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IEEE Std 1243-1997 IEEE Guide for Improving the Lightning Performance of Transmission LinesSponsorTransmission and Distribution Committeeof theIEEE Power Engineering SocietyApproved 26 June 1997IEEE Standards BoardAbstract: The effects of routing, structure type, insulation, shielding, and grounding on transmis-sion lines are discussed. The way these transmission-line choices will improve or degrade light-ning performance is also provided. An additional section discusses several special methods that may be used to improve lightning performance. Finally, a listing and description of the FLASH pro-gram is presented.Keywords: grounding, lightning protection, overhead electric power transmission, shieldingIEEE Standards documents are developed within the IEEE Societies and the Standards Coordinat-ing Committees of the IEEE Standards Board. Members of the committees serve voluntarily and without compensation. They are not necessarily members of the Institute. The standards developed within IEEE represent a consensus of the broad expertise on the subject within the Institute as well as those activities outside of IEEE that have expressed an interest in participating in the develop-ment of the standard.Use of an IEEE Standard is wholly voluntary. The existence of an IEEE Standard does not imply that there are no other ways to produce, test, measure, purchase, market, or provide other goods and services related to the scope of the IEEE Standard. Furthermore, the viewpoint expressed at the time a standard is approved and issued is subject to change brought about through developments in the state of the art and comments received from users of the standard. Every IEEE Standard is sub-jected to review at least every Þve years for revision or reafÞrmation. When a document is more than Þve years old and has not been reafÞrmed, it is reasonable to conclude that its contents, although still of some value, do not wholly reßect the present state of the art. Users are cautioned to check to determine that they have the latest edition of any IEEE Standard.Comments for revision of IEEE Standards are welcome from any interested party, regardless of membership afÞliation with IEEE. Suggestions for changes in documents should be in the form of a proposed change of text, together with appropriate supporting comments.Interpretations: Occasionally questions may arise regarding the meaning of portions of standards as they relate to speciÞc applications. When the need for interpretations is brought to the attention of IEEE, the Institute will initiate action to prepare appropriate responses. Since IEEE Standards rep-resent a consensus of all concerned interests, it is important to ensure that any interpretation has also received the concurrence of a balance of interests. For this reason, IEEE and the members of its societies and Standards Coordinating Committees are not able to provide an instant response to interpretation requests except in those cases where the matter has previously received formal consideration.Comments on standards and requests for interpretations should be addressed to:Secretary, IEEE Standards Board445 Hoes LaneP.O. Box 1331Piscataway, NJ 08855-1331USAAuthorization to photocopy portions of any individual standard for internal or personal use is granted by the Institute of Electrical and Electronics Engineers, Inc., provided that the appropriate fee is paid to Copyright Clearance Center. To arrange for payment of licensing fee, please contact Copyright Clearance Center, Customer Service, 222 Rosewood Drive, Danvers, MA 01923 USA; (508) 750-8400. Permission to photocopy portions of any individual standard for educational class-room use can also be obtained through the Copyright Clearance Center.Introduction(This introduction is not part of IEEE Std 1243-1997, IEEE Guide for Improving the Lightning Performance of Trans-mission Lines.)For most overhead power transmission lines, lightning is the primary cause of unscheduled interruptions.Several methods for estimating lightning-outage rates have been developed in the past, and many publica-tions have been written on how to design transmission lines that experience minimum interruptions.The methods for estimating the lightning performance of transmission lines show several approaches to a real-life engineering problem that is ill-deÞned. Precise constants are rarely known and are often not really con-stant, input data is difÞcult to describe mathematically except in idealized ways, and outputs may be depictable only by probabilities or average values. By its nature, lightning is difÞcult to study and model. Lightning tran-sients are so fast that air ionization time constants lead to a time- and waveshape-dependent insulation strength. Lightning peak currents may be ten times higher or lower than the median 31 kA value. Typical ground-ßash densities are between 1 and 10 ßashes/km 2 , so a typical 100 km long strip of width 20 m should receive 2Ð20 ßashes per year. A transmission line of normal height will receive ten times more ßashes because it is tall. Structure footing impedances vary with soil characteristics, ßash current, and time. Nonlinear corona and surge response effects change the waveshape and magnitude of stresses. As a further complication, light-ning ßash density varies widely from year to year and changes with location and season.Any method of judging the lightning performance of transmission lines must cope with these uncertainties.It is pointless, and indeed misleading, to promote a method that is more precise than the accuracy of the input data. The uncertainties of the problem do permit some simpliÞcation of the method; rough estimates are likely to be as correct as a much more detailed solution. It is in this spirit that this guide and the FLASH program have been prepared. If the transmission-line designer keeps these limitations in mind, the factors that most inßuence the lightning performance of a given transmission line may be evaluated.Knowledge has improved in recent years in such areas as shielding design, stroke characteristics, impulse current behavior of grounds, and lightning ground-ßash density. Work is continuing in these areas, as well as others. However, a simple design guide is needed now. It is the purpose of this publication to provide a sim-pliÞed guide that includes new advances in this Þeld, for use by transmission-line designers.The IEEE Working Group on Estimating the Lightning Performance of Overhead Transmission Lines had the following membership during the preparation of this guide:James T. Whitehead, Chair, 1985-1989William A. Chisholm, Chair, 1989-1997The Working Group would like to thank the many individuals who reviewed the text and provided comments in the review and balloting process.John G. AndersonJohn W. AndersonPhilip BarkerRalph BernsteinJames E. ChapmanPritindra ChowdhuriErnico CinieriRoger ClaytonLuigi DelleraHamid ElahiAndrew J. ErikssonGeorge GelaStanislaw Grzybowski Christopher Hickman Andrew R. Hileman Atsuyuki Inoue Marasu Ishii Wasyl Janischewskyj John Kappenman Robert C. Latham Duilio M. Leite Vito J. Longo Harry G. Mathews James McBride Thomas McDermott Hideki Motoyama Charles H. Moser Abdul M. Mousa Richard E. Orville Dee E. Parrish Charles Pencinger John B. Posey Joseph D. Renowden Farouk A. M. Rizk Thomas Short Mohamed H. Shwehdi Martin A. Uman Edward R. WhiteheadThis document is dedicated to the work and memory of Ed Whitehead:ÒLightning is the enemy,Trees are your friends,TheyÕll shield your linesHoweÕer the leader bends.ÓThe following persons were on the balloting committee:Hanna E. Abdallah Edward J. Adolphson Glenn AndersenJ. G. AndersonJames E. Applequist Edwin AverillMichael P. Baldwin Phil P. BarkerRon L. BarkerE. BetancourtJohn BoyleH. Steve Brewer Joseph F. BuchJames J. Burke Vernon L. Chartier Nisar ChaurdhryPrit ChowdhuriDon ChuEnrico CinieriSam ClutsThomas M. Compton F. Leonard Consalvo William T. CrokerPaul L. Dandeno Glenn A. DavidsonR. DeckerTom. Diamantis William K. DickDale A. Douglass Robert Engelken William E. FeeroJon M. Ferguson Harald FienJames FunkeGeorge GelaDonald A. Gillies Edwin J. Tip Goodwin I. S. GrantStan GrzybowskiAdel E. Hammad Jerome G. Hanson Donald G. Heald Roland Heinrichs Richard W. Hensel Steven P. Hensley Christopher W. Hickman Andrew Robert Hileman John Hunt Atsuyuki InoueWasyl JanischewskyjJohn G. KappenmanRichard KennonJeff J. KesterRobert O. KlugeEd KnappNestor KolcioAlan E. KollarDonald E. KoonceDavid J. KourySamy G. KrishnasamyBarin KumarStephen R. LambertRobert C. LathamBenny H. LeeGerald E. LeeAntonio L. LimJoseph MaK. T. MassoudaMike. McCaffertyJohn McDanielNigel P. McQuinRoss McTaggartJohn E. Merando Jr.Sam MichaelJ. David MitchellDaleep MohlaRichard K. MooreJ. H. MoranHideki MotoyamaAbdul M. MousaJay L. NichollsD. L. NickelGeorge B. NilesStig L. NilssonRonald J. OedemannRobert G. OswaldGerald A. PaivaBert ParsonsMohammad A. PashaJesse M. PattonDavid F. PeeloCarlos PeixotoThomas J. PekarekRobert C. PetersTrevor PfaffR. Leon PlasterPercy E. PoolSteven C. PowellPatrick D. QuinnParvez RashidJerry L. RedingJoseph RenowdenFrank RichensStephen J. RodickJohn R. RossettiG. W. RoweTim E. RoysterThomas J. RozekJohn S. RumbleAnne-Marie SahazizianMahesh P. SampatDonald SandellNeil P. SchmidtJohn SchneiderMinesh ShahDevki N. SharmaJohn SheaTom ShortMohamed H. ShwehdiHyeong Jin SimPritpal SinghTarkeshwar SinghRobert A. SmithStephen F. SmithBrian T. SteinbrecherGary E. StemlerL .R. StenslandRobert P. StewartMike F. StringfellowAndy SweetanaEva J. TarasiewiczSubhash C. TuliJ. G. TzimorangasJoseph J. VaschakJohn VithayathilReigh WallingDaniel J. WardJames T. WhiteheadJeffrey S. WilliamsFrank S. YoungLuciano E. ZaffanellaPeter ZhaoRich ZinserWilliam B. ZollarsWhen the IEEE Standards Board approved this guide on 26 June 1997, it had the following membership:Donald C. Loughry, Chair Richard J. Holleman, Vice ChairAndrew G. Salem, Secretary*Member EmeritusAlso included are the following nonvoting IEEE Standards Board liaisons:Satish K. AggarwalAlan H. CooksonErika H. MurphyIEEE Standards Project EditorNational Electrical Safety Code and NESC are registered trademarks and service marks of the Institute of Electrical and Electronics Engineers, Inc.Windows is a trademark of Microsoft Corporation.Clyde R. Camp Stephen L. Diamond Harold E. Epstein Donald C. Fleckenstein Jay Forster*Thomas F. Garrity Donald N. Heirman Jim Isaak Ben C. JohnsonLowell JohnsonRobert KennellyE. G. ÒAlÓ KienerJoseph L. KoepÞnger*Stephen R. LambertLawrence V . McCallL. Bruce McClungMarco W. Migliaro Louis-Fran•ois Pau Gerald H. Peterson John W. Pope Jose R. Ramos Ronald H. Reimer Ingo RŸsch John S. Ryan Chee Kiow TanHoward L. WolfmanContents1.Overview (1)1.1Scope (1)1.2Purpose (1)1.3Disclaimer (2)2.References (2)3.Definitions and Acronyms (2)3.1Definitions (2)3.2Acronyms (4)4.Route selection (4)4.1Lightning frequency of incidence (4)4.2Route effects (5)4.3Structure height (5)4.4Soil resistivity (6)4.5Adjacent environment (6)5.Shielding (6)5.1Shielding angle (7)5.2The final leader step (7)5.3Flashover from shielding failure (9)5.4Shielding and engineering (10)5.5Shielding of the center phase (12)5.6Overhead ground wire size and operating losses (12)6.Insulation (13)6.1Effect of voltage waveshapes (13)6.2Effect of insulation levels and insulation type (16)6.3Wood or fiberglass in series with insulators (17)6.4Effect of power-line voltage (17)7.Tower footing impedance (18)7.1Composite line performance (18)7.2Supplemental grounding (19)7.3Counterpoise (20)7.4Resistance of complex footings (21)7.5Special grounding effects (21)8.Special methods of improving lightning performance (22)8.1Additional shield wires (22)8.2Guy wires on transmission towers (23)8.3Ground wire on separate structures (23)8.4Line surge arresters (23)8.5Unbalanced insulation on double-circuit lines (24)8.6Active air terminals (24)Annex A (informative) Isolated bonding of wooden structures (26)Annex B (normative) The FLASH program (27)Annex C (informative) Bibliography (34)IEEE Guide for Improving the Lightning Performance of Transmission Lines1. Overview1.1 ScopeFor this guide, a transmission line is any overhead line with a phase-to-phase voltage exceeding 69 kV and an average conductor height of more than 10 m. The transmission line is usually shielded by one or more overhead ground wires (OHGWs), at least for a short distance from a substation. While reference is prima-rily made to ac transmission characteristics, the guide is also relevant for high-voltage direct-current (HVDC) overhead lines.The guide is written for the transmission-line designer. When given the problem of designing or redesigning a transmission line, the designer should consider certain limiting factors such as the voltage level, the begin-ning and ending points for the transmission line, and the desired ampacity of the line. Sometimes the exact route, and the type of conductor and structure have already been determined. Usually the designer may choose structural details, the geometry of the structure, the structure height, the exact placement of the OHGWs, the amount and type of insulation, the type of grounding, and other design features of a line. This guide is written to show the designer which choices will improve or degrade lightning performance. Sections of the guide discuss the effect of routing, structure type, insulation, shielding, and grounding. An additional section discusses several special methods, which may be used to improve lightning performance. Finally, in Annex B, a listing and description of the FLASH program is presented.The line designer should be aware that lightning performance is not of primary importance in the economics of line designing. Other factors, such as line length, right-of-way costs, construction costs, material costs, and losses affect the economics of a line design much more than lightning performance. The designer should always balance the costs of higher insulation levels, improved grounding, better shielding, or line relocation against the beneÞts of improved reliability.1.2 PurposeThis guide contains simple mathematical equations, tables, and graphs that provide the information needed to design an overhead power transmission line with minimum lightning interruptions. Versions 1.6 and 1.7 of the FLASH program are provided on the diskette included with this guide. Annex B includes a description of the program. The FLASH program uses the models in the design guide along with a description of transmission-line features to estimate the lightning outage rate that may be expected. These simpliÞed models may also be adapted to assess the beneÞts of novel methods for improving lightning performance.IEEEStd 1243-1997IEEE GUIDE FOR IMPROVING THE 1.3 DisclaimerThe FLASH program is included in this guide as a convenience to the user. Other numerical methods may be more appropriate in certain situations. The IEEE Working Group on Estimating the Lightning Performance of Overhead Transmission Lines of the Lightning and Insulator Subcommittee has made every effort to ensure that the program yields representative calculations under anticipated conditions. However, there may well be certain calculations for which the method is not appropriate. It is the responsibility of the user to check calculations against Þeld experience or other existing calculation methods.2. ReferencesThis guide shall be used in conjunction with the following standards. When the following standards are superseded by an approved revision, the revision shall apply.ANSI C2-1997, National Electrical Safety Code¨ (NESC¨).1ANSI C29.1-1988 (Reaff 1996), American National Standard for Electric Power InsulatorsÑTest Methods.2 ANSI C29.2-1992, American National Standard for InsulatorsÑWet Process Porcelain and Toughened GlassÑSuspension Type.ANSI C29.8-1985 (Reaff 1995), American National Standard for Wet-Process Porcelain Insulators (Appara-tus, Cap, and Pin Type).3. DeÞnitions and Acronyms3.1 DeÞnitions3.1.1 active air terminal: An air terminal which has been modiÞed to lower its corona inception gradient.3.1.2 air terminal (lightning protection): The combination of an elevation rod and brace, or footing placed on upper portions of structures, together with tip or point, if used.3.1.3 back ßashover (lightning): A ßashover of insulation resulting from a lightning stroke to part of a net-work or electric installation which is normally at ground potential. See also:direct-stroke protection.3.1.4 back-ßashover rate: The annual outage rate on a circuit or tower-line length basis caused by back ßashover on a transmission line.3.1.5 counterpoise: A conductor or system of conductors arranged beneath the line; located on, above, or most frequently below the surface of the earth; and connected to the grounding systems of the towers or poles supporting the transmission lines.3.1.6 critical current: The Þrst-stroke lightning current to a phase conductor which produces a critical impulse ßashover voltage wave.1The NESC is available from the Sales Department, American National Standards Institute, 11 West 42nd Street, 13th Floor, New York, NY 10036, USA. It is also available from the Institute of Electrical and Electronics Engineers, 445 Hoes Lane, P.O. Box 1331, Piscataway, NJ 08855-1331, USA.2ANSI publications are available from the Sales Department, American National Standards Institute, 11 West 42nd Street, 13th Floor, New York, NY 10036, USA.IEEE LIGHTNING PERFORMANCE OF TRANSMISSION LINES Std 1243-1997 3.1.7 critical impulse ßashover voltage (insulators) (CFO): The crest value of the impulse wave which, under speciÞed conditions, causes ßashover through the surrounding medium on 50% of the applications.3.1.8 direct stroke protection (lightning): Lightning protection designed to protect a network or electric installation against direct strokes.3.1.9 ßashover: A disruptive discharge through air around or over the surface of solid or liquid insulation, between parts of different potential or polarity, produced by the application of voltage wherein the break-down path becomes sufÞciently ionized to maintain an electric arc.3.1.10 ground electrode: A conductor or group of conductors in intimate contact with the ground for the purpose of providing a connection with the ground.3.1.11 ground ßash density (GFD): The average number of lightning strokes to ground per unit area per unit time at a particular location.3.1.12 lightning Þrst stroke: A lightning discharge to ground initiated when the tip of a downward stepped leader meets an upward leader from the earth.3.1.13 lightning ßash: The complete lightning discharge, most often composed of leaders from a cloud fol-lowed by one or more return strokes.3.1.14 lightning subsequent stroke: A lightning discharge that may follow a path already established by a Þrst stroke.3.1.15 lightning outage: A power outage following a lightning ßashover that results in system fault current, thereby necessitating the operation of a switching device to clear the fault.3.1.16 line lightning performance: The performance of a line expressed as the annual number of lightning ßashovers on a circuit mile or tower-line mile basis. See also:direct-stroke protection.3.1.17 line surge arrester: A protective device for limiting surge voltages on transmission-line insulation by discharging or bypassing surge current; it prevents continued ßow of follow-current to ground and is capable of repeating these functions.3.1.18 overhead ground wire (OHGW): Grounded wire or wires placed above the phase conductors for the purpose of intercepting direct strokes in order to prevent the phase conductors from the direct strokes. They may be grounded directly or indirectly through short gaps. See also:direct-stroke protection.3.1.19 shielding failure ßash-over rate (SFFOR): The annual number of ßashovers on a circuit or tower-line length basis caused by shielding failures.3.1.20 shielding failure rate (SFR): The annual number of lightning events on a circuit or tower-line length basis, which bypass the overhead ground/shield wire and terminate directly on the phase conductor. This event may or may not cause ßashover.3.2 shield wire: Grounded wire(s) placed near the phase conductors for the purposes ofa)Protecting phase conductors from direct lightning strokes,b)Reducing induced voltages from external electromagnetic Þelds,c)Lowering the self-surge impedance of an OHGW system, ord)Raising the mutual surge impedance of an OHGW system to the protected phase conductors.3.2.1 standard lightning impulse: A unidirectional surge having a 30Ð90% equivalent rise time of 1.2 m s and a time to half value of 50 m s.IEEEStd 1243-1997IEEE GUIDE FOR IMPROVING THE 3.2.2 transmission line: Any overhead line used for electric power transmission with a phase-to-phase volt-age exceeding 69 kV and an average conductor height of more than 10 m.3.2.3 underbuilt shield wires: Shield wires arranged among or below the average height of the protected phase conductors for the purposes of lowering the OHGW system impedance and improving coupling. Underbuilt shield wires may be bonded to the structure directly or indirectly through short gaps. Insulated earth return conductors on HVDC transmission lines, and/or faulted phases, both function as underbuilt shield wires.3.2 AcronymsACSR aluminum conductor, steel reinforcedAWG American Wire GageCFO critical ßashoverEGM electro-geometricEHV extra-high voltageGFD ground ßash densityHV high voltageHVDC high-voltage direct currentOHGW overhead ground wireRTS rated tensile strengthSFR shielding failure rateSFFOR shielding failure ßashover rate4. Route selectionMany factors play an important role in route selection. Power system considerations dictate where the trans-mission line should begin and end. Economic considerations require the line to be as short as possible, because construction costs and electrical losses are high. Certain environmental constraints dictate where and how a transmission line may be built. Even with these restrictions, there are still ways for the transmis-sion-line designer to make decisions that will affect the lightning performance of the line. It is the purpose of this clause to illustrate the ways that a designer may improve the lightning performance of a transmission line by selecting the proper route.4.1 Lightning frequency of incidenceLightning location systems and ßash-counter networks have been deployed in North America [B11], [B34]3 and elsewhere. With enough experience, these networks may provide detailed ground ßash-density (GDF) maps. Orographic and geographic features, such as proximity to large bodies of water or elevation changes, will affect the ßash density. Flash-density maps will provide much greater detail and accuracy than what was previously available with thunder data. A typical GDF map is shown in [B35]. Lightning severity maps may also be available to show where more damaging lightning strokes occur. When two routes with similar soil characteristics are being compared, the route through a region with lower density of severe ßashes will have fewer outages.With more detailed maps averaged over enough time, the designer may select a route with a minimum expo-sure to lightning. By way of guidance about the averaging time required, MacGorman et al. [B29] found that 3The numbers in brackets correspond to those of the bibliography in Annex C.IEEE LIGHTNING PERFORMANCE OF TRANSMISSION LINES Std 1243-1997 one-year average thunder data had roughly 35Ð40% standard deviations, Þve-year averages had 30% stan-dard deviations, and ten-year averages had 25% standard deviations. A similar variation would be expected for GFD. Thus, a minimum of 5Ð10 years of GFD observations are desirable. The natural scatter of lightning activity may make it impossible to estimate a mean value with more precision than the ten-year standard deviation.4.2 Route effectsThe transmission-line designer is often faced with the choice of routing a line through a valley, along the side of a mountain, or on the top of a mountain. This decision may affect the lightning performance of the line in two ways. First, the route may affect the exposure of the line to lightning. Second, soil resistivity may be different for alternate routes. The effects of soil resistivity are discussed in 4.4.Transmission line exposure is affected by both GFD and by the lineÕs physical relation to its environment. A structure that protrudes above the surrounding terrain is more likely to be struck by lightning than a structure shielded by natural features. Structures located along the top of mountains, ridges, or hills will be likely targets for lightning strikes. These locations should be avoided as much as possible. It is preferable to locate structures along mountain sides where the top of the structure does not appear higher than the top of the mountain or ridge. Locating lines in the ßoor of a narrow valley may provide the line with useful lightning protection.Detailed design of lines should consider the effects of different routes on structure height, soil, or overburden depth and the adjacent environment.4.3 Structure heightThe Þrst factor of a line route that affects lightning performance is structure height, especially if its towers are higher than the surrounding terrain. Increasing the tower height has two important effects: more ßashes are collected by the taller structure, and the shielding characteristics of overhead conductors change as height is increased, as explained in Clause 5.The ßash collection rate, N s , is given by the following equation [B18]:(1)wherehis the tower height (m); bis the OHGW separation distance (m); N gis the GFD (ßashes/km 2 /yr); N s is the ßashes/100 km/yr.From Equation (l), if the tower height is increased by 20%, the ßash rate to the line would increase by 12%.If a measured value of N g is not available, it may be estimated [B18], [B29] using(2)(3)whereT dis the number of thunderstorm days/yr (Keraunic Level);N S N g 28h 0.6b +10-----------------------èøæö=N g 0.04T d 1.25=N g 0.04T h1.1=IEEEStd 1243-1997IEEE GUIDE FOR IMPROVING THE T h is the number of thunderstorm hours/yr.Data for T d and T h may be obtained from several meteorological sources, (e.g., [B11], [B29]).4.4 Soil resistivityThe second factor of a line route that affects lightning performance is soil resistivity. Resistivity has a linear relationship to footing impedance, which is explained in Clause 7. Substantial voltages are generated on grounded members of the structure when either the OHGW or the structure are struck by lightning. High structure footing impedances cause increased voltages and more lightning outages for a given lightning exposure.A complete line design will specify the types and sizes of ground electrodes needed to achieve the required footing impedance. The electrode sizes and shapes will depend on the range of soil conductivities found on installation. In some geographic areas, surveys of apparent ground resistivity have been carried out for radio-frequency broadcast [B25] or geological purposes. In particular, airborne electromagnetic surveys [B36] at 10Ð50 kHz return a suitable conductivity-depth product which may be analyzed further or used directly for tower spotting.High footing impedances occur in rocky terrain, which should be avoided as much as possible. When rocky terrain may not be avoided, improved grounding methods should be used to lower footing impedances to acceptable values. These improved methods usually require large-ring or radial-crowfoot installation at con-siderable cost. Rocky terrain usually occurs on mountain tops and mountain sides, while river lowlands tend to have low soil resistivity. This is a second reason why transmission-line routes away from hill crests will tend to have better lightning performance.4.5 Adjacent environmentOne way to prevent structures from being a target for lightning is to take advantage of surrounding forestation. Tall trees located near the transmission line may intercept a lightning stroke which may have caused an outage if the tree had not been there. Most transmission lines have sufÞcient impulse strength to be immune to the resulting induced voltages. Reference [B31] provides additional information about the effects of forestation. When possible, lines may be routed through forestation and tall trees may be left in place near the line. Trees that may fall on the line or reduce operating clearances will require more frequent trimming, and forest Þres may degrade the line protection locally. However, the reduction in apparent line height and corresponding reduction in lightning outages may still be worthwhile.Tall structures located in ßat, open Þelds make excellent targets for lightning. In these conditions, the struc-ture height should be minimized, and the structure footing impedance should be reduced much as possible. Another way to use the surrounding environment to shield a transmission line from lightning is to route the line next to existing transmission line structures. Experience has shown that a line sharing right-of-way with another line having taller structures will have fewer lightning outages than if it were on a separate right-of-way. Two lines of identical design and on adjacent rights-of-way share lightning strikes resulting in lower than normal outage rates for both lines. This improvement should be balanced with the greater risk of multi-ple line outages.5. ShieldingWhen lightning strikes a phase conductor, no other object shares in carrying the lightning current. Most ßashes to an unprotected phase conductor are therefore capable of producing ßashovers. OHGWs may inter-cept the stroke and shunt the current to the ground through the tower impedance and footing resistance if they are properly located. The resultant voltages across the transmission-line insulation, and the likelihood。

The Differences and Solutions of Male and Female Students’English Learning【abstract】 in china, female students always perform better than male students in english learning. this thesis seeks to tackle the differences between male and female students，aiming to uncover the causes of the differences and explore the solutions for male and female students’ english learning.【关键词】 male and female students english learning differences solutionsin china, female students always have better performance than male students in english learning. what caused the differences between male and female students in english learning? and what can we do to deal with the differences? ⅰ. differences between male and female students1.1 biological differences between male and female students first, scientific research has found that the left brain dominates language and logical analysis, right brain dominates imaginable thinking and intuition. females’ left brain function better than males’, it specializes earlier and develops in a higher level. these make the female students’ language ability is superior to male students, andit makes female students be likely to pay attention to details. males’ right brain is superior to females’, so male students are good at dealing with spatial relationships of information and analyzing.1.2 psychological differences between male and female students1.2.1 intelligence differencethe male and the female are different in such areas as perception, memory and thinking. the female is good at listening and speech perception, while the male is good at vision and spatial perception. in addition, according to the characteristics of people in the process of perception, consciousness can be divided into three types: analytical, integrated and analytical-integrated analysis. it’s observed that female students lean to the analytical analysis, and male students lean to the integrated analysis. on the proportion of analytical-integrated analysis, female students’proportion is higher than male students’. generally speaking, female students are good at image memory, emotion and motor memory, male students are relatively good at logic memory. on the memory method, in general, girls are slightly dominant in unconscious memorizing and mechanical memorizing. inconscious and meaning memorizing, the male students are better than female students.on thinking, studies at home and abroad have the same views, that is female students have imaginable thinking, they are inclined to resolve the problem with images; male students think more logically, they are accustomed to solving problems with abstract thinking. on the quality of thinking, male students are slightly superior to female students. thinking quality includes depth, flexibility, ingenuity and agility. but from the view of english learning, female students’thinking is better than male students’ because female students prefer imaginable thinking which is used to fine processing, and it is cautious and slow, but more accurate. and male students are more impulsive.1.2.2 differences of learning motivationstudies have shown that female students are more interested in english learning, and they have obvious tendency to intrinsic motivation. male students are not very interested in english learning in general, they always urged by parents and teachers. they are more passive in english learning and have stronger external motivation.1.2.3 differences of personalitymales, in general, are confident, brave, independent, open-minded, careless, impatient, progressive, they have more emotional fluctuations. females generally are calm, patient, meticulous, serious, timid, shy, vulnerable and so on. on the personality types, males are mostly extroversive, independent, and strong; most females are introversive, submissive.male students are more willing to show them in the learning process, especially in spoken english, but sometimes they tend to be too impulsive. they think too hurriedly, and the answers are not accurate enough. while female students are not confident enough, they think carefully and their answers are more accurate.1.2.4 differences of cognitive stylefield-independent learners often perceive things with internal references rather than external references. they judg things in their own standard, and they are not very good at getting social information and feel free from the influence of external information. their emotional expressions are confident, self-reliant, brave and combative. they are not good at communicating with others. field-dependent learners are more dependent on external references when perceivethings. they like associate with others and pay more attention to social information. they are good at imitating and easy to be influenced by others, and they cannot differentiate things from the complex situation including a number of elements and components.1.2.5 differences of learning strategieslearning strategies can be divided into: cognitive strategies, metacognitive strategies, emotional strategies. the survey showed that: female students outperform male students in applying cognitive strategies and metacognitive strategies. about the emotional strategy, male students are slightly superior to female students. but sometimes, when facing difficulties that appear in the english learning, female students performance better at relieving their negative emotions, such as using words and so on. male students are typically not good at giving vent to their feelings. the long-term depression and anxiety will deepen their boredom feeling of english learning and even make them fear to english. on the whole, female students are more active than boys on the use of english learning strategies, in order to achieve better learning outcomes, female students use more learning strategies frequently.ⅱ. solutionsdue to physiological factors cannot be changed, here are some solutions that correspond to the psychological aspects.2.1 intelligence differencefemales are good at listening and speech perception, while males are good at vision and spatial perception. female students can listen to english songs, news and other listening materials to improve their english skills. females have a good sense of imitation, so female students should imitate more, and pay attention to the accuracy of and fluency of speech. male students can enhance english learning effects by using pictures, charts, videos, and actions, they need to listen and read more to enhance their english language sense. both male and female students should make more use ofanalytical-integrated analysis to solve problems in learning english.female students can use more conscious memory while using their mechanical memory to improve efficiency. male students need to make good use of the conscious memory and review frequently. in terms of thinking, male students should think comprehensively and carefully in english learning process. they should start from details to the whole when they arethinking, and they should pay more attention to the accuracy. female students should limit their thinking time, facilitating judgment by using intuition, improving the problem-solving speed and comprehensive problem-solving ability.2.2 learning motivationmale students are more easy to be influenced by extrinsic motivation, and they are more vulnerable to external incentives than female students. therefore male students should enhance their english learning purpose and stimulate their achievement motivation. teachers and parents should encourage them. and achieving success, is one of the best incentives. in addition, the female students should try to increase their interest in english through a variety of methods, and they should learn from excellent english learners in order to achieve greater success.2.3 differences of personalityas males are more impulsive, therefore they should keep calm when solve problems, and they should consider more deliberately before coming to the conclusion or solution to a problem. female students should be more confident without fear of errors.2.4 differences of cognitive styleas the male students tend to the field independent cognitive style in general, they are more rational than female students. when encountering problems, they prefer to solve them independently. so their interpersonal skills, discourse skills are weaker than femal students. so male students need to participate in more cooperative activities in english learning and communicate more with others. female students should overcome crowd and be more independent as well as be more active in the competition.2.5 differences of learning strategiesmale students need to improve their metacognitive strategies, they should plan, supervise and assess their own learning, reinforcing learning purpose and becoming initiative in english leraning. they should learn to groom their bad mood when encounter difficulties in the learning, for example, they can talk with others or ask for help. they should constantly encourage themselves to overcome difficulties and learn to use various learning strategies. female students should learn how to optimize their learning strategies to improve learning efficiency and use more emotional learning strategies, such as communication andco-operation.2.6 both male and female students should know how to use their strengths as well as avoid their weaknesses and learn from each other to make progressin practical english learning, everyone’s strengths and weaknesses vary from each other. we should recognize and respect the gender differences in english learning. at the same time, we should be aware of our special problems and adapt the appropriate solutions. by doing these, everyone can become a successful english learner.【references】[1] zhou chunfeng. research on gender differences and strategies of high school students ’ english learning[c]. master’s degree thesis, east china normal university, 2005.[2] wei li. comparative study of english learning differences between male and female students[m]. heilongjiang science and technology information, 2008。

Part Ⅱ Proofreading and Error Correction (15 min)The following passage contains TEN errors. Each line contains a maximum of ONE error. In each case, only ONE word is involved. You should proofread the passage and correct it in the following way.For a wrong word, underline the wrong word and write the correct one in the blank provided at the end of the line.For a missing word, mark the position of the missing word with a “∧” sign and write the word you believe to be missing in the blank provided at the end of the line.For an unnecessary word cross out the unnecessary word with a slash “/’ and put the word in the blank provided at the end of the line.ExampleWhen∧art museum wants a new exhibit, (1) anit never／buys things in finished form and hangs (2) neverthem on the wall. When a natural history museumwants an exhibition, it must often build it. (3) exhibitThe hunter-gatherer tribes that today live as our prehistoric 1.______ human ancestors consume primarily a vegetable diet supplementing 2._____with animal foods. An analysis of 58 societies of modem hunter-gatherers, including the Kung of southern Africa, revealed that onehalf emphasize gathering plant foods, one-third concentrate on fishingand only one-sixth are primarily hunters. Overall, two-thirdsand more of the hunter-gatherer’s calories come from plants. Detailed 3.______ studies of the Kung by the food scientists at the University ofLondon, showed that gathering is a more productive source of foodthan is hunting. An hour of hunting yields in average about 100 4.______ edible calories, as an hour of gathering produces 240. 5.______ Plant foods provide for 60 percent to 80 percent of the Kung 6._______ diet, and no one goes hungry when the hunt fails. Interestingly, ifthey escape fatal infections or accidents, these contemporaryaborigines live to old ages despite of the absence of medical care. 7._______ They experience no obesity, no middle-aged spread, little dentaldecay, no high blood pressure, on heart disease, and their bloodcholesterol levels are very low( about half of the average American 8._______ adult), if no one is suggesting what we return to an aboriginal life 9.________ style, we certainly could use their eating habits as a model for 10.________ healthier diet.The grammatical words which play so large a part in Englishgrammar are for the most part sharply and obviously different 1._______ from the lexical words. A rough and ready difference which mayseem the most obvious is that grammatical words have“ lessmeaning”, but in fact some grammarians have called them 2._______“empty” words as opposed in the “full” words of vocabulary. 3.________ But this is a rather misled way of expressing the distinction. 4._________ Although a word like the is not the name of something as man is,it is very far away from being meaningless; there is a sharp 5._________ difference in meaning between “man is vile and” “the man isvile”, yet the is the single vehicle of this difference in meaning. 6.________ Moreover, grammatical words differ considerably amongthemselves as the amount of meaning they have, even in the 7.________ lexical sense. Another name for the grammatical words has been“little words”. But size is by no mean a good criterion for 8._________ distinguishing the grammatical words of English, when weconsider that we have lexical words as go, man, say, car. Apart 9.________ from this, however, there is a good deal of truth in what somepeople say: we certainly do create a great number of obscurity 10.________ when we omit them. This is illustrated not only in the poetry ofRobert Browning but in the prose of telegrams and newspaper headlines.During the early years of this century, wheat was seen as thevery lifeblood of Western Canada. People on city streets watchedthe yields and the price of wheat in almost as much feeling as if 1._______ they were growers. The marketing of wheat became an increasing 2._______ favorite topic of conversation.War set the stage for the most dramatic events in marketingthe western crop. For years, farmers mistrusted speculative grainselling as carried on through the Winnipeg Grain Exchange.Wheat prices were generally low in the autumn, so farmers could 3._______ not wait for markets to improve. It had happened too often thatthey sold their wheat soon shortly after harvest when farm debts 4.________ were coming due, just to see prices rising and speculators getting rich. 5._______ On various occasions, producer groups, asked firmer control, 6._______ but the government had no wish to become involving, at 7.______ least not until wartime when wheat prices threatened to runwild.Anxious to check inflation and rising life costs, the federal 8.______ government appointed a board of grain supervisors to deal withdeliveries from the crops of 1917 and 1918. Grain Exchangetrading was suspended, and farmers sold at prices fixed by theboard. To handle with the crop of 1919, the government appointed 9.______ the first Canadian Wheat Board, with total authority to 10.______ buy, sell, and set prices.There are great impediments to the general use of a standardin pronunciation comparable to that existing in spelling (orthography).One is the fact that pronunciation is learnt “naturally”and unconsciously, and orthography is learnt 1__________ deliberately and consciously. Large numbers of us, in fact,remain throughout our lives quite unconscious with what our speech 2.__________ sounds like when we speak out, and it often comes as a shock 3.__________ when we firstly hear a recording of ourselves. It is not a voice we 4._________ recognize at once, whereas our own handwriting is somethingwhich we almost always know. We begin the natural learning 5.__________ of pronunciation long before we start learning to read or write,and in our early years we went on unconsciously imitating and 6.__________ practicing the pronunciation of those around us for many morehours per every day than we ever have to spend learning even our 7.___________ difficult English spelling. This is “natural”, therefore, that our 8.__________ speech-sounds should be those of our immediate circle; after all,as we have seen, speech operates as a means of holding a community 9.__________ and giving a sense of 'belonging'. We learn quite early torecognize a “stranger”, someone who speaks with anaccent of a different community-perhaps only a few miles far. 10.__________ 2003改错Demographic indicators show that Americans in the postwarperiod were more eager than ever to establish families. They quicklybrought down the age at marriage for both men and women and broughtthe birth rate to a twentieth century height after more than a hundred (1)______ years of a steady decline, producing the “baby boom.” These young(2)_______ adults established a trend of early marriage and relatively largefamilies that Went for more than two decades and caused a major (3)_______ but temporary reversal of long-term demographic patterns. Fromthe 1940S through the early 1960s, Americans married at a high rate (4)________ and at a younger age than their Europe counterparts. (5)________ Less noted but equally more significant, the men and women on who (6)________ formed families between 1940 and 1960 nevertheless reduced the (7)________ divorce rate after a postwar peak; their marriages remained intact toa greater extent than did that of couples who married in earlier as well (8)________ as later decades. Since the United States maintained its dubious (9)_________ distinction of having the highest divorce rate in the world, thetemporary decline in divorce did not occur in the same extent in (10)_________ Europe. Contrary to fears of the experts, the role of breadwinner andhomemaker was not abandoned.2004改错One of the most important non-legislative functions of the U.S Congressis the power to investigate. This power is usually delegated to committees - either standing committees, special committees set for a specific (1)________ purpose, or joint committees consisted of members of both houses. (2)________ Investigations are held to gather information on the need forfuture legislation, to test the effectiveness of laws already passed,to inquire into the qualifications and performance of members andofficials of the other branches, and in rare occasions, to lay the (3)________ groundwork for impeachment proceedings. Frequently, committeesrely outside experts to assist in conducting investigative hearings (4)_________ and to make out detailed studies of issues. (5)_________ There are important corollaries to the investigative power. Oneis the power to publicize investigations and its results. Most (6)_________ committee hearings are open to public and are reported (7)__________ widely in the mass media. Congressional investigationsnevertheless represent one important tool available to lawmakers (8)__________ to inform the citizenry and to arouse public interests in national issues.(9)________ Congressional committees also have the power to compeltestimony from unwilling witnesses, and to cite for contemptof Congress witnesses who refuse to testify and for perjurythese who give false testimony. (10)_________ 2005改错The University as BusinessA number of colleges and universities have announced steeptuition increases for next year much steeper than the current,very low, rate of inflation. They say the increases are needed becauseof a loss in value of university endowments heavily investing in common 1 stock. I am skeptical. A business firm chooses the price that maximizesits net revenues, irrespective fluctuations in income; and increasingly the 2 outlook of universities in the United States is indistinguishable from those of 3 business firms. The rise in tuitions may reflect the fact economic uncertainty 4 increases the demand for education. The biggest cost of beingin the school is foregoing income from a job (this is primarily a factor in 5 graduate and professional-school tuition); the poor one's job prospects, 6 the more sense it makes to reallocate time from the job market to education,in order to make oneself more marketable.The ways which universities make themselves attractive to students 7 include soft majors, student evaluations of teachers, giving studentsa governance role, and eliminate required courses. 8 Sky-high tuitions have caused universities to regard their students as customers. Just as business firms sometimes collude to shorten the 9 rigors of competition, universities collude to minimize the cost to them of the athletes whom they recruit in order to stimulate alumni donations, so the best athletes now often bypass higher education in order to obtain salaries earlierfrom professional teams. And until they were stopped by the antitrust authorities, the Ivy League schools colluded to limit competition for the best students, by agreeing not to award scholarships on the basis of merit rather than purelyof need-just like business firms agreeing not to give discounts on their best 10 customer.2006改错We use language primarily as a means of communication withother human beings. Each of us shares with the community in which welive a store of words and meanings as well as agreeing conventions as 1_______ to the way in which words should be arranged to convey a particular 2______ message: the English speaker has in his disposal vocabulary and a3_______ set of grammatical rules which enables him to communicate his4______ thoughts and feelings, in a variety of styles, to the other English 5_______ speakers. His vocabulary, in particular, both that which he uses activelyand that which he recognizes, increases in size as he growsold as a result of education and experience. 6______ But, whether the language store is relatively small or large, the systemremains no more, than a psychological reality for tike inpidual, unlesshe has a means of expressing it in terms able to be seen by another 7_______ member of his linguistic community; he bas to give tile system aconcrete transmission form. We take it for granted rice’ two m ost8_______ common forms of transmission-by means of sounds produced by ourvocal organs (speech) or by visual signs (writing). And these are 9___ ___ among most striking of human achievements. 10_______2007改错From what has been said, it must be clear that no one canmake very positive statements about how language originated.There is no material in any language today and in the earliest 1records of ancient languages show us language in a new and 2emerging state. It is often said, of course, that the language 3 ___ originated in cries of anger, fear, pain and pleasure, and the 4 necessary evidence is entirely lacking: there are no remotetribes, no ancient records, providing evidence ofa language with a large proportion of such cries 5than we find in English. It is true that the absenceof such evidence does not disprove the theory, but in 6other grounds too the theory is not very attractive.People of all races and languages make rather similarnoises in return to pain or pleasure. The fact that7such noises are similar on the lips of Frenchmenand Malaysians whose languages are utterly different,serves to emphasize on the fundamental difference8__________ between these noises and language proper. We maysay that the cries of pain or chortles of amusementare largely reflex actions, instinctive to large extent, 9whereas language proper does not consist of signsbut of these that have to be learnt and that are10__________ wholly conventional.2008年改错The desire to use language as a sign of national identityis a very natural one，and in result language has played a 1__________ prominent part in national moves．Men have often felt the need 2__________ to cultivate a given language to show that they are distinctive 3____________ from another race．whose hegemony they resent．At the time the 4.___________ United States split off from Britain，for example，therewere proposals that independence should be linguistically accepted by 5._________ the use of a different language from those of Britain． 6.__________ There was even one proposal that Americans should adopt Hebrew．Others favoured the adoption of Greek，though，as one man put it，things would certainly be simpler for Americans if they stuck on to 7.___________ English and made the British learn Greek．At the end，as everyone 8.___________ knows，the two countries adopted the practical and satisfactorysolution of carrying with the same language as before．Sincenearly two hundred years now，they have shown the 9.____________ world that political independence and national identity can be 10.___________ complete without sacrificing the enormous mutual advantages of a common language．2009年改错The previous section has shown how quickly a rhyme passesfrom one school child to the next and illustrates the further difference (1)__ ___ between shcool lore and nursery lore. In nursery lore a verse, learntin early childhood, is not usually passed on again when the little listener (2)__ ___ has grown up, and has children of their own, or even grandchildren. (3)___ __ The period between learning a nursery rhyme and transmittingIt may be something from twenty to seventy years. With the playground (4)__ ___ lore, therefore, a rhyme may be excitedly passed on whtin the very hour (5)__ ___it is learnt; and in the general, it passes between children of the (6)___ __ same age, or nearly so, since it is uncommon for the difference in agebetween playmates to be more than five years. If therefore, a playgroundrhyme can be shown to have been currently for a hundred years, or (7)___ __ even just for fifty, it follows that it has been retransmitting overand over; very possibly it has passed along a chain of two or three (8)__ ___ hundred young hearers and tellers, and the wonder is that it remains live (9)___ __ after so much handling, to let alone that it bears resemblance to the (10)__ __ original wording.2012PART IV PROOFREADING & ERROR CORRECTION (15 MIN)The passage contains TEN errors．Each indicated line contains a maximum of ONE error．In each case, only ONE word is involved．You should proof-read the passage and correct it in the following way:For a wrong word, underline the wrong word and write the correct one in the blank provided at the end of the line.For a missing word, mark the position of the missing word with a "L" sign and write the word you believe to be missing in the blank provided at the end of the line.For an unnecessary word, cross the unnecessary word with a slash "/" and put the word in the blank provided at the end of the line.EXAMPLEWhen A art museum wants a new exhibit, (1) anit never buys things in finished form and hangs (2) neverthem on the wall．When a natural history museumwants an exhibition, it must often build it．(3) exhibitProofread the given passage on ANSWER SHEET TWO as instructed.The central problem of translating has always been whether to translate literally or freely．The argument has been going since at least the first (1) ______ century B.C．Up to the beginning of the 19th century, many writersfavoured certain kind of “free” translation: the sp irit, not the letter; the (2) _______ sense not the word; the message rather the form; the matter not (3) _______ the manner．This is the often revolutionary slogan of writers who (4) _______ wanted the truth to be read and understood．Then in the turn of 19th (5) _______ century, when the study of cultural anthropology suggested thatthe linguistic barriers were insuperable and that the language (6) _______ was entirely the product of culture, the view translation was impossible (7) _______ gained some currency, and with it that, if was attempted at all, it must be as (8) _____ literal as possible．This view culminated the statement of the (9) _______ extreme “literalists” Walter Benjamin and Vladimir Nobokov.The argument was theoretical: the purpose of the translation, thenature of the readership, the type of the text, was not discussed．Toooften, writer, translator and reader were implicitly identified witheach other．Now, the context has changed, and the basic problem remains．(10)_____。

Performance AttributionAnalysisPrefacePerformance attribution interprets how investors achieve their performance and measures the sources of value added to a portfolio. This guide describes how returns, relative to a benchmark, are broken down into attribution effects to determine how investors achieve performance and measure the sources of value added to a portfolio.About Performance AttributionFor investment managers to evaluate their job performance, they need to know how they achieved their performance results. In particular, they need to know whether their success is the result of their ability to effectively allocate their portfolio’s assets to various segments, their ability to effectively select securities within a given segment or the combined effect of their selection and allocation within a segment. Performance attribution interprets how investors achieve their performance and measures the sources of value added to a portfolio. To determine success, investors establish a benchmark, which they seek to outperform. Value added is the amount the return achieves in excess of the benchmark.Different Attribution MethodsThere are generally considered to be three basic forms of attribution. These include multi-factor analysis, style analysis and return decomposition analysis. The highlights of each are as follows: Multi-Factor Analysis· Attributes performance to factors such as P/E ratio, economy, bond durations, etc· Used by academics and sophisticated firms· More difficult to calculate· Requires a great deal of data (economic/fundamental)· Difficult to explain· Not widely accepted in industryStyle Analysis· Developed by noble laureate William Sharpe (Sharpe ratio)· Uses portfolio rates of return to determine investment style· Easy to calculate (requires benchmark and portfolio returns)· Easy to explain (uses regression analysis)· Not widely accepted in industryReturn Decomposition Analysis· Attributes performance vs. benchmarks· Can focus on allocation (top/down approach) or selection (bottom up approach)· Easy to calculate ( requires benchmark and portfolio returns and weights)· Easy to understand and explain· Used by large portion of investment community· Widely accepted in industryAs the return decomposition analysis is most widely used and accepted, this is the model we will examine.About Attribution EffectsIn a return decomposition analysis model, value added to a portfolio’s return is commonly referred to as the active management effect. The active management effect is the difference between the total portfolio return and total benchmark return. It is also the sum of the following investment decisions or effects:• Allocation• Selection• InteractionDefining the Allocation EffectThe allocation effect measures an investment manager’s ability to effectively allocate their portfolio’s assets to various segments. A segment refers to assets or securities that are grouped within a certain classification such as Equity, Fixed, or Technology. The allocation effect determines whether the overweighting or underweighting of segments relative to a benchmark contributes positively or negatively to the overall portfolio return. Positive allocation occurs when the portfolio is overweighted in a segment that outperforms the benchmark and underweighted in a segment that underperforms the benchmark. Negative allocation occurs when the portfolio is overweighted in a segment that underperforms the benchmark and underweighted in a segment that outperforms the benchmark.Scenario 1A positive allocation effect occurred because the portfolio weight was greater than the benchmark weight and the benchmark return was greater than the total benchmark return. The investment manager overallocated assets to a segment that outperformed the total benchmark.Scenario 2A negative allocation effect occurred because the portfolio weight was greater than the benchmark weight and the benchmark return was less than the total benchmark return. The investment manager overallocated assets to a segment that underperformed the total benchmark. Scenario 3A negative allocation effect occurred because the portfolio weight was less than the benchmark weight and the benchmark return was greater than the total benchmark return. The investment manager underallocated assets to a segment that outperformed the total benchmark. Scenario 4A positive allocation effect occurred because the portfolio weight was less than the benchmark weight and the benchmark return was less than the total benchmark return. The investment manager underallocated assets to a segment that underperformed the benchmark. Calculating the Allocation EffectThe following calculation is used to calculate a portfolio’s allocation effect.The following example shows how return decomposition analysis calculates the portfolio’s allocation effect.Example:Benchmark = S&P 500Benchmark segment = S&P 500 TechnologyBenchmark return = 9%Benchmark weight = 7%Portfolio technology return = 8%Portfolio technology weight = 15%Total benchmark return = 7%The allocation effect can be expressed in percentages or basis points (bps) as follows:The allocation effect in this example is positive because the manager overweighted the Portfolio Technology segment, which performed better than the total benchmark for the portfolio.Defining the Selection EffectThe selection effect measures the investment manager’s ability to select securities within a given segment relative to a benchmark. The over or underperformance of the portfolio is weighted by the benchmark weight, therefore, selection is not affected by the manager’s allocation to the segment. The weight of the segment in the portfolio determines the size of the effect—the larger the segment, the larger the effect is, positive or negative.The table that follows displays two possible scenarios for the selection effect:Selection Effects TableScenario 1A positive selection effect occurred because the portfolio return was greater than the benchmark return. The investment manager made good decisions in selecting securities that, as a whole, outperformed similar securities in the benchmark.Scenario 2A negative selection effect occurred because the portfolio return was less than the benchmark return. The investment manager made poor decisions in selecting securities that, as a whole, underperformed similar securities in the benchmark.Calculating the Selection EffectThe return decomposition model uses the following calculation to calculate a portfolio’s selection effect:The following example shows how a portfolio’s selection effect is calculated.Example:Benchmark = S&P 500Benchmark segment = S&P 500 TechnologyBenchmark return = 9%Benchmark weight = 7%Portfolio technology return = 8%Portfolio technology weight = 15%Total benchmark return = 7%The selection effect can be expressed in percentages or basis points (bps) as follows:There is a negative selection effect in this example because the manager selected securities that did not perform as well as the securities in the benchmark for the same segment.Defining the Interaction EffectThe interaction effect measures the combined impact of an investment manager’s selection and allocation decisions within a segment. For example, if an investment manager had superior selection and overweighted that particular segment, the interaction effect is positive. If an investment manager had superior selection, but underweighted that segment, the interaction effect is negative. In this case, the investment manager did not take advantage of the superior selection by allocating more assets to that segment.Since many investment managers consider the interaction effect to be part of the selection or the allocation, it is often combined with the either effect.The table that follows displays four possible scenarios for the interaction effect:Interaction Effects TableScenario 1A positive interaction effect occurred because the portfolio weight was greater than the benchmark weight and the portfolio return was greater than the benchmark return. The investment manager exercised good selection and overallocated assets to that segment.Scenario 2A negative interaction effect occurred because the portfolio weight was greater than the benchmark weight and the portfolio return was less than the benchmark return. While the investment manager overweighted the portfolio securities for a given segment, that segment underperformed against the benchmark return for the same segment.Scenario 3A negative interaction effect occurred because the portfolio’s weight was less than the benchmark weight and the portfolio’s return was greater than the benchmark return. The investment manager underweighted the segment with good selection. The manager exercised good selection but poor allocation.Scenario 4A positive interaction effect occurred because the portfolio weight was less than the benchmark weight and the portfolio return was less than the benchmark return. The impact of the joint effects is positive because the manager’s decision to underweight a poor performing segment was a good decision.Calculating the Interaction EffectThe Performance Attribution module uses the following calculation to calculate a portfolio’s interaction effect:The following example shows how the return decomposition model calculates a portfolio’s interaction effect.Example:Benchmark = S&P 500Benchmark segment = S&P 500 TechnologyBenchmark return = 9%Benchmark weight = 7%Portfolio technology return = 8%Portfolio technology weight = 15%Total benchmark return = 7%The interaction effect can be expressed in percentages or basis points (bps) as follows:There is a negative interaction effect in this example because the manager overallocated securities for a segment that performed poorly relative to the benchmark.Defining the Active Management EffectThe active management effect is the sum of the selection, allocation, and interaction effects. It is also the difference between the total portfolio return and the total benchmark return. You can use the active management effect to determine the amount the investment manager has added to a portfolio’s return. If the active management effect is positive, the investment manager has contributed positively to the portfolio’s return. If the active management effect is negative, the investment manager has not contributed positively to the portfolio’s return.The return decomposition model uses the following calculation to calculate the active management effect:。

performance evaluation methodsPerformance evaluation is one of the most critical aspects of any organization. It refers to the process of assessing an employee's work performance against predetermined standards of performance. The primary objective of performance evaluation is to identify the strengths and weaknesses of an employee and provide feedback to help them improve their work.There are several methods of performance evaluation used in organizations. This article will discuss some of these methods in detail.1. Self-EvaluationSelf-evaluation is a method of performance evaluation in which employees assess their performance against predetermined goals and standards. This method involves employees reflecting on their work, identifying their strengths and weaknesses, and documenting their accomplishments.Self-evaluation can be a valuable tool, as it allows employees to take ownership of their performance, encourages self-reflection, and can improve communication between employee and employer.2. Peer EvaluationPeer evaluation is a method of performance evaluation in which an employee's performance is assessed by their colleagues. This method involves colleagues providing feedback on the employee's work, interpersonal skills, and overall contribution to the team.Peer evaluation can be a useful tool, as it provides employees with valuable feedback from their colleagues, encourages teamwork and collaboration, and can help identify areas for improvement.3. Manager EvaluationManager evaluation is a method of performance evaluation in which an employee's performance is assessed by their manager. This method involves the manager providing feedback on the employee's work performance, as well as their contribution to the organization.Manager evaluation can be effective if the manager has a good understanding of the employee's work and can provide constructive feedback. However, this method can also be biased, as managers may be influenced by personal biases or factors outside of work.4. 360-Degree Feedback360-degree feedback is a method of performance evaluation in which an employee's performance is assessed by multiple sources, including colleagues, managers, and customers. This method involves gathering feedback from a variety of perspectives to provide a comprehensive view of theemployee's performance.360-degree feedback can be an effective tool, as it provides a more complete picture of an employee's performance, encourages collaboration and teamwork, and can help identify areas for improvement.In conclusion, there are several methods of performance evaluation used in organizations. Each method has itsstrengths and weaknesses, and the most effective method will depend on the objectives of the evaluation and the culture of the organization. Employers should consider using a varietyof methods to provide a comprehensive view of an employee's performance and encourage ongoing development and improvement.。

Low Temperature Viscosity Measurements -Lovis for Battery ElectrolytesRelevant for: battery industry, electrochemical research, automotive industryPerform viscosity measurements down to -20 °C with Lovis 2000 M/ME with cooling option. Test even highly corrosive solvents for ion salts by using unbreakable PCTFE capillaries with small filling volumes (110 µL or 450 µL). Handling of the sample inside a glove box filled with inert gas and a hermetically closed system prevent contamination or evaporation of the sample.1 IntroductionSince the introduction of the lithium-ion batteries in 1990, the interest in this technology has emerged steadily, not only for portable devices but also for the automotive industry. Their high energy density as well as outstanding cycle stability are the main reasons for commercial success, but several problems arise with the usage of the most common non-aqueous electrolytes, which contain lithiumhexafluoro-phosphate (LiPF6) as conductive salt and a mixture of cyclic and non-cyclic organic carbonates.In addition to the high purity required of all used solvents (e.g. traces of protic impurities such as water can cause severe deterioration of the cell performance after a short life / cycle time) the cell performance has to be stable over a broad temperature range from arctic to tropical conditions without any significant degradation. Therefore, an exact characterization of newly developed electrolytes at different temperatures is an essential part in the lithium-ion cell research today. These challenges have to be considered for every other upcoming battery systems like magnesium ion cells or sulfur cells, too.Therefore, research companies use different standard electrochemical measurements for monitoring batteries. In this connection viscosity, conductivity and – if required – density measurements of the electrolytes support those investigations.The performance of the charge and discharge rate of a rechargeable battery, that is the ion transport, is characterized by the ion conductivity, which depends on the viscosity and the dielectric constant.The viscosity of the solvent, in which the ion salt is solved, affects the mobility of ions, as shown in the Stokes-Einstein equation; mobility is inversely proportional to the viscosity:r ... radius of the solvated ionBased on those viscosity measurements important conclusions on the wettability of the electrode /electro-lyte interface can be drawn, too. Fast, accurate and reproducible viscosity measurement over a wide temperature range is highly desirable for successful development of new electrolyte systems.This application report shows how the Lovis can be used for electrolyte measurements even at tempera-tures below zero. The Lovis, equipped with coolingoption and in combination with the capillary made of PCTFE, enables measurement of highly corrosive substances over a wide temperature range.2 Instrumentation2.1Lovis 2000 M/ME Microviscometer with Cooling OptionFigure 1: Lovis 2000 M with cooling optionThe Lovis 2000 M/ME Microviscometer measures the rolling time of a ball inside an inclined capillary.Variable inclination angles allow for measurements at different shear rates. Temperature control via Peltier elements is extremely fast and provides utmost accuracy.For measuring at temperatures below zero, the Lovis ME Module can be equipped with a lowtemperature option. In combination with a recirculating cooler, it is possible to measure at temperatures as low as -20 °C (lower temperatures down to -30 °C on request, depending on the cooling liquid of the recirculating cooling, ambient temperature and ambient air humidity).The integrated software calculates the kinematic or dynamic viscosity, provided the sample's density value is known.Figure 2: Lovis PCTFE capillariesWith the PCTFE capillaries it is possible to measure nearly every liquid, also corrosive, aggressive or hazardous solvents and electrolytes.The measuring viscosity of a PCTFE capillary ranges from 0.8 mPa.s to 160 mPa.s.Used material:▪ Capillary: PCTFE short (110 µL) ▪ Capillary diameter: 1.62mm ▪ Ball material: Steel ▪ Ball diameter: 1.5 mm2.3 Additional Equipment▪ Glove box filled with argon.▪Circulation cooler plus insulated hoses. How to set up the cooling is precisely described in the documentation of Lovis 2000 M/ME.3MeasurementAll determinations were performed manually without autosampler. The viscosity measurements were performed in a temperature range from -20 °C to +60 °C with steps of 5 °C or 10 °. For temperature table scans (TTS) two density values at two different reference temperatures were typed in manually in the "Quick Settings" ("Lovis Density TS/TTS") for every sample. The instrument automatically extrapolated the missing temperature / density values by linearextrapolation. The density values for the manual input were determined with the SVM™.Every scan was performed twice in order to obtain a repeat determination. To check the reproducibility, all measurements were performed with Lovis and SVM™ in parallel.3.1 Samples▪Different mixtures of organic carbonates, which contain lithiumhexafluorophosphate as conductive salt – for lithium ion batteries (LIB), either commercial available standardelectrolytes or newly developed electrolyte solutions.▪Solvents containing a polar organic solvent and dioxolane added with Li-sulfur-compounds as conductive salts – for future Li-S-cell systems (LiS).▪Solvents containing a polar organic solvent plus Mg-compounds as conductive salts – for prospective Mg-ion batteries.3.2 Instrument Settings Measuring Method: Temperature Table Scan (TTS) Measuring Settings:▪ Temperature: scan between -20 °C to +60 °C ▪ Equilibration Time: no ▪ Measurement Cycles: 3▪ Measuring Angle: Auto Angle * ▪ Variation Coefficient:0.4 % for standard electrolytes ▪ Measuring Distance: Short* Adjustment was performed over an angle range from of 20° to 70° in 10° steps3.3 Filling of the CapillaryAll samples were manually filled in an argon glove box under inert conditions. For each measurement a new steel ball was used to avoid any cross contamination from one measurement to the other. After closing the capillary with the appropriate plug, the hermetically sealed capillary was removed from the glove box.3.4 CleaningThe capillary was cleaned thoroughly with smallbrushes after every test sequence. Ethanol, deionized water and other appropriate solvents were used as cleaning liquids. If necessary, the capillary was placed into an ultrasonic bath (approximately 10 to 20 min, 30 °C, water plus standard detergent). Afterwards the capillary was dried under a pressure-less nitrogen stream.4 ResultsFigure 3: Reproducibility check; standard Li-ion electrolyte V24 measured with Lovis and SVM ™ from +20 °C to -20 °C4.4Temperature Profile of Li-polysulfide4.5Checking the Influence of Conducting Salt5ConclusionBy using the Lovis 2000 M/ME equipped with cooling option, it is possible to perform measurements from -20 °C up to +100 °C. In combination with the capillary made of PCTFE even extremely corrosive substances can be measured under hermetically sealed atmosphere. This allows users to measure theviscosity of electrolytes, which might be destroyed or changed in structure by air and/or air humidity.▪ The small capillary sizes require only littlesample volume (starting from 110 µL). ▪ The small diameter of the PCTFE capillary(1.62 mm) enables also the measurement of very low-viscosity samples (viscosity range from 0.8 mPa.s to 160 mPa.s).▪ The cooling option allows for viscositymeasurements down to -20 °C (lowertemperatures down to -30 °C are possible on request and depending on ambient conditions).▪ The closed system avoids any contaminationand evaporation.▪ The variable inclination angle of themeasurement allows for the variation of the shear rate.▪ Lovis 2000 M/ME is highly modular; it can becombined with DMA™ M Density Meters for automated calculation of dynamic andkinematic viscosity. It can also be combined with an Xsample™ sample changer (see Figure 8) for automatic filling and cleaning of the capillary and measurements with high sample throughput.6ReferencesSpecial thanks to DI Gisela Fauler and Ms. Katja Kapper from VARTA Micro Innovation GmbH who tested the Lovis with cooling option and the PCTFE capillaries and supported Anton Paar with their measurement data.Contact Anton Paar GmbH Tel: +43 316 257-0****************************|。

Performance Analysis of a Real-Time JavaExecution Environment for IEC 61499Kleanthis C. Thramboulidis, George S. Doukas, Alkiviadis ZoupasElectrical and Computer Engineering, University of Patras, 26500, Greece(e-mail: {thrambo, gdoukas}@ upatras.gr, azoupas@upnet.gr).Abstract: The IEC 61499 standard enhances the 1131 Function Block model to exploit the advantages of the object technology in the industrial automation domain. Several prototype development environments have been developed by various research groups and the first commercial tools that support this model are already in the market. However, the absence of mature run-time environments that will allow the execution of IEC 61499 compliant applications is still evident. Even for the existing ones, there is no evidence about their efficiency to meet real-time constraints imposed by this kind of applications. Benchmarking of run-time environment is required to prove that these environments can be considered for the development of real-time applications. In this paper, a benchmarking framework is described and it is used to analyze the performance of an IEC 61499 run-time environment that is based on the real-time Java Specification. The recently released IBM and Sun RTSJ implementations are used to demonstrate the effectiveness of the RTSJ based framework. Performance results prove the applicability of the proposed run-time environment and the model driven approach that was adopted, in the control and automation domain.Keywords: IEC 61499, Function Block, Real-Time Java, RTSJ, run-time environment, performance analysis, benchmark.1. INTRODUCTIONThe Function Block (FB) construct is a well-known and widely used by control engineers. It was first introduced by the IEC 61131 standard on programming languages for programmable logic controllers, and was later extended by the IEC’s 61499 standard (IEC 61499, 2005) to share many of the well defined and already widely acknowledged benefits of object technology. The IEC 61499 describes a methodology that utilizes the FB as the main building block and defines the way that FBs can be used to define robust, re-usable software components that constitute complex distributed control systems (DCSs). Complete control applications, can be defined by one or more FB Networks (FBNs) that specify event and data flow among function block or sub application instances. The event flow determines the scheduling and execution of the operations specified by each function block’s algorithm(s).The majority of control and automation software deals with applications with more or less strict timing constrains. Although IEC 61499 does not provide a way to capture these constraints, the final executable should be deterministic, thus runtime timing behaviour of execution environments should be provided. Moreover, the standard intentionally leaves a lot runtime issues open to be defined later by developers. This results in IEC 61499 runtime implementations that demonstrate various execution behaviours, which may confuse control application engineers. To address the above issues, benchmarking proves to be a very useful tool, enabling better understanding and evaluation of the runtime behavior of IEC 61499 compliant execution environments.In this paper a framework for the benchmarking of IEC 61499 FB applications is described. The FB instance and the FB network are analyzed and specific measurements for the benchmarking are defined. A real-time Java based run-time environment, the RTSJ-AXE, is used to demonstrate the proposed benchmark. This work also proves the efficiency of the RTSJ-AXE run time environment for the real-time domain.The remainder of this paper is organized as follows: Background and related work is presented in the next section. The real-time Java execution environment that is used as basis for this work is described in section 3. Performance analysis and results are presented in section 4, and finally the paper is concluded in section 5.2. BACKGROUND AND RELATED WORKThe IEC 61499 standard enhances the 1131 Function Block model to exploit the benefits of the object technology in the industrial automation domain. It also proposes a methodology for the development of control applications based on the component concept. The Function Block is defined as the main building block that can be used by the control engineer to define the model of his application. The FB type, which defines the structure and behaviour of FB instances, is composed of a head and a body. The body encapsulates the algorithms and internal data; it is connected to input eventsProceedings of the 13th IFAC Symposium on Information Control Problems in ManufacturingMoscow, Russia, June 3-5, 2009and generates output events. The head captures the dynamics of the FB; it consumes the input events, triggers the execution of algorithms and generates output events. The dynamics of the head are specified by a special kind of state transition diagram that is called execution control chart.Several research groups are working to provide run-timeenvironments for IEC 61499 based DCSs. The FBRT () is the first run-time environment for IEC 61499 based control applications. FBRT utilizes Java but it supports neither timeliness, nor the run-time re-configurability of the system. The method invocation paradigm, which is adopted for the implementation of event connections, and the non-determinism of the Java platform make the environment inappropriate for real-time applications and imposes many restrictions to its use in real world applications. IsaGraph (/), a well known commercially available toolset for the IEC 61131, includes in its latest version support for IEC 61499. The proposed execution environment, even though not completely compliant with the standard, provides the first commercially available tool that supports it. The Fuber execution environment is under development at Chalmers University of Technology (Cengic et al. 2006). The 4DIAC-RTE (/) is a runtime environment that is provided in a PC version and an embedded ARM7 based version (Sunder et al. 2007) . These environments are not currently described in publicly available documents regarding the adopted implementation policies; performance measurements are not available. A FB-based model to support configuration and reconfiguration of DCSs is proposed and its implementation on real-time Java is discussed in Brennan et al. (2002). However, no proof of concept neither a prototype implementation is provided for the above real-time Java based approach. Benchmarking of IEC61499-compliant runtime environments is an issue of interest for researchers for the last couple of years. Soundararajan et al. (2007), study an agent-based software design pattern, utilized for the benchmarking of real-time distributed control systems, such as IEC61499, and applied it on a hybrid physical/simulation environment. Sunder et al. (2007), proposed a set of benchmarks to evaluate the capabilities (performance) of different IEC 61499 runtime environments regarding the execution of basic FBs and FB networks. Key steps of the basic FB execution procedure were identified and utilized to produce timing characteristics of FB execution on two different environments the C++FBRT and the MARTE. However, the proposed benchmark does not address the reconfiguration related characteristics of the run time environment. Both run-time environments are not described in any publicly available publication so it is not possible to understand the different behaviours for the described execution application scenarios, i.e., mixed serial and parallel and tree structure. The C++ FBRT run time environment is executed on a C167 Infineon microcontroller without operating system so it is evident that the control application is in the form of a monolithic application. This means that the component based model that is in the heart of the IEC 61499 FB model is not supported bythis run-time environment and thus the providedmeasurements cannot be considered for comparison.3. THE REAL-TIME JAVA EXECUTION ENVIRONMENT3.1. The Real-Time Java SpecificationIt is widely accepted that Java applications running on a general-purpose JVM can only meet soft RT requirements that are at the level of hundreds of milliseconds. The most important reasons for this nondeterministic performance of Java run-time environment are the dynamic class loading, the garbage collection and the native code compilation. However, a wide variety of researchers recognized the potential of Java in real-time applications and invented techniques to address most of the above issues. The Real-time Java Specification for Java (RTSJ) (Bollela, G., et al., 2000) is the most important and systematic approach to provide an extension to address the limitations of the Java language. RTSJ defines modifications and new features to the semantics of the JVM such as scheduling properties suitable for real-time applications with provisions for periodic and sporadic tasks, support for deadlines and CPU time budgets, and means to avoid garbage collection (GC) delays. RTSJ, apart from favouring productivity and portability, allows programmers to write real-time programs in a type-safe language, reducing many opportunities for catastrophic failures.RTSJ received a high acceptance from vendors and a variety of real-time Java implementations and products were introduced in the market. The first reference implementation, the RI, was developed by TimeSys () and runs on all Linux versions. Jamaica () runs on a wide variety of real time operating systems. It provides the programmer with an optionally activated real-time GC, which makes memory programming a more relaxed process compared to the RTSJ one. Other JVM implementations claiming compliance with RTSJ are jRate (/) and the JVM developed by aJile Systems (), which running directly on hardware, opens new horizons on the application of RTSJ in embedded systems domain. The recently released IBM and Sun RTSJ-compliant implementations are expected to give a great push in the use of real-time Java. IBM’s real-time Java is part of the WebSphere Real Time V1.0 (/software/webservers/realtime/) that contains a stand-alone Java Standard Edition 5 Runtime Environment to support real-time applications. Sun real-time Java (/javase/technologies/realtime/index.jsp) is a standards-based extension of the J2SE 5.0 platform designed to address, according to Sun, the growing demand for predictable computing in industries such as aerospace, financial services, industrial automation and telecommunications, as well as in scientific research.3.2. The RTSJ-Archimedes Execution EnvironmentThe RTSJ-AXE is a run-time environment that was developed in the context of the Archimedes system platform to allow the exploitation of real-time Java implementations inthe industrial automation domain and specifically in 61499 based applications.The Archimedes system platform is composed of a methodology, a framework and an ESS. The Archimedes ESS that currently supports the design and deployment phases in the application and partially in the resource layers of the Model Integrated Mechatronics (MIM) architecture was developed utilizing the General Modelling Environment (GME). GME is a configurable toolset with generic functionality for graphical development that supports the easy creation of domain-specific modelling and program synthesisenvironments (Ledeczi, et al., 2001). The Archimedes system platform adopts the model driven development which means that the control engineer constructs the models of hisapplication using a domain specific language, as for example the IEC 61499 FB notation and the systems automatically generates the executable model that will be executed on an IEC 61499 compliant run-time environment. In the case of RTSJ-AXE, the 61499 models are automatically transformed to RTSJ compliant java specifications which are ready to be executed on the RTSJ-AXE run-time environment.In more detail, Archimedes ESS or any other 61499 compliant ESS, such as Corfu FBDK, can be used to define the FB model of the application. New FB types and FB networks can be defined or imported from IEC- compliant XML specifications that have been produced by other IEC-compliant ESSs. These FB design models are further refinedand enhanced to capture the real-time constraints of the control application. The so constructed platform independentmodels, utilizing the RTSJ-AXE model interpreters, are transformed to the RTSJ-based FB platform-specific implementation model that can be executed on the execution environment running on RTSJ compliant implementations.The RTSJ-AXE extends the functionality of Archimedessystem platform so as to exploit RTSJ in the model driven development process of distributed control applications. It is composed of: a) An FB implementation model framework, i.e. a set of classes that enable the re-use of all these design decisions that have been done for the proper use of RTSJ constructs in mapping FB based design specifications of control applications to executable real-time Java implementations. b) An execution environment that is required for the deployment and execution of the proposed FB implementation model. This environment provides the infrastructure required to meet deployment and re- deployment needs, as well as stringent non-functional requirements such as maximum permissible response times, minimum throughputs and deadlines usually imposed by the nature of DCSs. c) A set of interpreters to automaticallygenerate the implementation model from the FB design model. d) A tool, the RTSJ launcher, to support the preparation and launching of the application on the target environment.To eliminate the major cause of unpredictability of the Javaimplementation, the RTSJ defines theNoHeapRealtimeThread (NHRT) class. The NHRT class isused in the RTSJ-AXE to allow selected FB instances to pre-empt the GC without delay since it runs at a higher prioritythan the GC. This allows the control application to beindependent from the GC. Event connections between FB instances are implemented in RTSJ-AXE, using theAsynchronous Event Handling Mechanism. This mechanism was derived in RTSJ by generalizing the traditional asynchronous event handling mechanism of Java. Based on this, asynchronous event handlers become schedulable entities that act as real time threads and inherit all the scheduling characteristics of threads. A detailed description of the RTSJ-AXE can be found in (Thramboulidis et al. 2005).4. PERFORMANCE ANALYSIS AND RESULTS In this section the RTSJ execution environment used to analyze the timing behaviour of the proposed run-time environment are described. The timing behaviour of IEC 61499 implementations is analyzed and benchmarking results obtained running the proposed RTSJ-based 61499 run-time environment are presented. The objective of this performanceanalysis is to contribute to the performance analysis of IEC 61499 implementations, and demonstrate the timingcharacteristics of the proposed execution framework on different implementations environments of RTSJ. It is not the objective of this work to compare the various RTSJ implementations. Such evaluations can be found in other works, i.e. (Enery, et al., 2007). 4.1. Hardware and Software test bedsThree different real-time Java platforms, i.e., TimeSys RI, IBM WebSphere Real Time, and Sun Java Real-Time System, were used to analyze the timing behaviour of theproposed IEC 61499 run-time environment. More specifically the three software/hardware platforms used for are:TimeSys RI : TimeSys has developed the official RTSJ implementation. RI runs on any Linux platform and its threading model maps directly the Java threads onto Linux POSIX threads. For the test bed, version 1.0-547.1 of RI was used running on TimeSys Linux/RT (GPL version) 4.1 Kernel 2.4.21. A significant improvement in performance was identified using the new version 1.1-alpha. The hardware platform used was a PC with AMD 64 2.4 GHz and 2Gb RAM.IBM WebSphere Real Time : Ver 1.0 of IBM WebSphere Real Time was used, running over Red Hat Enterprise Linux 4 patched with RTkernel 2.6.16 on a IBM server 798452G, X3455, Opteron Dual-Core model 2218, 2X2.6GHz/ 2x1MB L2 SE, 4x512MB. Sun Java Real-Time System : Sun Java Real-Time System (Java RTS) 2.0 RC2 was used running over Sun Solaris 10, on a Sun Fire 280R Server, with 2x Sun UltraSPARC III CU1.2 GHz with 4 GB RAM. It is important to note that Sun Java RTS was designed for a dual-CPU system, but can alsorun on a single CPU system. This will result in higher latencyand jitter numbers, but is still an effective solution forapplications with high temporal requirements.Naturally, it would have been more favourable to run the tests on the same hardware platform. However, the IBM and Sun real-time Java implementations supported, when the tests were run, only their own hardware platforms. 4.2. FB instance performance analysisThe performance analysis of the FB instance is mainly based on the ECC that describes the dynamic behaviour of the basic FB type instance. The state transition diagram shown in Fig.1, which describes the behaviour of the FB instance, was used for the analysis of performance characteristics of the FB instance. The FB instance is blocked in the idle state (S0) waiting for an event at its event inputs. The presence of an event at the event inputs of the FB instance fires the transition t1. During this transition the sampling of input data of the FB instance is performed and the internal data input variables are updated so as to be used in subsequent calculations.S1S2S0t1t2t3t4Fig. 1. State machine for the execution of basic FB instance. During S1 the transitions of the current state of the ECC are evaluated. If a transition fires, i.e., its condition evaluates to true, the transition t3 is fired. This means that the duration of S1 depends on the number and the type of transitions. During S2 the actions that are associated with the new state of the FB instance’s ECC are executed. Each action is performed by executing the associated algorithm, if any, and issuing an event at the associated event output. The transition t4 is fired after the execution of the associated actions and the FB instance enters the S1 state where the transitions of the current state of the ECC are evaluated again. If a transition fires, t3 is activated, otherwise t2 is activated and the FB instance enters the S0 state.For the benchmark analysis of the execution timing characteristics of the FB instance, a dummy FB type was defined and used (Fig. 2). The DummyFB has: one inputevent (EI), seven data inputs (D1, D2, … D7) of type Boolean, one event output (EO) and one data output (DO). The event input EI is used to activate the FB instance, which executes 1, 2, 3 or up to 7 subsequent state transitions depending on the values of associated data. One ‘dummy’ algorithm is assumed with just a return statement and only one event per EC action. The performance results of the DummyFB can be used to calculate the execution time of any basic FB type of the application, assuming that the worst-case-execution-times of its algorithms are available.Read Data Test (t1): This test measures the time required forthe transition t1 of the state machine that describes theexecution of the basic FB instance. For each platform usedfor the execution of the proposed run-time environment 1.000samples were collected and the average, standard deviation,min, max and 99,5 % are reported. Fig. 3 illustrates the read-data duration for IBM, Sun and TimeSys RI implementationsfor the DummyFB. In our implementation all input data areread from the DataConnectionManager independent of theevent WITH specifier. Better results can be obtained using the WITH specifier to read only the data related to the specific event that fired the transition.Fig. 2. The dummy FB type used in our benchmark.Evaluating Transitions Test (S1): This test measures the duration of the S1 state. The S1 time depends on the number and the type of transitions. Transition expressions in the DummyFB are: a) ei && d1, for the transition from START to the ST1 state, and dn for the transition from START to the STn state, where n=2..7. There is a transition which always fires from any STn state to the START state. Fig. 4 shows the results obtained using the DummyFB with one output event and up to seven transitions.Fig. 3. Read Data Duration performance measurements.Fig. 4. Evaluating Transitions (S1) Test results.Performing Actions (S2) Test: This test measures the durationof the S2 state. The S2 duration depends on the number ofactions, the algorithm execution time, and the output eventfiring time. Fig. 5 shows the results obtained using theDummyFB with one output event and up to seven associatedactions in a state. It should be noted that a great jitter in theexecution times was noticed until the IBM and Sunimplementations reach a steady state. This is why we had the systems to reach the steady state before starting the measurements of our tests (Enery, et al., 2007).Fig. 5. Performing Actions (S2) Test results. 4.3. FB Network performance analysisFor the FB network performance analysis, dispatch-latency,event-connection latency and the event-handler computation-time are presented. The Read data latency has no meaning forthe FB network, since for the proposed implementation this time is the one that was measured in the FB instance performance analysis.Dispatch latency Test:This test measures the time from when the output event of the event producer FB is fired, to when its handler, which is an instance of the EventHandler class, isinvoked. Fig. 6 illustrates the dispatch latency for the three platforms used to run the proposed run-time environment. Itshould be noted that standard deviation for allimplementations is quite small and better performance isachieved when a BoundAsyncHandler is used (Fig. 7). Event-connection latency Test: This test measures the time from when the event is fired, to when the corresponding consumer FB instance becomes ready for execution. To ensure that each event firing causes a complete execution cycle of the consumer FB instance, the producer FB instance fires the next event only after the processing of the previous event has terminated. Measurements for the event-connection latency on the three platforms used are quite similar to the Dispatch Latency ones.Event-handler computation-time (EHCT) Test: This testmeasures the time needed for the event handler to perform itstask without interruption, i.e., to update the events in the inputEventMonitors that have already been subscribed for the specific event (Fig. 8). The EHCT depends on the number of monitors to update. There is one monitor per FB instance that accepts the corresponding event. It should be noted that Std. Dev. is greatly improved in the case of the Sun execution environment using BoundAsyncHandler (Fig. 9), whereas it remains the same for the case of IBM and RI.4.4. Deployment and re-deployment performance analysis For the deployment and re-deployment performance analysis the Festo MPS example application was used. Festo MPS is a laboratory system widely used in IEC 61499 related papers. It is composed of three units: the distribution unit, the testingunit and the processing unit. Cylindrical work pieces are forwarded from the distribution unit to the testing unit andnext to the processing unit, where a drill performs the most important processing of this mechanical unit. A detaileddescription of this case study can be found inhttp://seg.ee.upatras.gr/seg/dev/FestoMPS.htm. The whole control application for the Festo MPS system in the form of FB notation can be downloaded from the same site.Fig. 6. Dispatch latency for Async Event Handler Fig. 7. Dispatch latency for BoundAsync Event HandlerFig. 8. Event-handler computation-time for Async EventHandlerFig. 9. Event-handler computation-time for BoundAsync Event Handler.Table 1. Deployment timing characteristics for the Festo MPS application.Action # of timesperformedWebSphere RealTimeRI(1.1-alpha)Java RTSJava RTS(Dedicated CPU)FB instantiation 5 64,314 ms 21,564 ms 38,181 ms 38,934 msData Connection 10 2,160 ms 0,736 ms 1,390 ms 1,516 msEvent Connection 8 8,797 ms 20,845 ms 5,103 ms 5,456 ms Table 2. Re-deployment timing characteristics for the Festo MPS application.Action # of timesperformedWebSphere RealTimeRI(1.1-alpha)Java RTSJava RTS(Dedicated CPU)FB instantiation 2 4,065 ms 5,705 ms 7,033 ms 6,816 ms Create DataConnection1 0,023 ms 0,028 ms 0,076 ms 0,043 ms Create EventConnection5 1,227ms 14,543 ms 2,939 ms 2,732 ms Delete EventConnection2 0,106 ms 0,130 ms 0,379 ms 0,274 msTable 1 presents the timing characteristics of the deployment process for 5 FB instances. It must be noted that the 5 instances were all of different FB types so the load time for 5 different classes is responsible for the long total instantiation time. Deployment was executed in two steps: The first step is executed with priority 20 whereas the second step is executed with high priority (35). Times are for the given number of repetitions of the corresponding action. It is evident that our framework does not exploit the second CPU of the hardware platform when this is dedicated to RT threads.After the deployment of the first version of the Festo MPS example application that does not count the illegal workpieces, a re-deployment scenario was executed to demonstrate the applicability of the proposed approach regarding re-configuration. Two FB instances, one CounterFB and one PrintInt, were appended to the control application during runtime to get the final version, which has exactly the same behavior as the first one, except that the number of illegal workpieces is printed on the operator’s display. Table 2 illustrates the timing characteristics of the proposed run-time environment for the various RTSJ implementations for the above re-deployment scenario. It should be noted that the create event connection operation isa time consuming action for RI.5. CONCLUSIONSA benchmark for the IEC 61499 function block model is described and performance measurements for a run-time environment based on real-time Java are provided. Performance results prove the applicability of the real-time Java execution environment for hard real-time applications. These measurements can be used to calculate the response time of the various transactions of any FB application. This work should be further extended to analyze the timing characteristics of an FB network that is deployed on a network of interconnected nodes.REFERENCESInternational Electro-technical Commission (IEC), 2005, International Standard IEC 61499, Function Blocks, Part 1 - Part 4Thramboulidis K. (2005), “Model Integrated Mechatronics – Towards a new Paradigm in the Development ofManufacturing Systems”, IEEE Transactions onIndustrial Informatics, vol. 1, No. 1. Feb 2005 Thramboulidis, K., and Zoupas, A. (2005), “Real-Time Java in Control and Automation: A Model DrivenDevelopment Approach”, 10th IEEE Int. Conf. onEmerging Technologies and Factory Automation,Catania, Italy, Sept. 2005Sünder, C., Zoitl, A., Strasser, T., and Brunnenkreef, J.(2007). Benchmarking of IEC 61499 runtime environments. In Name(s) of editor(s) (ed.), EmergingTechnologies and Factory Automation, ETFA. IEEEConference on, 474-481.Soundararajan, K., and Brennan, R. (2008). Design patterns for real-time distributed control system benchmarking.Robotics and Computer-Integrated Manufacturing, 606-615. Pergamon Press.Bollela, G., B. Brosgol, et al., 2000, The Real-Time Specification for Java, /Addison Wesley/.Cengic, G., Ljungkrantz, O., Akesson, K. (2006), “Formal Modeling of Function Block Applications Running inIEC 61499 Execution Runtime”, /11th IEEE International Conference on Emerging Technologies andFactory Automation/, September 20-22, 2006, CzechRepublic.Sunder, C., Zoitl, A., Rofner, H, Strasser, T., Brunnenkreef, J. (2007), “Benchmarking of IEC61499 runtimeenvironments”, /12th IEEE Int. Conf. on EmergingTechn. and Factory Automation/, Sept 2007, Patras,Greece.Brennan, R., Fletcher, M., Norrie, D. (2002), “An Agent-Based approach to reconfiguration of real-timedistributed control systems”, /IEEE transactions onRobotics and Automation/, vol. 18, No. 4, pp. 444-451,August 2002Ledeczi, A., M. Maroti, A. Bakay , G. Karsai, J. Garrett, C.Thomason , G. Nordstrom, J. Sprinkle and P. Volgyesi.“The Generic Modeling Environment”. Proc. ofWISP’2001, Budapest, 2001.Enery Mc Enery, D. Hickey, and M. Boubekeur, “Empirical Evaluation of Two Main-Stream RTSJ Implementations”, 5th International Workshop on JavaTechnologies for Real-time and Embedded Systems(JTRES ’07) Sept. 26-28, 2007 Vienna, Austria.。

Performance of low-complexity code acquisition for direct-sequence spread spectrum systemsK.Yu and I.B.CollingsAbstract:The authors consider a low-complexity technique for direct-sequence spread spectrum code acquisition.They present performance analysis in Rayleigh fading channels,and with carrier frequency offset in the receiver.They derive analytical expressions for acquisition probabilities,and use them to determine the mean and variance of the acquisition time.It is shown that acceptable performance can be achieved with significant reduction in hardware requirements,compared to conventional detectors.Simulations confirm the analytical results.Performance comparisons are presented under a range of scenarios including significant frequency offsets and fading channels.1IntroductionTiming recovery is a vital task which fundamentally affects the performance of digital communications receivers.For direct-sequence spread spectrum communications,one of the most important timing tasks is code acquisition.This is especially the case for high mobility communications,with fading channels.Such a problem is of course well known and studied,given the widespread use of CDMA systems [1].However,recently the development of wireless LANs has raised new problems.In particular,these systems use very short spreading sequences,and experience large frequency offsets in the receiver carrier recovery circuitry.These two factors have sparked renewed interest in problems of code acquisition.In this paper we are interested in examining low-complexity acquisition techniques required for practical WLAN systems with low spreading gain.Of particular interest is the effect of fading and frequency offset.Our analysis will show that acceptable performance can be achieved with considerably lower computational complexity than is commonly thought to be required.Traditionally,code acquisition designs are divided into two stages,namely detection (code search)and verification.In a parallel acquisition mode,the detection stage simultaneously evaluates all the possible code offsets and picks the one with the highest correlation [2–5].In a serial search,the detection stage evaluates the possible code offsets one after another until an acceptable correlation is found [6–11].Hybrid schemes have also been considered including partial parallel searches [2,12].Performance analyses for these acquisition structures have been pre-sented,in the case of fading channels in [8,9,11,12],and in the case of frequency offset in [13–16].In fact,it is well known that for a conventional correlation detector,a frequency offset of half the data rate causes a performance loss of about 4dB.In this paper we consider the combinedeffect of simultaneous channel fading and receiver frequency offset.Of course,the task of code acquisition is intimately linked to the transmission modulation and coding format,and to the transmission channel characteristics.The IEEE stan-dard (802.11)for WLANs [17]must be able to operate in a wide variety of environments,providing a tough challenge for the code acquisition detector.The standard specifies a direct sequence spreading code of only 11chips,and the frequency offset can be up to 62.5kHz.The task of code acquisition is significantly more difficult than for current mobile standards such as IS95and 3G systems.This is particularly the case with increasing user mobility,and with the pressure to reduce the computational complexity.A key contribution of this paper is that we consider and analyse the use of a low-complexity sign correlator in the code acquisition system,as opposed to conventional correlation.Sign operations (including sign quantisation,sign multiplication,and sign summation),significantly reduce the complexity of hardware implementation.We derive analytical performance measures for both the low-complexity and conventional schemes,in the presence of fading and frequency offset.Expressions are derived for mean acquisition times and associated variances,for both serial and parallel search schemes,and are given as a function of the fading,SNR and frequency offset.The results show that the sign-correlator technique can provide acceptable performance within 1.6dB of the conventional approach,and do so with significantly reduced complexity.Simulated acquisition probabilities are generated which demonstrate the validity of the assumptions made in our analysis.The simulated results are seen to match closely our analytic predictions.2Signal model and detectorConsider the following transmitted signalx ðt Þ¼s ðt Þcos 2p f 0twhere f 0is the carrier frequency ands ðt Þ¼s rem b tT c c N þ1h t ÀtT c "#T cThe authors are with the Telecommunications Laboratory,School of Electrical and Information Engineering,University of Sydney,NSW 2006,Australia r IEE,2003IEE Proceedings online no.20030742doi:10.1049/ip-com:20030742Paper first received 22nd August 2002and in revised form 28th May 2003where rem denotes remainder,ands i 2Æ1ffiffiffiffiNp &'for i ¼1;...;Nis the spreading code.Also,T c ¼T /N ,where N is the spreading gain,T c is the chip duration,T is the symbol period,and h (t )¼1/T c for 0o t o T c and 0otherwise.We further define the spreading code vector s ¼[s 1s 2?s N ]T ,and the cyclic k -position shifted code vector s (k )¼[s N Àk +1?s 1?s N Àk ]T ,where 77s 772¼77s (k )772¼1.Alsodenote s ðk Þi as the i th element of s (k ).For the WLAN applications of interest,we assume the usual complex-valued fading Rayleigh channel,constant over a symbol interval but independent from symbol to symbol.Also,in this paper we consider frequency offset in the transmitter (or receiver).The received signal is thereforer ðt Þ¼ffiffiffiP p j a j s ðt Àk T c ÞÂcos ð2p ðf 0þD f Þðt Àk T c Þþy Þþu ðt Þwhere P is the signal power,7a 7is the channel amplitude (with E [7a 72]¼1),and k T c is the transmission delay with k a random integer uniformly distributed between 1and k max (i.e.chip synchronisation).Also y is the random phase (uniformly distributed),D f is the carrier frequency offset,and v (t )is Gaussian noise.Now consider the low-complexity sign-correlator code acquisition detector.This detector has all correlation calculations performed as binary operations,significantly reducing hardware requirements.We do not suggest that this detector is particularly novel,but rather our aim is to provide analysis which we will then use to demonstrate that such a low-complexity scheme can prove successful even in fading channels with high levels of frequency offset,typical of current WLAN systems.The first stage in the detector is shown in Fig.1,with quadrature baseband (two-level)quantized outputs,where sgn (x )models a simple 1-bit logic gate.The second stage of the detector is shown in Fig.2,where the binary outputs from Fig.1are binary correlated with the local spreading code,with the appropriate test timing offset.Turning to the mathematical details we have [Note 1]r f ðt Þ¼2ffiffiffiP p j a j s ðt Àk T c Þ½cos ð2p ð2f 0þD f Þt À2p ðf 0þD f Þk T c þy Þþcos ð2pD ft À2p ðf 0þD f Þk T c þy Þ þv I ðt Þandy I ð‘Þ¼sgn Z ‘T c ð‘À1ÞT cr I ðt Þd t 0B@1CANow since f 0c 1/T c we can writey I ð‘Þ¼sgn ffiffiffiP p j a j s rem ‘À1Àk N ÀÁþ1T c½I I ð‘Þþn I ð‘Þ!ð1Þwhere n I (c )is an independent zero-mean Gaussian variable with variance s 2n ,andI I ð‘Þ91T cZ ‘T c ð‘À1ÞT ccos ð2pD ft þf Þd t ð2Þf 9À2p ðf 0þD f Þk T c þy ð3ÞNow,when D f {1/T c ,I I (c )is approximately constantfor all c in the same symbol,so let us instead consider the average integration over an entire symbol interval,as in [18].Consider c 0to be a chip sampling instance which corresponds to the first chip of a symbol.Now,for c A [c 0,N +c 0À1],we defineJ 91N X ‘0þN À1‘¼‘I I ð‘Þ¼1NT cZ ð‘0þN À1ÞT c ð‘0À1ÞT ccos ð2pD ft þf Þd t¼12p NT c D f½sin ð2pD f ð‘0À1þN ÞT c þf ÞÀsin ð2pD f ð‘0À1ÞT c þf Þ ¼a cos cð4Þwherea ¼sin ðpD fNT c ÞpD fNT cc ¼pD fNT c þ2pD f ð‘0À1ÞT c þf By replacing I I (c )in (1),with this average expression (4),we havey I ð‘Þ%sgnffiffiffiP p a j a j cos c s rem‘À1Àk NÂÃþ1þZ I ð‘Þ We can now write an ‘average’expression for z I (i )(defined in Fig.2).At the i th symbol,y I (c )correlates with test-offsetcode s (d )whose j th element is s ðd Þj ,where c take valuesfrombinary chip matched filter0ty I (l )(binary)y Q (l )(binary)Fig.1Binary chip matchedfilterbinary correlatory I (l )y Q (l )offset codeFig.2Code offset detector using binary correlatorNote 1:We present only the in-phase component for notational simplicity since the quadrature component is independent and therefore can be derived in the same way.c 0¼((i À1)N +1)+k to iN +k .Thereforez I ði Þ¼X N j ¼1y I ðði À1ÞN þk þj Þsgn ðs ðd Þj Þð5Þ%X N j ¼1sgn ðffiffiffiP p a j a j cos c s ðk ÞjþZ I ðði À1ÞN þk þj ÞÞsgn ðs ðd Þj Þ¼X N j ¼1ð1Àr j Þsgn ðffiffiffiP p a j a j cos c s ðk Þj Þsgn ðs ðd Þj Þð6Þwherer j ¼2n I ðði À1ÞN þk þj Þs ðk Þj cos c o 0;and j n I ðði À1ÞN þk þj Þj 4ffiffiffiffiffiffiffiffiffiffiP =N p a j a jj cos c j 0elsewhere8><>:And the conditional probability for r j ¼2isP ðr j ¼2j j a j ;c Þ¼Z1f ða ;c Þ1ffiffiffiffiffiffi2p p s nexp Àx 22s n !d x ¼Qf ða ;c Þs n ð7Þwhere Q (x )is the Q-function andf ða ;c Þ¼ffiffiffiffiPN r a j a j cos c Now (6)can be further written asz I ði Þ¼sgn ðcos c ÞX N j ¼1ð1Àr i Þsgn ðs ðk Þj Þsgn ðs ðd Þj ÞDue to non-coherent detection,we only need consider the magnitude of z I (i )and hence the term ~z I ði Þas follows:~z I ði Þ¼X N j ¼1ð1Àr j Þsgn ðs ðk Þj Þsgn ðs ðd Þj Þð8ÞThis model will be used in the following Section to derive the detection probabilities and acquisition times in each case studied.3Parallel search structure3.1Mean and variance of acquisition timeFigure 3shows the state diagram of the parallel codeacquisition detector,similar to that in [12].Let hypothesis H 1denote that the codes are in synchronisation while hypothesis H 0means codes are not in synchronisation.A maximum selection criterion (MSC)is applied in the search (or selection)stage using a parallel code offset search (usually performed by parallel hardware)which simulta-neously tests all possible offsets.At verification stage a threshold-crossing criterion is applied to the selected offset A times.The A tests are performed over disjoint NT c second intervals and are thus statistically independent.If the threshold is exceeded at least B times out of the A tests,then the hypothesis is accepted,otherwise the search recommences.It is assumed that the channel-decoder will detect false alarms and in that case the search recommences after a JNT c delay.Flow graph theory gives the mean and variance ofacquisition time as follows [1,6,12]E ½T ACQ ¼d d z ln G ðz Þ !z ¼1¼1P D ð1þA þJP F ÞT ð9ÞVar ½T ACQ ¼d 2d z ln G ðz Þþdd z ln G ðz Þ!z ¼1¼1P D½J 2P F ÀðA þ1Þ2 T 2þ1P D½1þA þJP F 2T 2ð10Þwhere G (z )is the generating function from Fig.3,T is thesymbol period,P D ¼P D 1P D 2and P F ¼P F 1P F 2.Clearly,we need now to obtain these acquisition probabilities for the low-complexity sign-correlation detector which is the focus of this paper.3.2Detection probabilities3.2.1Sign-correlator detector with frequency offset :For hypothesis H 1(codes in synchronisation)wehave,from (8),that~z I ði Þ¼N ÀX N j ¼1r jLet m 1and s 21be defined to be the mean and variance of ~z I ði Þ.Then using (7)m 1¼N ÀX N j ¼1E ½r j¼N À2N E Qf ða ;c Þs n!ð11ÞRecall that 7a 7is Rayleigh,and since y is uniform,so is c .Definingg ðc Þ¼ffiffiffiP p a cos cffiffiffiffiN p sn(1−P )z ATFig.3State-transition diagram of the acquisition processSEL is the parallel selection stage.The verification stages are denoted V.V1indicates that the correct offset was tentatively selected,V2indicates that an incorrect offset was tentatively selected.FA indicates false alarmgivesE Qfða;cs n!¼12pZ2pZ1QðgðcÞxÞÂ2x exp½Àx2 d x d cð12Þ¼12pZ2p12ÀZ1exp½Àx2Â1ffiffiffiffiffiffi2pp gðcÞexpÀg2ðcÞ2x2!d x d c¼14pZ2p1Àffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffig2ðcÞ2þgðcÞs!d c¼12À1parcsin uð13Þwhereu¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP a2P aþ2N s n sSubstituting(13)into(11)yieldsm1¼2Nparcsin uð14ÞLikewise the variance of~z IðiÞiss2 1¼N2À2NX Nj¼1E½r j þX Nj¼1E½r2jþX Ni¼1X Nj¼1;j¼1E½r i E½r j Àm21¼N2À4NðNÀ1ÞE Qfða;cs n!þ4NðNÀ1ÞE Qfða;cn!2Àm21ð15ÞSubstituting(13)into(15)producess2 1¼N1À4p2arcsin2uð16ÞAlso,note that the quadrature correlator output,~z Q,has the same mean and variance as this in-phase component.We now propose to model~z fðiÞand~z QðiÞas twoGaussian random variables with mean m1and variance s21.Of course,the actual ranges of~z IðiÞand~z QðiÞarefinite on [ÀN,N],therefore the variance of our Gaussian model mustbe greater than s21in order to achieve a good match forspreading gains of practical interest.Based on num-erical studies it turns out that a modified variance,s2 1c ¼s21þ0:75m21,provides an accurate analytical expres-sion.This modified variance was found to provide accurate theoretical predictions of performance in all cases studied, and therefore validates our Gaussian assumption. Clearly,under the Gaussian assumptions,the testvariable(from Fig.2)RðiÞ¼~z2I ðiÞþ~z2QðiÞhas a non-centralchi-square distribution with non-centrality parameter m2R¼2m21.The pdf of R(i)conditioned on hypothesis H1isp RðR j H1Þ¼12s21cexpÀ2m21þR2s21c!I0ffiffiffi2pm1ffiffiffiRps21c!ð17ÞFor hypothesis H0(codes not in synchronisation),we againmodel~z IðiÞand~z QðiÞas two Gaussian random variables.With random spreading,we can easilyfind that their meansare zero in this case,and their variances are N,which givesp RðR j H0Þ¼12NeÀR=2Nð18ÞNow let us move on to derive the probabilities ofacquisition.At search stage(‘SEL’in Fig.3)the detectionprobability P D1is given byP D1¼Z1p Rðy j H1ÞZ yp Rðx j H0Þd x2435NÀ1d yð19ÞSubstituting(17)and(18)into(19)givesP D1¼Z112s21cexpÀ2m21þy2s21c!I0m1ffiffiffiffiffiffi2yps21Â1ÀexpÀy2Nh iNÀ1d y¼12snexpÀm21s1c!X NÀ1n¼0ðÀ1ÞnNÀ1nÂZ1expÀ12s1cþn2Ny!I0m1ffiffiffiffiffiffi2yps1cd y¼X NÀ1n¼0ðÀ1Þn NNþn s1cNÀ1nexpÀnm21Nþn s1c!ð20ÞThe incorrect detection probability at search stage can beobtained in terms of P D1:P F1¼1ÀP D1ð21ÞAt verification stage(‘V1’and‘V2’in Fig.3)it can beshown thatP D2¼X Ai¼BAiP i1ð1ÀP1ÞAÀið22ÞP F2¼X Ai¼BAiP i2ð1ÀP2ÞAÀið23ÞHere P1and P2are the probabilities that(respectively)H1test sample and H0test sample cross the threshold.ThereforeP1¼Z1R112s21cexpÀ2m21þx2s21c!I0ffiffiffi2pm1ffiffiffix ps21c!d x¼Qffiffiffiffiffiffiffiffi2m21s1cs;ffiffiffiffiffiffiffiR1s1cs!ð24ÞP2¼Z1R112NexpÀx2Nh id x¼expÀR12N!ð25Þwhere R1is the threshold value which is selected numericallyto minimize the mean acquisition time at each SNR[11,12],and Q(x1,x2)is the Marcum Q-function[19].Substituting(20)–(23)into(9)and(10)will now give the requiredperformance measures for the reduced complexity sign-correlation detector with frequency offset.3.2.2Conventional correlator detector with frequency offset:In this Section we consider the conventional correlator which requires high-resolution correlation in contrast to the binary correlator in the previous section.Our analysis here follows the procedures in[12],but here we extend the analysis to include the effect of constant frequency offset.In this case the in-phaseoutput isz IðiÞ¼ffiffiffiPpa j a j cos cðsðdÞÞT sðkÞþw IðiÞð26Þwhere w I(i)is an independent zero mean Gaussianrandom variable with variance s2w .Conditioned on7a7and H1,we havep RðR j H1;j a jÞ¼1n expÀP a2j a j2þRw "#ÂI0ffiffiffiffiffiffiPRpa j a jswNote that this function does not depend on c.Averaging with respect to7a7givesp RðR j H1Þ¼12s22expÀR2s22!ð27Þwhere s22¼0:5a2Pþs2w.For hypothesis H0we havep RðR j H0Þ¼12s2wexpÀR2s2w!ð28ÞAt search stage,P D1is again given by(19).Substituting(27) and(28)givesP D1¼Z112s22expÀx2s22!1ÀexpÀx2s2w!NÀ1d x¼X NÀ1i¼0ðÀ1ÞiNÀ1iZ112s2expÀs2wþi s222s2ws2x!d x¼X NÀ1i¼0ðÀ1ÞiNÀ1is2ws2wþi s2P:ð29Þand P F1¼1ÀP D1.At verification stage the detection and false alarm probabilities can be respectively computed using(22)and (23),but where in this caseP1¼Z1R212s2expÀx2s2!d x¼expÀR22s2!ð30ÞP2¼Z1R212s2wexpÀx2s2w!d x¼expÀR22s2n!ð31Þwhere R2is the detection threshold determined in the same way as R1.4Serial search structureIn this Section we consider the case when a serial search and threshold-crossing criterion is applied at search stage,in contrast to the parallel search in the previous Section.The verification stage is the same as in the preceding Section. While serial search takes longer to achieve acquisition compared to the parallel search,it requires less hardware to implement.4.1Mean and variance of acquisition timeA general expression for the mean acquisition time in this case is given by(1.183)in[1],and here we extend it to include the false-alarm penalty time J.We therefore have the following expressions for mean and variance of the acquisition time in this serial-detection caseE½T ACQ ¼1P D1þAP D1þðNÀ1Þ½Â1À12P DðAP F1þJP FÞ!Tð32ÞVar½T ACQ ¼À12À1þ2AP Dþ2ð1þAP D1ÞPDf0þNþ712ÀNP DþNÀ1P2Df2þÀ12þ1P Df1!ðNÀ1ÞT2þ1P2Dð1þAP D1Þ2À1P Dð1þ2AþA2P D1Þ!T2ð33Þwheref0¼1þAP F1þJP Ff1¼AðAþ1ÞP F1þð1þ2AþJÞJP Fand where the remaining variables in(32)and(33)have analogous definitions to those in the previous Section.4.2Detection probabilitiesSince the search stage in this serial case only tests one code offset at a time,the analysis can be viewed in the same way as that for the verification stage previously,but with A¼1 and B¼1.In other words for the sign correlator the search-stage probabilities,P D1and P F1are given in this case by P1 and P2as given in(24)and(25)respectively.For the conventional correlator the search-stage probabilities are likewise given by(30)and(31)respectively.For both correlators,the verification-stage probabilities are the same as in the parallel search case of the previous Section.5Complexity comparisonTable1shows the computational requirements for the serial and parallel acquisition approaches during one symbol duration,under the assumption that the output of the conventional chip matchedfilter is quantised into q levels. Generally conventional correlation uses(q¼16)-bit opera-tions(or higher),but sign correlation only needs(q¼1)-bitTable1:Computational requirements per symbol duration for processing gain N(for sign-correlation q¼1,while for conventional-correlation q is typically16or more)Search stage Serial q-bit multiplications:N+1q-bit additions:N+1Parallel q-bit multiplications:N(N+1)q-bit additions:N(N+1) VerificationstageSerial q-bit multiplications:N+1q-bit additions:NParallel q-bit multiplications:N+1q-bit additions:Noperations.In other words the sign-correlator only requires one-sixteenth of the hardware.6Numerical and simulation resultsEach of the simulation studies in this Section assumes the modified Jakes model was used to generate the Rayleigh fading channel with normalised fading rate 0.02(normalised to the symbol rate).In each case,the curves relating to the conventional detector are generated assuming infinite precision,(i.e.there is no quantisation).The data symbol rate was 1Msymbol/s.Figure 4shows the detection probabilities,P D 1,for the conventional correlator and the sign correlator,with and without carrier frequency offset.Clearly the analytic expressions we have derived in this paper accurately match the simulated results for practical SNR.Using these simulated and analytical detection probabil-ities,we then computed the corresponding mean acquisition time and standard deviation.The results are shown in Figs.5and 6(note that the curves for the conventional correlator with 0kHz frequency offset can be related to the simulation figures in [12],however in that paper they considered a partial parallel structure,and had a slightly different fading model).Here we set the verification-stage parameters to check for B ¼2threshold crossings out of A ¼3tests.We also set the false-alarm penalty variable to J ¼100.Actually this is somewhat arbitrary since it depends on the particular equaliser–decoder used further down the processing chain (e.g.J 4100in [8,12]).However our main interest is to characterise and examine the performance of the low-complexity sign correlator,and as such we are primarily interested in the relative performance compared to the conventional approach.The figures show that the low-complexity sign-correlation-based acquisition scheme achieves close to the performance of the conventional method (with and without the presence of frequency offset).The performance degradation is about 1.6dB.Figures 7and 8show P D 1and the mean acquisition time,for two different spreading gains.The plots are without the presence of carrier frequency offset.As the spreading increases the detection probability decreases since of course there are more code offsets to test.The results show that the performance degradation of the low-complexity sign-correlator technique is slightly less for lower WLAN−5510152000.20.40.60.81.0average SNR per symbol, dBd e t e c t i o n p r o b a b i l i t i e sFig.4Parallel-search analytical and simulated selection-stage detection probabilities (P D1)of the sign-correlator and conventional approaches for two values of frequency offset (0and 500kHz),with a spreading gain of 31−551015201010101010average SNR per symbol, dBm e a n , µsFig.5Parallel-search analytical and simulated mean acquisition time of the sign-correlator and conventional approaches for two values of frequency offset (0and 500kHz),with a spreading gain of 31−55101520100101102103104average SNR per symbol, dBs t a n d a r d d e v i a t i o n , µsFig.6Parallel-search analytical and simulated standard deviation of acquisition time of the sign-correlator and conventional approaches for two values offrequency offset (0and 500kHz),with a spreading gain of 3100.20.40.60.81.0average SNR per symbol, dBd e t e c t i o n p r o b a b i l i t i e sFig.7Parallel-search selection-stage detection probabilities (P D1)of the sign-correlator and conventional approaches for two different spreading gains (SG ¼15,127)without frequency offsetspreading gains.We also see acceptable performance for high spreading,which is interesting since the implementa-tion savings increase significantly as the spreading gain increases.Figure 9shows the mean acquisition time when the serial search structure is applied (note that the curves for the conventional correlator with 0kHz frequency offset can be related to the simulation figures in [8],however in that paper they considered much longer spreading sequences,and plotted their curves normalised to the chip rate,rather than the symbol rate for fading and SNR).As can be seen from the figures here,the performance degradation of the sign-based technique is even smaller than those in Figs.5and 6.Figures 10and 11compare the four different combina-tions of correlators and search structures with and without frequency offset respectively.7ConclusionThis paper generated accurate closed form analytical expressions for both low-complexity and conventional codeacquisition schemes,in fading channels and with frequency offset.We also presented performance comparisons to verify the analysis,and showed that the low-complexity scheme performs with only a 1.6dB performance degrada-tion.8AcknowledgmentThe first author was supported in part by Southern Poro Communications (SPC).9References1Simon,M.,Omura,J.K.,Scholtz,R.A.,and Levitt,B.K.:‘Spread spectrum communications handbook’(McGraw-Hall,New York,USA,1995)2Su,Y.T.:‘Rapid code acquisition algorithms employing PN matched filters’,IEEE mun.,June 1988,36,pp.724–7333Milstein,L.B.,Gevargiz,J.,and Das,P.K.:‘Rapid acquisition for direct sequence spread-spectrum communications using parallel SAW convolvers’,IEEE mun.,July 1985,33,pp.593–6004Chawla,K.K.,and Sarwate, D.V.:‘Parallel acquisition of PN sequences in DS/SS systems’,IEEE mun.,May 1994,42,pp.2155–2164−55101520100101102103104average SNR per symbol, dBm e a n , µsFig.8Parallel-search mean acquisition time of the sign-correlator and conventional approaches for two different spreading gains (SG ¼15,127)without frequency offset101102103104average SNR per symbol, dBm e a n , µsFig.9Serial-search analytical and simulated mean acquisition time of the sign-correlator and conventional approaches for two values of frequency offset (0and 500kHz)and SG ¼31100101102103104average SNR per symbol, dBm e a n , µsFig.10Mean acquisition times for the four different combinations of correlators and search structures without frequency offset,with SG ¼31−50510152010101102103104average SNR per symbol, dBm e a n , µsFig.11Mean acquisition times for the four different combinations of correlators and search structures with a frequency offset of 500kHz,and with SG ¼31。