LINKPersonal Communications Programme

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1 LINKPersonal Communications Programme COLLABORATIVE RESEARCH Electromagnetic Compatibility Aspects of Radio-based Mobile Telecommunications Systems FINAL REPORT Engineering and Physical Sciences Research Council Department of Trade and Industry

2 Industrial sponsors: Technical Partners: Produced in 1999 for the LINK Personal Communications Programme by ERA Technology Ltd, Cleeve Road, Leatherhead, Surrey, KT22 7SA, England

3 Page 3 of 93 List of Contents LIST OF CONTENTS... 3 LIST OF FIGURES... 5 LIST OF TABLES... 7 EXECUTIVE SUMMARY... 9 ABBREVIATIONS BACKGROUND INTRODUCTION Electromagnetic Compatibility Trends in the Mobile Telecommunications Market OVERVIEW OF CURRENT AND FUTURE SYSTEMS OBJECTIVES APPROACH RESULTS DETERMINING THE THREAT ENVIRONMENT Defining the Threat Parameters Modulation Schemes Amplitude Modulation Effects Models Signal Statistics Measurements INTERACTION BETWEEN THE EM ENVIRONMENT AND EQUIPMENT Direct Coupling to Stand-alone Equipment Aperture Coupling Interaction with Internal Electronics Coupling to Wired Networks Basic Concepts Common Mode Effects Transfer Functions Twisted and Shielded Cables Statistical Measurements on Ethernet Cables EQUIPMENT IMMUNITY Equipment Level Radiated Susceptibility Testing Component Level Susceptibility REPRESENTATIVE SCENARIOS Hospital Railway Office IT Installation... 58

4 Page 4 of ANALYSIS ANALYSIS OF RESULTS The Hospital Scenario The IT Scenario The Railway Scenario THREAT PROBABILITY Method and Parameters Relation to Modeling and Measurements Statistical Determination of the Threat Relation to Existing EMC Standards and Test Methods Threat Determination for the Representative Scenarios The Hospital Scenario The IT Scenario The Railway Scenario Summary of Threat Probability MITIGATION TECHNIQUES Additional EM Immunity Tests Proposed Specific Free Field Tests Levels Proposed Generic Free Field Tests Conducted Immunity Tests Limit/Control Use of Mobile Phones IMPLICATIONS FOR FUTURE SYSTEMS CONCLUSIONS General For Equipment Designers and Manufacturers For Mobile Telecommunication Systems Operators For Standards Organisations For Administrators and Managers REFERENCES LIST OF APPENDICES... 93

5 Page 5 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 the Aperture...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 Nine GSM900 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 & Figure 27: Envelope of Signal from 3 GSM Phones Operating on Channels 1 3 & Figure 28: TF Between a Source and a Cable, Differential and Common Mode...65

6 Page 6 of 93 Figure 29: Envelope of Signal from 5 GSM Phones Operating on Channels 1, 13, 59, 101 & Figure 30: Envelope of Signal from 5 GSM Phones Operating on Channels 1, 3, 5, 7 & 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

7 Page 7 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

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9 Page 9 of 93 Executive Summary Electromagnetic Compatibility Aspects of Radio-based Mobile Telecommunications Systems is a research study conducted under the Link Personal Communications Programme (PCP). The aim of this study was to quantify the risk associated with unwanted EM interactions between mobile communications devices, such as mobile phones, and surrounding electrical/electronic equipment. In addition to the current situation, emphasis was placed on considering the perceived future position, to do this mobile communications technology and market trends were considered along with predicted developments in general electronics technology. The general approach to this study was to define the EM threat produced by single and multiple mobile communications devices and then determine how this would interact/couple with stand alone and distributed electrical/electronic systems. The predicted threat generated at/within the victim equipment was then compared with current EMC immunity test techniques and levels to determine if they adequately covered the threat posed by mobile communication devices. Susceptibility experiments were conducted on a selection of medical equipment using simulated mobile communications EM environments. To understand and demonstrate typical/realistic situations where mobile communications devices and electronic systems coexist, three different scenarios were developed. The EM threat posed by mobile communications devices has been defined in terms of the main parameters that constitute it and these include; frequency, power, range(s), number of devices and modulation/multiplexing scheme. The threat itself will obviously have characteristics and these 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 and multiple sources and wired/distributed systems. These models have shown that at mobile phone frequencies considerable common mode current can be induced into interconnecting wiring and this can lead to potential interference signals in the cable loads due to mode conversion. It has been determined that the EM threat generated by mobile communication devices will, in many realistic situations, exceed the immunity test limits currently defined in most EMC standards. Limited susceptibility experiments were conducted and it was found that some medical equipment failed when exposed to the EM signals that can be generated by mobile communications devices. Although this highlights potential risks this study has shown that the probability of this situation occurring is relatively small and this is supported by the low number of reported incidences. When assessing the potential future risk both electronic technology within victim equipment and anticipated future mobile communications techniques need to be addressed.

10 Page 10 of 93 All the information suggests that the speed of electronics will continue to increase and the power consumption will continue to reduce as will operating voltages. This advancement in electronics has always been considered as detriment to the EMI performance. This is because higher speed is associated with greater bandwidth, thus more interference energy (noise) will be seen by the electronics, and this coincides with lower signal power and voltage levels and both these effects will reduce the signal to noise ratio. On a more positive note modern electronics is becoming more sophisticated and is likely to be better at handling and/or recovering from transient interference effects. It is believed that the EM threat produced by future mobile communications systems will be less than that associated with current systems. This is based on the assumption that future systems will rely more on base-band signal processing techniques and thus use lower transmission power. This is true for CDMA, which is widely accepted as being the third generation access scheme and will be used in UMTS. Compared to TDMA systems, CDMA will 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 risk associated with unwanted EM interactions between mobile communication devices and electronic systems. Also advice and recommendations have been made for equipment designers, network operators, users and standardisations groups.

11 Page 11 of 93 Abbreviations AM BPSK CDMA CT DART DECT DCS DQPSK DTI DTX EM EMC EMI EPSRC ETSI FDD FDMA FM GFSK GMSK GSM IT LAN PCN PCP PMR PCS QPSK RF RFI TACS TDD TDMA TETRA UMTS Amplitude Modulation Binary Phase Shift Keying Code Division Multiple Access Cordless Telephone Digital Advanced Radio for Trains Digital European Cordless Telecommunications Digital Cellular System Differential Quadrature Phase Shift Keying Department of Trade and Industry Discontinuous Transmission Electromagnetic Electromagnetic Compatibility Electromagnetic Interference Engineering and Physical Sciences Research Council European Telecommunications Standards Institute Frequency Division Duplex Frequency Division Multiple Access Frequency Modulation Gaussian Frequency Shift Keying Gaussian Minimum Shift Keying Global System for Mobile communications Information Technology Local Area Network Personal Communications Network Personal Communications Programme Private Mobile Radio Personal Communications System Quadrature Phase Shift Keying Radio Frequency Radio Frequency Interference Total Access Communications System Time Division Duplex Time Division Multiple Access Trans-European Trunked Radio Architecture Universal Mobile Telecommunication System

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13 Page 13 of Background 1.1 Introduction The Link Personal Communications Programme (PCP) was established to investigate the implications and impact of an expanding mobile communications market. This report addresses Electromagnetic Compatibility Aspects of Radio-based Mobile Telecommunications Systems, which was one of the areas investigated under this programme. EMC Aspects of Radio-based Mobile Telecommunications Systems originally gained approval in March 1995 and work commenced in April The project was funded under the Link PCP with the DTI and EPSRC providing half of the funding and the industrial partners providing the other half. The total funding was approximately 370,000 over the three and a half year project. The industrial partners were BT Cellnet, Nokia, Orange and Vodefone who in addition to providing funding also provided invaluable information about existing and emerging telecommunication 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 Electromagnetic Compatibility The ability of electrical systems to operate in an electromagnetic (EM) environment without adverse effects is known as electromagnetic compatibility (EMC) and this is an established phenomenon in electronics engineering. The reality is that all electrical systems can be disturbed by EM energy if sufficient power is available. For this reason EMC is achieved by limiting/controlling EM emissions in addition to ensuring that electrical systems are sufficiently immune to EM interactions. Mobile communications devices are intentional EM emitters and as such their radiation characteristics (frequency, power, etc) are tightly regulated, by organisations such as the European Telecommunications Standards Institute (ETSI). However, by their very nature the operating location will be uncontrolled and thus the distance between a mobile communications device and a potential victim electrical system is undefined. The power density generated by an EM emitter in free space is proportional to 1/d 2, where d is the distance from the emitter. This means 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 of devices operated in a given area is not and is only limited by the capacity that the service providers offer. EM emissions from separate emitters can combine together, either in free space or within an electrical system, to produce a more severe EM threat.

14 Page 14 of 93 The number of mobile communications devices in use is rapidly increasing and service providers are constantly investigating methods to increase network capacity to meet the demand. Given the known electromagnetic interference (EMI) problems associated with electrical systems it is apparent that the uncontrolled use of mobile communications devices could present potential EMC problems Trends in the Mobile Telecommunications Market In the past 15 years the mobile telecommunications market has expanded from almost nothing into a multi billion pound industry. In the UK the current market penetration is quoted as being about 20%, that is one in five people owns a mobile communications device of some description, most likely a mobile phone. To put it another way there are approximately 10 million mobile phones in use in the UK at the present time. This may seem to be an incredible large market, however the UKs use of mobile phones looks quite limited when compared with other 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 is considered that the use of mobile phones within the UK will increase significantly over the next few years. From section it may be concluded that this increase in mobile phones would be detrimental to the EMC of other electrical systems. Although this is true to an extent, it may be mitigated by the anticipated lower transmission power, in future mobile communications devices.

15 Page 15 of Overview of Current and Future Systems There are many types of mobile radio systems in use today, each designed to meet the requirements 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 (mainly hand-held portables) which are capable of being brought into close proximity to electronic systems in normal usage, producing a variable and unpredictable EM threat. Abnormal usage of systems, or special interactions such as that between a base-station and its immediate environment are not considered, since these interactions are likely to be the subject of installation guide lines and normal EMC requirements. The effects of base-stations may however need to be considered in future systems where cell sizes may be reduced to increase system capacity, thus increasing the density of base-stations. This overview is therefore limited to systems in common, current use and to future systems whose parameters are either known or may be sensibly estimated. One of the main aims of this brief overview is to identify the general system parameters that need inclusion for a meaningful EMC analysis. The major defining parameters of a radio system are the frequency band of operation, radiated power level, type of modulation, multi-user access technique and duplex technique. The EM threat posed by the simultaneous use of several such systems is dependent upon the nature of the time-varying ensemble received signal as observed in a particular (non-radio) bandwidth as well as the susceptibility characteristics of the victim system. It is therefore necessary to evaluate the variation of this ensemble signal on a range of time-scales, including those corresponding to carrier cycles (individual carrier frequencies), phase interactions (total system operating band), system time frame (time division multiple access, time division duplex, packet transmission etc.), signal fading (automatic power level control) and transmitted message characteristics (discontinuous transmission etc). Three "generations" of mobile radio communications systems are commonly defined: first generation analogue systems are still in widespread use but are increasingly being replaced by second generation digital systems. Third generation digital systems, offering major advances in service provision, including and broadband data, are now in the final phase of standardisation. Cellular systems divide the coverage area into a large number of cells, whose size ranges from typically a few hundred metres to a few kilometres, in order to increase radio spectrum utilisation by reusing frequencies in non-adjacent cells. The radio spectrum resource (usually in the form of a number of carrier frequency channels) allocated to each cell may be shared allowing multiple-user access, in a number of ways: Frequency Division Multiple Access (FDMA): Each communications channel occupies a different carrier frequency and hence the number of available carriers must be at least equal to the number of simultaneous users.

16 Page 16 of 93 Time Division Multiple Access (TDMA): Each communications channel is allocated a carrier frequency for only part of the time. Usually the available time is shared into a number of equal time-slots. The system has to remain synchronised throughout. Inclusion of synchronisation and 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 of N times that of a single communication channel. Code Division Multiple Access (CDMA): The available spectrum is divided into a small number (possibly even one, but usually around ten) of high bandwidth channels. Many users are allocated the same channel simultaneously, each being able to decipher their own communications by the use of a large number of non-interacting (orthogonal) digital codes. An essential requirement for successful operation of CDMA is that all the signals for communication with a particular base station must arrive at that base station with approximately equal power levels. Some systems use a combination of the above methods to improve capacity. Simultaneous two way communication is achieved by one of the following techniques: Time Division Duplex (TDD): The mobile station and base station each transmit in different time intervals. Frequency Division Duplex (FDD): The mobile station and base station each transmit on different carrier frequencies. The main technical parameters of a number of systems both in use and proposed are summarised in Table 1.

17 Page 17 of 93 UK TACS PMR Trunked Digital PMR GSM900 CT-2 IS95 US CDMA CT3 DCT900 DECT DCS1800 TETRA UMTS Generation PCS Primary Use Analogue Cellular Analogue Telephony Digital Data Digital Cellular Digital Cordless Digital Cellular Digital Cordless Handset Freq (MHz) Peak Power (mw) Mobile Power Control Yes No Yes (Slow) 2dB steps Multi-Access FDMA FDMA FDMA 4TDMA TDMA FDMA 250 Yes Yes No FDMA CDMA(DS) FDMA No TDMA FDMA Digital Cellular PCS Digital Cellular Digital Cellular Yes (Slow) 2dB steps TDMA FDMA TDMA FDMA Yes (Fast) 0.25/1.5dB steps W-CDMA FDMA Duplexing FDD FDD FDD FDD TDD FDD TDD FDD FDD/TDD Modulation (Up-Link) FM FM QPSK GMSK GFSK BPSK QPSK Frame Duration (ms) N/A N/A N/A Channel Spacing (khz) Table 1: Summary of Current and Emerging Mobile Telecommunications Systems GFSK GMSK DQPSK Dual Chan QPSK µs

18 Page 18 of 93 Table 1 summarises the main systems in use in the UK at the present time. The main threats to be 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. For example, in the GSM900 system, mobile channels may be spaced by up to 25MHz giving rise 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 the specified maximum power ramping rate and the maximum power level change between time-slots. GSM, DCS and TETRA employ power management to conserve battery power. For the GSM systems the mobile transmitted power is adjustable in steps of 2dB between each time-slot, with a dynamic range of 30dB. Note that for a system using 1:N TDMA the average mobile transmitted power is 1/N. For example GSM handsets with a peak power of 2W have an average 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 power level. Also summarised in Table 1 are later, second generation systems, some of which are often termed generation two-and-a-half systems, these offer enhanced personal communication services. Details of the third generation UMTS have not yet been finalised although it is known that 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 anticipated that the ensemble signal received by a victim circuit would be noise-like, lacking in any of the coherent effects, which may be observed in narrow-band systems. In UMTS there will, however, be a received power variation associated with the time-slot duration and the fast closed-loop power control. The power control maintains a constant received signal/noise ratio at the base-station. However at other locations the received signal will consist of the power transmitted by the mobile handset (controlled by fading on the mobile to 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 been reviewed with emphasis on the parameters which, when the systems are used in a congested environment, are likely to contribute towards an EM threat.

19 Page 19 of Objectives The objective of this study was to determine and quantify the potential for unwanted EM interactions from mobile communication devices to surrounding electrical systems and suggest practical mitigation techniques where appropriate. The study was also to consider the projected future usage of mobile communications and technology such that the future threat could be assessed. Only mobile communication devices were to be considered in this study as fixed installations should be controlled and regulated in accordance with existing guide lines. Also only interactions with electrical systems were to be considered, personal safety issues were only considered 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 three separate 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 be accurately characterised. However if more than one device is operated in close proximity the individual emissions will interact to generate a combined EM environment. This combined environment is unlikely to be a simple addition of the individual sources it represents as relative distances, signal frequency and phasing will all determine the resultant environment. Being able to define the EM environment when more than one mobile communications device is in operation is considered to be a major component of this work programme.

20 Page 20 of Approach Originally the project was divided into five work packages which could initially be investigated separately, 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 for the first two years of the project these packages were essentially investigated separately. At this stage the project was reviewed and subsequently restructured to ensure that the various work packages would converge, thus allowing the programme objectives to be fulfilled. The modified structure was intended to act as a framework for the final report as this would provide clear direction and allow shortcomings to be identified early. The outline project plan implemented during the final 18 months of the project is shown in Figure 1, and from this it can be 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 electronic equipment 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 potential victim systems had been established, equipment immunity tests were conducted. The three initial topics listed above and the equipment immunity tests therefore form the main results section of the final report. Using the information and results obtained from the four areas highlighted above, it was possible to analyse different situations to determine if there were any potential risks. Also by understanding how the threat is generated and coupled into systems it is possible to suggest mitigation techniques to reduce any potential threats.

21 Page 21 of 93 Project Plan Determine the threat environment Frequency Band(s) Worst case ERP Modulation characteristics Min distance from victim equipment Constructive / destructive interference Consider future systems Identify Relevant coupling channels / mechanisms Dominant channel for each scenario/equip type Coupling channels / mechanisms (& Levels) Consider frequency band(s) Source - Free field radiation Direct coupling to PCB / devices Coupling to interconnecting cables Coupling to local cables Coupling directly to antennas (out / in band) Consider a representative selection of scenarios Hospital Railway (track, station, concourse) IT Network Type / amount of interconnecting cables Type / amount of electronic equipment Importance of equipment Typical layout Consider worst case scenario + probability Immunity/Susceptibility assessment Use modeling to reduce scope / amount of testing Test representative selection of equipment Conduct a risk assessment for typical scenarios Consequence of a failure Hospital - (loss of life) Railway - (loss of life / economic) IT Network - (economic) Likelihood of a failure Reported incidences in known environment Immunity/susceptibility tests on various equipment Consider future technology Identify/Quantify risk areas Make recommendations on risk reduction Identify predominant interference parameter Provide guidance on test techniques Figure 1: Project Plan

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23 Page 23 of Results 2.1 Determining the Threat Environment In this section the parameters of the RF environment which constitute an EMC threat to nonradio 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 defining individual features of the RF threat The summation of individual threat components in realistic situations is discussed in section 2.4 whilst the probability that these threats will add up to an actual EMC problem is the subject of section 3.2. The concept of an elementary isotropic source of radio waves is used throughout this report and is therefore defined here. An isotropic source is one which emits radiowaves equally in all directions. 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 and provides approximate estimates of field strength for radiating elements which are short compared with the wavelength of radiated waves. Radiation from an isotropic source may be visualised as follows: Figure 2: Radiation from an Isotropic Source For an isotropic source in free space the radiated power spreads out equally in all directions and therefore the power density, P d W/m 2 at a distance d m from the source is given as: P d PT = 2 4 πd Equation 1 Where P T = total radiated power in Watts

24 Page 24 of 93 For free field impedance the corresponding electric field strength (E) is given by: E( V / m) = 30 P d T Equation 2 A typical "threat element" may be envisaged as shown in Figure 3. z VICTIM LOAD SOURCE y x Figure 3: An Element of a Threat Scenario (Threat Element) In section the parameters of the above threat element are summarised and in section 2.4 a number of typical scenarios are constructed using ensembles of these elements. A general description of the features of a modulated RF carrier which may lead to EMC problems in wired systems is followed by discussion of modulation schemes in common usage as well as those planned for future systems. The methods used to model and simulate these modulation schemes are presented along with a discussion of likely signal fading and statistics of a single RF emitter, such as a mobile radio handset. A method of assessing the probability associated with individual and combined risk parameters is presented in section 3.2. Finally, a series of measurements is described and related to the modelling. It is concluded that the models used are capable of simulating the ensemble signal due to a realistic disposition of mobile radio handsets, provided that suitable input parameters are used Defining the Threat Parameters In Section 2.2 it will be shown in detail how energy from EM waves can couple into conductors to produce EMC problems. A simplified appreciation of the main characteristics of these coupling methods is required in order that modulated RF carriers may be assessed in terms of their threat parameters. Without elaborating on the details of coupling mechanisms at this point, it may be shown that, in general, the coupling between a RF field and a simple wire conductor 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.

25 Page 25 of 93 Each of these parameters is itself dependent upon a large number of other factors. The first obviously depends upon the length of the conductor but is also dependent upon the detailed spectrum of the radiated field which is in turn dependent upon signal modulation and other amplitude and phase effects. The incident field strength is dependent upon all of the factors related to transmitted power, amplitude modulation effects, distance between the conductor and the RF source and the relative geometry of the field and conductor. Many of these parameters can only be accurately described in a statistical manner. Equation 3 illustrates the relationship between the main parameters that contribute to the threat. t = k N P. p n n n= 1 Zone. n c n. dl Equation 3 Where: t = threat k = constant of proportionality N = number of emitters (handsets) P = transmission power of handset n p = propagation loss, including statistics c = coupling from field to load, including statistics Zone = length over which a significant interaction may occur The following paragraphs describe each threat parameter in more detail and relate each to observable characteristics of real radio systems. Frequency band, channel and system bandwidth: The overall spectrum allocated to the system governs the bandwidth of possible phase interference effects since the maximum beat frequency 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 inversely proportional to this bandwidth. In GSM, for example, this bandwidth is 25MHz, giving the possibility of pulse-like events of 40ns duration. The distributed nature of coupling into fixed conductors and of propagation along those conductors may further compress this minimum feasible pulse duration, see appendix G. Distance of RF source from victim conductor: For communication between two isotropic antennas in free space the received power is inversely proportional to the square of the distance between the antennas. If both antennas are situated above a simple infinite reflecting ground the received power varies in a more complex way due to interference between the direct and reflected wave and at large distances become inversely proportional to the fourth power of the distance. In the scenarios considered in this study (see section 2.4) the following conditions generally 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 of the victim conductor.

26 Page 26 of 93 The models used in this study consider the radiation field and include reflections, close range induction effects have not been considered, see appendix B. Transmitted Power: Since the coupling techniques considered here are linear, the threat magnitude will be related to the transmitted power for an individual emitter. In determining the feasibility 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 specifically considered in this study since although they have much higher power levels, they are fixed in position and their installation is controlled by established EMC and radio regulatory legislation. Modulation scheme: Each type of modulation has its own characteristic radiated spectrum and those used in modern cellular systems are chosen for their excellent radio spectrum utilisation characteristic. In this study we are not concerned with amplitude modulation schemes whose spectrum may contain data-specific components. Modulation schemes considered include GMSK, GFSK and DPSK, which are all constant-envelope schemes. Appendix D contains a detailed analysis of GMSK modulation with emphasis on the phase addition for signals from a number of emitters. The potential threat is will be related to the carrier frequency and the overall system bandwidth; higher carrier frequencies and bandwidths give rise to greater rates of 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. In a simple FDMA system the overall system bandwidth has to be large enough to accommodate the 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 channels have in excess of N times the bandwidth of one message channel but there are less radio channels available. More importantly, a receiver in the vicinity of a mobile station will typically only receive power from one or two time-slots within the TDMA frame, causing an amplitude modulation effect. More complex systems may use a combined CDMA/TDMA approach, again resulting in amplitude modulation of the signal received at a non-intended location. Fast power control will also 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 modulation bandwidth. For example in UMTS, transmitted power increments may be between 0.25dB and 1.5dB depending upon cell size. Control signals are sent at a rate of 800 per second, resulting in 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 of 100Hz 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 the up-link and down-link channels. The use of TDD (in CT2 and CT3 for example) generally increases the required traffic channel bandwidth by a factor of two with consequent EMC implications as described above.

27 Page 27 of 93 There are a range of other modulation-related effects that have implications for the EMC threat posed by a congested RF environment including DTX, power management and the rate of power ramping. These effects, often causing amplitude modulation of the received signal, are discussed in more detail in Section Modulation Schemes Throughout the history of radio communications, modulation schemes have been continuously developed in order to improve the performance and spectral utilisation. In recent years smaller scales of circuit integration and faster DSP devices and techniques have meant that very complex 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 the modulating signal. In more complex systems the introduction of separate channels for signalling and multiple access techniques, means that in practice the behaviour of the RF carrier cannot be described using simple, theoretical types of modulation. In order to examine the 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 are discussed in this section, and other sources of amplitude modulation which may arise due to system considerations, which are discussed in the following section. Constant envelope techniques such as PSK and FSK have been refined by stages to reduce spectral spreading due to abrupt instantaneous phase or frequency changes, leading to minimum shift keying (MSK) and gaussian minimum shift keying (GMSK). In GMSK, the bandwidth of the gaussian filter (B), relative to the data rate (1/T), can be chosen to reach a desired 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 3.69µs and a filter bandwidth of 90.3kHz. A typical spectrum of a GMSK signal (derived from simulation) is shown in Figure 4. Frequency (khz) Figure 4: Simulated Spectrum of a GMSK Signal

28 Page 28 of 93 In simple terms the spectrum of Figure 4 can be considered to repeat every 200kHz across the spectrum allocated to GSM. It will be noted that this spectrum differs from the smooth curves often shown in text books, which are normally a theoretical average over all possible combinations of input data. It may be seen that low frequency structure may be introduced into the signal spectrum by repetition of data sequences but this effect does not involve significant envelope amplitude changes and is therefore not considered. In the case where each carrier may be considered to be a narrow band signal, amplitude changes would be expected due to phase effects between multiple carriers when the receiver (in this 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 interactions which 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 produce a 40kHz beat frequency (N=5, K=20kHz, m=2, N-m=3). In general there would be no welldefined phase relationship between the carriers and the simultaneous addition of all carriers in phase becomes progressively less likely, although of greater effect, as N increases, see appendix K. In practice, the signals are not narrow band and each is spread over approximately 200kHz by the (independent) modulating signals. Multi path propagation and motion of mobile handsets further 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 a victim conductor. These practicalities are discussed when defining realistic scenarios (section 2.4) and presenting results for each scenario (section 3.1). Measurements and realistic simulations are described in appendices E and F respectively. Amplitude modulation due to system TDMA and power control is described in section Other forms of constant envelope modulation used in modern mobile radio systems, such as GFSK, 4-QPSK and DQPSK have spectra similar to that of Figure 4 with the bandwidth depended upon the transmitted symbol rate. A further class of systems deserving particular consideration is that including planned thirdgeneration 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 Table 1. 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 is spread over a wide bandwidth and the desired channel signal-to-noise ratio is achieved as a consequence of coherent, matched, detection. The spectral density of signal energy due to a single channel is therefore much less than in a narrow band system (In simple terms, the energy is reduced by the processing gain, which in UMTS is 128, i.e. 21dB). Whilst, for comparable fully occupied narrow band and CDMA systems the signal power spectral density may be comparable, for a lightly loaded CDMA system the signal power spectral

29 Page 29 of 93 density will be much less, possibly comparable with natural or man-made radio noise levels. 2. The probability of significant phase coherent addition of signals from multiple users is also much reduced compared with the conventional case and the coupled energy associated with any such additions is much smaller due to the higher chip rate. CDMA systems are discussed further in section 3.4 and the TDMA aspects of UMTS are discussed in section Amplitude Modulation Effects Radio systems utilising constant envelope modulation schemes can produce amplitude modulation of the RF carrier in a number of ways, most of which are consequences of system operational 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 the operational frequency range of the victim electronic equipment. In the cellular mobile radio systems considered here, amplitude modulation may be produced due to the TDMA framing structure and due to adaptive power control. The TDMA structure may produce amplitude modulation with a fundamental frequency equal to the frame rate, due to differing amounts of power received from each time-slot in general. In a fully utilised system, at the base station (only) the power control will strive to maintain equal received power in each slot, this is particularly critical in CDMA systems. However, if some slots are not used, or if, as is the general case, the unintentional receiver is not co-incident with the base station, amplitude changes of a repetitive nature will occur, this effect has been verified by experiment, see section The frame rate associated with commonly used systems is therefore 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 medical equipment. In general there will always be the possibility of specific interactions between individual frame rates and equipment although, due to the noise like nature of CDMA, it is likely 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 the power transmitted from a mobile to maintain constant power at the base station, a receiver close to the mobile will observe the resulting amplitude changes in their entirety. It is therefore necessary 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 800 changes in transmitted power per second. If these power steps are all in the same direction and are of maximum allowed size, the signal amplitude could be changing at 1200dB per second.

30 Page 30 of 93 Again in a worst case, if a mobile was moving through a faded field at 60mph it would experience 60 fades per second (it may be shown that fading at the 10dB level occurs at a rate which 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 TDMA structure. However, in this study we are concerned predominantly with close range effects and it has been shown that the zone of interaction, which may produce EMC effects of the type considered, is limited to a few metres from the victim conductor. Whilst the effects of the above fading and consequent power control may be experienced by vehicle-borne systems, it is not physically possible to move at the speed described above and still remain within the zone of interaction of a fixed conductor for any significant time. It is therefore concluded that power control as a direct effect is of secondary importance, and that amplitude modulation, in the context of this report, will arise predominantly from the TDMA structure and associated slow power control. Other effects, such as DTX may also be considered using the same argument since the operation of DTX will be different in different time-slots. Randomisation of the usage of time-slots by frequency hopping will also tend to suppress the effect of the fundamental frame rate Models The study involves the modeling of individual and ensemble signals transmitted from mobile handsets, with a view to characterising the features which pose a threat to other electronic/electrical devices operating in the surrounding environment. With this in mind a GMSK modulation simulator was implemented. Since this would be representative of the majority of current second-generation digital systems using the GSM standard. At the time the study was undertaken it seemed likely that the third generation systems would also adopt a GSM type protocol but the decision has since been made in favour of W-CDMA. The implications of CDMA have been discussed above. The GMSK simulator was based on a quadrature, baseband architecture and a representation of this is shown in Figure 5. Figure 5: GMSK Modulator, based on a Quadrature, Baseband Architecture