carried by Thomas Theiler. Thomas is greatly acknowledged ...... [39] Arnold S. Rosenbaum, Frederick E. Glave, âAn Error-Probability. Upper Bound for Coherent ...
On Digital Radio Receiver Performance in Electromagnetic Interference Environments Peter Stenumgaard
RADIO COMMUNICATION SYSTEMS LABORATORY
On Digital Radio Receiver Performance in Electromagnetic Interference Environments Peter Stenumgaard May 1999 TRITA - S3 - RST - 9906 ISSN 1400-9137 ISRN KTH/RST/R--99/06 --SE
RADIO COMMUNICATION SYSTEMS LABORATORY DEPARTMENT OF SIGNALS, SENSORS AND SYSTEMS
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Abstract adiated emission from electronic equipment, co-located to a digital radio receiver, can affect receiving performance. It is therefore of great importance that this undesired emission be considered in the early design phase of a system containing radio equipment. For this purpose, methods to estimate the performance degradation on digital radio receivers in such environment must be available. From a military point of view, such methods are necessary for at least two important situations; 1) The communication system is not subject to interference from hostile jammers. In this case, undesired electromagnetic interference will decrease the operating range of the radio link. 2) The communication system is subject to interference from hostile jammers. In this case, the ability to withstand jamming is degraded by the undesired interference, as the latter degrades the signal protection devices in the communication system. A tactical consequence of this is that the jammer can obtain the same result at a larger distance than if no inherent interference is present. In this thesis, a method of performing this kind of performance analysis is proposed when the interference from co-located equipment is expressed in parameters from international standards concerning maximum allowed radiated interference limits. As existing standards were developed with respect to analog radio receivers, a method of relating these standards to digital radio receivers is needed. A method to relate an interference level to the bit error probability of a digital radio receiver exposed to that interference is proposed and evaluated. It is shown that the method proposed gives an estimated bit error probability that is close enough to the true value to be used in system design applications. The method is shown to give satisfactory results for both the non-jamming and the jamming cases With this method it is possible to compare the impact of different standard emission limits and, for instance, make a trade-off between economics and the tactical demands on a complex system.
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Acknowledgments
efore I started the work towards this thesis, I had heard that research is not a one-man job. Now that statement has become a part of my own experience. A number of people have inspired and supported me during this work. First of all, I would like to thank my adviser, Professor Jens Zander. Without his scientific guidance and enthusiastic encouragement, this thesis would never had been written. All of my colleagues at the Department of Communication Systems at the Swedish Defence Research Establishment provide a positive and stimulating environment to work in and are acknowledged for that. I would like to thank Lars Ahlin, the head of the Department, for giving me the opportunity and encouragement to perform this work. My project manager, Björn Johansson, and near colleague, Kia Wiklundh, are also greatly acknowledged. The vision that resulted in the problem investigated in this thesis was carried by Thomas Theiler. Thomas is greatly acknowledged for his never-ending enthusiasm that finally influenced me to start working with this problem. The encouragement and support from my wife, Helena, cannot be overemphasized. Furthermore, my two children, Sara and Jacob, have provided me with the necessary relaxation from my research work. Finally, I would like to thank the Swedish Defence Materiel Administration for financing part of this work.
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Contents 1 INTRODUCTION...................................................................................... 9 1.1 DIGITAL RADIO SYSTEMS IN ELECTROMAGNETIC INTERFERENCE ENVIRONMENTS ........................................................................................ 9 1.2 PROBLEM OVERVIEW AND THE INTENDED STRATEGY FOR SOLVING THE PROBLEM .............................................................................................…14 1.3 CONTRIBUTIONS ...................................................................................... 20 1.4 THESIS OUTLINE ...................................................................................... 22
2 INTERFERENCE SOURCE MODELS................................................ 23
3 EMI DETECTORS .................................................................................. 29 3.1 GENERAL ................................................................................................. 29 3.2 THE QUASI-PEAK DETECTOR .................................................................... 30 3.3 THE PEAK DETECTOR ............................................................................... 32 3.4 THE DEVELOPMENT TOWARDS NEW DETECTORS ...................................... 35 3.5 SUMMARY OF DETECTOR CHARACTERISTICS ............................................ 35
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4 METHODS FOR THE ESTIMATION OF BEP.................................. 37 4.1 APPROACH ...............................................................................................37 4.2 METHOD FOR BROAD BAND INTERFERENCE ............................................ 41 4.3 METHOD FOR NARROW BAND CW INTERFERENCE .................................. 44
5 ANALYSIS FOR THE NON-JAMMING CASE.................................... 45 5.1 INTRODUCTION ........................................................................................ 45 5.2 THE MSK MODULATION SCHEME ............................................................45 5.3 ESTIMATION OF BEP FOR MSK IN A MIXTURE OF GAUSSIAN AND CW INTERFERENCE ........................................................................................ 48 5.4 RESULTS FOR THE NON-JAMMING CASE ................................................... 49 5.4.1 Comparison of BEP for the ideal receiver with measured BEP on a real system....................................................................... 49 5.4.2 Results for the non-jamming case .................................................. 53 5.5 SENSITIVITY ANALYSIS ........................................................................... 57
6 ANALYSIS FOR THE JAMMING CASE.............................................. 61 6.1 INTRODUCTION ........................................................................................ 61 6.2 JAMMERS ................................................................................................. 62 6.3 IMPACT OF INTERFERENCE ON THE DUEL BETWEEN JAMMER AND COMMUNICATION SYSTEM ....................................................................... 63 6.3.1 The narrow band method in the CW tone jamming situation ...... 63 6.3.2 The narrow band method and Gaussian approximated jamming signal ................................................................................................65 6.4 RESULTS FOR THE JAMMING CASE............................................................66
7 PERFORMANCE MEASURES FOR TACTICAL CONSIDERATIONS ................................................................................ 69 7.1 BACKGROUND ......................................................................................... 69 7.2 THE IMPACT ON OPERATING RANGE AS A MEASURE OF PERFORMANCE DEGRADATION IN THE NON-JAMMING CASE ............................................ 70 7.3 THE ”JAMMING LOSS” AS A MEASURE OF AN INCREASED VULNERABILITY TO A HOSTILE JAMMER ........................................................................... 72 7.3.1 Definition of jamming loss..............................................................72
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7.3.2 Example of jamming loss for a system using error correcting code................................................................................................... 73
8. DISCUSSION ............................................................................................83
APPENDIX A ................................................................................................87
APPENDIX B................................................................................................. 91
APPENDIX C ................................................................................................95
APPENDIX D ................................................................................................97
REFERENCES ............................................................................................101
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1 Introduction 1.1 Digital radio systems in electromagnetic interference environments
echnological advances in the electronics industry are extremely rapid, resulting in far-reaching consequences for practically every single individual and activity in society. These developments are largely based on the demands and possibilities of civilian society. This is a situation that will also, to a large extent, guide military development and the feasibility of designing command and control systems within the armed forces in years to come. From a historical point of view, military technology has always retained a pole position in the application of new technology. In the current situation, the technical developments on the civilian market have caught up on the military technology in a number of fields. Together with reduced defence budgets, this situation opens up completely new possibilities for the armed forces to use civilian electronics in military applications. This so-called dual use technology will probably be a reality in the future, leading to an increased amount of different civilian electronics, such as information technology equipment (ITE), in the vicinity of military radio systems. In military applications, the use of ITE in the vicinity of communication systems is rapidly increasing. This is due to the increased need of quick and accurate information for command and control in a battlefield characterized by fast changes. After the end of the cold war, a shift in military philosophy has
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occurred. Military planners now realize that the era of prolonged wars is long gone. Increasingly, military forces are relying on superior technology and less on manpower to detect and combat hostile threats [1]. One key issue is Dominant Battlefield Awareness (DBA), which in simple words means having the best knowledge of what is going on in the battlefield. The future battlefield will require several new services to support the battle command. The ability to visualize the battlefield by accessing broadcast data is one example of requirements considered in the development of future command and control systems, see figure 1.1. This requires the ability to manage sensors efficiently, so that sensor data can be collected, processed and fused before displaying these to the commanders.
Figure 1.1: The use of ITE in the vicinity of communication systems is rapidly increasing due to the increased need of quick and accurate information for command and control.
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The goal is to create a common picture of the battlefield so that a rapid and effective concentration of combat power can be provided. This vision is often described in terms of digitizing the battle field as an increased share of overall defence spending will be paid on systems for control, command, communications, computers and intelligent sensors. From a communication performance point of view, these different electronics will contribute to the electromagnetic interference (EMI) environment at the radio receiver. In general, the electromagnetic environment surrounding a digital radio receiver consists of different kinds of interference sources. The total interference is a mixture of natural interference, such as atmospheric interference, and man-made interference. Roughly speaking, man-made interference sources can be divided into intentional and unintentional, see figure 1.2. Intentional sources are other transmitting equipment which typically works with some kind of modulated signals and whose interference typically consists of harmonics and intermodulation products. As intentional sources consist of colocated transmitters, hostile jammers are not included in this category. Unintentional sources are other electronic systems which are not intended to produce any radiated electromagnetic energy. As radiated emission from electronic equipment, co-located to a digital radio receiver, can affect receiving performance, it is of great importance that this undesired emission be considered in the early design phase of a system containing radio equipment. For this purpose, methods of estimating the performance degradation on digital radio receivers in such environment must be available. From a military point of view, such methods are necessary for at least two important situations; 1) The communication system is not subjected to interference from hostile jammers. In this case, undesired EMI will decrease the operating range of the radio link [2]. 2) The communication system is subjected to interference from hostile jammers. In this case, the ability to withstand jamming is degraded by the undesired interference, as the latter degrades the signal protection devices ( e.g. error correcting codes) in the communication system. A tactical consequence of this is that the jammer can obtain the same result at a larger distance than if no inherent EMI was present [3]. In this thesis, a method of performing these kinds of performance analyses is proposed.
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Most published system design methods considering the impact of radiated interference on digital radio systems have so far been focused on the impact from other transmitting systems in the vicinity of the radio receiver. The most extensive work concerning complex systems, such as aircraft and ships, has been carried out under the sponsorship of the Department of Defense (DoD) in the United States. The work was initially performed by the DoD Electromagnetic Compatibility Analysis Center (ECAC). During the 1970s, an automated procedure for topside communications RF system design was developed for the Naval Command Control and Ocean Surveillance Center (NOSC) [4] [5]. The first published algorithms to be used in EMC design tools was the Co-Site Analysis Model (COSAM) [6]. COSAM was the basic design tool on which several further developments were based. COSAM handled interference from intentional transmitters on analog victims, see figure 1.2. Since then, the co-location problem between intentional transmitters and receivers has been well investigated [7] - [14]. In the late 1970s, a more
Sources intentional transmitter
Victim
unintentional transmitter Figure 1.2: The definition of interference sources and the victim.
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general EMC design tool called the Intrasystem Electromagnetic Compatibility Program (IEMCAP) [15] was developed. In this design tool, it was possible to consider interference from electronics which are not intentional transmitters. The victim was analog communication systems and the basic interference criterion was to compare the interference power to a certain power threshold level in the receiver of the victim. The analog victim was treated in publications as late as in the middle of the 1990s [16] [17] [52]. At the same time, the digital radio receiver as a victim began to be treated in published EMC system design tools [17]. Even if a lot of theoretical work has been carried out on interference on digital receivers since the 1960s, methods suitable for engineering purposes did not start to show up until the 1990s. This work was a further development of the system design algorithms developed during the 1970s and 1980s. In [34], the problem of estimating the bit error probability (BEP) for a digital radio receiver, for a known interference signal is treated. This is done by determining the probability density function of the interference signal and then calculating the BEP. How to determine the BEP for a digital radio receiver subjected to a known interference signal has been well known for a long time, but during the 1990s methods suitable for others than scientists started to be published. However, no research has been published on how to estimate the BEP for an interference waveform, only specified as the result of a standard emission measurement. Previous work relies on the basic assumption that the system designer has detailed knowledge about the different interference levels and wave forms present in the actual system. In this thesis, the case when no such information is available is treated. This case corresponds to the early design phase of a complex military system, when not all hardware has been developed. Up to only a few years ago, the financial situation for the military forces allowed the designer to always put the strongest emission requirements on all electronics in a military system. Thus, several potential interference problems caused by unintentional interference sources were solved by the wallet. Today, the financial situation does not allow this solution, which is why a method for quantification of the impact from radiated emission on digital radio receivers is needed.
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1.2 Problem overview and the intended strategy for solving the problem In the early design phase of electronic systems, different electromagnetic compatibility (EMC) requirements are chosen for all subsystems and equipment. The purpose of applying these requirements is to minimize the risk of interference problems in the system. These requirements are normally divided into two groups; emission and immunity requirements. In this thesis, emission requirements are treated. Emission requirements control the maximum allowed levels of electromagnetic interference produced, while immunity requirements control how much electromagnetic interference the systems must be able to withstand without performance degradation. From the designer’s point of view, it is very important that all requirements be thoroughly chosen so that an accurate trade-off is made between economics, and the risk of running into interference problems. Furthermore, when all requirements have been chosen, and the production phase is going, it is normally very difficult to change EMC requirements without considerably increasing the cost. A lot of standardized EMC requirements have been developed during recent decades, including both civilian and military standard EMC requirements. In general, the military standards have higher requirements than civilian ones, which is why military-specified electronics require more expensive measures to fulfill such requirements. From a military point of view, the interest in civilian radiated emission limits has increased as a result of the dual use situation. As the choice of radiated emission requirements is of major concern in system design, interference levels that equal such limits are considered in this thesis. This implies a worst case analysis with respect to the emission level. The background of emission standards started in the 1920s, when broadcasting services started to reach a larger part of the society. Quite early it became obvious that radiated interference had to be limited in order to create good conditions for the reception of these new services. However, imposing limitations on electric machines, household appliances, etc. could cause trading problems if different countries applied different requirements. This problem was soon realized on national levels, which led to the foundation of the International Special Committee on Radio Interference (CISPR). The International Electrotechnical Commission (IEC) and the International Telecommunication Union (ITU) were cofounders. The first goal was to reach an agreement on measurement pro-
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cedures. This work was carried out during the 1930s. After that the work of developing standard emission limits could start [18], [19]. The first standard produced was at a national level when the BS613 (British Standard) concerning components for radio interference suppression devices was published in England. In 1937, the BS727 concerning characteristics of radio interference measuring apparatus was published. This standard had a large impact on the standardization work within CISPR. Today, a large variety of radiated emission standards exists. Standards for intentional transmitters differ from standards applicable to unintentional transmitters. Depending on the frequency region of interest, maximum limits on magnetic or electric field is usually specified . In general, magnetic field is specified at the lower frequency regions while electric field is specified in the higher frequency bands. The frequency limit between magnetic and electric field specification depends on the specific standard. In the commonly used military standard , MILSTD-461D, electric field is specified at frequencies above 10 kHz, while the civilian European EN55022, required for electronics sold in the European Union, specifies the electric field at frequencies above 30 MHz. Every standard has special requirements on the measurement setup and measurement procedure. In figure 1.3, a simplified description of the test setup for the emission test RE102, in MIL-STD-461D, is shown. The equipment under test (EUT) is placed on a ground plane. Cables for powering and monitoring are connected according to special requirements. During the test the EUT is powered up and set into normal operation modes while the radiated interference is measured with the antenna. The antenna is connected to a measurement device supplying a special measurement detector. The measurement device often consists of a spectrum analyzer which works with a superheterodyne receiver. In order to make a performance analysis, a performance measure must be selected. In this thesis, the BEP of the digital radio system is used. The reason is that the BEP is a common measure of the quality of the information received in the radio, and is furthermore used in system specifications. Other related measures such as block error rate and message error rate can also be specified depending on the specific system properties. However, the bit error probability is often the basic system performance measure, from which other measures can be determined. Another type of performance measures is, what could be called the hardware dependent measures. This means performance degradations caused by hardware limitations, due to the fact that in reality ideal components do not exist. An example is reciprocal mixing which is due to interference caused by mixing of the receiver’s local oscillator noise with an
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interfering signal in the receiver front end pass band [20]. Another kind of performance degradation is desensitization. Desensitization occurs when a strong interference signal causes an apparent decrease in receiver sensitivity [21]. As methods of analyzing these kinds of hardware-dependent effects are available, they will not be treated in this thesis. Consequently, choosing BEP as the performance measure of interest, it must be assumed that the hardware related degradations can either be analyzed with known methods or be neglected. If the BEP caused by a certain emission level, can be estimated, this value can be compared to certain acceptable limits, depending on the kind of information transferred on the radio channel. Examples of typical limits for the BEP can be 10−3 for speech and 10−5 for data transfer. If an interference problem occurs, there are generally two basic principles for solving the problem. The first is just to increase the physical distance between the interference source and radio receiver. The second is to decrease the emission level by means of an electromagnetic shielding device. As this is just a practical question, assuming that the BEP can be estimated, the variation in separation distance will be used to evaluate the results. In some cases, the interference problem could be handled on a higher system level by the use of forward error correcting codes. However, allowing inherent interference from co-located electronics to be handled in that way, is hazardous as the error correcting code is always implemented to take care of other problems such as hostile jammers or changes in the radio channel. Antenna 1m
EUT Ground plane
120 cm
80 - 90 cm
Ground plane Figure 1.3: Simplified figure of the measurement setup during a standard radiated emission measurement according to the MILSTD-461D.
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If the error correcting code can handle the interference from co-located equipment, the user will not recognize that an interference problem is present, until the problem, for which the error correcting code is intended to handle, shows up. Thus, in this thesis we will not count on the possibilities to handle interference from co-located equipment by the use of error correcting codes. The assumptions above lead to the conclusion that the issue of interest is how to translate an emission level to BEP for a digital radio receiver at a given distance r from the interference source (see figure 1.4 and 1.5). No method for this specific purpose has been published. Present radiated standard emission limits are in most cases specified as maximum allowed electric (or magnetic) field strength as a function of frequency and at a certain distance r from the interference source, see Figure 1.4 and 1.5. Furthermore, the standards are specified for a special laboratory test setup with a certain measurement detector (EMI-detector). In this thesis the emission level expressed as the output from a standard emission measurement will be used as input to the analysis. Present emission standards are developed with respect to analog radio receivers, which is why an immediate connection to digital radio receivers does not exist. Current measurement procedures and detectors used are based on the work carried out in the standardization organizations during 1930 - 1939 [19]. The work of developing measurement procedures considering a digital radio receiver as victim has started both in CISPR [22] and ITUR [23].
Electric field strength
BEP at a certain frequency Estimation method
Frequency
Distance
Figure 1.4: The estimation problem.
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r Desired signal Pb
Electric field strength RF LO
Estimation of Pb
EMI detector IF DISPLAY
Frequency Figure 1.5: Problem overview. This work is, however, only in its beginning and it will probably take a long time before new standards including a new EMI detector can be presented. Furthermore, as long as analog systems exist, present standards will be used, which means that the knowledge of how to handle present standards for a digital victim will be needed not only today, but probably for at least one or two decades. Another practical problem is that even when new standards are available, manufacturers comply with the old standards cannot be expected to automatically verify ”off the shelf” equipment against new standards. Thus, the knowledge of how to handle the relation between present EMI detectors and digital radio receivers will be of great importance for system designers for at least the next one or two decades. Among several difficulties with the problem analyzed in this thesis, one basic difficulty is that the wave form of interference signal, which potentially could reach the emission limit, is unknown in an early design phase [24]. This complicates the situation since the performance of a digital radio receiver is affected by the wave form, not only the amplitude, of the interference signal.
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Furthermore, the EMI detector used gives different responses for different wave forms of interference signals. Throughout this thesis, the basic assumption is that the method is intended to be used in an early design phase of a system when no detailed information about the time domain properties of the interfering signals is available. For system design applications, such a method should be fairly simple to use and, at the same time, deliver useful results. This requires a trade-off between the degree of method complexity and the accuracy obtained in the results. The strategy for solving this problem is to perform the estimation of BEP in two steps. The first step is to estimate the signal to interference ratio γ I at the input to the radio receiver. Secondly, the estimated value γ I is used to estimate the BEP as a function of the separation distance r. Thus, the estimated BEP can be written as
Pb = f(γ I(r)).
(1.1)
Furthermore, in a military application, the increase in BEP must be interpreted to a measure that can be used for tactical considerations. If the radio system is not subjected to hostile jamming, a useful measure is how the operating range of a radio system is affected by the interference it produces itself. This case is referred to as the non-jamming case. If the radio system is subjected to hostile jamming, it is important to know how its own interference will contribute to the jammer’s effort to damage the communication link. This case is referred to as the jamming case. A measure of this kind of performance degradation will be suggested and discussed in this thesis. As undesired radiated emission can vary a lot in signal wave forms and levels, depending on the type of equipment, some assumptions have to be made to be able to create a basic model for our analyses. Here, the focus will be on information technology equipment supplying internal clocks and a video display as interference sources, because ITE is one common type of equipment that is co-located to military radio systems. The frequency region of main interest, for the specific examples analyzed, will be the lower part of the VHF band, where current combat radio systems typically work. However, the method proposed can be used in other frequency bands as long as the basic assumptions used are not violated.
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1.3 Contributions The original contributions in this thesis are the following; 1) A method for the estimation of performance degradation for a digital radio receiver in the presence of inherent interference, from co-located ITE equipment, is proposed. The intended use of this method is in system applications when radiated emission standards are to be chosen in the early design phase of a larger system. The method provides fairly simple calculations suitable for engineers to use, for instance when the impact from different radiated emission standards is to be compared. This method is new in that it makes it possible to relate radiated emission levels, developed for analog radio receivers, to the performance degradation of digital radio receivers. 2) Depending on the tactical situation for a military digital radio system, different performance evaluation measures are proposed. The purpose of these measures is to translate the estimated BEP to parameters of use for a military officer. A measure both for the non-jamming and for the jamming case is proposed. The result when these measures are applied to a specific situation with typical system parameters is presented. 3) The lack of previously published useful mathematical models of the interference from ITE requires that a proposed model be used in this thesis. Therefore a model is suggested that, for the purpose of the analyses, describes the important basic behavior of this interference. The model is based on published measurements of this kind of interference. The work on this thesis has resulted in a number of publications. The most important publications are listed below. Publication no. 4 is a joint paper with Kia C. Wiklundh. Kia has contributed with the basic theoretical analyses concerning the performance of differential MSK subjected to CW interference. Furthermore, she has provided the necessary basic understanding of the properties of the differential MSK modulation scheme. Publication no. 1: Peter F. Stenumgaard, ”A Simple Method to Estimate the Impact of Different Emission Standards on Digital Radio Receiver Performance, ”
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IEEE Transactions on Electromagnetic Compatibility, no. 4, November 1997. Main content: The problem definition and the basic ideas of how to solve the problem. Results for the case when the interference are of AWGN type are presented. This is the first published paper proposing a solution of the problem of how to relate emission standards to the impact on digital radio receiver performance. Publication no. 2: Peter Stenumgaard, ”Impact of Intersystem Interference on the Duel between Jammer and Communication System, ” FOA-report: FOA-R—98-00794-504--SE, May 1998. Main content: The basic ideas of how to analyze the impact of unintentional interference in a jamming situation. Results from a system example are given. Publication no. 3: Peter F. Stenumgaard, ”Digital Radio System Range Reduction Due to Radiated Electromagnetic Interference, ” Proceedings of EMC’98 Roma International Symposium on Electromagnetic Compatibility, pp 843846. Main content: Shows the impact of unintentional interference on the operating range of a radio system. Publication no. 4: Peter F. Stenumgaard, Kia C. Wiklundh, ”An Improved Method to Estimate the Impact on Digital Radio Receiver Performance of Radiated Electromagnetic Interference, ” Conditionally accepted for publication in the IEEE Transactions on Electromagnetic Compatibility Main content: An extension of the results in publication 1 to cover the case where the interference source is information technology equipment. A model for the radiated interference of ITE is proposed.
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1.4 Thesis outline In the thesis, a method of estimating the impact of inherent electromagnetic interference on digital radio receivers is developed. In addition, methods of evaluating the performance degradation in tactical terms are presented and applied to some chosen system examples. More specifically, in chapter 2 a model of the inherent interference is determined. This model is intended to represent the basic behavior of radiated electromagnetic interference from information technology equipment. In chapter 3, standard detectors for electromagnetic interference measurements are analyzed to create some basic knowledge required to translate a detector output to useful signal parameters of the interference measured during a standard measurement. In chapter 4, the method of how to translate an interference level, measured by a standard procedure, to the bit error probability of a digital radio receiver exposed to this interference is developed. In chapter 5, the suggested method is evaluated for the non-jamming case, i.e. no hostile jammer involved. It is shown that the suggested estimation method gives results close enough to the real values to be useful in practical design work. Furthermore, the sensitivity of the method proposed is analyzed to see the sensitivity of the estimated BEP to different interference wave forms. In chapter 6, the estimation method is investigated for the multitone jamming case. Here the combined effect of inherent interference and the jamming interference is considered. It is shown that the basic idea of the estimation method can be used for performance analyses of the more complex jamming case where several interference wave forms are involved. In chapter 7, the estimated BEP is related to evaluation parameters that are useful for tactical considerations in military applications. The suggested evaluation parameters are applied to system examples to show the usefulness of such parameters and to give some results immediately that are applicable to existing systems. Finally, in chapter 8, the conclusions and suggestions for further work are given.
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2 Interference source models s mentioned in chapter 1, man-made interference sources surrounding a communication system can vary a lot in the time domain properties of the waveforms. With the rapid technological advances in electronics, new types of interference sources are continuously created which contribute to the total interference environment. Before the second world war, roughly speaking, ignition interference from cars, interference from electric motors and corona interference from power lines were the only man-made sources of major concern. Today, man-made noise consists of a lot more interference sources, to which information technology equipment is one important contributor. In military applications, the use of ITE in the vicinity of communication systems is rapidly increasing. This is due to the increased need of quick and accurate information for command and control in a battlefield characterized by rapid changes. As the focus in this thesis will be on interference from ITE, a model which gives a proper description of this interference must be used. A lot of radiated emission measurements are daily produced all around the world, therefore there is a widespread knowledge of what this kind of interference looks like in general. Surprisingly, however, there are no detailed interference models published that describe the interference from ITE in useful mathematical terms. Middleton [25] [26] carried out extensive work in formulating general mathematical models for manmade interference. The goal was to create useful models that are based on a general physical mechanism and with parameters that are possible
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to measure. A consequence of connecting the models to general physical mechanisms is that the focus is placed on non-gaussian amplitude distributions. Earlier models were mostly concerned with natural noise, such as atmospheric noise, and although they were characterized by mathematical simplicity they were severely limited in usefulness [25]. The problem with Middleton’s models is that they are mathematically difficult, and even though they are based on familiar physical situations, the connection to the physical scenario is not apparent [27]. This is probably the reason why these models have not been adopted as standard tools in describing man-made noise. Furthermore, Middleton’s models have been shown to be limited in describing lots of physical situations [27], which is another contributing factor to why these models are not widely used. The lack of previously published useful mathematical models of the interference from ITE requires that a model be proposed for use in this thesis. Earlier publications concerning measured interference from ITE are used to determine a model for our purpose, describing the important basic behavior of this kind of interference. In general, the characteristics of the interference from ITE supplying internal high-speed clocks and a video display unit (monitor) can be divided into two basic parts. These parts are one narrow band ip() t and one t . The narrow band consists of harmonics from periodic broad band, n() signals, see for instance [28], [29], [30]. The broad band is caused by non-periodic or random signals [31], [29]. Furthermore, the narrow band interference is typically of much higher levels than the broad band. A useful interference model must contain these two basic properties of the interference. In this thesis the proposed mathematical description of the interference amplitude iI() t from our interference source is iI() t = n() t + ip() t = n() t +
∞
∑ ck e j(2πf k t + ϕ)
(2.1)
k = −∞
where n() t denotes the broad band random noise, here assumed to be Gaussian amplitude distributed, and the sum is the Fourier series of the periodic interference ip() t . The coefficients ck are defined as
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ck =
1 Tp
Tp / 2
te ∫− Tp / 2 ip()
− j 2πf k t
dt .
(2.2)
The Fourier coefficients, if ip() t consists of rectangular pulses, are ck =
Ap πf k
sin πf k T
(2.3)
where f k = k / Tp , Tp is the period time of the pulse train and Ap is the pulse amplitude, see figure 5.1. In the frequency domain, ip() t consists of discrete sine wave components separated with distance 1 / Tp . The first main lobe is schematically shown in figure 5.2. As long as the pulse repetition frequency is greater than the bandwidth of the radio receiver, at most one spectral component will enter the receiver. The conclusion is that under these circumstances, the expression for the BEP derived for a pure sine wave can be used. The frequency distance between adjacent spectral components in the Fourier series of the periodic interference is assumed to be larger than the bandwidth of the radio receiver. Typical pulse repetition frequencies today could vary from approximately 30 kHz, from the horizontal scanning of the monitor, to several hundreds of MHz from internal clocks.
ip(t)
Ap
t
T Tp Figure 5.1: Periodic pulsed signal.
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ck
1/Tp
f
0 1/T
Figure 5.2: A simplified figure of the main lobe of the frequency t spectrum of ip() As the frequency region where this narrow band interference dominates depends on the pulse repetition frequencies used, there is some upper limit in frequency where this assumption is valid. Present ITE has clock frequencies up to 400-500 MHz, thus this assumption should be valid in the VHF region and the lower part of the UHF region. However, as the development towards faster processors is rapid, this assumption will soon be valid for the complete UHF band. The use of the terms ”broad band” and ”narrow band” could sometimes be a bit confusing as these terms always are related to some bandwidth of a receiver. A commonly used definition of broad band interference is that the total power within a bandwidth will increase if the bandwidth is increased. For narrow band noise, the total received power remains constant if the bandwidth is increased. In our case, the harmonics in the Fourier series are denoted narrow band as we have assumed a bandwidth smaller than the pulse repetition frequency. For larger bandwidths, however, this interference component should be denoted broad band as the total interference power within the bandwidth would increase with the bandwidth. The Gaussian noise is assumed to have an approximately constant power spectral density within the bandwidth of the receiver. Thus this interference can be treated as additive white Gaussian noise
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(AWGN). Under the assumption that the power levels from the narrow band interference are much larger than from the broad band, the analysis can be divided into two cases, one concerning interference from AWGN and one concerning interference from sinusoidal interference. Furthermore the thermal noise level, also assumed to be AWGN, in the radio receiver will also be considered. The conclusion of the interference model used is that the performance analysis for a digital radio receiver will concern how this radio receiver is affected by either pure AWGN or a combination of AWGN and a sine wave assumed to have a uniformly distributed random phase.
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3 EMI detectors 3.1 General
ypical standard EMI measurement equipment is based on a superheterodyne receiver, e.g. a spectrum analyzer, see figure 3.1. The output from the antenna is fed into a radio frequency (RF) amplifier and is then mixed with the signal from the local oscillator (LO). The output from the intermediate frequency (IF) filter is then fed into the detector. Emission measurements are always automated to reduce the measurement time. Often, a computer is used to monitor the measurement receiver and to present the measurement result. A common method is to scan the frequency region of interest in discrete frequency steps. At each frequency, a certain dwell time is required to make a correct measurement. The step size and dwell time are generally specified in some way. For instance, in MIL-STD-461D, the step size must be one half increment of the measurement bandwidth or less. The requirements of these measurement parameters result in a maximum sweep time [Hz/s] which determines the total measurement time for scanning a certain frequency region. Two basic types of detectors are used in standard measurements. The civilian Euronorm (EN) standard, required for all commercial equipment sold in the European Union, as well as the American Federal Communi-
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B
C
D
