The Experts below are selected from a list of 46158 Experts worldwide ranked by ideXlab platform
Jing Deng - One of the best experts on this subject based on the ideXlab platform.
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can multiple subchannels improve the delay performance of RTS cts based mac schemes
IEEE Transactions on Wireless Communications, 2009Co-Authors: Jing Deng, Yunghsiang S. Han, Sanjeev R. KulkarniAbstract:We analyze the delay performance of RTS/CTS-based (Request-To-Send/Clear-To-Send) multi-channel MAC (Medium Access Control) schemes for wireless networks. These schemes usually employ multiple data subchannels for data transmission and one control subchannel to send the RTS/CTS dialogue for channel reservation. Through theoretical analysis and simulations, we show that, in fully-connected networks, such multi-channel MAC schemes suffer longer delays than the corresponding single channel MAC scheme, that puts the RTS/CTS dialogue on the same channel as data packet transmissions. This conclusion holds even when data packets have different priorities and higher priority traffic is sent ahead of lower priority traffic.
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dual busy tone multiple access dbtma a multiple access control scheme for ad hoc networks
IEEE Transactions on Communications, 2002Co-Authors: Zygmunt J Haas, Jing DengAbstract:In ad hoc networks, the hidden- and the exposed-terminal problems can severely reduce the network capacity on the MAC layer. To address these problems, the ready-to-send and clear-to-send (RTS/CTS) dialogue has been proposed in the literature. However, MAC schemes using only the RTS/CTS dialogue cannot completely solve the hidden and the exposed terminal problems, as pure "packet sensing" MAC schemes are not safe even in fully connected networks. We propose a new MAC protocol, termed the dual busy tone multiple access (DBTMA) scheme. The operation of the DBTMA protocol is based on the RTS packet and two narrow-bandwidth, out-of-band busy tones. With the use of the RTS packet and the receive busy tone, which is set up by the receiver, our scheme completely solves the hidden- and the exposed-terminal problems. The busy tone, which is set up by the transmitter, provides protection for the RTS packets, increasing the probability of successful RTS reception and, consequently, increasing the throughput. This paper outlines the operation rules of the DBTMA scheme and analyzes its performance. Simulation results are also provided to support the analytical results. It is concluded that the DBTMA protocol is superior to other schemes that rely on the RTS/CTS dialogue on a single channel or to those that rely on a single busy tone. As a point of reference, the DBTMA scheme out-performs FAMA-NCS by 20-40% in our simulations using the network topologies borrowed from the FAMA-NCS paper. In an ad hoc network with a large coverage area, DBTMA achieves performance gain of 140% over FAMA-NCS and performance gain of 20% over RI-BTMA.
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dual busy tone multiple access dbtma a new medium access control for packet radio networks
IEEE International Conference on Universal Personal Communications, 1998Co-Authors: Jing Deng, Zygmunt J HaasAbstract:In packet radio networks, the hidden terminal problem and the exposed terminal problem can severely reduce the utilization of a medium access control (MAC) protocols. To avoid these problems, request-to-send/clear-to-send (RTS/CTS)-based schemes were proposed. However, as shown in this paper, utilization of these schemes is still degraded, especially in the cases in which the propagation and the transmission delays are long. We propose a new MAC protocol, termed dual busy tone multiple access (DBTMA), and we evaluate its performance. In DBTMA, two busy tones are used to separate the use of the forward and the reverse communication directions. the simulations show that the network utilization of DBTMA is about twice as that of RTS/CTS-based schemes. We also discuss the effect of nodal mobility on the network utilization in packet radio networks, concluding that it is negligible under normal operational conditions.
Y Iwasa - One of the best experts on this subject based on the ideXlab platform.
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ybco and bi2223 coils for high field lts hts nmr magnets hts hts joint resistivity
IEEE Transactions on Applied Superconductivity, 2013Co-Authors: Youngjae Kim, Juan Bascunan, Seungyong Hahn, Thibault Lecrevisse, John P Voccio, Dong Keun Park, Y IwasaAbstract:This paper presents our latest experimental results on high-temperature superconducting (HTS) splice joints for HTS insert coils made of YBCO and Bi2223, that comprise a 1.3 GHz low-temperature superconducting/HTS nuclear magnetic resonance magnet currently under development at Francis Bitter Magnet Laboratory. HTS splice joint resistivity at 77 K in these insert coils must be reproducible and <; 100 Ω·cm2. Several YBCO tape to YBCO tape (YBCO-YBCO) splice joint samples were fabricated, and their resistivity and Ic were measured at 77 K. First, we describe the joint splicing setup and discuss the parameters that affect joint resistivity: pressure over joint surface, solder, and YBCO spool batch. Second, we report results on YBCO-YBCO joints at 77 K in zero field. Measurements have shown that spool batch and solder are primary sources of a wide range of variation in YBCO-YBCO joint resistivity. By controlling these parameters, we expect to reproducibly achieve HTS-HTS resistive joints of resistance <; 100 nΩ·cm2.
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a 1 3 ghz lts hts nmr magnet a progress report
IEEE Transactions on Applied Superconductivity, 2011Co-Authors: Juan Bascunan, Seungyong Hahn, Dong Keun Park, Y IwasaAbstract:In this paper we present details of a 600 MHz HTS insert (H600) double pancake (DP) windings. It will first be operated in the bore of a 500 MHz LTS magnet, achieving a frequency of 1.1 GHz. Upon completion of H600, we will embark on the final phase (Phase 3B) of a 3-Phase program began in 2000: completion of a high resolution 1.3 GHz LTS/HTS magnet. In Phase 3B, the H600 will be coupled to a 700 MHz LTS magnet to achieve the ultimate frequency of 1.3 GHz. The HTS insert is composed of two concentric stacks of double pancakes, one wound with high strength BSCCO-2223 tape, the other with YBCO coated conductor. Details include conductor and coil parameters, winding procedure, DPs mechanical support and integration to the background 500 MHz LTS magnet. Test results of individual DPs in LN2 are also presented.
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an analytical technique to elucidate field impurities from manufacturing uncertainties of an double pancake type hts insert for high field lts hts nmr magnets
IEEE Transactions on Applied Superconductivity, 2009Co-Authors: Seungyong Hahn, Min Cheol Ahn, Juan Bascunan, E S Bobrov, Y IwasaAbstract:This paper addresses adverse effects of dimensional uncertainties of an HTS insert assembled with double-pancake coils on spatial field homogeneity. Each DP coil was wound with Bi2223 tapes having dimensional tolerances larger than one order of magnitude of those accepted for LTS wires used in conventional NMR magnets. The paper presents: 1) dimensional variations measured in two LTS/HTS NMR magnets, 350 MHz (LH350) and 700 MHz (LH700), both built and operated at the Francis Bitter Magnet Laboratory; and 2) an analytical technique and its application to elucidate the field impurities measured with the two LTS/HTS magnets. Field impurities computed with the analytical model and those measured with the two LTS/HTS magnets agree quite well, demonstrating that this analytical technique is applicable to design a DP-assembled HTS insert with an improved field homogeneity for a high-field LTS/HTS NMR magnet.
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nonlinear behavior of a shim coil in an lts hts nmr magnet with an hts insert comprising double pancake hts tape coils
IEEE Transactions on Applied Superconductivity, 2009Co-Authors: Seungyong Hahn, Min Cheol Ahn, Juan Bascunan, Weijun Yao, Y IwasaAbstract:This paper repoRTS results, experimental and analytical, of the nonlinear behavior of a shim coil in the presence of an HTS coil assembled with double-pancake (DP) HTS-tape coils. The experimental results are from: 1) operation of a 700 MHz LTS/HTS NMR magnet (LH700) consisting of a 600 MHz LTS NMR magnet (L600) equipped with superconducting shim coils and a 100 MHz DP-assembled HTS insert (H100) and; 2) an experiment with a room-temperature (RT) Z1 shim coil coupled to a 50 MHz DP-assembled HTS insert (H50). A field mapping theory with a harmonic analysis is applied to interpret both results. Based on experimental results and analyses, we conclude that the screening-current-induced field (SCF) generated by a DP-assembled HTS insert is primarily responsible for the nonlinear behavior, including strength degradation, of a shim coil.
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stability and protection of superconducting magnets a discussion
IEEE Transactions on Applied Superconductivity, 2005Co-Authors: Y IwasaAbstract:The paper discusses stability and protection, two key issues for design and operation of superconducting magnets, both LTS and HTS. A rather optimistic view on stability is presented first, followed by discussion of protection issue, focusing on its sub-issues-overheating and internal voltage-for three types of magnets: 1) LTS cryostable magnet; 2) LTS adiabatic magnet; and 3) HTS adiabatic magnet.
Zygmunt J Haas - One of the best experts on this subject based on the ideXlab platform.
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dual busy tone multiple access dbtma a multiple access control scheme for ad hoc networks
IEEE Transactions on Communications, 2002Co-Authors: Zygmunt J Haas, Jing DengAbstract:In ad hoc networks, the hidden- and the exposed-terminal problems can severely reduce the network capacity on the MAC layer. To address these problems, the ready-to-send and clear-to-send (RTS/CTS) dialogue has been proposed in the literature. However, MAC schemes using only the RTS/CTS dialogue cannot completely solve the hidden and the exposed terminal problems, as pure "packet sensing" MAC schemes are not safe even in fully connected networks. We propose a new MAC protocol, termed the dual busy tone multiple access (DBTMA) scheme. The operation of the DBTMA protocol is based on the RTS packet and two narrow-bandwidth, out-of-band busy tones. With the use of the RTS packet and the receive busy tone, which is set up by the receiver, our scheme completely solves the hidden- and the exposed-terminal problems. The busy tone, which is set up by the transmitter, provides protection for the RTS packets, increasing the probability of successful RTS reception and, consequently, increasing the throughput. This paper outlines the operation rules of the DBTMA scheme and analyzes its performance. Simulation results are also provided to support the analytical results. It is concluded that the DBTMA protocol is superior to other schemes that rely on the RTS/CTS dialogue on a single channel or to those that rely on a single busy tone. As a point of reference, the DBTMA scheme out-performs FAMA-NCS by 20-40% in our simulations using the network topologies borrowed from the FAMA-NCS paper. In an ad hoc network with a large coverage area, DBTMA achieves performance gain of 140% over FAMA-NCS and performance gain of 20% over RI-BTMA.
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dual busy tone multiple access dbtma a new medium access control for packet radio networks
IEEE International Conference on Universal Personal Communications, 1998Co-Authors: Jing Deng, Zygmunt J HaasAbstract:In packet radio networks, the hidden terminal problem and the exposed terminal problem can severely reduce the utilization of a medium access control (MAC) protocols. To avoid these problems, request-to-send/clear-to-send (RTS/CTS)-based schemes were proposed. However, as shown in this paper, utilization of these schemes is still degraded, especially in the cases in which the propagation and the transmission delays are long. We propose a new MAC protocol, termed dual busy tone multiple access (DBTMA), and we evaluate its performance. In DBTMA, two busy tones are used to separate the use of the forward and the reverse communication directions. the simulations show that the network utilization of DBTMA is about twice as that of RTS/CTS-based schemes. We also discuss the effect of nodal mobility on the network utilization in packet radio networks, concluding that it is negligible under normal operational conditions.
Seungyong Hahn - One of the best experts on this subject based on the ideXlab platform.
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ybco and bi2223 coils for high field lts hts nmr magnets hts hts joint resistivity
IEEE Transactions on Applied Superconductivity, 2013Co-Authors: Youngjae Kim, Juan Bascunan, Seungyong Hahn, Thibault Lecrevisse, John P Voccio, Dong Keun Park, Y IwasaAbstract:This paper presents our latest experimental results on high-temperature superconducting (HTS) splice joints for HTS insert coils made of YBCO and Bi2223, that comprise a 1.3 GHz low-temperature superconducting/HTS nuclear magnetic resonance magnet currently under development at Francis Bitter Magnet Laboratory. HTS splice joint resistivity at 77 K in these insert coils must be reproducible and <; 100 Ω·cm2. Several YBCO tape to YBCO tape (YBCO-YBCO) splice joint samples were fabricated, and their resistivity and Ic were measured at 77 K. First, we describe the joint splicing setup and discuss the parameters that affect joint resistivity: pressure over joint surface, solder, and YBCO spool batch. Second, we report results on YBCO-YBCO joints at 77 K in zero field. Measurements have shown that spool batch and solder are primary sources of a wide range of variation in YBCO-YBCO joint resistivity. By controlling these parameters, we expect to reproducibly achieve HTS-HTS resistive joints of resistance <; 100 nΩ·cm2.
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a 1 3 ghz lts hts nmr magnet a progress report
IEEE Transactions on Applied Superconductivity, 2011Co-Authors: Juan Bascunan, Seungyong Hahn, Dong Keun Park, Y IwasaAbstract:In this paper we present details of a 600 MHz HTS insert (H600) double pancake (DP) windings. It will first be operated in the bore of a 500 MHz LTS magnet, achieving a frequency of 1.1 GHz. Upon completion of H600, we will embark on the final phase (Phase 3B) of a 3-Phase program began in 2000: completion of a high resolution 1.3 GHz LTS/HTS magnet. In Phase 3B, the H600 will be coupled to a 700 MHz LTS magnet to achieve the ultimate frequency of 1.3 GHz. The HTS insert is composed of two concentric stacks of double pancakes, one wound with high strength BSCCO-2223 tape, the other with YBCO coated conductor. Details include conductor and coil parameters, winding procedure, DPs mechanical support and integration to the background 500 MHz LTS magnet. Test results of individual DPs in LN2 are also presented.
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an analytical technique to elucidate field impurities from manufacturing uncertainties of an double pancake type hts insert for high field lts hts nmr magnets
IEEE Transactions on Applied Superconductivity, 2009Co-Authors: Seungyong Hahn, Min Cheol Ahn, Juan Bascunan, E S Bobrov, Y IwasaAbstract:This paper addresses adverse effects of dimensional uncertainties of an HTS insert assembled with double-pancake coils on spatial field homogeneity. Each DP coil was wound with Bi2223 tapes having dimensional tolerances larger than one order of magnitude of those accepted for LTS wires used in conventional NMR magnets. The paper presents: 1) dimensional variations measured in two LTS/HTS NMR magnets, 350 MHz (LH350) and 700 MHz (LH700), both built and operated at the Francis Bitter Magnet Laboratory; and 2) an analytical technique and its application to elucidate the field impurities measured with the two LTS/HTS magnets. Field impurities computed with the analytical model and those measured with the two LTS/HTS magnets agree quite well, demonstrating that this analytical technique is applicable to design a DP-assembled HTS insert with an improved field homogeneity for a high-field LTS/HTS NMR magnet.
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nonlinear behavior of a shim coil in an lts hts nmr magnet with an hts insert comprising double pancake hts tape coils
IEEE Transactions on Applied Superconductivity, 2009Co-Authors: Seungyong Hahn, Min Cheol Ahn, Juan Bascunan, Weijun Yao, Y IwasaAbstract:This paper repoRTS results, experimental and analytical, of the nonlinear behavior of a shim coil in the presence of an HTS coil assembled with double-pancake (DP) HTS-tape coils. The experimental results are from: 1) operation of a 700 MHz LTS/HTS NMR magnet (LH700) consisting of a 600 MHz LTS NMR magnet (L600) equipped with superconducting shim coils and a 100 MHz DP-assembled HTS insert (H100) and; 2) an experiment with a room-temperature (RT) Z1 shim coil coupled to a 50 MHz DP-assembled HTS insert (H50). A field mapping theory with a harmonic analysis is applied to interpret both results. Based on experimental results and analyses, we conclude that the screening-current-induced field (SCF) generated by a DP-assembled HTS insert is primarily responsible for the nonlinear behavior, including strength degradation, of a shim coil.
David Starobinski - One of the best experts on this subject based on the ideXlab platform.
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on false blocking in RTS cts based multihop wireless networks
IEEE Transactions on Vehicular Technology, 2007Co-Authors: S Ray, David StarobinskiAbstract:The request-to-send/clear-to-send (RTS/CTS) mechanism is widely used in wireless networks in order to avoid packet collisions and, thus, achieve high network throughput. In multihop settings, however, current implementations of the RTS/CTS mechanism may lead to interdependencies that unnecessarily prohibit nodes from transmitting over long periods of time. We refer to this problem as "false blocking." In this paper, we describe and analyze the false blocking problem in detail. We show that false blocking can lead to a significant performance degradation in a variety of topologies and, possibly, to network-wide congestion as well. We propose a backward-compatible solution to the false blocking problem, called the RTS validation. We model and analyze the performance of RTS validation under general traffic and topology settings and show that it achieves a considerable reduction in the probability of false blocking. Furthermore, we carry out extensive simulations that validate our analysis and show that the RTS validation stabilizes the throughput at high load and increases its peak value, sometimes by as much as 50%
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RTS cts induced congestion in ad hoc wireless lans
Wireless Communications and Networking Conference, 2003Co-Authors: S Ray, J B Carruthers, David StarobinskiAbstract:The RTS/CTS mechanism is widely used in wireless networks in order to avoid packet collisions and, thus, achieve high throughput. In ad hoc networks, however the current implementation of the RTS/CTS mechanism may lead to interdependencies so that nodes become unable to transmit any packets during long periods of time. This effect manifests itself in the form of congestion where, after a certain point, the network throughput decreases with increasing load instead of maintaining its peak value. In this paper, we describe and analyze this problem in detail and provide a backward-compatible solution, called RTS validation. Our simulations show that this solution leads to a 60% gain in the peak throughput in addition to stabilizing the throughput at high load.
