Muhammad Usman - Academia.edu (original) (raw)

Papers by Muhammad Usman

A model for realistic InAs quantum dot composition profile is proposed and analyzed, consisting o... more A model for realistic InAs quantum dot composition profile is proposed and analyzed, consisting of a double region scheme with an In-rich internal core and an In-poor external shell, in order to mimic the atomic scale phenomena such as In-Ga intermixing and In segregation during the growth and overgrowth with GaAs. The parameters of the proposed model are derived by reproducing the experimentally measured polarization data. Further understanding is developed by analyzing the strain fields which suggests that the two-composition model indeed results in lower strain energies than the commonly applied uniform composition model.

Journal of Entrepreneurship in Emerging Economies, 2014

ABSTRACT Purpose ‐ This study aims to compare the entrepreneurial activity in China and Pakistan ... more ABSTRACT Purpose ‐ This study aims to compare the entrepreneurial activity in China and Pakistan based on these factors: fear of failure, perceived capabilities, perceived opportunities, and knowledge of other entrepreneurs. Design/methodology/approach ‐ The authors use the data from global entrepreneurship monitor (GEM). In this study, the authors use the cross-sectional data for survey year 2010, both for China and Pakistan. The authors employed the sequential logistic regression model in order to predict the likelihood of involvement in entrepreneurial activity. Findings ‐ Chinese are not significantly influenced by their fear of failure to engage in entrepreneurial activity. Gender is also not a significant predictor of entrepreneurial activity in China. In Pakistan, perception of opportunities does not significantly predict whether to involve in entrepreneurial activity or not, while other factors do. Gender is a significant predictor of entrepreneurial activity in Pakistan. Originality/value ‐ Previously literature is lacking in this kind of a unique comparison based upon GEM data evidence. There are many lessons for Pakistan in this study. There is a need to create awareness among people about the fruits of self-employment. Government should create a conducive environment for doing business. Efforts should be made to enhance the women participation in business.

2012 International Conference on Emerging Technologies, 2012

2013 IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), 2013

Network simulators have gained an increased importance in the research community to an extent tha... more Network simulators have gained an increased importance in the research community to an extent that approximately 11% of the research papers that appeared in the proceedings of leading international conferences used network simulators to test their claims. It is anticipated that due to the several advantages network simulators will become more prevalent in the research community in future. However, with the growing popularity of the network simulators, challenges in the form of consistent revelations about their inaccuracies have started to seriously question the use of such simulators and simulation based research. This paper attempts to answer the questions (1) How can assumptions/simplifications in modeling affect realism of simulations? (2) How credibility of simulation based research be improved?, and (3) What necessary steps must be taken to make network simulators credible? The paper provides deliberated recommendations for improving the authenticity of simulation based experiments. The authors believe that the discussion in this paper will greatly facilitate the repeatability of simulation based research and will help researchers to select the most suitable (credible) simulator.

International Journal of Electrical Engineering Education, 2013

ABSTRACT This article researches the influence of IJEEE on electrical engineering and electrical ... more ABSTRACT This article researches the influence of IJEEE on electrical engineering and electrical engineering education as a discipline. For this purpose, the history of this journal has been presented from a citation perspective. To identify leading and evolving research areas within IJEEE the authors conducted keyword analysis, which additionally showed how IJEEE contains both educational and technical contributions. The authors also studied the temporal evolution and distribution of keywords. Word co-occurrence was analysed to discover the main context in which the keywords have been used. The analysis also revealed the prominent contributors within the community of IJEEE based on various authorship and citation criteria. It was observed that the influential authors appear in multiple ways, i.e. most of the authors who were influential by one criterion also made to the top list of other criteria. The authors concluded that the single-author pattern is quite prominent within this community, and very little work has been done between the same co-authors. Therefore, there is a need to encourage IJEEE authors to write more collaborative publications so that the authorship/co-authorship network may grow.

2011 IEEE International Conference on Communications (ICC), 2011

Identifying the most influential nodes in social networks is a key problem in social network anal... more Identifying the most influential nodes in social networks is a key problem in social network analysis. However, without a strict definition of centrality the notion of what constitutes a central node in a network changes with application and the type of commodity flowing through a network. In this paper we identify social hubs, nodes at the center of influential neighborhoods, in massive online social networks using principal component centrality (PCC). We compare PCC with eigenvector centrality's (EVC), the de facto measure of node influence by virtue of their position in a network. We demonstrate PCC's performance by processing a friendship graph of 70, 000 users of Google's Orkut social networking service and a gaming graph of 143, 020 users obtained from users of Facebook's 'Fighters' Club' application.

2010 44th Annual Conference on Information Sciences and Systems (CISS), 2010

We present principal component centrality (PCC) as a measure of centrality that is more general a... more We present principal component centrality (PCC) as a measure of centrality that is more general and encompasses eigenvector centrality (EVC). We explain some of the difficulties in applying EVC to graphs and networks that contain more than just one neighborhood of nodes with high influence. We demonstrate the shortcomings of traditional EVC and contrast it against PCC. PCC's ranking procedure is based on spectral analysis of the network's graph adjacency matrix and identification of its most significant eigenvectors.

2008 42nd Annual Conference on Information Sciences and Systems, 2008

Page 1. Abstract—Bit errors are orders of magnitude more frequent in wireless channels than in wi... more Page 1. Abstract—Bit errors are orders of magnitude more frequent in wireless channels than in wired channels. Design of network protocols and other architectural components of wireless networking systems entail a better ...

2007 41st Annual Conference on Information Sciences and Systems, 2007

Page 1. Adaptive Network Coding With Noisy Packets Shirish S. Karande, Kiran Misra, Muhammad U. I... more Page 1. Adaptive Network Coding With Noisy Packets Shirish S. Karande, Kiran Misra, Muhammad U. Ilyas and Hayder Radha, Senior Member, IEEE Abstract-In this paper we study the problem of employing network coding in presence of bit corruptions in the packets. ...

2011 Asia-Pacific Power and Energy Engineering Conference, 2011

Humera Shahid School of Management, Huazhong University of Science and Technology, PO box 430074,... more Humera Shahid School of Management, Huazhong University of Science and Technology, PO box 430074, Wuhan, PRChina. Tel: 008615623520701 E-mail: mano_hailian@hotmail.com ... Faiq Mahmood School of Management, Huazhong University of Science and ...

Materials Science, 2011

n-and p-type 4H-SiC epilayers were grown on heavily doped SiC substrates. The thickness of the p-... more n-and p-type 4H-SiC epilayers were grown on heavily doped SiC substrates. The thickness of the p-type layer was 7 μm and the doping level around 10 17 cm -3 , while the n-type epilayers were 15 μm thick and had a doping concentration of 3 -5×10 15 cm -3 . Several different surface treatments were then applied on the epilayers for surface passivation: SiO 2 growth, Al 2 O 3 deposited by atomic layer deposition, and Ar-ion implantation. Using collinear pump -probe technique the effective carrier lifetimes were measured from various places and statistical lifetime distributions were obtained. For surface recombination evaluation, two models are presented. One states that surface recombination velocity (SRV) is equal on both the passivation/epi layer interface (S 2 ) and the deeper interface between the epilayer and the SiC substrate (S 1 ), i. e. (S 1 = S 2 ). The other model is simulated assuming that SRV in the epilayer/substrate (S 1 ) interface is constant while in the passivation layer/epilayer (S 2 ) interface SRV can be varied S 2 < S 1 . Empirical nomograms are presented with various parameters sets to evaluate S 2 values. We found that on the investigated 4H-SiC surfaces S 2 ranges from 3×10 4 to 5×10 4 assuming that the bulk lifetime is 4 µs. In Ar + implanted surfaces S 2 is between (10 5 -10 6 ) cm/s.

InAs/GaAs quantum dot systems can emit light at the wavelengths above 1.3µm by covering the InAs ... more InAs/GaAs quantum dot systems can emit light at the wavelengths above 1.3µm by covering the InAs quantum dots with an In x Ga 1-x As strain reducing capping layer (SRCL). The presence of the SRCL relaxes the strain in the growth direction while the in-plane compressive strain remains nearly unchanged. This results in an aspect ratio increase of the quantum dot. Both the strain relaxation in the growth direction and the aspect ratio change induce a non-linear red shift. This work studies the dependence of the emission wavelength on the thickness and the indium composition of the SRCL. Experimental topologies have been simulated and a close quantitative match is found.

2008 10th International Conference on Advanced Communication Technology, 2008

Abstract In the last decade, several effective algorithms have been developed in the areas of con... more Abstract In the last decade, several effective algorithms have been developed in the areas of content based video retrieval and copy detection. However, the task of video copy detection becomes more difficult when the target video sequences available are in degraded or edited ...

TENCON 2005 - 2005 IEEE Region 10 Conference, 2005

... [1] R. Lopamudra, A. Ehab, H. Hazem, GB ... and Observations,” [2] F. Beritelli, S. Casale, G... more ... [1] R. Lopamudra, A. Ehab, H. Hazem, GB ... and Observations,” [2] F. Beritelli, S. Casale, G. Ruggeri, “Performance Comparison between VBR speech Coders for Adaptive VoIP Applications,” [3] JW Seo, SJ Woo, KS Bae, “ A Study on the Application of an AMR Speech Codec to ...

2009 IEEE 13th International Multitopic Conference, 2009

Distributed heterogeneous data sources need to be queried uniformly using global schema. Query on... more Distributed heterogeneous data sources need to be queried uniformly using global schema. Query on global schema is reformulated so that it can be executed on local data sources. Constraints in global schema and mappings are used for source selection, query optimization, and querying partitioned and replicated data sources. The provided system is all XML-based which poses query in XML form, transforms, and integrates local results in an XML document. Contributions include the use of constraints in our existing global schema which help in source selection and query optimization, and a global query distribution framework for querying distributed heterogeneous data sources.

2005 Student Conference on Engineering Sciences and Technology, 2005

The paper before you is an evaluation of the IS-95A CDMA forward link and reverse link performanc... more The paper before you is an evaluation of the IS-95A CDMA forward link and reverse link performance. We have devised some numerical results based on the real time simulation of the forward link and the reverse link under constraints imposed by the regular AWGN and ...

2009 Third International Conference on Electrical Engineering, 2009

VoIP has emerged from the advancements and revolutions in packet-based networks and has seen trem... more VoIP has emerged from the advancements and revolutions in packet-based networks and has seen tremendous improvements over the years. Normally there are two approaches to the implementation of VoIP, one in the form of soft phone and other in the form of dedicated IP phone. In this paper we describe a new scheme to the implementation of VoIP which is

Wireless Networks, 2011

This paper presents the 'Generalized Poor Man's SIMO System' (gPMSS) which combines two approache... more This paper presents the 'Generalized Poor Man's SIMO System' (gPMSS) which combines two approaches, cooperative communication and diversity combination, to reduce packet losses over links in wireless sensor networks. The proposed gPMSS is distinct from previous cooperative communication architectures in wireless sensor networks which rely on a relay channel, and also distinct from implementations in 802.11 networks that require a wired infrastructure or hardware changes for cooperation. gPMSS foregoes the need for any changes to mote hardware and it works within the current IEEE 802.15.4 standard. We describe the gPMSS protocol that governs the cooperation between receivers. Three variants are evaluated including selection diversity, equal gain and maximal ratio combining. First, we demonstrate gPMSS on bit error traces in a fully reproducible manner. This is followed by an implementation of gPMSS in C# on the .NET Micro Framework edition of the recently released Imote2 mote platform. We demonstrate by means of experiments an increase in the packet reception rate from 22-30% to 73-76%, a relative increase of 150-245%. We also analyzed the power consumed by the transmitter per delivered packet and observe a reduction of up to 68%. We also take into account the retry limit of the IEEE 802.15.4 protocol and demonstrate that gPMSS is able to provide 99% packet delivery at the protocol's default retry parameters against 65-75% without it.

The Journal of Physical Chemistry C, 2013

Contradicting results about the origin of room temperature ferromagnetism (RTFM) from measurement... more Contradicting results about the origin of room temperature ferromagnetism (RTFM) from measurements on different forms of transition metal (TM)-doped ZnO nanostructured materials lead to strong debates on whether RTFM could be an intrinsic property to TM-doped ZnO or not. Through careful synthesis and extensive characterizations, we have excluded the extrinsic contaminations as the cause of RTFM. Our experimental study confirms that defects such as oxygen vacancies lie on surface of nanorods and are likely a source of RTFM. X-ray absorption and emission spectroscopy (XAS and XES) suggest that the doped Co ions, primarily in the divalent state, replace the Zn ions inside the tetrahedral without introducing Co clustering or Zn-related defects. Band gap narrowing upon Co doping is observed in both optical reflectance and O K-edge XAS/XES and is in agreement with the presence of oxygen vacancies and strong sp−d hybridization. Furthermore, such a trend can be nicely reproduced in GGA+U band structure calculations. Calculations also suggest that these oxygen vacancies are likely to congregate at low-energy (101) and (100) surfaces, instead of inside the bulk. Our findings highlight the importance of using the nanocrystalline surfaces to enhance the impurity concentrations and stabilize the ferromagnetism without post-sample annealing in an oxygen-deficient environment.

Solid-State Electronics, 2014

ABSTRACT A theoretical design assessment is presented using two dimensional numerical computer ai... more ABSTRACT A theoretical design assessment is presented using two dimensional numerical computer aided design (TCAD) tool for 15–20 kV 4H–SiC IGBTs. Physical parameters of the layer structures such as drift layer thickness, doping in the drift layer, JFET region width and interface charges underneath the gate region are varied to predict the device performance. Performance is further assessed at different temperatures and with different carrier lifetime in the drift layer. Using identical set of physical device parameters (doping, thicknesses), simulated structure was first calibrated with the experimental data. Simulations show that a minority carrier lifetime in the drift layer of 1.0–1.6 μs produces a close match with the experimental device. An on-resistance first decays with temperature (i.e., increased in ionization level, and increase in minority carrier lifetime), stays nearly constant with further increase in the temperature (may be all carriers are now fully ionized and increase in carrier lifetime is compensated with decrease in the carrier mobility) and finally increases linearly with temperature (&gt;450 K) due to decrease in the carrier mobility. A significant increase in the forward voltage drop is observed with the presence of interface trap charges. Increasing JFET region width slightly decreases the forward on-state voltage. A drift layer of at least 175 μm thick with a doping concentration of &lt; 2.0 × 1014 cm−3 is required to get a blocking voltage of 20 kV for 4H–SiC IGBTs assuming a minority carrier lifetime of 1 μs in the drift layer.

A model for realistic InAs quantum dot composition profile is proposed and analyzed, consisting o... more A model for realistic InAs quantum dot composition profile is proposed and analyzed, consisting of a double region scheme with an In-rich internal core and an In-poor external shell, in order to mimic the atomic scale phenomena such as In-Ga intermixing and In segregation during the growth and overgrowth with GaAs. The parameters of the proposed model are derived by reproducing the experimentally measured polarization data. Further understanding is developed by analyzing the strain fields which suggests that the two-composition model indeed results in lower strain energies than the commonly applied uniform composition model.

Journal of Entrepreneurship in Emerging Economies, 2014

ABSTRACT Purpose ‐ This study aims to compare the entrepreneurial activity in China and Pakistan ... more ABSTRACT Purpose ‐ This study aims to compare the entrepreneurial activity in China and Pakistan based on these factors: fear of failure, perceived capabilities, perceived opportunities, and knowledge of other entrepreneurs. Design/methodology/approach ‐ The authors use the data from global entrepreneurship monitor (GEM). In this study, the authors use the cross-sectional data for survey year 2010, both for China and Pakistan. The authors employed the sequential logistic regression model in order to predict the likelihood of involvement in entrepreneurial activity. Findings ‐ Chinese are not significantly influenced by their fear of failure to engage in entrepreneurial activity. Gender is also not a significant predictor of entrepreneurial activity in China. In Pakistan, perception of opportunities does not significantly predict whether to involve in entrepreneurial activity or not, while other factors do. Gender is a significant predictor of entrepreneurial activity in Pakistan. Originality/value ‐ Previously literature is lacking in this kind of a unique comparison based upon GEM data evidence. There are many lessons for Pakistan in this study. There is a need to create awareness among people about the fruits of self-employment. Government should create a conducive environment for doing business. Efforts should be made to enhance the women participation in business.

2012 International Conference on Emerging Technologies, 2012

2013 IEEE 24th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), 2013

Network simulators have gained an increased importance in the research community to an extent tha... more Network simulators have gained an increased importance in the research community to an extent that approximately 11% of the research papers that appeared in the proceedings of leading international conferences used network simulators to test their claims. It is anticipated that due to the several advantages network simulators will become more prevalent in the research community in future. However, with the growing popularity of the network simulators, challenges in the form of consistent revelations about their inaccuracies have started to seriously question the use of such simulators and simulation based research. This paper attempts to answer the questions (1) How can assumptions/simplifications in modeling affect realism of simulations? (2) How credibility of simulation based research be improved?, and (3) What necessary steps must be taken to make network simulators credible? The paper provides deliberated recommendations for improving the authenticity of simulation based experiments. The authors believe that the discussion in this paper will greatly facilitate the repeatability of simulation based research and will help researchers to select the most suitable (credible) simulator.

International Journal of Electrical Engineering Education, 2013

ABSTRACT This article researches the influence of IJEEE on electrical engineering and electrical ... more ABSTRACT This article researches the influence of IJEEE on electrical engineering and electrical engineering education as a discipline. For this purpose, the history of this journal has been presented from a citation perspective. To identify leading and evolving research areas within IJEEE the authors conducted keyword analysis, which additionally showed how IJEEE contains both educational and technical contributions. The authors also studied the temporal evolution and distribution of keywords. Word co-occurrence was analysed to discover the main context in which the keywords have been used. The analysis also revealed the prominent contributors within the community of IJEEE based on various authorship and citation criteria. It was observed that the influential authors appear in multiple ways, i.e. most of the authors who were influential by one criterion also made to the top list of other criteria. The authors concluded that the single-author pattern is quite prominent within this community, and very little work has been done between the same co-authors. Therefore, there is a need to encourage IJEEE authors to write more collaborative publications so that the authorship/co-authorship network may grow.

2011 IEEE International Conference on Communications (ICC), 2011

Identifying the most influential nodes in social networks is a key problem in social network anal... more Identifying the most influential nodes in social networks is a key problem in social network analysis. However, without a strict definition of centrality the notion of what constitutes a central node in a network changes with application and the type of commodity flowing through a network. In this paper we identify social hubs, nodes at the center of influential neighborhoods, in massive online social networks using principal component centrality (PCC). We compare PCC with eigenvector centrality's (EVC), the de facto measure of node influence by virtue of their position in a network. We demonstrate PCC's performance by processing a friendship graph of 70, 000 users of Google's Orkut social networking service and a gaming graph of 143, 020 users obtained from users of Facebook's 'Fighters' Club' application.

2010 44th Annual Conference on Information Sciences and Systems (CISS), 2010

We present principal component centrality (PCC) as a measure of centrality that is more general a... more We present principal component centrality (PCC) as a measure of centrality that is more general and encompasses eigenvector centrality (EVC). We explain some of the difficulties in applying EVC to graphs and networks that contain more than just one neighborhood of nodes with high influence. We demonstrate the shortcomings of traditional EVC and contrast it against PCC. PCC's ranking procedure is based on spectral analysis of the network's graph adjacency matrix and identification of its most significant eigenvectors.

2008 42nd Annual Conference on Information Sciences and Systems, 2008

Page 1. Abstract—Bit errors are orders of magnitude more frequent in wireless channels than in wi... more Page 1. Abstract—Bit errors are orders of magnitude more frequent in wireless channels than in wired channels. Design of network protocols and other architectural components of wireless networking systems entail a better ...

2007 41st Annual Conference on Information Sciences and Systems, 2007

Page 1. Adaptive Network Coding With Noisy Packets Shirish S. Karande, Kiran Misra, Muhammad U. I... more Page 1. Adaptive Network Coding With Noisy Packets Shirish S. Karande, Kiran Misra, Muhammad U. Ilyas and Hayder Radha, Senior Member, IEEE Abstract-In this paper we study the problem of employing network coding in presence of bit corruptions in the packets. ...

2011 Asia-Pacific Power and Energy Engineering Conference, 2011

Humera Shahid School of Management, Huazhong University of Science and Technology, PO box 430074,... more Humera Shahid School of Management, Huazhong University of Science and Technology, PO box 430074, Wuhan, PRChina. Tel: 008615623520701 E-mail: mano_hailian@hotmail.com ... Faiq Mahmood School of Management, Huazhong University of Science and ...

Materials Science, 2011

n-and p-type 4H-SiC epilayers were grown on heavily doped SiC substrates. The thickness of the p-... more n-and p-type 4H-SiC epilayers were grown on heavily doped SiC substrates. The thickness of the p-type layer was 7 μm and the doping level around 10 17 cm -3 , while the n-type epilayers were 15 μm thick and had a doping concentration of 3 -5×10 15 cm -3 . Several different surface treatments were then applied on the epilayers for surface passivation: SiO 2 growth, Al 2 O 3 deposited by atomic layer deposition, and Ar-ion implantation. Using collinear pump -probe technique the effective carrier lifetimes were measured from various places and statistical lifetime distributions were obtained. For surface recombination evaluation, two models are presented. One states that surface recombination velocity (SRV) is equal on both the passivation/epi layer interface (S 2 ) and the deeper interface between the epilayer and the SiC substrate (S 1 ), i. e. (S 1 = S 2 ). The other model is simulated assuming that SRV in the epilayer/substrate (S 1 ) interface is constant while in the passivation layer/epilayer (S 2 ) interface SRV can be varied S 2 < S 1 . Empirical nomograms are presented with various parameters sets to evaluate S 2 values. We found that on the investigated 4H-SiC surfaces S 2 ranges from 3×10 4 to 5×10 4 assuming that the bulk lifetime is 4 µs. In Ar + implanted surfaces S 2 is between (10 5 -10 6 ) cm/s.

InAs/GaAs quantum dot systems can emit light at the wavelengths above 1.3µm by covering the InAs ... more InAs/GaAs quantum dot systems can emit light at the wavelengths above 1.3µm by covering the InAs quantum dots with an In x Ga 1-x As strain reducing capping layer (SRCL). The presence of the SRCL relaxes the strain in the growth direction while the in-plane compressive strain remains nearly unchanged. This results in an aspect ratio increase of the quantum dot. Both the strain relaxation in the growth direction and the aspect ratio change induce a non-linear red shift. This work studies the dependence of the emission wavelength on the thickness and the indium composition of the SRCL. Experimental topologies have been simulated and a close quantitative match is found.

2008 10th International Conference on Advanced Communication Technology, 2008

Abstract In the last decade, several effective algorithms have been developed in the areas of con... more Abstract In the last decade, several effective algorithms have been developed in the areas of content based video retrieval and copy detection. However, the task of video copy detection becomes more difficult when the target video sequences available are in degraded or edited ...

TENCON 2005 - 2005 IEEE Region 10 Conference, 2005

... [1] R. Lopamudra, A. Ehab, H. Hazem, GB ... and Observations,” [2] F. Beritelli, S. Casale, G... more ... [1] R. Lopamudra, A. Ehab, H. Hazem, GB ... and Observations,” [2] F. Beritelli, S. Casale, G. Ruggeri, “Performance Comparison between VBR speech Coders for Adaptive VoIP Applications,” [3] JW Seo, SJ Woo, KS Bae, “ A Study on the Application of an AMR Speech Codec to ...

2009 IEEE 13th International Multitopic Conference, 2009

Distributed heterogeneous data sources need to be queried uniformly using global schema. Query on... more Distributed heterogeneous data sources need to be queried uniformly using global schema. Query on global schema is reformulated so that it can be executed on local data sources. Constraints in global schema and mappings are used for source selection, query optimization, and querying partitioned and replicated data sources. The provided system is all XML-based which poses query in XML form, transforms, and integrates local results in an XML document. Contributions include the use of constraints in our existing global schema which help in source selection and query optimization, and a global query distribution framework for querying distributed heterogeneous data sources.

2005 Student Conference on Engineering Sciences and Technology, 2005

The paper before you is an evaluation of the IS-95A CDMA forward link and reverse link performanc... more The paper before you is an evaluation of the IS-95A CDMA forward link and reverse link performance. We have devised some numerical results based on the real time simulation of the forward link and the reverse link under constraints imposed by the regular AWGN and ...

2009 Third International Conference on Electrical Engineering, 2009

VoIP has emerged from the advancements and revolutions in packet-based networks and has seen trem... more VoIP has emerged from the advancements and revolutions in packet-based networks and has seen tremendous improvements over the years. Normally there are two approaches to the implementation of VoIP, one in the form of soft phone and other in the form of dedicated IP phone. In this paper we describe a new scheme to the implementation of VoIP which is

Wireless Networks, 2011

This paper presents the 'Generalized Poor Man's SIMO System' (gPMSS) which combines two approache... more This paper presents the 'Generalized Poor Man's SIMO System' (gPMSS) which combines two approaches, cooperative communication and diversity combination, to reduce packet losses over links in wireless sensor networks. The proposed gPMSS is distinct from previous cooperative communication architectures in wireless sensor networks which rely on a relay channel, and also distinct from implementations in 802.11 networks that require a wired infrastructure or hardware changes for cooperation. gPMSS foregoes the need for any changes to mote hardware and it works within the current IEEE 802.15.4 standard. We describe the gPMSS protocol that governs the cooperation between receivers. Three variants are evaluated including selection diversity, equal gain and maximal ratio combining. First, we demonstrate gPMSS on bit error traces in a fully reproducible manner. This is followed by an implementation of gPMSS in C# on the .NET Micro Framework edition of the recently released Imote2 mote platform. We demonstrate by means of experiments an increase in the packet reception rate from 22-30% to 73-76%, a relative increase of 150-245%. We also analyzed the power consumed by the transmitter per delivered packet and observe a reduction of up to 68%. We also take into account the retry limit of the IEEE 802.15.4 protocol and demonstrate that gPMSS is able to provide 99% packet delivery at the protocol's default retry parameters against 65-75% without it.

The Journal of Physical Chemistry C, 2013

Contradicting results about the origin of room temperature ferromagnetism (RTFM) from measurement... more Contradicting results about the origin of room temperature ferromagnetism (RTFM) from measurements on different forms of transition metal (TM)-doped ZnO nanostructured materials lead to strong debates on whether RTFM could be an intrinsic property to TM-doped ZnO or not. Through careful synthesis and extensive characterizations, we have excluded the extrinsic contaminations as the cause of RTFM. Our experimental study confirms that defects such as oxygen vacancies lie on surface of nanorods and are likely a source of RTFM. X-ray absorption and emission spectroscopy (XAS and XES) suggest that the doped Co ions, primarily in the divalent state, replace the Zn ions inside the tetrahedral without introducing Co clustering or Zn-related defects. Band gap narrowing upon Co doping is observed in both optical reflectance and O K-edge XAS/XES and is in agreement with the presence of oxygen vacancies and strong sp−d hybridization. Furthermore, such a trend can be nicely reproduced in GGA+U band structure calculations. Calculations also suggest that these oxygen vacancies are likely to congregate at low-energy (101) and (100) surfaces, instead of inside the bulk. Our findings highlight the importance of using the nanocrystalline surfaces to enhance the impurity concentrations and stabilize the ferromagnetism without post-sample annealing in an oxygen-deficient environment.

Solid-State Electronics, 2014

ABSTRACT A theoretical design assessment is presented using two dimensional numerical computer ai... more ABSTRACT A theoretical design assessment is presented using two dimensional numerical computer aided design (TCAD) tool for 15–20 kV 4H–SiC IGBTs. Physical parameters of the layer structures such as drift layer thickness, doping in the drift layer, JFET region width and interface charges underneath the gate region are varied to predict the device performance. Performance is further assessed at different temperatures and with different carrier lifetime in the drift layer. Using identical set of physical device parameters (doping, thicknesses), simulated structure was first calibrated with the experimental data. Simulations show that a minority carrier lifetime in the drift layer of 1.0–1.6 μs produces a close match with the experimental device. An on-resistance first decays with temperature (i.e., increased in ionization level, and increase in minority carrier lifetime), stays nearly constant with further increase in the temperature (may be all carriers are now fully ionized and increase in carrier lifetime is compensated with decrease in the carrier mobility) and finally increases linearly with temperature (&gt;450 K) due to decrease in the carrier mobility. A significant increase in the forward voltage drop is observed with the presence of interface trap charges. Increasing JFET region width slightly decreases the forward on-state voltage. A drift layer of at least 175 μm thick with a doping concentration of &lt; 2.0 × 1014 cm−3 is required to get a blocking voltage of 20 kV for 4H–SiC IGBTs assuming a minority carrier lifetime of 1 μs in the drift layer.