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- Tim Rogers, Isaac Pérez Castillo, Reimer Kühn, Koujin Takeda
- Physical review. E, Statistical, nonlinear, and…
- 2008

The spectral density of various ensembles of sparse symmetric random matrices is analyzed using the cavity method. We consider two cases: matrices whose associated graphs are locally treelike, and sparse covariance matrices. We derive a closed set of equations from which the density of eigenvalues can be efficiently calculated. Within this approach, the… (More)

- Koujin Takeda, Shinsuke Uda, Yoshiyuki Kabashima
- ArXiv
- 2006

{ An approach to analyze the performance of the code division multiple access (CDMA) scheme, which is a core technology used in modern wireless communication systems, is provided. The approach characterizes the objective system by the eigenvalue spectrum of a cross-correlation matrix composed of signature sequences used in CDMA communication, which enable… (More)

- Koujin Takeda, Yoshiyuki Kabashima
- ArXiv
- 2010

We provide a scheme for exploring the reconstruction limits of compressed sensing by minimizing the general cost function under the random measurement constraints for generic correlated signal sources. Our scheme is based on the statistical mechanical replica method for dealing with random systems. As a simple but non-trivial example, we apply the scheme to… (More)

- Koujin Takeda, Yoshiyuki Kabashima
- 2010 IEEE International Symposium on Information…
- 2010

We investigate a reconstruction limit of compressed sensing for a reconstruction scheme based on the L<inf>1</inf>-norm minimization utilizing a correlated compression matrix with a statistical mechanics method. We focus on the compression matrix modeled as the Kronecker-type random matrix studied in research on multiple-input multiple-output wireless… (More)

- Atsushi Hatabu, Koujin Takeda, Yoshiyuki Kabashima
- Physical review. E, Statistical, nonlinear, and…
- 2009

The Kronecker channel model of wireless communication is analyzed using statistical mechanics methods. In the model, spatial proximities among transmission/reception antennas are taken into account as certain correlation matrices, which generally yield nontrivial dependence among symbols to be estimated. This prevents accurate assessment of the… (More)

- Koujin Takeda, Atsushi Hatabu, Yoshiyuki Kabashima
- ArXiv
- 2007

Abstract. A statistical mechanical framework to analyze linear vector channel models in digital wireless communication is proposed for a large system. The framework is a generalization of that proposed for code-division multiple-access systems in Europhys. Lett., 76 (2006) 1193 and enables the analysis of the system in which the elements of the channel… (More)

- Koujin Takeda
- 2003

We study the four-dimensional Z2 random-plaquette lattice gauge theory as a model of topological quantum memory, the toric code in particular. In this model, the procedure of quantum error correction works properly in the ordered (Higgs) phase, and phase boundary between the ordered (Higgs) and disordered (confinement) phases gives the accuracy threshold of… (More)

- Toshifumi Jittoh, Shigeki Matsumoto, Joe Sato, Yoshio Sato, Koujin Takeda
- 2005

We study the decay process of an unstable quantum system, especially the deviation from the exponential decay law. We show that the exponential period no longer exists in the case of the s-wave decay with small Q value, where the Q value is the difference between the energy of the initially prepared state and the minimum energy of the continuous eigenstates… (More)

- Atsushi Hatabu, Koujin Takeda, Yoshiyuki Kabashima
- ArXiv
- 2009

The Kronecker channel model of wireless communication is analyzed using statistical mechanics methods. In the model, spatial proximities among transmission/reception antennas are taken into account as certain correlation matrices, which generally yield non-trivial dependence among symbols to be estimated. This prevents accurate assessment of the… (More)

Abstract. We study a random code ensemble with a hierarchical structure, which is closely related to the generalized random energy model with discrete energy values. Based on this correspondence, we analyze the hierarchical random code ensemble by using the replica method in two situations: lossy data compression and channel coding. For both the situations,… (More)