The Experts below are selected from a list of 101262 Experts worldwide ranked by ideXlab platform

Mohammad Shikh-bahaei - One of the best experts on this subject based on the ideXlab platform.

  • Beamforming Design for Multiuser Transmission Through Reconfigurable Intelligent Surface
    IEEE Transactions on Communications, 2021
    Co-Authors: Zhaohui Yang, Chongwen Huang, Jianfeng Shi, Mohammad Shikh-bahaei
    Abstract:

    This article investigates the problem of resource allocation for multiuser communication networks with a reconfigurable intelligent surface (RIS)-assisted wireless transmitter. In this network, the sum transmit power of the network is minimized by controlling the phase beamforming of the RIS and transmit power of the base station. This problem is posed as a joint optimization problem of transmit power and RIS control, whose goal is to minimize the sum transmit power under signal-to-Interference-plus-noise ratio (SINR) constraints of the users. To solve this problem, a dual method is proposed, where the dual problem is obtained as a semidefinite programming problem. After solving the dual problem, the phase beamforming of the RIS is obtained in the closed form, while the optimal transmit power is obtained by using the standard Interference Function. Simulation results show that the proposed scheme can reduce up to 94% and 27% sum transmit power compared to the maximum ratio transmission (MRT) beamforming and zero-forcing (ZF) beamforming techniques, respectively.

  • Beamforming Design for Multiuser Transmission Through Reconfigurable Intelligent Surface
    arXiv: Signal Processing, 2020
    Co-Authors: Zhaohui Yang, Chongwen Huang, Jianfeng Shi, Mohammad Shikh-bahaei
    Abstract:

    This paper investigates the problem of resource allocation for multiuser communication networks with a reconfigurable intelligent surface (RIS)-assisted wireless transmitter. In this network, the sum transmit power of the network is minimized by controlling the phase beamforming of the RIS and transmit power of the BS. This problem is posed as a joint optimization problem of transmit power and RIS control, whose goal is to minimize the sum transmit power under signal-to-Interference-plus-noise ratio (SINR) constraints of the users. To solve this problem, a dual method is proposed, where the dual problem is obtained as a semidefinite programming problem. After solving the dual problem, the phase beamforming of the RIS is obtained in the closed form, while the optimal transmit power is obtained by using the standard Interference Function. Simulation results show that the proposed scheme can reduce up to 94% and 27% sum transmit power compared to the maximum ratio transmission (MRT) beamforming and zero-forcing (ZF) beamforming techniques, respectively.

Frederic Leroy - One of the best experts on this subject based on the ideXlab platform.

  • probing surface and interface morphology with grazing incidence small angle x ray scattering
    Surface Science Reports, 2009
    Co-Authors: G Renaud, Remi Lazzari, Frederic Leroy
    Abstract:

    Abstract Nanoscience and nanotechnology are tremendously increasing fields of research that aim at producing, characterizing and understanding nanoobjects and assemblies of nanoobjects. Their new physical or chemical properties, which arise from confinement effects, intimately depend on their morphological properties, i.e. their shapes, their sizes and their spatial organization. This calls for dedicated morphological characterization tools, among which is the Grazing Incidence Small Angle X-Ray Scattering (GISAXS). This reciprocal space technique has emerged in the last two decades as a powerful tool that allows investigating in a non-destructive way the morphological properties from one to billions of nanoparticles, either on a surface, or embedded in a matrix, with sizes ranging from 1 nm to several microns. The advantages of the technique are that it is non-destructive; it yields statistical information averaged on a large number of nanoparticles; it allows probing both the surface or deep below it, by changing the incident angle of the X-ray beam; it can be used in very different sample environments, in particular in situ in the course of a given process such as growth, annealing, gas exposure; and it may be given chemical sensitivity by use of anomalous scattering. This report presents a review of the GISAXS technique, from experimental issues to the theories underlying the data analysis, with a wealth of examples. The physical morphological information contained in GISAXS data and its analysis are presented in simple terms, introducing the notions of particle form factor and Interference Function, together with the different cases encountered according to the size/shape dispersion. The theoretical background of X-ray diffuse scattering under grazing incidence is presented in a general way, and then applied to the particular case of grazing incidence small angle X-ray scattering from assemblies of particles either on a substrate, or buried below it. Most of the GISAXS measurements published to date are reported, covering the fields of ex situ studies of embedded metallic nanoparticles, granular multilayered systems, implanted systems, embedded or stacked or deposited semi-conductor nanostructures, porous materials and copolymer thin films. A special emphasis is brought on in situ experiments, performed either in ultra-high vacuum during nanoparticle growth by molecular beam epitaxy, or in gas-reactors during catalytic reactions. This covers a very broad field, from (i) the 3D island (Volmer–Weber) growth of metals on oxides surfaces to (ii) the organized growth of metals on surfaces that are nanopatterned either by surface reconstruction or by underlying dislocation networks or by deposit-induced nanofacetting, to (iii) the in situ investigation of the self-organized Stranski–Krastanow hetero-epitaxial growth of semi-conductor quantum dots on semi-conductor surfaces, or (iv) the in situ surface nanopatterning by ion bombardment. Many examples are discussed in detail, to illustrate the large diversity of systems and morphologies that can be addressed as well as the different analysis issues and the conclusions of the technique in terms of growth mode.

  • quantitative analysis of grazing incidence small angle x ray scattering pd mgo 001 growth
    Physical Review B, 2004
    Co-Authors: C Revenant, Frederic Leroy, Remi Lazzari, G Renaud, Claude R Henry
    Abstract:

    The present paper focuses on the analysis of grazing incidence small-angle x-ray scattering (GISAXS) of islands on a substrate. Getting accurate morphological parameters relevant for the elaboration process, i.e., growth curves, island equilibrium shape, and interfacial energy, implies a quantitative data analysis. The emphasis is put on the island form factor, i.e., the Fourier transform of the island shape. It is shown that the island shape and size can be obtained through the island symmetry, the presence of island facets, the asymptotic behavior at high momentum transfer for large polydispersity, and the zeros or minima of the intensity for small polydispersity. The specificity brought by the grazing incidence scattering geometry is highlighted by a careful comparison between the Born approximation and the more accurate distorted wave Born approximation. The interplay between the form factor and the Interference Function is all the more important in the total scattering intensity when incoherent diffuse scattering comes into play at small momentum transfer for disordered systems. Getting rid of these interpretation difficulties requires accurate measurements of the scattered intensity far in the reciprocal space. This analysis methodology is illustrated through recently acquired GISAXS patterns during the in situ molecular beam epitaxy of Pd nanoislands on MgO(001) single crystals for different thicknesses and temperatures. The morphological parameters obtained agree very well with subsequent transmission electron microscopy-results. Finally, GISAXS diffuse scattering has been shown, originating from the growth-coalescence process and from the size dependence of the island capture area.

Zhaohui Yang - One of the best experts on this subject based on the ideXlab platform.

  • Beamforming Design for Multiuser Transmission Through Reconfigurable Intelligent Surface
    IEEE Transactions on Communications, 2021
    Co-Authors: Zhaohui Yang, Chongwen Huang, Jianfeng Shi, Mohammad Shikh-bahaei
    Abstract:

    This article investigates the problem of resource allocation for multiuser communication networks with a reconfigurable intelligent surface (RIS)-assisted wireless transmitter. In this network, the sum transmit power of the network is minimized by controlling the phase beamforming of the RIS and transmit power of the base station. This problem is posed as a joint optimization problem of transmit power and RIS control, whose goal is to minimize the sum transmit power under signal-to-Interference-plus-noise ratio (SINR) constraints of the users. To solve this problem, a dual method is proposed, where the dual problem is obtained as a semidefinite programming problem. After solving the dual problem, the phase beamforming of the RIS is obtained in the closed form, while the optimal transmit power is obtained by using the standard Interference Function. Simulation results show that the proposed scheme can reduce up to 94% and 27% sum transmit power compared to the maximum ratio transmission (MRT) beamforming and zero-forcing (ZF) beamforming techniques, respectively.

  • Beamforming Design for Multiuser Transmission Through Reconfigurable Intelligent Surface
    arXiv: Signal Processing, 2020
    Co-Authors: Zhaohui Yang, Chongwen Huang, Jianfeng Shi, Mohammad Shikh-bahaei
    Abstract:

    This paper investigates the problem of resource allocation for multiuser communication networks with a reconfigurable intelligent surface (RIS)-assisted wireless transmitter. In this network, the sum transmit power of the network is minimized by controlling the phase beamforming of the RIS and transmit power of the BS. This problem is posed as a joint optimization problem of transmit power and RIS control, whose goal is to minimize the sum transmit power under signal-to-Interference-plus-noise ratio (SINR) constraints of the users. To solve this problem, a dual method is proposed, where the dual problem is obtained as a semidefinite programming problem. After solving the dual problem, the phase beamforming of the RIS is obtained in the closed form, while the optimal transmit power is obtained by using the standard Interference Function. Simulation results show that the proposed scheme can reduce up to 94% and 27% sum transmit power compared to the maximum ratio transmission (MRT) beamforming and zero-forcing (ZF) beamforming techniques, respectively.

G Renaud - One of the best experts on this subject based on the ideXlab platform.

  • probing surface and interface morphology with grazing incidence small angle x ray scattering
    Surface Science Reports, 2009
    Co-Authors: G Renaud, Remi Lazzari, Frederic Leroy
    Abstract:

    Abstract Nanoscience and nanotechnology are tremendously increasing fields of research that aim at producing, characterizing and understanding nanoobjects and assemblies of nanoobjects. Their new physical or chemical properties, which arise from confinement effects, intimately depend on their morphological properties, i.e. their shapes, their sizes and their spatial organization. This calls for dedicated morphological characterization tools, among which is the Grazing Incidence Small Angle X-Ray Scattering (GISAXS). This reciprocal space technique has emerged in the last two decades as a powerful tool that allows investigating in a non-destructive way the morphological properties from one to billions of nanoparticles, either on a surface, or embedded in a matrix, with sizes ranging from 1 nm to several microns. The advantages of the technique are that it is non-destructive; it yields statistical information averaged on a large number of nanoparticles; it allows probing both the surface or deep below it, by changing the incident angle of the X-ray beam; it can be used in very different sample environments, in particular in situ in the course of a given process such as growth, annealing, gas exposure; and it may be given chemical sensitivity by use of anomalous scattering. This report presents a review of the GISAXS technique, from experimental issues to the theories underlying the data analysis, with a wealth of examples. The physical morphological information contained in GISAXS data and its analysis are presented in simple terms, introducing the notions of particle form factor and Interference Function, together with the different cases encountered according to the size/shape dispersion. The theoretical background of X-ray diffuse scattering under grazing incidence is presented in a general way, and then applied to the particular case of grazing incidence small angle X-ray scattering from assemblies of particles either on a substrate, or buried below it. Most of the GISAXS measurements published to date are reported, covering the fields of ex situ studies of embedded metallic nanoparticles, granular multilayered systems, implanted systems, embedded or stacked or deposited semi-conductor nanostructures, porous materials and copolymer thin films. A special emphasis is brought on in situ experiments, performed either in ultra-high vacuum during nanoparticle growth by molecular beam epitaxy, or in gas-reactors during catalytic reactions. This covers a very broad field, from (i) the 3D island (Volmer–Weber) growth of metals on oxides surfaces to (ii) the organized growth of metals on surfaces that are nanopatterned either by surface reconstruction or by underlying dislocation networks or by deposit-induced nanofacetting, to (iii) the in situ investigation of the self-organized Stranski–Krastanow hetero-epitaxial growth of semi-conductor quantum dots on semi-conductor surfaces, or (iv) the in situ surface nanopatterning by ion bombardment. Many examples are discussed in detail, to illustrate the large diversity of systems and morphologies that can be addressed as well as the different analysis issues and the conclusions of the technique in terms of growth mode.

  • quantitative analysis of grazing incidence small angle x ray scattering pd mgo 001 growth
    Physical Review B, 2004
    Co-Authors: C Revenant, Frederic Leroy, Remi Lazzari, G Renaud, Claude R Henry
    Abstract:

    The present paper focuses on the analysis of grazing incidence small-angle x-ray scattering (GISAXS) of islands on a substrate. Getting accurate morphological parameters relevant for the elaboration process, i.e., growth curves, island equilibrium shape, and interfacial energy, implies a quantitative data analysis. The emphasis is put on the island form factor, i.e., the Fourier transform of the island shape. It is shown that the island shape and size can be obtained through the island symmetry, the presence of island facets, the asymptotic behavior at high momentum transfer for large polydispersity, and the zeros or minima of the intensity for small polydispersity. The specificity brought by the grazing incidence scattering geometry is highlighted by a careful comparison between the Born approximation and the more accurate distorted wave Born approximation. The interplay between the form factor and the Interference Function is all the more important in the total scattering intensity when incoherent diffuse scattering comes into play at small momentum transfer for disordered systems. Getting rid of these interpretation difficulties requires accurate measurements of the scattered intensity far in the reciprocal space. This analysis methodology is illustrated through recently acquired GISAXS patterns during the in situ molecular beam epitaxy of Pd nanoislands on MgO(001) single crystals for different thicknesses and temperatures. The morphological parameters obtained agree very well with subsequent transmission electron microscopy-results. Finally, GISAXS diffuse scattering has been shown, originating from the growth-coalescence process and from the size dependence of the island capture area.

Remi Lazzari - One of the best experts on this subject based on the ideXlab platform.

  • probing surface and interface morphology with grazing incidence small angle x ray scattering
    Surface Science Reports, 2009
    Co-Authors: G Renaud, Remi Lazzari, Frederic Leroy
    Abstract:

    Abstract Nanoscience and nanotechnology are tremendously increasing fields of research that aim at producing, characterizing and understanding nanoobjects and assemblies of nanoobjects. Their new physical or chemical properties, which arise from confinement effects, intimately depend on their morphological properties, i.e. their shapes, their sizes and their spatial organization. This calls for dedicated morphological characterization tools, among which is the Grazing Incidence Small Angle X-Ray Scattering (GISAXS). This reciprocal space technique has emerged in the last two decades as a powerful tool that allows investigating in a non-destructive way the morphological properties from one to billions of nanoparticles, either on a surface, or embedded in a matrix, with sizes ranging from 1 nm to several microns. The advantages of the technique are that it is non-destructive; it yields statistical information averaged on a large number of nanoparticles; it allows probing both the surface or deep below it, by changing the incident angle of the X-ray beam; it can be used in very different sample environments, in particular in situ in the course of a given process such as growth, annealing, gas exposure; and it may be given chemical sensitivity by use of anomalous scattering. This report presents a review of the GISAXS technique, from experimental issues to the theories underlying the data analysis, with a wealth of examples. The physical morphological information contained in GISAXS data and its analysis are presented in simple terms, introducing the notions of particle form factor and Interference Function, together with the different cases encountered according to the size/shape dispersion. The theoretical background of X-ray diffuse scattering under grazing incidence is presented in a general way, and then applied to the particular case of grazing incidence small angle X-ray scattering from assemblies of particles either on a substrate, or buried below it. Most of the GISAXS measurements published to date are reported, covering the fields of ex situ studies of embedded metallic nanoparticles, granular multilayered systems, implanted systems, embedded or stacked or deposited semi-conductor nanostructures, porous materials and copolymer thin films. A special emphasis is brought on in situ experiments, performed either in ultra-high vacuum during nanoparticle growth by molecular beam epitaxy, or in gas-reactors during catalytic reactions. This covers a very broad field, from (i) the 3D island (Volmer–Weber) growth of metals on oxides surfaces to (ii) the organized growth of metals on surfaces that are nanopatterned either by surface reconstruction or by underlying dislocation networks or by deposit-induced nanofacetting, to (iii) the in situ investigation of the self-organized Stranski–Krastanow hetero-epitaxial growth of semi-conductor quantum dots on semi-conductor surfaces, or (iv) the in situ surface nanopatterning by ion bombardment. Many examples are discussed in detail, to illustrate the large diversity of systems and morphologies that can be addressed as well as the different analysis issues and the conclusions of the technique in terms of growth mode.

  • quantitative analysis of grazing incidence small angle x ray scattering pd mgo 001 growth
    Physical Review B, 2004
    Co-Authors: C Revenant, Frederic Leroy, Remi Lazzari, G Renaud, Claude R Henry
    Abstract:

    The present paper focuses on the analysis of grazing incidence small-angle x-ray scattering (GISAXS) of islands on a substrate. Getting accurate morphological parameters relevant for the elaboration process, i.e., growth curves, island equilibrium shape, and interfacial energy, implies a quantitative data analysis. The emphasis is put on the island form factor, i.e., the Fourier transform of the island shape. It is shown that the island shape and size can be obtained through the island symmetry, the presence of island facets, the asymptotic behavior at high momentum transfer for large polydispersity, and the zeros or minima of the intensity for small polydispersity. The specificity brought by the grazing incidence scattering geometry is highlighted by a careful comparison between the Born approximation and the more accurate distorted wave Born approximation. The interplay between the form factor and the Interference Function is all the more important in the total scattering intensity when incoherent diffuse scattering comes into play at small momentum transfer for disordered systems. Getting rid of these interpretation difficulties requires accurate measurements of the scattered intensity far in the reciprocal space. This analysis methodology is illustrated through recently acquired GISAXS patterns during the in situ molecular beam epitaxy of Pd nanoislands on MgO(001) single crystals for different thicknesses and temperatures. The morphological parameters obtained agree very well with subsequent transmission electron microscopy-results. Finally, GISAXS diffuse scattering has been shown, originating from the growth-coalescence process and from the size dependence of the island capture area.