultiple radio sources into the equipment at the ends of the cables. Significant effort wasinvested in the construction of EM models for this coupling path, backed up by practicalexperiments. The details of this work are contained in appendix G, H & I, and this sectionsummarises the main findings.

2.2.2.1 Basic Concepts

Currents induced on cables by external EM fields can be decomposed into two modes ofpropagation: differential mode (DM) and common mode (CM). Figure 11 illustrates this

decomposition. The currents i1 and i2 flowing on the two wires at distance l from one end ofthe cable can be described by Equation 6.

of 93

dc

dc

iii

iii

-=+=

2

1

Equation 6

Where id is the differential mode current and ic is the common-mode current.

i i

i ii

i

= +

1 c d

dc2

Total Common Differential

Figure 11: Differential and Common Mode Currents on a Twin Wire Cable

Thus at any cross-section of the cable the differential mode currents on the two wires are equaland opposite whereas the common mode currents are equal and in the same direction. Thedifferential mode currents are almost always the functional currents responsible for carryingthe signal along the cable. The differential mode currents can be approximated as giving rise toa transverse-electromagnetic wave (TEM) along the cable and hence predicted using standardtransmission line theory. However, the common mode currents cannot, in general, be describedby transmission line equations and must be predicted using other techniques. Frequencydomain, moment method techniques are particularly suited to the modelling of distributed wirestructures and two codes based on this method, NEC and CONCEPT were used extensively inthe research for this study.

-40

-30

-20

-10

0

10

20

30

40

50

60

0 2 4 6 8 10 12 14 16 18 20

Position Along Cable (m)

Magnit

ude O

f C

urr

ent

(dB

uA

)

i1

i2

id

ic

Figure 12: Differential and Common Mode Currents on a 20m Twin Wire Cable

of 93

Figure 12 shows the currents induced on a 20m length of 200W parallel wire transmission lineat 1GHz, as predicted by the NEC moment method code. The cable is in free space andmatched at each end. A 1GHz dipole with unity excitation voltage is positioned half way alongthe cable at a range of 1m with optimum polarisation for coupling to the cable. As can be seenthe currents on the two conductors are slightly different and this leads to a small differentialmode current on the cable. However the dominant current along the cable is common mode.

This study investigated many, relative, geometries of cable and source. It was determined thatcommon mode pickup was dominant in the vast majority of configurations with typicalmagnitudes 20 to 30dB above the direct coupled differential mode currents. A single wiremodel of the cable, with a radius characteristic of the overall diameter of the cable, was foundto be adequate for predicting the common mode currents, significantly improving the efficiencyof the modelling process.

2.2.2.2 Common Mode Effects

To understand how common mode currents are generated on a cable, consider Figure 13 whichshows a twin wire cable over an ideal ground plane excited by an incident field. Using imagetheory the cable can be represented as four conductors, with an incident field and its image, asshown. If the space (s) between the wires of a cable is small compared to the height (h) of thecable above the ground plane and the cable is much longer than h, each pair of conductors inthe image model can be represented with a single conductor of radius s. This again produces atwo conductor model, which is open circuit at each end and excited by the original field and itsimage. We can solve this problem using transmission line theory to give the differential modecurrent on the image conductors that are then interpreted as the common mode currents on theoriginal cable (assuming the current is distributed equally among the conductors of the originalcable).

Note that the effective noise sources driving the common mode current are related to the fieldsbetween the image conductors and are therefore much greater than the differential modecurrents. The superposition of the incident field and its image (i.e. ground reflections) will alsocreate an interference pattern in the common mode currents.

Ei

Ei

Ei

Eiimage E

iimage

Ground Plane

Physical Situation Image Model Single Wire Model

Figure 13: Image Theory Model of Common Mode Currents

of 93

The attenuation of the common mode current as it propagates along the cable is governed bythe parameters of the image problem and is in general much lower than that for the differentialmode current. Differential mode loss on data cables, designed for use at less than 100MHz, aretypical several decibels per metre at frequencies above 1GHz, whereas the common mode losscan be less than 1dB/m, see appendix H. The common mode current propagation was found tobe strongly dependent on the installation of the cable. The proximity of the ground plane andpresence of other metallic objects near to the cable provide guidance for the common modecurrent which reduces loss due to re-radiation. Inhomogeneities in the common modeimpedance of the cable can have a large impact on reflections and the propagation of commonmode currents.

The presence of common mode current on a cable does not in itself pose a threat to the integrityof the differential mode data signals. However if any mechanisms exist via which energy canbe converted from common mode to differential mode, then the common mode current canbecome a dominant interference signal. The fundamental mechanism, which gives rise to modeconversion, is electrical or physical imbalance between the signal carrying conductors in acable. Consider Figure 14 that shows how an imbalance in the terminations of a twin wirecable can lead to mode conversion.

ic

ic

id i -c

ci +Z

RL

1

Z2

Ground Plane

ic2id

id

Figure 14: Schematic of Mode Conversion Mechanism at a Cable Termination

The common mode currents Ic are incident on a cable termination with resistance RL. If thecommon mode return paths are perfectly balanced, including parasitic elements, then therewould be no differential mode current through the load. However, if the impedance of the

common mode return paths are different (Z Z1 2 ) then a differential mode current can begenerated through the load. The degree of imbalance between the return paths of the two wiresdetermines the amount of common mode current that appears in the cable load. Thus anyasymmetry in the conductors of a twin wire cable can lead to common mode currentsinterfering with the data signals.

EM models for cable terminations with varying degrees of imbalance were constructed for twinwire cables. The worst case situation was considered to be when one of the cable's wires has alow impedance path onto the chassis of the equipment near the cable load, as this creates alarge imbalance. The amount of mode conversion was then bounded using a model of thissituation and a model with no asymmetry in the termination.

of 93

2.2.2.3 Transfer Functions

The frequency domain modelling results are generally represented as transfer functions betweenthe input terminals of the source antenna (mobile handset) and the loads of network cables andrepresented in dB, as shown in Equation 7. Appendix G describes the transfer functionbetween the input terminals of the source antenna and the threat.

source

load10log20)dB( V

VTF =

Equation 7

These transfer functions encapsulate all the complex electromagnetic effects in a given couplingpath, including mode conversion and scattering.

-100

-90

-80

-70

-60

-50

-40

875 880 885 890 895 900 905 910 915 920 925

Frequency (MHz)

Tra

nsf

er

Funct

ion (

dB

)

Free Space

Ground Plane

Ground Plane & Box

Figure 15: Effect of the Environment and Termination Practice on Mode Conversion

Figure 15 shows transfer functions between the input terminals of a 1GHz dipole and the load

of a 5m length of matched 115W parallel wire cable, for different physical environments acrossthe GSM up-link frequency band. These transfer functions were predicted using NEC and thesource was positioned 1.5m from the cable, half way along its length. The free space case iscompared to the situation in which a ground plane is present 1m below the cable and also whena metallic box is present at the observed load end of the cable. The model of the connectionbetween the cable and the box was unbalanced by allowing one of the cable's wires to have alow impedance path onto the surface of the box (i.e., grounding it to the chassis). The presenceof the ground plane enhances the load voltage due to ground reflections and a small amount ofmode conversion due to the different capacitance between the two wires and ground. Thepresence of a poorly balanced connector is likely to have a considerable effect on the immunityof a system, as it increasing the load voltage by 20dB. For a 2W GSM source this corresponds

of 93

to a load voltage of approximately 90mV. This enhancement of the load voltage, due to modeconversion, was typical of all of the results obtained from the modelling.

-110

-100

-90

-80

-70

-60

-50

-40

-30

-20

200 300 400 500 600 700 800 900 1000

Frequency (MHz)

Tra

nsf

er

Funct

ion (

dB

)

Measured

Simulated

Figure 16: Transfer Function of a Category 5 Ethernet Cable and Wall Socket

A more realistic configuration is considered in Figure 16 which shows the transfer function fora Category 5 (CAT-5) ethernet cable which is terminated at each end by an approved wallsocket. The source was placed half way along the cable at a range of 1m and the cable was 1mabove a ground plane. The results obtained from a NEC simulation are compared tomeasurements on an open area test site (OATS). The results are in good agreement belowaround 850MHz with a maximum coupling of approximately -50dB at 700-800MHz and acoupling of -65dB in the PMR (400MHz) and GSM900 bands.

The discrepancy in the measured and simulated results above 850MHz was traced to theproperties of the ethernet socket and RJ45 plug, which were not modelled in the simulations.When these were taken into account good agreement was achieved up to 1GHz. In fact theelectromagnetic properties of the ethernet plug and socket are highly variable and unstableabove 800MHz. Slight movement of the wires inside the socket can lead to over a 10dBvariation in the transfer function in the GSM900 band.

The transfer functions exhibit much structure at high frequencies due to ground reflections andcable resonances. For sources far from the cable the transfer functions fall off at 20dB/decadewith distance. Closer to the cable the non-planar incident field and distributed pick up alongthe cable make a simple relationship between range of the source from the cable and thetransfer function difficult to establish, particularly when considering relatively narrowfrequency bands.

of 93

2.2.2.4 Twisted and Shielded Cables

Two common methods of reducing the coupling of EMI to cables are the use of twisted cablesand shielded cables. The basic idea of twisted cables is to provide cancellation of any noisefrom external fields or cross talk with other cables by periodically altering the phase of theinduced noise. This is accomplished by altering the orientation of the conductors with respectto the noise source by uniformly twisting them.

Un-shielded twisted pair (UTP) cables were modelled using a bifilar helix in NEC. This was amuch greater numerical task than parallel wire structures so only short runs of cable could bemodelled. However the pick-up of common mode current was found to be insensitive to thetwisting and a single wire model was again adequate to predict these currents. As expected theinduced differential mode current was suppressed by the twist. However the NEC models werefound to be limited for determining differential mode currents.

Shielded cables were also conside

Anh Huong Cua He Thong TTDD

Documents

Transcript of Anh Huong Cua He Thong TTDD