A comparison of SLM and PTS peak-to-average power ratio reduction schemes for OFDM systems

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High power pulsed magnetron sputtering A review on scientific and engineering state of the art

High power pulsed magnetron sputtering:A review on scienti ﬁc and engineering state of the artK.Sarakinos a ,⁎,1,J.Alami b ,1,S.Konstantinidis c ,1a Materials Chemistry,RWTH Aachen University,52074Aachen,Germanyb Sulzer Metaplas GmbH,Am Böttcherberg 30-38,51427,Bergisch Gladbach,GermanycLaboratoire de Chimie Inorganique et Analytique,Universitéde Mons,Avenue Copernic 1,7000Mons,Belgiuma b s t r a c ta r t i c l e i n f o Article history:Received 31March 2009Accepted in revised form 6November 2009Available online 19January 2010Keywords:HPPMS HiPIMSIonized PVDHigh power pulsed magnetron sputtering (HPPMS)is an emerging technology that has gained substantial interest among academics and industrials alike.HPPMS,also known as HIPIMS (high power impulse magnetron sputtering),is a physical vapor deposition technique in which the power is applied to the target in pulses of low duty cycle (b 10%)and frequency (b 10kHz)leading to pulse target power densities of several kW cm −2.This mode of operation results in generation of ultra-dense plasmas with unique properties,such as a high degree of ionization of the sputtered atoms and an off-normal transport of ionized species,with respect to the target.These features make possible the deposition of dense and smooth coatings on complex-shaped substrates,and provide new and added parameters to control the deposition process,tailor the properties and optimize the performance of elemental and compound ﬁlms.©2009Elsevier B.V.All rights reserved.Contents 1.Introduction ..............................................................16622.HPPMS:power supplies and processes .................................................16623.Plasma dynamics in HPPMS ......................................................16643.1.On the HPPMS plasma .....................................................16643.2.Determination of plasma properties:a theoretical overview ....................................16653.3.The HPPMS plasma properties ..................................................16663.3.1.The effect of the pulse on/off time con ﬁguration on the plasma properties .........................16683.3.2.Rarefaction in the HPPMS plasma ............................................16693.4.Instabilities in the HPPMS plasma ................................................16713.4.1.Plasma instabilities:an overview ............................................16713.4.2.Plasma initiation and the relationship between plasma instabilities and the chargetransport in the HPPMS plasma .....16714.Plasma-surface interactions and deposition rate .............................................16734.1.Non-reactive HPPMS ......................................................16734.1.1.Target-plasma interactions and target erosion rate ....................................16734.1.2.Transport of ionized sputtered species ..........................................16754.2.Reactive HPPMS ........................................................16755.Growth of thin ﬁlms by HPPMS ....................................................16765.1.HPPMS use for deposition on complex-shaped substrates .....................................16765.2.Interface engineering by HPPMS .................................................16775.3.The effect of HPPMS on the ﬁlm microstructure .........................................16785.4.Phase composition tailoring of ﬁlms deposited using HPPMS ...................................16796.Towards the industrialization of HPPMS ................................................16816.1.Examples of industrially relevant coatings by HPPMS .................................. (1681)Surface &Coatings Technology 204(2010)1661–1684⁎Corresponding author.Tel.:+492418025974,+492204299390,+3265554956.E-mail address:sarakinos@mch.rwth-aachen.de (K.Sarakinos).1All authors have contributed equally to the preparation of themanuscript.0257-8972/$–see front matter ©2009Elsevier B.V.All rights reserved.doi:10.1016/j.surfcoat.2009.11.013Contents lists available at ScienceDirectSurface &Coatings Technologyj o u r n a l h om e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /s u r fc o a t6.2.Up-scaling of the HPPMS process (1682)6.3.Positioning HPPMS in the coating and components market (1682)Acknowledgements (1682)References (1682)1.IntroductionThinﬁlms are used in diverse technological applications,such as surface protection and decoration,data storage,and optical and microelectronic devices.The increasing demand for new functional ﬁlms has been a strong incentive for research towards not only understanding the fundamentals and technical aspects of thinﬁlm growth,but also developing new deposition techniques which allow for a better control of the deposition process.Among the various methods employed forﬁlm growth,Physical Vapor Deposition(PVD)techniques, such as magnetron sputtering,are widely used[1].Magnetron sputtering is a plasma-based technique,in which inert gas(commonly Ar)atoms are ionized and accelerated as a result of the potential difference between the negatively biased target(cathode)and anode.The interactions of the ions with the target surface cause ejection(sputtering)of atoms which condensate on a substrate and form aﬁlm[1,2].The condensation and ﬁlm growth processes frequently occur far from the thermodynamic equilibrium,due to kinetic restrictions[2,3].Thus,a good control of both the thermodynamic and the kinetic conditions has implications on the growth dynamics and enables the tailoring of the structural,optical, electrical,and mechanical properties of theﬁlms[4].One way to control the growth dynamics is by heating the substrate during the deposition [2].The magnitude of the deposition temperature affects the energy transferred to theﬁlm forming species(adatoms)[2].This energy is,for instance,decisive for the activation of surface and bulk diffusion processes[5,6]and enables control over theﬁlm morphology[7–10]. Another source of energy are the plasma particles that impinge onto the growingﬁlm transferring energy and momentum to the adatoms[4,5]. The bombardingﬂux consists both of neutral and charged gas particles,as well as sputtered species.Numerous studies have shown that the energy, theﬂux,the angle of incidence,and the nature of the bombarding species are of importance for the properties of the depositedﬁlms[4,5,11,12].In general,the plasma particles exhibit a relatively broad energy distribu-tion with a mean value of several eV[13].The particle energy,andﬂux are affected by the target-to-substrate distance and the working pressure[1]. When the particles are charged the control of their energy can additionally be achieved by the use of electricﬁelds,e.g.by applying a bias voltage to the substrate[4,14],while theirﬂux depends on a number of factors including the plasma density,the target power,and the magneticﬁeld conﬁguration at the target[4,14].It is therefore evident, that a high ion fraction in a depositingﬂux facilitates a more efﬁcient and accurate control of the bombardment conditions providing,thus,added means for the tuning of theﬁlm properties.In magnetron sputtering processes the degree of ionization of the plasma particles is relatively low[13],resulting in a low total ionﬂux towards the growingﬁlm[13].As a consequence,in many cases bias voltage values of several tenths or even hundreds of V are required in order to increase the average energy provided to the deposited atoms and signiﬁcantly affect theﬁlm properties[5].Moreover,the degree of ionization of sputtered species is typically less than1%[13,15,16].As a result,the majority of the charged bombarding particles is made of Ar+ions[13].This fact in combination with the relatively high bias voltages may cause subplantation of the Ar atoms in theﬁlm[17,18], leading to a generation of lattice defects,[18,19]high residual stresses [20–23],a deterioration of the quality of theﬁlm/substrate interface [21],and a poorﬁlm adhesion[24].Thus,the increase of the fraction of the ionized sputtered species has been an objective of many research works during the recent decades.Some of the approaches that have been demonstrated include the use of an inductively coupled plasma (ICP)superimposed on a magnetron plasma[25],the use of a hollow cathode magnetron[25],and the use of an external ion source[26]. Parallel to the above mentioned approaches Mozgrin et al.[27], Bugaev et al.[28]and Fetisov et al.[29]demonstrated in the mid90s that the operation of a conventional sputtering source in a pulsed mode,with a pulse duration ranging from1µs to1s and a frequency less than1kHz,allowed for pulsed target currents two orders of magnitude higher than the average target current in a conventional sputtering technique,such as direct current magnetron sputtering (dcMS)[27–29].These high pulse currents resulted,in turn,in ultra-dense plasmas with electron densities in the order of1018m−3[27–29],which are much higher than the values of1014–1016m−3 commonly obtained for dcMS[13,30].A few years later,Kouznetsov et al.[31]demonstrated in a publication,that received much acclaim,the high power pulsed operation of the sputtered target showing that in the case of Cu the high plasma densities obtained using this pulsed technique resulted in a total ionﬂux two orders of magnitude higher than that of a dcMS plasma,and a sputtered material ionization of ∼70%.The new deposition technique was called High Power Pulsed Magnetron Sputtering(HPPMS).In the years after Kouzsetov's report, HPPMS received extensive interest from researchers and led to a substantial increase of the HPPMS-related publications(Fig.1).Later, several research groups[25]adopted the alternative name High Power Impulse Magnetron Sputtering(HiPIMS)for this technique.The aim of this review paper is to describe the scientiﬁc and engineering state of the art of HPPMS.For this purpose,the principle and the basic structure of the existing HPPMS power supplies are described and the various existing approaches to produce high power pulses are demonstrated.The spatial and the temporal evolution of the HPPMS plasma and its interactions with the target and the growingﬁlm are discussed both from a theoretical and an experi-mental perspective.Furthermore,the effect of the HPPMS operation on the deposition rate and theﬁlm properties is presented.Finally,the use of HPPMS in industry and the challenges that the latter faces regarding the use of HPPMS are brieﬂy addressed.2.HPPMS:power supplies and processesThe experimental realization of HPPMS requires power supplies different than those used in conventional magnetron sputtering processes. Those power supplies must be able to provide the target with pulses of high power density(typically in the range of a few kW cm−2),while maintaining the time-averaged target power density in values similarto Fig.1.Number of HPPMS-related publications in the period1999–2008.1662K.Sarakinos et al./Surface&Coatings Technology204(2010)1661–1684those during dcMS (i.e.a few W cm −2).The low average target power density is necessary to prevent overheating of the cathode and damage of the magnets and the target.A number of research groups and companies have developed HPPMS arrangements for both laboratorial and industrial use.These devices exhibit similarities in their basic structure which is schematically depicted in Fig.2.A dc generator is used to load the capacitor bank of a pulsing unit,which is connected to the magnetron.The charging voltage of the capacitor bank ranges typically from several hundreds of V up to several kV.The stored energy is released in pulses of de ﬁned width and frequency using transistors with a switching capability in the µs range,located between the capacitors and the cathode.The pulse width (also referred to as pulse on-time)ranges,typically,from 5to 5000µs,while the pulse repetition frequency spans from 10Hz to 10kHz.Under these conditions,the peak target current density may reach values of up to several Acm −2,which are up to 3orders of magnitude higher than the current densities in dcMS [25].The high target current densities during the pulse on-time are accompanied by differences in the electrical characteristics of the discharge,as manifested by the current –voltage (I –V )curves of a magnetron operating in a dcMS and an HPPMS mode (Fig.3).According to Thornton [32]the target and the voltage of the magnetron are linked by the power law I ∝Vnð1ÞIn dcMS processes the exponent n in Eq.(1)ranges from 5up to 15(Fig.3).In HPPMS the exponent n changes from values similar to dcMS at low target voltages to values close to the unity when high voltages are applied (Fig.3)[33,34].Despite their common basic architecture,the available HPPMS power supplies exhibit differences primarily in the width and the shape of the pulses they can deliver,and in whether they can sustain a constant voltage during the pulse on-time.Arrangements able to provide high power pulses were already developed in the mid 90s,in order to explore the feasibility of producing high-current quasi-stationary magnetron glow discharges for high rate deposition [27–29].However,it was in 1999when Kouznetsov et al.[31]emphasizedthat the high power pulsing could allow for a high ionization degree of copper vapor (70%)as well as a homogeneous ﬁlling of 1:2aspect ratio trenches (see Fig.4).The time-dependent target current and target voltage curves of the HPPMS power supply developed by Kouznetsov et al.[31]are plotted in Fig.5where it is shown that the ignition of the plasma is accompanied by a drop of the voltage from ∼1000V to ∼800V,as the current increases to its peak value ranging from 200to 230A at the highest applied working pressure [35,36].A characteristic of this power supply is that the loading voltage is not maintained constant during the pulse on-time,and its temporal evolution is rather determined by the size of the capacitor bank and the time-dependent plasma impedance.Other power generators were designed to deliver voltage pulses with a rectangular shape (see Fig.6),i.e.a constant voltage value during the pulse on-time [34,37–39].It is to be noted that pulses with widths larger than about 50µs allow for a saturation of the discharge current and establishment of a steady-state plasma [40]provided that enough energy is stored in the capacitor bank.A common problem encountered in sputtering processes is the occurrence of arcs.This phenomenon is particularly pronounced during the deposition from conducting targets covered by insulating layers and can be detrimental for the quality of the ﬁlms,due to the ejection of µm sized droplets from the target [41,42].The high peak target currents during the HPPMS operation may enhance the frequency of the arc events [43].Therefore,sophisticated electronics can be used in conjunction with the HPPMS power supplies to limit their effect if formed [43].An alternative way to alleviate this problem is to operate HPPMS using short pulses with widths ranging from 5to 20µs [44].A waveform during this mode of operation is presented in Fig.7.Within this short period of time,the glow discharge remains in a transient regime [40](as manifested by the triangular form of the target current)so that an eventual glow-to-arc transition is prevented.These short pulses have been,for instance,shown to allow for a stable and an arc-free reactive deposition of metal oxide ﬁlms [45,46].However,in order to eliminate the time lag for plasma ignition which would allow for the production of high-current pulses within this short period of time,a pre-ionization stage has to be implemented [44,47].The pre-ionization of the discharge ensures that a low density plasma is maintained between the pulses as the charge carriers are already present before the voltage is turned on.The plasma pre-ionization step may be achieved by super-imposing a secondary plasma (such as a dc,a microwave,or an inductively coupled plasma)on the HPPMS plasma or by running the discharge at relatively high frequencies [44,47–49].The latter pre-ionization method is extensively implemented in mid-frequency pulsed plasmas [50–53].In contrast to the short pulses described above,long pulses with a width of several thousands of µs can also be used [54](Fig.8).In this case the pulse is composed of two (or more)stages.TheFig.2.Basic architecture of an HPPMS power supply.The dc generator charges the capacitor bank of a pulsing unit.The energy stored in the capacitors is dissipated into the plasma in pulses of well-de ﬁned width and frequency using ultra fastswitches.Fig.3.Current –voltage curves of a magnetron during operation in dcMS and HPPMS mode.The change of the slope from 7to 1at a voltage value of 650V indicates loss of the electron con ﬁnement (data taken from [33]).Fig. 4.Cross-section SEM image of two via holes with an aspect ratio 1:2homogeneously ﬁlled by Cu using HPPMS (reprinted from [31]after permission,1999©Elsevier).1663K.Sarakinos et al./Surface &Coatings Technology 204(2010)1661–1684ﬁrst step of the discharge is made of a weakly ionized regime,while the second part of the pulse brings the glow discharge to the high ionization state.The weakly ionized regime allows for the formation of a stable discharge prior to entering the highly ionized regime which helps to suppress the arc formation during the high power mode of operation [54].3.Plasma dynamics in HPPMS 3.1.On the HPPMS plasmaPlasmas are partially ionized gases characterized by,for example,the charge (electron)density or the electron energy distribution function [55].A classi ﬁcation of different plasmas could be based on the charge density as shown in Fig.9,where the magnetron sputtering plasmas are shown to exhibit densities ranging from ∼1014up to 1020m −3.It is known that high density plasmas such as arc plasmas exhibit high ionization fractions [25,31].This is not the case for conventional magnetron plasmas where the electron density does not exceed the value of 1016m −3,penning ionization is the main ionization mechanism [30]and therefore the ionization of the sputtered species is at the best of few percent [13].In order to achieve a high ion fraction in a discharge it is necessary to promote the electron impact ionization [55]which is best realized by increasing the electron density and temperature.Different approaches have been made to achieve this,for example by using an electron cyclotron resonance (ECR)plasma as a plasma electron source [56],or by employing an inductively coupled plasma (ICP)[57,58].A higher frequency of electron impact ionization collisions can alsobeFig.5.Target current and voltage waveforms produced by the power supply developed by Kouznetsov et al.at working pressures of (a)0.06Pa,(b)0.26Pa,(c)1.33Pa and (d)2.66Pa.The charging voltage of 1000V drops down to 800V and the plasma is ignited.Peak target currents of up to 230A are achieved.The time lag for the ignition of the plasma decreases when the pressure is increased (reprinted from [35]after permission,2002©Elsevier).Fig.6.Rectangular shape voltage waveforms generated by the HPPMS power supply developed by MELEC GmbH.The data were recorded from an Ar –Al HPPMS discharge operating at pulse on/off time con ﬁguration 25/475µs (K.Sarakinos,unpublisheddata).Fig.7.Temporal evolution of target voltage and current during operation using a 10µs long pulse.The target current is interrupted before the plasma enters into the steady-state regime.The data were recorded from an Ar –Ti HPPMS discharge (S.Konstantinidis unpublished data).1664K.Sarakinos et al./Surface &Coatings Technology 204(2010)1661–1684achieved by increasing the power to the sputter source and therewith the electron density.This approach is,however,limited since a power excess can result in an overheating of the target or an exceeding of the Curie temperature of the magnetron's magnets.Thus,the work of Kouznetsov et al.[31]opened a new era of magnetron sputtering deposition and plasma studies,since it allowed for the generation of highly ionized plasmas using a conventional magnetron source.The early analyses of the HPPMS plasma showed that its temporal characteristics are complex and unique [35,59,60].Understanding the discharge evolution and the mechanisms that take place in the plasma is therefore of utmost importance as this sheds light on the growth conditions during the thin ﬁlm deposition.To achieve this,modeling and a number of analytical studies have been carried out,using Langmuir probes as well as optical emission,mass,and absorption spectroscopy techniques.These studies are reviewed in the following chapters and the basic properties of the HPPMS plasma are thus outlined.3.2.Determination of plasma properties:a theoretical overview Plasmas are commonly modeled as ﬂuids,and each species is described with its ﬂuid equation [61].The ﬂuid approximation is suf ﬁciently accurate to describe the majority of observed plasma phenomena.When this is not viable,the accumulated behavior of anensemble of charged particles is described using a statistical approach,in the form of the velocity distribution function which is given as f (r,u,t )d 3r d 3u ,i.e.the number of particles inside a six-dimensional phase space volume d 3r d 3u at (r ,u )and time t [55].Here,r is the position vector and u is the speed vector.Knowing the distribution function,fundamental features of a large ensemble can be quanti ﬁed,i.e.by integrating over the velocity,the average (macroscopic)quantities associated with the plasma are de ﬁned as:n ðr ;t Þ=∫f ðr ;u ;t Þd u plasmanumber ðion ;electron ;orneutral Þdensityð2Þu Àðr ;t Þ=1n∫u f ðr ;u ;t Þd u plasmabulkfluid velocity ð3ÞεÀðr ;t Þ=m 2n∫ðu À−u Þ2f ðr ;u ;t Þd u plasmameankineticenergy ð4Þwhere m is the particle (ion,electron,or neutral)mass.This procedure of integrating over velocity space is referred to as taking velocity moments,with each one yielding a physically signi ﬁcant quantity.Knowledge of the electron energy (velocity)distribution function (EEDF)permits us to determine important parameters.The EEDF can be measured using a Langmuir probe,as was demonstrated by Druyvesteyn [55]:g e ðV Þ=2m e A pr 2eV m12d 2I edV ð5Þwhere A pr is the probe area,I e is the electron current,m is the electronmass,and V =V pl −V b is the potential difference between the plasma potential and the probe potential.With the change of variables ε=eV ,the electron density n e is determined as:n e =∫∞0g e ðεÞd εð6ÞPlasma parameters,such as the electron density,are easily obtained if the mathematical description of the EEDF is Maxwellian.The Maxwellian distribution is characterized by its single temperature and is obtained when plasma electrons undergo enough collisions and are in equilibrium with other plasma components and with the electron ensemble itself.At a low pressure,the EEDF is generally non-Maxwellian,and the electron temperature is thought of as aneffectiveFig.8.Target voltage and current waveforms during the long-pulse operation.In the ﬁrst 500µs a weakly ionized plasma is generated which is followed by a highly ionized steady-state plasma (reprinted from [54]after permission,Society of Vacuum Coaters ©2007).Fig.9.Plasma density is used in order to classify plasmas.The HPPMS plasma spans over a large density range depending on the pulse on/off time con ﬁguration used for a process and presents therefore a tool for better choosing the plasma dynamics needed for the deposition process.The abbreviation ECR stands for Electron Cyclotron Resonance (taken from [36]).1665K.Sarakinos et al./Surface &Coatings Technology 204(2010)1661–1684electron temperature,T eff∼23〈ε〉,representing the mean electron energy determined from the EEDF according to[55]:〈ε〉=1n e ∫∞εg eðεÞdεð7Þ3.3.The HPPMS plasma propertiesOne of the earliest works related to the study of the properties of the HPPMS plasma was carried out by Gudmundsson et al.[35,59]. Using a Langmuir probe[62]in an Ar–Ta plasma,they measured the temporal behavior of the EEDF by recording the time-dependent probe current curves.It was shown that the EEDF evolved from a Druyvesteyn-like distribution with a broad energy distribution(high average energy)during the pulse to a double Maxwellian distribution (i.e.a distribution that is composed of two-distinct energy-distribu-tions)toward the end of the pulse,andﬁnally a Maxwellian-like distribution hundreds ofµs after the pulse had been switched off.This is shown in Fig.10,where the temporal evolution of the EEDF is also seen to be affected by the working pressure[35].Similar results were obtained by Pajdarova et al.[63],although the double Maxwellian distribution was observed from the beginning of the pulse,which could be explained by the nature of the target material(Cu in this case)and the HPPMS parameters used in the experiments.Further-more,it was found that the effective electron temperature peaked a fewµs after the pulse start,and at the same time,independent of measurement position[35].This is indicative of the existence of high energy electrons accelerated by the target voltage,and thus escaping the conﬁnement region in the vicinity of the cathode[35].InaFig.10.The EEDF(electron energy distribution function)for an HPPMS plasma with a100µs on-time and a50Hz frequency at three chamber pressures(a)0.25,(b)1.33,and(c) 2.66Pa.The distribution function changes during the pulse from a Druvesteyn distribution to a double-Maxwellian distribution(reprinted from[35]after permission,Elsevier©2002).1666K.Sarakinos et al./Surface&Coatings Technology204(2010)1661–1684different study by Seo et al.[64],Langmuir probes were utilized to determine the properties of a HPPMS plasma at a higher pulsing frequency of 10kHz and a duty cycle of 10%.By using such a high frequency,the electron density reached a value of 1.5×1017m −3,which could be seen as a relatively low density HPPMS plasma.The temporal evolution of the energy distribution in this plasma showed ef ﬁcient electron heating,which took place within the ﬁrst few µs from the pulse start.This work revealed that the plasma dynamics presented in the studies by Gudmundsson et al.[35,59]are to a large extent general features of the low duty cycle pulsed plasmas regardless of the frequency.The early studies on the HPPMS plasma characteristics also showed the potentially high ionization fraction of the sputtered material that could be achieved by using this technique [16,25,33,60,65,66].Spectroscopic analyses,such as optical emission spectroscopy [67–69],mass spectroscopy [69],and absorption spectroscopy [68,69]were,therefore,employed in order to better quantify and understand the average and time-dependent evolution of the ion population in the HPPMS plasma.The optical emission studies [33,65,70–72],were performed both for the HPPMS and the dcMS discharges and showed the much higher metal ion emission intensity in HPPMS (Fig.11).However,in order to better quantify these observations,absorption spectroscopy [70,71,73,74]and mass spectroscopy [75,76]studies were performed,con ﬁrming the much higher ionization of the sputtered material and of the sputtering gas achieved in HPPMS.One well studied material in this regard is Ti.Mass spectroscopy measurements of the peak ion compositions in an Ar –Ti discharge for the pulse on-time of 100µs and a frequency of 50Hz [75]showed that the ionized part of the plasma contained 50%of Ti 1+,24%of Ti 2+,23%of Ar 1+,and 3%of Ar 2+ions.When longer pulse on-times of several hundreds of µs were used even Ti 3+and Ti 4+could also be detected [77].It is deduced that for the formation of multiply charged ions certain conditions should be ful ﬁlled [77];(i)the discharge should contain a high enough number of metal ions,(ii)the pulse on-time should be long enough (and the capacitance of the used power supply should be large enough to store enough energy for the whole pulse),(iii)the self-sputtering yield (i.e.the sputtering yield of the target material from ionized target species)should be low,and (iv)the ionization energy of each ionization step should not be too high.These two examples show clearly that the HPPMS plasma properties depend not only on the plasma density and energy distribution but also,to a large degree,on the pulse on/off time con ﬁguration.Since the introduction of the HPPMS technique,most of the discharge studies have been performed for pulses with pulses on-time longer than 50µs.Exceptions to this can be found in the works by Konstantinidis et al.[70,73],Ganciu et al.[44],and Vasina et al.[48],where shorter pulses ranging between 5and 30µs have been used,an example of which is shown in ing time-resolved optical emission spectroscopy in a Ti –Ar discharge,an increase in the line intensity ratio of the ion-to-neutral Ti and Ar species with increasing the pulse duration was observed [70].This means that the ionization degree increases with increasing pulse length,indicating the necessity for the sputtered atoms to reside a suf ﬁcient time in the target vicinity for the vapor to become ionized.The increase of the AremissionFig.11.Optical emission spectroscopy from a typical HPPMS plasma.The emission from the Ti and Ar species increases greatly in HPPMS indicating the high ionization of the metal and gas species (data taken from [72]).Fig.10(continued ).1667K.Sarakinos et al./Surface &Coatings Technology 204(2010)1661–1684。

《Broadcasting, IEEE Transactions on》期刊第3页200条数据

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Implementation》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html16.《An Efficient Nonlinear Companding Transform for Reducing PAPR of OFDM Signals》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html17.《Saliency Inspired Full-Reference Quality Metrics for Packet-Loss-Impaired Video》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html18.《Improved CIR-Based Receiver Design for DVB-T2 System in Large Delay Spread Channels: Synchronization and Equalization》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html19.《High Power Amplifier Pre-Distorter Based on Neural-Fuzzy Systems for OFDM Signals》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html20.《Performance Analysis of Inter-Layer Prediction in Scalable Video Coding Extension of H.264/AVC》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html21.《Study of Rating Scales for Subjective Quality Assessment of High-Definition Video》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html22.《Planning Factors for Digital Local Broadcasting in the 26 MHz Band》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html23.《Peak-to-Average Power Ratio Reduction of OFDM Signals Using PTS Scheme With Low Computational Complexity》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html24.《Feedback Cancellation for T-DMB Repeaters Based on Frequency-Domain Channel Estimation》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html25.《Efficient Multi-Reference Frame Selection Algorithm for Hierarchical B Pictures in Multiview Video Coding》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html26.《Performance Comparisons and Improvements of Channel Coding Techniques for Digital Satellite Broadcasting to Mobile Users》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html27.《Burst-Aware Dynamic Rate Control for H.264/AVC Video Streaming》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html28.《Helicopter-Based Digital Electronic News Gathering (H-DENG) System: Case Study and System Solution》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html29.《Transmit Diversity for TDS-OFDM Broadcasting System Over Doubly Selective Fading Channels》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html30.《Interference Cancellation Techniques for Digital On-Channel Repeaters in T-DMB System》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html31.《Field Measurements of EM Radiation From In-House Power Line Telecommunications (PLT) Devices》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html32.《A Novel Scheme of Joint Channel and Phase Noise Compensation for Chinese DTMB System》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html33.《Path Loss Prediction for Mobile Digital TV Propagation Under Viaduct》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html34.《Efficient Motion Vector Interpolation for Error Concealment of H.264/AVC》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html35.《3D-TV Content Creation: Automatic 2D-to-3D Video Conversion》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html36.《A Novel Rate Control Technique for Multiview Video Plus Depth Based 3D Video Coding》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html37.《The Effect of Crosstalk on the Perceived Depth From Disparity and Monocular Occlusions》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html38.《Semi-Automatic 2D-to-3D Conversion Using Disparity Propagation》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html39.《Display-Independent 3D-TV Production and Delivery Using the Layered Depth Video Format》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html40.《3DTV Roll-Out Scenarios: A DVB-T2 Approach》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html41.《PAPR Reduction Using Low Complexity PTS to Construct of OFDM Signals Without Side Information》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html42.《Quality-Oriented Multiple-Source Multimedia Delivery Over Heterogeneous Wireless Networks》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html43.《Efficient PAPR Reduction in OFDM Systems Based on a Companding Technique With Trapezium Distribution》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html44.《Objective Video Quality Assessment Methods: A Classification, Review, and Performance Comparison》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html45.《Pixel Interlacing Based Video Transmission for Low-Complexity Intra-Frame Error Concealment》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html46.《Fountain Codes With PAPR Constraint for Multicast Communications》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html47.《RF Watermark Backward Compatibility Tests for the ATSC Terrestrial DTV Receivers》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html48.《61st Annual IEEE Broadcast Symposium — Save the Date》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html49.《Evaluation of Asymmetric Stereo Video Coding and Rate Scaling for Adaptive 3D Video Streaming》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html50.《Stereoscopic Perceptual Video Coding Based on Just-Noticeable-Distortion Profile》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html51.《A Depth Information Based Fast Mode Decision Algorithm for Color Plus Depth-Map 3D Videos》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html52.《3D-TV Production From Conventional Cameras for Sports Broadcast》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html53.《A Digital Blind Watermarking for Depth-Image-Based Rendering 3D Images》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html54.《Object-Based 2D-to-3D Video Conversion for Effective Stereoscopic Content Generation in 3D-TV Applications》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html55.《3D-TV Content Storage and Transmission》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html56.《New Depth Coding Techniques With Utilization of Corresponding Video》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html57.《3DTV Broadcasting and Distribution 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Systems》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html64.《Perceptual Issues in Stereoscopic Signal Processing》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html65.《Performance Evaluation of Multimedia Content Distribution Over Multi-Homed Wireless Networks》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html66.《The Relationship Among Video Quality, Screen Resolution, and Bit Rate》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html67.《Corrections to “Efficient Motion Vector Interpolation f or Error Concealment of H.264/AVC”》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html68.《PAPR Reduction of OFDM Signals by PTS With Grouping and Recursive Phase Weighting Methods》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html69.《Improve the Performance of LDPC Coded QAM by Selective Bit Mapping in Terrestrial Broadcasting System》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html70.《The Importance of Visual Attention in Improving the 3D-TV Viewing Experience: Overview and New Perspectives》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html71.《Co-Channel Analog Television Interference in the TDS-OFDM-Based DTTB System: Consequences and Solutions》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html72.《Prediction and Transmission Optimization of Video Guaranteeing a Bounded Zapping-Delay in DVB-H》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html73.《IBC2011 Experience the Future》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html74.《61st Annual IEEE Broadcast Symposium — Save the 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Future》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html80.《Evaluation of Stereoscopic Images: Beyond 2D Quality》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html81.《An Evaluation of Parameterized Gradient Based Routing With QoE Monitoring for Multiple IPTV Providers》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html82.《Three-Dimensional Displays: A Review and Applications Analysis》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html83.《Guest Editorial Special Issue on 3D-TV Horizon: Contents, Systems, and Visual Perception》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html84.《LIVE: An Integrated Production and Feedback System for Intelligent and Interactive TV 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Modulated Symbols for Advanced T-DMB System》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html96.《Pooling-Based Intra Prediction Mode Coding for Mobile Video Applications》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html97.《A Suboptimal Tone Reservation Algorithm Based on Cross-Entropy Method for PAPR Reduction in OFDM Systems》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html98.《A Measurement Method of the DTMB Modulator》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html99.《Interference Elimination for Chinese DTMB System With Transmit Diversity》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html100.《61st Annual IEEE Broadcast Symposium》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html101.《IBC2011 Experience the Future》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html102.《A Low-Complexity SLM Scheme Using Additive Mapping Sequences for PAPR Reduction of OFDM Signals》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html103.《Illumination-Sensitive Background Modeling Approach for Accurate Moving Object Detection》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html104.《Coordinating Allocation of Resources for Multiple Virtual IPTV Providers to Maximize Revenue》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html105.《Inter-Sequence Error Concealment Techniques for Multi-Broadcast TV Reception》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html106.《Performance Evaluation of the DVB-RCT Standard for Interactive Services》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html107.《An Efficient Predistorter Design for Compensating Nonlinear Memory High Power Amplifiers》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html108.《Accurate BER Analysis of OFDM Systems Over Static Frequency-Selective Multipath Fading Channels》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html109.《A Frame-Related Approach for Performance Improvement of MPE-FEC in DVB-H》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html110.《Balanced Multiple Description Coding for 3D DCT Video》原文链接:https:///academic-journal-foreign_broadcasting-ieee-transactions_thesis/020*********.html111.《Performance Validation of the DVB-SH Standard for Satellite/Terrestrial Hybrid Mobile Broadcasting 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极谱值英文专业表达

极谱值英文专业表达Polarographic Values: A Technical Overview.Polarography, often referred to as voltammetry, is an electrochemical analytical technique used to determine the concentration of various substances in a solution. Itrelies on the measurement of the current-voltagerelationship as a working electrode is scanned through a range of potentials in the presence of the analyte. The resulting polarogram, which is a plot of current against potential, provides information about the electrochemical behavior of the analyte and can be used to quantify its concentration.Polarographic values, or more specifically, the peak current and peak potential values obtained from polarograms, are crucial parameters in the analysis of substances using this technique. These values are directly related to the electrochemical properties of the analyte and can be usedto identify and quantify different compounds in a sample.Peak Current in Polarography.Peak current, denoted as Ip, is the maximum current value observed in a polarogram when the working electrode passes through the potential at which the analyte undergoes an electrochemical reaction. The magnitude of the peak current is dependent on several factors, including the concentration of the analyte, the nature of the electrochemical reaction, and the rate of electron transfer at the electrode surface.The peak current is proportional to the concentration of the analyte, assuming that other conditions such as temperature, electrode surface area, and solution composition remain constant. This relationship can be expressed as:Ip = nFAvC.where:Ip is the peak current.n is the number of electrons transferred in the electrochemical reaction.F is Faraday's constant (96,485 C/mol)。

手性样品前处理

MIP provides a unique opportunity for the creation of 3-dimensional cavities.
MIP-based chiral recognition sets an exotic trend in development of chiral sensors.
Analytica Chimica Acta 853 (2015) 1–18
Contents lists available at ScienceDirect
Analytica Chimica Acta
journal homepage: /locate/aca
Molecularly imprinted polymer based enantioselective sensing devices: A review
We present about rational design of chiral sensors as selective and sensitive devices.
GRAPArticle history: Received 20 December 2013 Received in revised form 8 June 2014 Accepted 9 June 2014 Available online 12 June 2014
ã 2014 Elsevier B.V. All rights reserved.
* Corresponding author at: N 16/62 E-K, Sudamapur, Vinayaka, Varanasi 221010, Uttar Pradesh, India. Tel.: +91 9450239133. E-mail addresses: mahavirtiwari@, krineshtiwari@ (M.P. Tiwari).

等离激元共振峰英文

等离激元共振峰英文全文共四篇示例，供读者参考第一篇示例：Plasmon Resonance PeakIntroductionPlasmon resonance is a collective oscillation of free electrons in a material that occurs when the frequency of incident light matches the natural frequency of the electrons in the material. This phenomenon is often observed in metallic nanoparticles, where the conduction electrons can be excited by incident electromagnetic radiation. One of the most prominent features of plasmon resonance is the appearance of a distinct peak in the absorption or scattering spectra of the material, known as the plasmon resonance peak or plasmon resonance band.第二篇示例：Plasmon resonance refers to the collective oscillation of free electrons in a metal when it is subjected to electromagnetic radiation. This phenomenon, also known as surface plasmon resonance (SPR), has been extensively studied and applied invarious fields such as sensing, imaging, and light manipulation. One of the key features of plasmon resonance is the emergence of a characteristic peak in the absorption or scattering spectrum, known as the plasmon resonance peak or plasmon resonance band. In this article, we will focus on a specific type of plasmon resonance peak – the localized surface plasmon resonance peak, which is commonly referred to as the plasmon resonance peak.第三篇示例：Plasmonic resonance peak, also known as localized surface plasmon resonance (LSPR) peak, is a phenomenon in which free electrons in a metal nanoparticle oscillate collectively in response to incident light. This oscillation creates a strong electromagnetic field enhancement around the nanoparticle, leading to enhanced light-matter interactions. The spectral position of the plasmonic resonance peak, known as the plasmon resonance wavelength, depends on the size, shape, composition, and surrounding environment of the nanoparticle.第四篇示例：One specific type of surface plasmon resonance that has attracted attention is the localized surface plasmon resonance (LSPR) peak. LSPR peaks manifest as sharp extinction peaks inthe absorption or scattering spectra of metal nanoparticles due to the resonance between incident light and the localized surface plasmons on the nanoparticle surface. These peaks are highly sensitive to the size, shape, and composition of the nanoparticle, making them an excellent candidate for various applications such as chemical sensing, biological detection, and single molecule analysis.。

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Power spectrum: 4th order LP delta sigma transformed to 8th order BP delta−sigma 10
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REALIZATION OF A SIGMA-DELTA MODULATOR IN FPGA
Bachelor thesis 1999 at Ericsson Microwave Systems AB by Michael Melin Magnus Nilsson
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Supervisor, EMW: Rune Olsson Supervisor, CTH: Lars Bengtsson Examinator: Bert Lanne Institution of electronics Chalmers university of tecnology Gothenburg 1999
Bachelor Thesis
Uppgjord (även faktaansvarig om annan) - Prepared (also subject responsible if other) Nr - No.
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Magnus Nilsson & Michael Melin
Dokansv/Godkänd - Doc respons/Approved Kontr - Checked

T.W. ANDERSON (1971). The Statistical Analysis of Time Series. Series in Probability and Ma

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OFDM系统PAPR减小技术综述

文章编号：１００２－８６９２（２００６）０１－００４１－０３ＯＦＤＭ系统ＰＡＰＲ减小技术综述＊韩艳春，杨士中（重庆大学通信工程学院，四川重庆４０００４４）【摘要】描述了一些减小ＰＡＰＲ的主要方法：限幅类技术、编码类技术、概率类技术［包括部分传输序列（ＰＴＳ）、选择映射（ＳＬＭ）、交织、ＴＲ（ＴｏｎｅＲｅｓｅｒｖａｔｉｏｎ），ＴＩ（ＴｏｎｅＩｎｊｅｃｔｉｏｎ）和ＡＣＥ（ＡｃｔｉｖｅＣｏｎｓｔｅｌｌａｔｉｏｎＥｘｔｅｎｓｉｏｎ）］等。

【关键词】正交频分复用；峰均功率比；互补累积分布函数；选择映射方法；传输序列方法【中图分类号】ＴＮ９１１．３；ＴＮ９４１．１【文献标识码】ＡＡＮＯＶＥＲＶＩＥＷＯＦＰＡＰＲＲＥＤＵＣＴＩＯＮＴＥＣＨＮＩＱＵＥＳＦＯＲＯＦＤＭＨＡＮＹａｎ－ｃｈｕｎ，ＹＡＮＧＳｈｉ－ｚｈｏｎｇ（ＣｏｍｍｕｎｉｃａｔｉｏｎＥｎｇｉｎｅｅｒｉｎｇＳｃｈｏｏｌ，ＣｈｏｎｇｑｉｎｇＵｎｉｖｅｒｓｉｔｙ，Ｃｈｏｎｇｑｉｎｇ４０００４４，Ｃｈｉｎａ）【Ａｂｓｔｒａｃｔ】ＴｈｉｓａｒｔｉｃｌｅｄｅｓｃｒｉｂｅｓｓｏｍｅｏｆｔｈｅｉｍｐｏｒｔａｎｔＰＡＰＲｒｅｄｕｃｔｉｏｎｔｅｃｈｎｉｑｕｅｓｆｏｒＯＦＤＭｉｎｃｌｕｄｉｎｇａｍｐｌｉｔｕｄｅｃｌｉｐｐｉｎｇｔｅｃｈ－ｎｉｑｕｅ，ｃｏｄｉｎｇｔｅｃｈｎｉｑｕｅａｎｄｐｒｏｂａｂｉｌｉｔｙｔｅｃｈｎｉｑｕｅ，ｓｕｃｈａｓｐａｒｔｉａｌｔｒａｎｓｍｉｔｓｅｑｕｅｎｃｅ，ｓｅｌｅｃｔｅｄｍａｐｐｉｎｇ，ｉｎｔｅｒｌｅａｖｉｎｇ，ｔｏｎｅｒｅｓｅｒｖａ－ｔｉｏｎ，ｔｏｎｅｉｎｊｅｃｔｉｏｎ，ａｎｄａｃｔｉｖｅｃｏｎｓｔｅｌｌａｔｉｏｎｅｘｔｅｎｓｉｏｎ．【Ｋｅｙｗｏｒｄｓ】ＯＦＤＭ；ＰＡＰＲ；ＣＣＤＦ；ＳＬＭ；ＰＴＳ・综述・１引言ＯＦＤＭ的提出已经有近４０年的历史了，近年来，由于数字信号处理技术的飞速发展，ＯＦＤＭ作为一种可以有效对抗符号间干扰（ＩＳＩ）的高速传输技术被广泛关注。

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A Comparison of SLM and PTS Peak-to-AveragePower Ratio Reduction Schemes for OFDMSystemsYasmin Hassan#1, Mohammed El-Tarhuni#2#Electrical Engineering Department, American University of SharjahUnited Arab Emirates, Sharjah1g00013853@2mtarhuni@Abstract- In this paper, we compare the performance of two peak-to-average power ratio (PAPR) reduction schemes. Specifically, selected mapping (SLM) and partial transmit sequence (PTS) schemes are investigated. The analysis was performed for an OFDM system aligned with the IEEE 802.16e standard with 256 and 1024 subcarriers. The results indicate that SLM has superior performance over PTS in reducing PAPR for the same number of subcarriers but at the expense of higher computational complexity. We have also investigated a modified selected mapping scheme with lower complexity. Index Terms ------OFDM; PAPR; Selected Mapping; Partial Transmit SequenceI. INTRODUCTIONRecently, there has been a steady growth in using wireless communication systems for a wide range of services including voice, data, and multimedia applications. However, there are many challenges imposed by the wireless channel on such systems such as severe multipath conditions leading to inter-symbol interference (ISI) and fading causing significant loss of the received signal. One of the key techniques that effectively handles both of these issues is to use orthogonal frequency division multiplexing (OFDM). There has been a wide use of OFDM in different systems such as wireless LANs, digital audio broadcasting (DAB), digital video broadcasting (DVB), and recently in WiMAX and fourth generation systems [1].OFDM mitigates the multipath conditions by dividing the data to be transmitted over a large number of relatively narrowband channels using orthogonal subcarriers. However, OFDM systems suffer from a major drawback because the transmitted signal has a high peak-to-average power ratio (PAPR). This leads to a significant degradation of power efficiency since the signal has to be protected against the distortion introduced by non-liner devices such as power amplifiers. Hence, PAPR reduction schemes are very important for practical deployment of OFDM-based systems [1].Different schemes have been proposed in the literature for PAPR reduction in OFDM systems [2] [3]. These schemes can be classified into two types: multiplicative or additive. In multiplicative schemes, the OFDM data sequence is multiplied by a specific phase sequence to reduce the PAPR of the transmitted signal. Examples of this type are selected mapping (SLM), windowing and partial transmit sequence (PTS) schemes. The latter type, however, is based on adding a reference signal to the input sequence to keep it below a certain threshold. Clipping and tone reservation are examples of the additive type. Another criterion for classification is whether the PAPR reduction scheme is deterministic or probabilistic. In deterministic schemes, clipping is applied to limit the signal peaks below a certain threshold causing distortion to the signal and lead to degradation in bit error rate (BER). On the other hand, probabilistic schemes improve the characteristics of the PAPR distribution without signal distortion.In this paper, we compare the performance of two of the probabilistic PAPR reduction schemes; namely SLM and PTS schemes [2] [3] [4]. A low complexity SLM scheme was also simulated and compared to the typical SLM scheme and PTS. This modified scheme achieves comparable performance to that of SLM with reduced computational complexity.This reset of the paper is organized as follows. In Section II, a brief description of the OFDM system and PAPR reduction problem is presented. Analysis and comparison of the PAPR schemes are discussed in Section III. In Section IV, simulation results are given and the performance of the evaluated techniques is compared. Finally, conclusions are presented in Section V.II. OFDM SYSTEMSIn OFDM systems, the available spectrum bandwidth is divided into smaller narrower sub-bands with frequency spacing ∆ / . The signal transmitted in each sub-band can be independently modulated using phase-shift keying (PSK) or quadrature amplitude modulation (QAM) at a symbol rate of 1/∆ . The high data stream is divided into smaller data streams that are transmitted simultaneously. For each sub-band, we can associate a sinusoidal carrier at frequency equal to the mid frequency in the th subband. The subcarriers can be made orthogonal over a symbol interval by making the separation between adjacent carriers to be equal to / , where n is a positive integer.The OFDM signal is generated by applying inverse discrete Fourier transform (IDFT) to the modulated signal [5]. A cyclic prefix is usually inserted at the beginning of each OFDM symbol before transmission. A block diagram of OFDM transmitter is illustrated by Fig. 1. The input information bits are referred to as data points and the modulated symbol is referred to as a symbol for the subcarrier. Moreover, vectors … and … denote the frequency domain and time domain OFDM symbols, respectively, where1/0,…, 1 1Which can be expressed in a matrix format as:2 Where W is the IDFT matrix whose elements are ,/ . The main disadvantage in OFDM systems as well as multicarrier modulation systems is the high peak to average power ratio inherent in the transmitted signal. When the modulated sub-carriers are added up coherently to be transmitted, the signals from different sub-channel add up constructively in phase leading to large signal peaks. The occurrence of such large peaks leads to clipping of the OFDM signal in the digital-to-analog (D/A) converter and/or it may saturate the amplifier leading to distortion in the transmitted signal. This causes signal distortion including in-band distortion and out-of-band emissions and hence inducing degradation to the bit error rates (BER). The PAPR of the transmitted signal is defined asPAPR| || | 3III. PAPR REDUCTION ALGORITHMSA. WindowingThe simplest way to reduce the PAPR in OFDM systems is to apply clipping to the OFDM signal. However, this leads to nonlinear distortion to the signal and increase the out-of- band emissions. Instead of clipping the amplitude of the signal, the signals' large peaks can be scaled such that it is maintained below a certain level. This can be done by applying a window function to the transmitted signal. This window function can be used to maintain the out-of-band emission below a certain level by increasing the length of the window. However, the length of the window should also be small enough in order not to degrade the BER performance. The weighting coefficients should be chosen in such a way that the resulting windowed signal does not exceed a certain threshold. However, when two or more peaks occur within half of a window size, windows will overlap resulting in further reduction of the peak amplitude of the signal below the required threshold which leads to BER degradation [3]. To avoid such a problem, the large signal peaks could be detected and suppressed to the exact threshold value even if successive peaks occur.B. Selected Mapping Scheme (SLM)SLM is a PAPR reduction scheme for the OFDM system that does not lead to in-band distortion or out-of-band radiation [4]. Assume … represents an input symbol sequence in frequency domain, is the complex data of the k th subcarrier and is the number of subcarriers. Then, the OFDM sequence … is the inverse fast Fourier transform of . In SLM, alternative input symbol sequences are generated at the transmitter bearing the same input symbol sequence and the sequence resulting in the minimum PAPR is transmitted. These sequences are generated via the multiplication of the input sequence and U phase sequences , , … , for 1 as shown below:, , … ,, , , , , ,…, , , 4where corresponds to component-wise multiplication of two vectors. Moreover, the phase vector consists of unit-magnitude complex numbered elements of the form , , , where , 0,2 . Note that is the alternative input symbol data in frequency domain; thus, IFFT needs to be performed for , 1 to generate the U alternative OFDM signal sequences as follows IFFT IFFT , 1 5For each alternative OFDM sequence, PAPR is computed as in (3) by substituting , . The sequence with the minimum PAPR among the U sequences is selected for transmission. The problem with SLM is that as the number of alternative sequences U increases, the PAPR reduction increases in the expense of computational complexity. This computational complexity occurs due to the large number of IFFT required to generate each alternative OFDM sequence of the U sequences. It is desirable to reduce the required number of IFFTs without degrading the performance of PAPR reduction.A modified SLM scheme was developed in [2] that considerably reduces the computational complexity while keeping comparable performance to that of the conventional SLM scheme. The basic idea of the modified scheme is to use a linear combination of the alternative OFDM sequences generated to obtain new sequences without the need of performing IFFT. This concept can be explained as follows. Let and be two of the alternative sequences generated in (4). The linear combination of the two sequences can be written as:,IFFT IFFTIFFT ,1 6If the elements of the vector are unity in magnitude, this vector can be considered as another phase sequence in the SLM scheme, where , represents a corresponding OFDM signal sequence. Hence another OFDM sequence can be obtained without the need of performing IFFT. The values of and needs to be chosen such that elements in vector given the initial condition that all elements of and respectively have unit magnitude as well. Satisfying these conditions, we can obtain additional 1 phase sequences from binary phase sequences. Hence, a total of phase sequences are obtained, where only IFFTs is required in addition to summations of 1 pairs of OFDM signal sequences [2].Comparing the modified selected mapping (MSLM) scheme with phase sequences with the conventional SLM scheme with phase sequences, the two schemes perform comparably in PAPR reduction for small . However, as the number of phase sequences increase the performance of the modified scheme deteriorates compared the conventional SLM. The reason is that the additional 1 phasesequences in the modified scheme are statistically correlated with the original phase sequences and hence no additional gain is achieved as becomes large.C. Partial Transmit Sequence (PTS)Partial transmit sequence is one of the commonly used techniques for PAPR reduction in OFDM systems. It is a distortion-less phase optimization scheme that is based on merging phase shifted signal sub-blocks by certain phase factors that results in a candidate signal with minimum PAPR to be transmitted [5][6] [7].In conventional PTS OFDM systems, the input data block , 0 1 is partitioned into disjoint sub-blocks , , , ,…, , ,1 such that ∑ . Since each signal blockhas N signal points, then each sub-block will haveexistingsignal points and others are padded with zeros. In other words, the th sub-block will have the first 1 / and the last / signal points to be equal to zero. If we let ,…, ,…, be a set of phase factor with 0,2 , then the time domain signal after phase combining becomes… , 1,…, 7where is N-point IFFT of each sub-block ,,…, ,…, with , and L is the number of candidate signal with . A block diagram describing OFDM system with PTS is shown in Fig. 2.Finally, the phase sequence that minimizes the transmitted PAPR can be determined as follows,|| , 01 8Since all candidate signals have same average power, we choose the sequence with smallest peak and consequently smallest PAPR.IV. SIMULATION AND NUMERICAL ANALYSISIn this section, we present simulation results for PAPR reduction performance of SLM, MSLM and PTS schemes. In order to satisfy the unity magnitude condition in (6), the phase sequence used in SLM and MSLM are binary phase sequences i.e . , 1,-1}. The rows of Hadamard matrix are used in phase factor generation in this case. However, phase factors used in the SLM and PTS analysis weregenerated such that, , , ,…, . The performance metric utilized in evaluating various PAPRreduction schemes is the complementary cumulative distribution function (CCDF) of the PAPR of the transmitted continuous time signal . It specifies the probability that the transmitted signal will be clipped by the power amplifier at the transmitter, which in turns determines the level of deterioration in BER resulting from this clipping. The numerical analysis for the mentioned PAPR reduction schemes was performed for the OFDM systems specified in IEEE802.16e standard, which use 256 and 1024 subcarriers and 16-QAM modulation.The CCDFs of PAPR (the probabilities that the PAPR of OFDM signal exceeds a given ) are numerically obtained for the SLM with 16 and 25 and MSLM with 4 and 5. The resulting CCDF curves are presented in Fig.3 for 10,000 input symbol sequences and 1024. It is clear that both schemes improve the original OFDM PAPR significantly for different phase sequence length. It can also be observed that MSLM with 4,5 is worse than the conventional SLM with 16 and 25 by about 0.2dB. However, this deterioration in performance is negligible compared to the reduction in computational complexity offered by MSLM.In Fig .4 and Fig. 5, similar results were obtained for PTS with 4 and 16 candidate signals and SLM with 9 and 16 signals applied to OFDM with 256 and 1024, respectively. It is clear that conventional SLM outperforms PTS scheme by 0.5dB more reduction in PAPR. This advantage in SLM scheme is countered with further computational complexity since it requires IFFTs and complex multiplications, while PTS requires only IFFTs and complex multiplications.Finally, the effect of the total number of sub-blocks in PTS scheme was analyzed. As the number of sub-blocks is increased from 4 to 8, the PAPR is further reduced by about 0.6dB. This suggests that the higher the number of blocks, the larger the more the phase variations in each candidate signal which may lead to further reduction in PAPR. However, increasing the number of sub-blocks comes at the expense of increasing computational complexity of PTS scheme.V. CONCLUSIONDifferent PAPR reduction schemes in OFDM systems including SLM, modified SLM and PTS were investigated and their performance was compared. All the investigated techniques provide sufficient improvement in PAPR reduction over original OFDM signal. As the number of phase sequences in the applied scheme increase, the PAPR reduction increases in the expense of computational complexity. It was shown that modified SLM performs comparably to conventional SLM with much lower complexity. Moreover, it was shown that SLM has a performance improvement of 0.5dB in PAPR reduction that PTS scheme.REFERENCES[1] J. Proakis and M. Salehi, Digital Communications , 5th ed., McGraw-Hill, 2008.[2] S. Heo, H. Noh, S, J. No, and D. Shin, "A Modified SLM Scheme withLow Complexity for PAPR Reduction in OFDM Systems," IEEE Transactions on Broadcasting, vol.53, pp.404-408, 2007.[3] S. Cha, M. Park, S. Lee, K. Bang, and D. Hong, "A New PAPRReduction Technique for OFDM Systems Using Advanced Peak Windowing Method," IEEE Transactions on Consumer Electronics, vol.54, pp.405-410, 2008.[4] D.-W.Lim, C.-W.Lim, J.-S.No, and H.Chung, "A New SLM OFDMwith Low Complexity for PAPR Reduction," IEEE Signal Proc.Lett.,vol.12, pp.93-96, 2005.[5] A. Ghassemi, and T. A. Gulliver, "A Low Complexity IFFT-Based PTSTechniques for PAPR Reduction in OFDM Systems," Proc. Of IEEE Vehicular Technology Conference, pp.616-619, 2007.[6] Y. J. Kou, W.-S. Lu, and A. Antoniou, "A New Peak-to-AveragePower-Ratio Reduction Algorithm for OFDM Systems viaConstellation Extension," IEEE Transaction on Wireless Communications, vol.6, pp.1823-1832, 2007.[7] Y. Xiao, X. Lei, Q. Wen, and S.Li, "A Class of Low Complexity PTSTechniques for PAPR Reduction in OFDM Systems," IEEE Signal Processing Letters, vol.14, pp.680-683, 2007.Fig. 1. OFDM TransmitterFig. 2. System block diagram of partial transmit sequence algorithm.Fig. 3. PAPR reduction performance of SLM and MSLM with =1024.Fig. 4. PAPR reduction performance of SLM and PTS with =256.Fig. 5. PAPR reduction performance of SLM and PTS with=1024.1010101010PAPRo (dB)P r (P A P R >P A P R o )10101010PAPRo (dB)P r (P A P R >P A P R o )101010-110PAPRo (dB)P r (P A P R >P A P R o )Pr(PAPR>PAPRo) vs PAPRo。