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A New Multicarrier Multicode DS-CDMA Scheme for Time and Frequency Selective Fading Channels
Cao Yewen,Tjhung Tjeng Thiang,Ko Chi Chung The Korea Institute of Information and Commucation 2005 Journal of communications and networks Vol.7 No.1
In this paper, a new multi carrier, direct sequence code division multiple access (MC-DS-CDMA) system is proposed. Our new signal construction is based on convolutional encoding of the transmitted data, serial-to-parallel (S/P) conversion of the encoded data, Walsh-Hadamard-transformation (WHT), a second S/P conversion of the WHT outputs, spread spectrum (SS) modulation with a common pseudo-noise (PN) sequence, and then multicarrier transmission. The system bit error rate (BER) performance in frequency selective fading channel in the presence of additive white Gaussian noise (AWGN) and a jamming tone is analyzed and simulated. The numerical results are compared with those from an orthogonal MC-DS-CDMA system of Sourour and Nakagawa [7]. It is shown that the two systems have almost the same BER performance, but the proposed scheme has better anti-jamming ability.
Maximum Likelihood Receivers for DAPSK Signaling
Xiao Lei,Dong Xiaodai,Tjhung Tjeng T. The Korea Institute of Information and Commucation 2006 Journal of communications and networks Vol.8 No.2
This paper considers the maximum likelihood (ML) detection of 16-ary differential amplitude and phase shift keying (DAPSK) in Rayleigh fading channels. Based on the conditional likelihood function, two new receiver structures, namely ML symbol-by-symbol receiver and ML sequence receiver, are proposed. For the symbol-by-symbol detection, the conventional DAPSK detector is shown to be sub-optimum due to the complete separation in the phase and amplitude detection, but it results in very close performance to the ML detector provided that its circular amplitude decision thresholds are optimized. For the sequence detection, a simple Viterbi algorithm with only two states are adopted to provide an SNR gain around 1 dB on the amplitude bit detection compared with the conventional detector.
Sun, Hainan,Xu, Xiaomin,Hu, Zhiwei,Tjeng, Liu Hao,Zhao, Jie,Zhang, Qin,Lin, Hong-Ji,Chen, Chien-Te,Chan, Ting-Shan,Zhou, Wei,Shao, Zongping The Royal Society of Chemistry 2019 Journal of Materials Chemistry A Vol.7 No.16
<P>If different active sites in a catalyst have optimal binding to different reaction intermediates and short reaction paths among them, they may work cooperatively to enhance the oxygen evolution reaction (OER) activity. Based on this design principle, in this study, we start with a B-site ordered double perovskite Sr2FeMoO6−δ with poor OER activity as the host material to fulfill the requirement of a short pathway, and then, replace Mo with Ni and Fe with Co to optimize the synergistic interplay of the multi-active sites. Replacing Mo with Ni indeed dramatically enhances the OER activity and structural/operating stability. Further improvement in OER performance is realized by partial substitution of Fe with Co, leading to the development of a material with the nominal composition of Sr2Fe0.8Co0.2Mo0.65Ni0.35O6−δ, which outperforms the noble metal oxide IrO2 and is better than most of the electrocatalysts developed based on a single descriptor, such as Ba0.5Sr0.5Co0.8Fe0.2O3−δ (eg occupancy close to unity), PrBaCo2O5+δ (O 2p-band center relative to the Fermi level), and La0.5Sr0.5CoO3−δ (charge-transfer energy) in many aspects. As a universal method, combined structural and compositional tuning to create a cooperative effect among different active sites for intermediate adsorption and reaction in an ordered structure may provide a new way for the design of superior electrocatalysts for various applications.</P>
Hybridization gap and Fano resonance in SmB<sub>6</sub>
Rö,ßler, Sahana,Jang, Tae-Hwan,Kim, Dae-Jeong,Tjeng, L. H.,Fisk, Zachary,Steglich, Frank,Wirth, Steffen National Academy of Sciences 2014 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF Vol.111 No.13
<P>Hybridization between conduction electrons and the strongly interacting <I>f</I>-electrons in rare earth or actinide compounds may result in new states of matter. Depending on the exact location of the concomitant hybridization gap with respect to the Fermi energy, a heavy fermion or an insulating ground state ensues. To study this entanglement locally, we conducted scanning tunneling microscopy and spectroscopy (STS) measurements on the “Kondo insulator” SmB<SUB>6</SUB>. The vast majority of surface areas investigated were reconstructed, but infrequently, patches of varying sizes of nonreconstructed Sm- or B-terminated surfaces also were found. On the smallest patches, clear indications for the hybridization gap with logarithmic temperature dependence (as expected for a Kondo system) and for intermultiplet transitions were observed. On nonreconstructed surface areas large enough for coherent cotunneling, we were able to observe clear-cut Fano resonances. Our locally resolved STS indicated considerable finite conductance on all surfaces independent of their structure, not proving but leaving open the possibility of the existence of a topologically protected surface state.</P>
A Unified Framework for Transmitter Power Control in Cellular Radio Systems
Chin Choy Chai,Ying Lu,Yong Huat Chew,Tjeng Thiang Tjhung 한국전자통신연구원 2004 ETRI Journal Vol.26 No.5
This paper investigates the impacts of array weight errors (AWE) in an antenna array (AA) on a parallel interference cancellation (PIC) receiver in uplink synchronous and asynchronous direct sequence code division multiple access (DS-CDMA) systems. The performance degradation due to an AWE, which is approximated by a Gaussian distributed random variable, is estimated as a function of the variance of the AWE. Theoretical analysis, confirmed by simulation, demonstrates the tradeoffs encountered between system parameters such as the number of antennas and the variance of the AWE in terms of the achievable average bit error rate and the user capacity. Numerical results show that the performance of the PIC with the AA in the DS- CDMA uplink is sensitive to the AWE. However, either a larger number of antennas or uplink synchronous transmissions have the potential of reducing the overall sensitivity, and thus improving its performance.
Sun, Hainan,Hu, Zhiwei,Xu, Xiaomin,He, Juan,Dai, Jie,Lin, Hong-Ji,Chan, Ting-Shan,Chen, Chien-Te,Tjeng, Liu Hao,Zhou, Wei,Shao, Zongping American Chemical Society 2019 Chemistry of materials Vol.31 No.15
<P>The development of cost-effective, non-noble metal electrocatalysts for the oxygen evolution reaction (OER) is of paramount importance for sustainable technologies. Efficient screening strategies for electrocatalysts can greatly increase the commercialization speed of these advanced technologies. Here, ternary phase diagrams with large-scale tuning and designated-scale tuning strategies are applied for the first time to provide a new method for screening perovskite oxide-based electrocatalysts for OERs. Specifically, the family of double perovskites (Sr<SUB>2</SUB>Fe<SUB>1.5-<I>x</I>-<I>y</I></SUB>Co<SUB><I>x</I></SUB>Ni<SUB><I>y</I></SUB>Mo<SUB>0.5</SUB>O<SUB>6−δ</SUB>, 0 ≤ <I>x</I>, <I>y</I> ≤ 1.5) was utilized to understand the role of transition metals in perovskite oxides. Ternary phase diagrams can facilitate a rapid screening process, provide a straightforward relationship between phase structures and catalytic activities, and help to confirm the effects of various combinations of transition metals on the OER activity. The Fe-Co system (Sr<SUB>2</SUB>Fe<SUB>1.5-<I>x</I></SUB>Co<SUB><I>x</I></SUB>Mo<SUB>0.5</SUB>O<SUB>6−δ</SUB>) improves the catalytic activities, as demonstrated by the reduced Tafel slope and enhanced stability, while the Fe-Ni system (Sr<SUB>2</SUB>Fe<SUB>1.5-<I>y</I></SUB>Ni<SUB><I>y</I></SUB>Mo<SUB>0.5</SUB>O<SUB>6−δ</SUB>) improves the surface kinetic properties of the OER, as demonstrated by its reduced overpotential. Significantly, the Co, Ni, and Fe ternary phase systems can serve as the synergistic coactive sites (Sr<SUB>2</SUB>Fe<SUB>1.5-<I>x</I>-<I>y</I></SUB>Co<SUB><I>x</I></SUB>Ni<SUB><I>y</I></SUB>Mo<SUB>0.5</SUB>O<SUB>6−δ</SUB>) to catalyze the OER, resulting in an improved overall OER performance. This systematic study not only demonstrates a new strategy to allow the rapid screening of double perovskite OER catalysts based on large-scale tuning and designated-scale tuning strategies but, more importantly, also provides an insightful understanding of the use of multitransition metal-based double perovskites for catalysis of the OER.</P> [FIG OMISSION]</BR>
Agrestini, S.,Kuo, C.-Y.,Moretti Sala, M.,Hu, Z.,Kasinathan, D.,Ko, K.-T.,Glatzel, P.,Rossi, M.,Cafun, J.-D.,Kvashnina, K. O.,Matsumoto, A.,Takayama, T.,Takagi, H.,Tjeng, L. H.,Haverkort, M. W. American Physical Society 2017 Physical review. B Vol.95 No.20
<P>We have investigated the electronic structure of Sr2IrO4 using core-to-core resonant inelastic x-ray scattering. The experimental spectra can be well reproduced using ab initio density functional theory based multiplet ligand field theory calculations, thereby validating these calculations. We found that the low-energy, effective Ir t(2g) orbitals are practically degenerate in their crystal-field energy. We uncovered that Sr2IrO4 and iridates in general are negative charge transfer systems with large covalency and a substantial oxygen ligand hole character in the Ir t(2g) Wannier orbitals. This has far reaching consequences, as not only the on-site crystal-field energies are determined by the long-range crystal structure, but, more significantly, magnetic exchange interactions will have long-range distance dependent anisotropies in the spin direction. These findings set constraints and show pathways for the design of d(5) materials that can host compasslike magnetic interactions.</P>
Bulk and surface electronic properties of SmB6 : A hard x-ray photoelectron spectroscopy study
Utsumi, Y.,Kasinathan, D.,Ko, K.-T.,Agrestini, S.,Haverkort, M. W.,Wirth, S.,Wu, Y-H.,Tsuei, K-D.,Kim, D-J.,Fisk, Z.,Tanaka, A.,Thalmeier, P.,Tjeng, L. H. American Physical Society 2017 Physical Review B Vol.96 No.15
Electronic structure and evolution of the orbital state in metallicCa2−xSrxRuO4
Noh, Han-Jin,Oh, S.-J.,Park, B.-G.,Park, J.-H.,Kim, J.-Y.,Kim, H.-D.,Mizokawa, T.,Tjeng, L. H.,Lin, H.-J.,Chen, C. T.,Schuppler, S.,Nakatsuji, S.,Fukazawa, H.,Maeno, Y. American Physical Society 2005 Physical review. B, Condensed matter and materials Vol.72 No.5
Electronically highly cubic conditions for Ru in α−RuCl3
Agrestini, S.,Kuo, C.-Y.,Ko, K.-T.,Hu, Z.,Kasinathan, D.,Vasili, H. B.,Herrero-Martin, J.,Valvidares, S. M.,Pellegrin, E.,Jang, L.-Y.,Henschel, A.,Schmidt, M.,Tanaka, A.,Tjeng, L. H. American Physical Society 2017 Physical Review B Vol.96 No.16