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Sugiura S.,Toyota Central Research and Development Laboratories Inc. | Chen S.,University of Southampton | Hanzo L.,University of Southampton
IEEE Transactions on Wireless Communications | Year: 2011

In this paper, motivated by the recent concept of Spatial Modulation (SM), we propose a novel Generalized Space-Time Shift Keying (G-STSK) architecture, which acts as a unified Multiple-Input Multiple-Output (MIMO) framework. More specifically, our G-STSK scheme is based on the rationale that P out of Q dispersion matrices are selected and linearly combined in conjunction with the classic PSK/QAM modulation, where activating P out of Q dispersion matrices provides an implicit means of conveying information bits in addition to the classic modem. Due to its substantial flexibility, our G-STSK framework includes diverse MIMO arrangements, such as SM, Space-Shift Keying (SSK), Linear Dispersion Codes (LDCs), Space-Time Block Codes (STBCs) and Bell Lab's Layered Space-Time (BLAST) scheme. Hence it has the potential of subsuming all of them, when flexibly adapting a set of system parameters. Moreover, we also derive the Discrete-input Continuous-output Memoryless Channel (DCMC) capacity for our G-STSK scheme, which serves as the unified capacity limit, hence quantifying the capacity of the class of MIMO arrangements. Furthermore, EXtrinsic Information Transfer (EXIT) chart analysis is used for designing our G-STSK scheme and for characterizing its iterative decoding convergence. © 2011 IEEE. Source

Inagaki M.,Toyota Central Research and Development Laboratories Inc.
International Journal of Heat and Fluid Flow | Year: 2011

A new wall-damping function, based on the Kolmogorov velocity scale, for large eddy simulation (LES) is proposed, which accounts for the near-wall effect. To calculate the Kolmogorov velocity scale, uε, the dissipation rate of turbulent energy, ε, is needed. In LES, however, the dissipation rate is generally not solved, unlike in the Reynolds averaged Navier-Stokes (RANS) simulations, e.g., k-ε models. Although, in some previous studies, the dissipation rate of the subgrid-scale (SGS) turbulent energy, εSGS, is used instead of ε in calculating the Kolmogorov velocity scale, the scale obtained using such a method overly depends on the grid resolution employed and is generally inappropriate. Accordingly, the wall-damping function using the incorrect velocity scale also depends on the grid resolution and gives an inadequate wall effect. This is because εSGS contains only the components in the scale smaller than the grid-filter width, which obviously varies with the grid resolution employed. In this study, to overcome this problem, we propose a method for estimating the Kolmogorov velocity scale with a technique of conversion in LES, and the estimated one is utilized in the wall-damping function. The revised wall-damping function for LES is tested in channel flows and a backward-facing step flow. The results show that it yields a proper near-wall effect in all test cases which cover a wide range of grid resolution and Reynolds numbers. It is also shown that all three kinds of SGS models incorporating the present wall-damping function provide good predictions, and it is effective both in one-equation and 0-equation SGS models. These results suggest that the use of the proposed wall-damping function is a refined and versatile near-wall treatment in LES with various kinds of SGS models. © 2010 Elsevier Inc. Source

Kuriki R.,Tokyo Institute of Technology | Sekizawa K.,Tokyo Institute of Technology | Sekizawa K.,Toyota Central Research and Development Laboratories Inc. | Ishitani O.,Tokyo Institute of Technology | Maeda K.,Tokyo Institute of Technology
Angewandte Chemie - International Edition | Year: 2015

A heterogeneous photocatalyst system that consists of a ruthenium complex and carbon nitride (C3N4), which act as the catalytic and light-harvesting units, respectively, was developed for the reduction of CO2 into formic acid. Promoting the injection of electrons from C3N4 into the ruthenium unit as well as strengthening the electronic interactions between the two units enhanced its activity. The use of a suitable solvent further improved the performance, resulting in a turnover number of greater than 1000 and an apparent quantum yield of 5.7% at 400 nm. These are the best values that have been reported for heterogeneous photocatalysts for CO2 reduction under visible-light irradiation to date. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA. Source

Takeda Y.,Toyota Central Research and Development Laboratories Inc. | Motohiro T.,Toyota Central Research and Development Laboratories Inc.
Solar Energy Materials and Solar Cells | Year: 2011

We propose a novel type of highly efficient solar cells: intermediate-band-assisted hot-carrier solar cells (IB-HC-SCs). Carriers are generated by two-step photo-excitation via intermediate bands, and extracted through energy-selective contacts before they are completely thermalized. The two-step excitation dramatically reduces entropy generation associated with hot-carrier extraction. As a result, limiting conversion efficiency is significantly improved to be around 60% (0.1 sun)70% (1000 sun), even though considering a finite thermalization time of hot carriers being 1 ns. This improvement is contrasting to the fact that the limiting conversion efficiency of usual hot-carrier cells under 1 sun irradiation is higher only slightly than the ShockleyQueisser limit (34%), because of the remarkable entropy generation. © 2011 Elsevier B.V. Source

Takeda Y.,Toyota Central Research and Development Laboratories Inc. | Motohiro T.,Toyota Central Research and Development Laboratories Inc.
Solar Energy Materials and Solar Cells | Year: 2010

We have calculated the limiting conversion efficiency of solar cells utilizing carrier multiplication (CM), using the detailed balance theory. The solar cells were assumed to comprise quantum dots (QDs) embedded in another material. It has been elucidated that three requisites must be fulfilled, so that a sufficient number of photons in the solar spectrum contribute to CM, resulting in significantly higher conversion efficiency than the values of conventional cells. These requisites are as follows: (1) the effective mass of electrons in the QDs should be much lighter than that of holes, so that the threshold photon energy above which CM can occur is close to the energy gap of the QDs. In this respect, InAs is a promising candidate for the QD material, but PbSe and Si are not. (2) The potential barrier height for electrons in the QDs, which determines the upper limit of the quantum yield of photon-to-carrier conversion (γlimit), should be slightly larger than the energy gap of the QDs to achieve a γlimit value of 2, when the solar cells are used under the non-concentrated insolation. InAs QDs embedded in AlxGa1-xAsySb1-y is a possible candidate to fulfill these two criteria. A higher barrier does not contribute to generation of more carriers, but likely disturbs electron transport. In contrast, under the concentrated insolation, a potential barrier slightly higher than twice the energy gap to achieve a γlimit value of 3 leads to higher conversion efficiency. (3) The quantum yield of photon-to-carrier conversion as a function of photon energy should rise as steeply as possible at the threshold photon energy. The experimentally observed quantum yield with a sloping rise leads to little improvement in conversion efficiency due to CM, under the non-concentrated insolation. Although it could be improved under the concentrated insolation, the conversion efficiency cannot reach the limiting value for triple-junction solar cells. © 2010 Elsevier B.V. All rights reserved. Source

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