The Experts below are selected from a list of 77460 Experts worldwide ranked by ideXlab platform
L K Rasmussen - One of the best experts on this subject based on the ideXlab platform.
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a near ideal noise whitening filter for an asynchronous time varying cdma system
IEEE Transactions on Communications, 1996Co-Authors: L K RasmussenAbstract:A near ideal noise whitening filter for a time-varying code-division multiple-access (CDMA) system is examined. First, the structure of the ideal noise whitening filter is determined. The ideal noise whitening filter for a time-varying CDMA system is dependent on unknown future system parameters and is, therefore, impractical. A near ideal, realizable whitening filter is introduced as a solution. The convergence of the factorization method for a time-varying CDMA system is considered. The truncation of the number of taps of the ideal noise whitening filter is studied and a Metric Function based on the near ideal noise whitening filter for a tree search detection algorithm is formulated. Simulation results are obtained for five and ten-user time-varying CDMA systems with binary random signature sequences of length 10 and a rectangular chip waveform. The results show that the near ideal noise whitening filter can accurately approximate the ideal noise whitening filter at a low complexity level. The performance degradation of a suboptimal tree search detector for a time-varying, asynchronous CDMA system using a typical near ideal noise whitening filter is minimal compared to an identical system using the ideal noise whitening filter.
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a near ideal noise whitening filter for an
1996Co-Authors: L K RasmussenAbstract:In this paper, a near ideal noise whitening filter for a time-varying code-division multiple-access (CDMA) system is examined. First, the structure of the ideal noise whitening filter is determined. The ideal noise whitening filter for a time- varying CDMA system is dependent on unknown future system parameters and is, therefore, impractical. A near ideal, realizable whitening filter is introduced as a solution. The convergence of the factorization method for a time-varying CDMA system is considered. The truncation of the number of taps of the ideal noise whitening filter is studied and a Metric Function based on the near ideal noise whitening filter for a tree search detection algorithm is formulated. Simulation results are obtained for five and ten-user time-varying CDMA systems with binary random signature sequences of length 10 and a rectangular chip waveform. The results show that the near ideal noise whitening filter can accurately approximate the ideal noise whitening filter at a low complexity level. The performance degradation of a suboptimal tree search detector for a time-varying, asynchronous CDMA system using a typical near ideal noise whitening filter is minimal compared to an identical system using the ideal noise whitening filter.
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a near ideal whitening filter for m algorithm detection in an asynchronous time varying cdma system
International Symposium on Information Theory, 1995Co-Authors: L K RasmussenAbstract:A near ideal noise whitening filter for a time-varying CDMA system is considered. The structure of the ideal noise whitening filter is studied and the Metric Function for tree search detection is derived. The ideal noise whitening filter for a time-varying CDMA system depends on unknown, future system parameters and is therefore difficult to realize. A near ideal, realizable noise whitening filter is proposed as a solution.
Xiaobao Wang - One of the best experts on this subject based on the ideXlab platform.
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critical phenomena in gravitational collapse of husain martinez nunez scalar field
arXiv: General Relativity and Quantum Cosmology, 2019Co-Authors: Xiaobao Wang, Sijie GaoAbstract:We construct analytical models to study the critical phenomena in gravitational collapse of the Husain-Martinez-Nunez massless scalar field. We first use the cut-and-paste technique to match the conformally flat solution ($c=0$ ) onto an outgoing Vaidya solution. To guarantee the continuity of the Metric and the extrinsic curvature, we prove that the two solutions must be joined at a null hypersurface and the Metric Function in Vaidya spacetime must satisfy some constraints. We find that the mass of the black hole in the resulting spacetime takes the form $M\propto (p-p^*)^\gamma$, where the critical exponent $\gamma$ is equal to $0.5$. For the case $c\neq 0$, we show that the scalar field must be joined onto two pieces of Vaidya spacetimes to avoid a naked singularity. We also derive the power-law mass formula with $\gamma=0.5$. Compared with previous analytical models constructed from a different scalar field with continuous self-similarity, we obtain the same value of $\gamma$. However, we show that the solution with $c\neq 0$ is not self-similar. Therefore, we provide a rare example that a scalar field without self-similarity also possesses the features of critical collapse.
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critical phenomena in gravitational collapse of husain martinez nunez scalar field
European Physical Journal C, 2019Co-Authors: Xiaobao Wang, Sijie GaoAbstract:We construct analytical models to study the critical phenomena in gravitational collapse of the Husain-Martinez-Nunez massless scalar field. We first use the cut-and-paste technique to match the conformally flat solution ($$c=0$$ ) onto an outgoing Vaidya solution. To guarantee the continuity of the Metric and the extrinsic curvature, we prove that the two solutions must be joined at a null hypersurface and the Metric Function in Vaidya spacetime must satisfy certain constraints. We find that the mass of the black hole in the resulting spacetime takes the form $$M\propto (p-p^*)^\gamma $$, where the critical exponent $$\gamma $$ is equal to 0.5. For the case $$c\ne 0$$, we show that the scalar field must be joined onto two pieces of Vaidya spacetimes to avoid a naked singularity. We also derive the power-law mass formula with $$\gamma =0.5$$. Compared with previous analytical models which were constructed from a different scalar field with continuous self-similarity, we obtain the same value of $$\gamma $$. However, we show that the solution with $$c\ne 0$$ is not self-similar. Therefore, we provide a rare example that a scalar field without self-similarity also possesses the features of critical collapse.
Sijie Gao - One of the best experts on this subject based on the ideXlab platform.
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critical phenomena in gravitational collapse of husain martinez nunez scalar field
arXiv: General Relativity and Quantum Cosmology, 2019Co-Authors: Xiaobao Wang, Sijie GaoAbstract:We construct analytical models to study the critical phenomena in gravitational collapse of the Husain-Martinez-Nunez massless scalar field. We first use the cut-and-paste technique to match the conformally flat solution ($c=0$ ) onto an outgoing Vaidya solution. To guarantee the continuity of the Metric and the extrinsic curvature, we prove that the two solutions must be joined at a null hypersurface and the Metric Function in Vaidya spacetime must satisfy some constraints. We find that the mass of the black hole in the resulting spacetime takes the form $M\propto (p-p^*)^\gamma$, where the critical exponent $\gamma$ is equal to $0.5$. For the case $c\neq 0$, we show that the scalar field must be joined onto two pieces of Vaidya spacetimes to avoid a naked singularity. We also derive the power-law mass formula with $\gamma=0.5$. Compared with previous analytical models constructed from a different scalar field with continuous self-similarity, we obtain the same value of $\gamma$. However, we show that the solution with $c\neq 0$ is not self-similar. Therefore, we provide a rare example that a scalar field without self-similarity also possesses the features of critical collapse.
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critical phenomena in gravitational collapse of husain martinez nunez scalar field
European Physical Journal C, 2019Co-Authors: Xiaobao Wang, Sijie GaoAbstract:We construct analytical models to study the critical phenomena in gravitational collapse of the Husain-Martinez-Nunez massless scalar field. We first use the cut-and-paste technique to match the conformally flat solution ($$c=0$$ ) onto an outgoing Vaidya solution. To guarantee the continuity of the Metric and the extrinsic curvature, we prove that the two solutions must be joined at a null hypersurface and the Metric Function in Vaidya spacetime must satisfy certain constraints. We find that the mass of the black hole in the resulting spacetime takes the form $$M\propto (p-p^*)^\gamma $$, where the critical exponent $$\gamma $$ is equal to 0.5. For the case $$c\ne 0$$, we show that the scalar field must be joined onto two pieces of Vaidya spacetimes to avoid a naked singularity. We also derive the power-law mass formula with $$\gamma =0.5$$. Compared with previous analytical models which were constructed from a different scalar field with continuous self-similarity, we obtain the same value of $$\gamma $$. However, we show that the solution with $$c\ne 0$$ is not self-similar. Therefore, we provide a rare example that a scalar field without self-similarity also possesses the features of critical collapse.
Sayan Kar - One of the best experts on this subject based on the ideXlab platform.
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Revisiting a family of wormholes: geometry, matter, scalar quasinormal modes and echoes
The European Physical Journal C, 2020Co-Authors: Poulami Dutta roy, S. Aneesh, Sayan KarAbstract:We revisit a family of ultra-static Lorentzian wormholes which includes Ellis–Bronnikov spacetime as a special case. We first show how the required total matter stress energy (which violates the local energy conditions) may be split into a part due to a phantom scalar and another extra piece (which vanishes for Ellis–Bronnikov) satisfying the Averaged Null Energy Condition (ANEC) along radial null geodesics. Thereafter, we examine the effective potential for scalar wave propagation in a general setting. Conditions on the Metric Function, for which the effective potential may have double barrier features are written down and illustrated (using this class of wormholes). Subsequently, using numerous methods, we obtain the scalar quasinormal modes (QNMs). We note the behaviour of the QNMs as a Function of n (the Metric parameter) and $$b_0$$ b 0 (the wormhole throat radius). Thus, the shapes and sizes of the wormholes, governed by the Metric parameter n and the throat radius $$b_0$$ b 0 are linked to the variation and the values of the QNMs. Finally, we demonstrate how, for large n , the time domain profiles exhibit, expectedly, the occurence of echoes. In summary, our results suggest that this family of wormholes may indeed be used as a template for further studies on the gravitational wave physics of exotic compact objects.
Poulami Dutta roy - One of the best experts on this subject based on the ideXlab platform.
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Revisiting a family of wormholes: geometry, matter, scalar quasinormal modes and echoes
The European Physical Journal C, 2020Co-Authors: Poulami Dutta roy, S. Aneesh, Sayan KarAbstract:We revisit a family of ultra-static Lorentzian wormholes which includes Ellis–Bronnikov spacetime as a special case. We first show how the required total matter stress energy (which violates the local energy conditions) may be split into a part due to a phantom scalar and another extra piece (which vanishes for Ellis–Bronnikov) satisfying the Averaged Null Energy Condition (ANEC) along radial null geodesics. Thereafter, we examine the effective potential for scalar wave propagation in a general setting. Conditions on the Metric Function, for which the effective potential may have double barrier features are written down and illustrated (using this class of wormholes). Subsequently, using numerous methods, we obtain the scalar quasinormal modes (QNMs). We note the behaviour of the QNMs as a Function of n (the Metric parameter) and $$b_0$$ b 0 (the wormhole throat radius). Thus, the shapes and sizes of the wormholes, governed by the Metric parameter n and the throat radius $$b_0$$ b 0 are linked to the variation and the values of the QNMs. Finally, we demonstrate how, for large n , the time domain profiles exhibit, expectedly, the occurence of echoes. In summary, our results suggest that this family of wormholes may indeed be used as a template for further studies on the gravitational wave physics of exotic compact objects.
