| 1. |
- Abdelgaid, Mona, et al.
(författare)
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Multiscale modeling reveals aluminum nitride as an efficient propane dehydrogenation catalyst
- 2023
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Ingår i: Catalysis Science and Technology. - 2044-4753 .- 2044-4761. ; 13:12, s. 3527-3536
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Tidskriftsartikel (refereegranskat)abstract
- Nonoxidative propane dehydrogenation (PDH) is a promising route to meet the steadily increasing demand for propylene, an important building block in the chemical industry. Wurtzite group-IIIA metal nitrides are potential catalysts for PDH with high chemical, thermal, and mechanical stability alongside inherent Lewis acid-base properties that can activate the C-H bond of alkanes. Herein, we investigate the catalytic behavior of pristine (AlN) and gallium-doped (Ga/AlN) aluminum nitride for PDH via concerted and various stepwise mechanisms using density functional theory (DFT) calculations and microkinetic modeling (MKM). The reaction profiles investigated with DFT calculations are used in MKM, which reveals that the stepwise mechanisms produce >99% of propylene on both AlN and Ga/AlN. AlN has approximately one order of magnitude higher activity than Ga/AlN due to lower barriers along the dominant PDH reaction pathway. In summary, we propose the potential application of AlN as an efficient dehydrogenation catalyst for the conversion of light alkanes into valuable olefins. In addition, we show that multiscale simulations are essential to evaluate the catalytic behavior of complex alkane conversion reaction networks and obtain activity trends for dehydrogenation catalysts.
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| 2. |
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| 3. |
- Andersen, JR, et al.
(författare)
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Small-x phenomenology - summary and status 2002
- 2004
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 35:1, s. 67-98
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Forskningsöversikt (refereegranskat)abstract
- A second workshop on small-x physics, within the Small-x Collaboration, was held in Lund in June 2002 with the aim of over-viewing recent theoretical progress in this area and summarizing the experimental status.
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| 4. |
- Andersson, Bo, et al.
(författare)
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Small x phenomenology: summary and status
- 2002
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 25:1, s. 77-101
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Forskningsöversikt (refereegranskat)abstract
- The aim of this paper is to summarize the general status of our understanding of small-x physics. It is based on presentations and discussions at an informal meeting OIL this topic held in Lund, Sweden, in March 2001. This document also marks the founding of an informal collaboration between experimentalists and theoreticians with a special interest in small-x physics. This paper is dedicated to the memory of Bo Andersson. who died unexpectedly from a heart attack on March 4th, 2002.
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| 5. |
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| 6. |
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| 7. |
- Gustafson, Gösta, et al.
(författare)
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A simple model for the BFKL-DGLAP transition in deep inelastic scattering
- 2002
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Ingår i: European Physical Journal C. Particles and Fields. - : Springer Science and Business Media LLC. - 1434-6044. ; 23:2, s. 267-274
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Tidskriftsartikel (refereegranskat)abstract
- A simple model is presented which interpolates between the DGLAP and BFKL regimes in deep inelastic cp scattering. This model is based on the CCFM and LDC models, and it is simple enough to provide an intuitive picture of the transition region between the two domains. Results are presented for both fixed and running coupling; for fixed coupling the transition between the domains occurs at a constant ratio between Ink 2 L and In 1/x, while for a running coupling it occurs for constant ratio between lnlnk(2) and ln 1/x.
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| 8. |
- Gustafson, Gösta, et al.
(författare)
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Gluon distribution functions in the k(perpendicular to) -factorization approach
- 2002
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Ingår i: Journal of High Energy Physics. - : Springer Science and Business Media LLC. - 1029-8479. ; 33:10, s. 2989-2993
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Tidskriftsartikel (refereegranskat)abstract
- At small x, the effects of finite transverse momenta of partons inside a hadron become increasingly important, especially in analyses of jets and heavy-quark production. These effects can be systematically accounted for in a formalism based on k(perpendicular to)-factorization and unintegrated distribution functions. We present results for the unintegrated distribution function, together with the corresponding integrated one, obtained within the framework of the Linked Dipole Chain model. Comparisons are made to results obtained within other approaches.
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| 9. |
- Gustafson, Gösta, et al.
(författare)
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Hadronic collisions in the linked dipole chain model
- 2003
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Ingår i: Physical Review D (Particles, Fields, Gravitation and Cosmology). - 1550-2368. ; 67:034020:3
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Tidskriftsartikel (refereegranskat)abstract
- We apply the Linked Dipole Chain model to hadronic collisions using a modified version of the LDCMC Monte Carlo program. In particular we investigate the effects of multiple scatterings, which in this framework are reformulated in terms of the production of multiple chains of dipoles. We find, among other things, that this way of describing multiple scatterings is less sensitive to the treatment of soft, non-perturbative, scatterings. Although the results presented here are only at parton level, by comparing with corresponding results from PYTHIA, we are confident that our model will be able to describe essential features of high energy hadronic collisions, including underlying events and jet-pedestal effects in high pT events. The model should also be applicable to estimates of transverse energy flow in high energy heavy ion collisions, of interest in analyses of signals for quark-gluon plasma formation.
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| 10. |
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| 11. |
- Hong, Sungil, et al.
(författare)
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First-principles microkinetic modeling of partial methane oxidation over graphene-stabilized single-atom Fe-catalysts
- 2023
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Ingår i: Catalysis Science and Technology. - 2044-4753 .- 2044-4761. ; 13:24, s. 6999-7010
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Tidskriftsartikel (refereegranskat)abstract
- Catalytic conversion of CH4 to transportable liquid hydrocarbons via partial oxidation is a promising avenue towards efficient utilization of natural gas. Single Fe atoms on N-functionalized graphene (FeN4/GN) have recently been shown to be active for partial CH4 oxidation with H2O2 at room temperature. Here, density functional theory (DFT) calculations combined with mean-field microkinetic modeling (MKM) have been applied to obtain kinetic understanding of partial CH4 oxidation with H2O2 to CH3OH and CH3OOH over FeN4/GN. CH3OH and CH3OOH are found to be minor and major reaction products, respectively, with a selectivity in agreement with reported experimental data. The kinetic modeling reveals two pathways for CH3OH production together with a dominant catalytic cycle for CH3OOH formation. The selectivity is found to be sensitive to the temperature and H2O2 concentration, with the CH3OH selectivity increasing with increasing temperature and decreasing H2O2 concentration. Turnover frequencies of both CH3OH and CH3OOH are found to decrease over time, due to a change in the Fe formal oxidation state from +6 to +4; Fe(+6) is more active, but less stable than Fe(+4). The present work unravels the detailed reaction mechanism for partial oxidation of methane by FeN4/GN, rationalizes experimental observations and provides guidance for efficient room-temperature methane conversion by single-atom Fe-catalysts.
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| 12. |
- Khatri, C, et al.
(författare)
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Outcomes after perioperative SARS-CoV-2 infection in patients with proximal femoral fractures: an international cohort study
- 2021
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Ingår i: BMJ open. - : BMJ. - 2044-6055. ; 11:11, s. e050830-
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Tidskriftsartikel (refereegranskat)abstract
- Studies have demonstrated high rates of mortality in people with proximal femoral fracture and SARS-CoV-2, but there is limited published data on the factors that influence mortality for clinicians to make informed treatment decisions. This study aims to report the 30-day mortality associated with perioperative infection of patients undergoing surgery for proximal femoral fractures and to examine the factors that influence mortality in a multivariate analysis.SettingProspective, international, multicentre, observational cohort study.ParticipantsPatients undergoing any operation for a proximal femoral fracture from 1 February to 30 April 2020 and with perioperative SARS-CoV-2 infection (either 7 days prior or 30-day postoperative).Primary outcome30-day mortality. Multivariate modelling was performed to identify factors associated with 30-day mortality.ResultsThis study reports included 1063 patients from 174 hospitals in 19 countries. Overall 30-day mortality was 29.4% (313/1063). In an adjusted model, 30-day mortality was associated with male gender (OR 2.29, 95% CI 1.68 to 3.13, p<0.001), age >80 years (OR 1.60, 95% CI 1.1 to 2.31, p=0.013), preoperative diagnosis of dementia (OR 1.57, 95% CI 1.15 to 2.16, p=0.005), kidney disease (OR 1.73, 95% CI 1.18 to 2.55, p=0.005) and congestive heart failure (OR 1.62, 95% CI 1.06 to 2.48, p=0.025). Mortality at 30 days was lower in patients with a preoperative diagnosis of SARS-CoV-2 (OR 0.6, 95% CI 0.6 (0.42 to 0.85), p=0.004). There was no difference in mortality in patients with an increase to delay in surgery (p=0.220) or type of anaesthetic given (p=0.787).ConclusionsPatients undergoing surgery for a proximal femoral fracture with a perioperative infection of SARS-CoV-2 have a high rate of mortality. This study would support the need for providing these patients with individualised medical and anaesthetic care, including medical optimisation before theatre. Careful preoperative counselling is needed for those with a proximal femoral fracture and SARS-CoV-2, especially those in the highest risk groups.Trial registration numberNCT04323644
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| 13. |
- Leong, Miu Yoong, et al.
(författare)
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BCH Codes for Coherent Star DQAM Systems with Laser Phase Noise
- 2017
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Ingår i: Journal of optical communications. - : Walter de Gruyter. - 0173-4911 .- 2191-6322. ; 38:1, s. 47-56
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Tidskriftsartikel (refereegranskat)abstract
- Coherent optical systems have relatively high laser phase noise, which affects the performance of forward error correction (FEC) codes. In this paper, we propose a method for selecting Bose-Chaudhuri-Hocquenghem (BCH) codes for coherent systems with star-shaped constellations and M-ary differential quadrature amplitude modulation (DQAM). Our method supports constellations of any order M which is a power of 2, and includes differential M-ary phase shift keying as a special case. Our approach is straightforward, requiring only short pre-FEC simulations to parameterize a statistical model, based on which we select codes analytically. It is applicable to pre-FEC bit error rates (BERs) of around 10-3. We evaluate the accuracy of our approach using numerical simulations. For a target post-FEC BER of 10-5, codes selected with our method yield BERs within 2× target. Lastly, we extend our method to systems with interleaving, which enables us to use codes with lower overhead.
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| 14. |
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| 15. |
- Leong, Miu Yoong
(författare)
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Coherent Optical Transmission Systems : Performance and Coding Aspects
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Since the invention of fiber-optic systems in the 1970s, user demand has driven innovation forward, and each new generation of products has achieved higher data rates than its predecessor. Today, the most promising approach for further increasing data rates is coherent transmission with multi-level modulation and digital signal processing (DSP). By using multi-level modulation, data rates can be increased without increasing the spectral bandwidth of the signal. Digital signal processing has a highly-predictable design flow, and solutions are likely to become more attractive in the future as technology scales. As multi-level modulation is very susceptible to noise and distortions, these systems typically include forward error correction (FEC), which fits well with the DSP structure.In this thesis, we focus on two aspects of DSP-based coherent systems. First, we use a unified approach to analyze theoretical performance limits of coherent optical receivers and microwave receivers, in terms of signal-to-noise ratio (SNR) and bit error rate (BER). By using our general framework, we directly compare the performance of ten coherent optical receiver architectures and five microwave receiver architectures. In addition, we put previous publications into context, and identify areas of agreement and disagreement between them.Second, we consider simple Bose-Chaudhuri-Hocquenghem (BCH) codes for such systems. While most of coding theory is based on the assumption of additive white Gaussian noise (AWGN) channels, fiber-optic systems have other channel impairments in addition to AWGN. For example, there is relatively high phase noise (PN) from the transmitter and local oscillator (LO) lasers. We present a family of straightforward methods for selecting BCH codes for systems with PN. These codes are highly predictable and systematic to construct. They have low-complexity implementations and no error floor. Our methods are based on simple statistical models that can be parameterized from pre-FEC simulations, thus requiring only modest simulation effort. They are suitable for correcting pre-FEC BERs of around 10^−3. We consider differential quadrature phase-shift keying (DQPSK) modulation and higher-order differential quadrature amplitude modulation (DQAM) with star-shaped constellations.This thesis is an extension of our licentiate thesis, and improves upon the latter in two significant ways. First, the methods for code selection that were previously limited to DQPSK are now generalized to higher-order star-shaped DQAM formats, which can potentially deliver higher data rates. Second, we consider block interleavers which yield practical low-complexity implementations. These complement our earlier analysis of uniform interleavers, which provide general theoretical insight.
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| 16. |
- Leong, Miu Yoong, et al.
(författare)
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Dimensioning BCH Codes for Coherent DQPSK Systems With Laser Phase Noise and Cycle Slips
- 2014
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Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 32:21, s. 3446-3450
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Tidskriftsartikel (refereegranskat)abstract
- Forward error correction (FEC) plays a vital role in coherent optical systems employing multi-level modulation. However, much of coding theory assumes that additive white Gaussian noise (AWGN) is dominant, whereas coherent optical systems have significant phase noise (PN) in addition to AWGN. This changes the error statistics and impacts FEC performance. In this paper, we propose a novel semianalytical method for dimensioning binary Bose-Chaudhuri-Hocquenghem (BCH) codes for systems with PN. Our method involves extracting statistics from pre-FEC bit error rate (BER) simulations. We use these statistics to parameterize a bivariate binomial model that describes the distribution of bit errors. In this way, we relate pre-FEC statistics to post-FEC BER and BCHcodes. Our method is applicable to pre-FEC BER around 10(-3) and any post-FEC BER. Using numerical simulations, we evaluate the accuracy of our approach for a target post-FEC BER of 10(-5). Codes dimensioned with our bivariate binomial model meet the target within 0.2-dB signal-to-noise ratio.
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| 17. |
- Leong, Miu Yoong, et al.
(författare)
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Dimensioning RS codes for mitigation of phase noise induced cycle slips in DQPSK systems
- 2014
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Ingår i: Asia Communications and Photonics Conference, ACP. - Washington, D.C. : Optical Society of America. - 9781557528520
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Konferensbidrag (refereegranskat)abstract
- We present a semi-analytical method for dimensioning Reed-Solomon codes for coherent DQPSK systems with laser phase noise and cycle slips. We evaluate the accuracy of our method for a 28 Gbaud system using numerical simulations.
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| 18. |
- Leong, Miu Yoong
(författare)
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DSP-based Coherent Optical Systems : Receiver Sensitivity and Coding Aspects
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- User demand for faster access to more data is at a historic high and rising. One of the enabling technologies that makes the information age possible is fiber-optic communications, where light is used to carry information from one place to another over optical fiber. Since the technology was first shown to be feasible in the 1970s, it has been constantly evolving with each new generation of fiber-optic systems achieving higher data rates than its predecessor.Today, the most promising approach for further increasing data rates is digital signal processing (DSP)-based coherent optical transmission with multi-level modulation. As multi-level modulation formats are very susceptible to noise and distortions, forward error correction (FEC) is typically used in such systems. However, FEC has traditionally been designed for additive white Gaussian noise (AWGN) channels, whereas fiber-optic systems also have other impairments. For example, there is relatively high phase noise (PN) from the transmitter and local oscillator (LO) lasers.The contributions of this thesis are in two areas. First, we use a unified approach to analyze theoretical performance limits of coherent optical receivers and microwave receivers, in terms of signal-to-noise ratio (SNR) and bit error rate (BER). By using our general framework, we directly compare the performance of ten coherent optical receiver architectures and five microwave receiver architectures. In addition, we put previous publications into context, and identify areas of agreement and disagreement between them. Second, we propose straightforward methods to select codes for systems with PN. We focus on Bose-Chaudhuri-Hocquenghem (BCH) codes with simple implementations, which correct pre-FEC BERs around 10−3. Our methods are semi-analytical, and need only short pre-FEC simulations to estimate error statistics. We propose statistical models that can be parameterized based on those estimates. Codes can be selected analytically based on our models.
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| 19. |
- Leong, Miu Yoong, et al.
(författare)
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Interleavers and BCH Codes for Coherent DQPSK Systems With Laser Phase Noise
- 2015
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Ingår i: IEEE Photonics Technology Letters. - 1041-1135 .- 1941-0174. ; 27:7, s. 685-688
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Tidskriftsartikel (refereegranskat)abstract
- The relatively high phase noise of coherent optical systems poses unique challenges for forward error correction (FEC). In this letter, we propose a novel semianalytical method for selecting combinations of interleaver lengths and binary Bose-Chaudhuri-Hocquenghem (BCH) codes that meet a target post-FEC bit error rate (BER). Our method requires only short pre-FEC simulations, based on which we design interleavers and codes analytically. It is applicable to pre-FEC BER similar to 10(-3), and any post-FEC BER. In addition, we show that there is a tradeoff between code overhead and interleaver delay. Finally, for a target of 10(-5), numerical simulations show that interleaver-code combinations selected using our method have post-FEC BER around 2x target. The target BER is achieved with 0.1 dB extra signal-to-noise ratio.
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| 20. |
- Leong, Miu Yoong, et al.
(författare)
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Low-complexity BCH codes with optimized interleavers for DQPSK systems with laser phase noise
- 2017
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Ingår i: Photonic network communications. - : Springer Science and Business Media LLC. - 1387-974X .- 1572-8188. ; 33:3, s. 328-333
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Tidskriftsartikel (refereegranskat)abstract
- The presence of high phase noise in addition to additive white Gaussian noise in coherent optical systems affects the performance of forward error correction (FEC) schemes. In this paper, we propose a simple scheme for such systems, using block interleavers and binary Bose-Chaudhuri-Hocquenghem (BCH) codes. The block interleavers are specifically optimized for differential quadrature phase shift keying modulation. We propose a method for selecting BCH codes that, together with the interleavers, achieve a target post-FEC bit error rate (BER). This combination of interleavers and BCH codes has very low implementation complexity. In addition, our approach is straightforward, requiring only short pre-FEC simulations to parameterize a model, based on which we select codes analytically. We aim to correct a pre-FEC BER of around . We evaluate the accuracy of our approach using numerical simulations. For a target post-FEC BER of , codes selected using our method result in BERs around 3 target and achieve the target with around 0.2 dB extra signal-to-noise ratio.
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| 21. |
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| 22. |
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| 23. |
- Leong, Miu Yoong, et al.
(författare)
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Receiver sensitivity in optical and microwave, heterodyne and homodyne systems
- 2014
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Ingår i: Journal of optical communications. - : Walter de Gruyter GmbH. - 0173-4911 .- 2191-6322. ; 35:3, s. 221-229
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we analyze the sensitivities of coherent optical receivers and microwave receivers. We derive theoretical limits of signal-to-noise ratio and bit error rate. By applying a generic approach to a broad range of receivers, we can compare their performance directly. Other publications have considered some of these receivers. However, their diverse nature obscures the big picture. Using our results as a unifying platform, previous publications can be compared and discrepancies between them identified.
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| 24. |
- Miu, Gabriela
(författare)
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Analyses of Electron-Proton and Hadron-Hadron Collisions Using the Linked Dipole Chain Model
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we perform phenomenological studies of Quantum Chromodynamics, as manifested in deeply inelastic electron-proton (DIS) and high-energy hadron-hadron interactions. Most of the results are obtained using the Linked Dipole Chain model (LDC). This model was originally developed for studies of DIS and is formulated in terms of kT-factorization and un-integrated parton distribution functions. We develop a simple model, based on the CCFM and LDC models, which interpolates between the DGLAP and BFKL regimes in DIS. This model gives an intuitive qualitative picture of the transition region between the DGLAP and BFKL domains. We use an approximate version of the LDC model to obtain qualitative results for the inclusive minijet cross section and transverse energy flow, expected to be relevant in analyses of future high-energy hadron-hadron or nucleus-nucleus collisions. Our results contain a dynamical suppression at small transverse momenta, which corresponds to an effective cutoff which is slowly increasing with the total collision energy. This is in contrast to the traditional approach based on collinear factorization, which results in an inclusive cross section that diverges at small transverse momenta and implies the introduction of a cutoff parameter whose value needs to be adjusted to data. The LDC model is particularly suitable for a description of e.g. the cross section for jets and heavy quarks at very high energies, where the effects of finite transverse momenta of partons inside the hadrons become increasingly important. We obtain results for the LDC integrated and un-integrated gluon distribution functions and compare them with those of other formalisms, e.g. those based on the CCFM model, demonstrating how to make a relevant comparison between the different formalisms. We exploit the symmetric formulation of the LDC model with respect to the photon and proton ends and develop a version of the LDC model for hadronic interactions. In this preliminary analysis, we focus on features of jet and minijet production which are less sensitive to hadronization effects and compare our results to those of the PYTHIA event generator.
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| 25. |
- Miu, Gabriela, et al.
(författare)
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W production in an improved parton-shower approach
- 1999
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Ingår i: Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics. - 0370-2693. ; 449:3-4, s. 313-320
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Tidskriftsartikel (refereegranskat)abstract
- In the description of the production properties of gauge bosons (W±, Z0, γ*) at colliders, the lowest-order graph normally is not sufficient. The contributions of higher orders can be introduced either by an explicit order-by-order matrix-element calculation, by a resummation procedure or by a parton-shower algorithm. Each approach has its advantages and disadvantages. We here introduce a method that allows the parton-shower algorithm to be augmented by higher-order information, thereby offering an economical route to a description of all event properties. It is tested by comparing with the P⊥ spectrum of W bosons at the Tevatron.
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