### Browsing by Department "University of California Davis"

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Item Barbalinardo et al. Reply(American Physical Society, 2022-06-24) Barbalinardo, Giuseppe; Chen, Zekun; Dong, Haikuan; Fan, Zheyong; Donadio, Davide; University of California Davis; Multiscale Statistical and Quantum Physics; Department of Applied PhysicsItem Electron–phonon coupling in copper intercalated Bi 2 Se 3(Nature Publishing Group, 2022-07-15) Wiesner, Maciej; Koski, Kristie; Laitinen, Antti; Manninen, Juuso; Zyuzin, Alexander A.; Hakonen, Pertti; Adam Mickiewicz University Poznan; University of California Davis; Centre of Excellence in Quantum Technology, QTF; Department of Applied PhysicsWe report charge and heat transport studies in copper-intercalated topological insulator Bi2Se3 hybrid devices. Measured conductivity shows impact of quantum corrections, electron–electron and electron–phonon interactions. Our shot noise measurements reveal that heat flux displays a crossover between T2 and T4 with the increase of temperature. The results might be explained by a model of inelastic electron scattering on disorder, increasing the role of transverse acoustic phonons in the electron–phonon coupling process.Item Interplay between vertical sectorization and user distribution for urban NB-IoT networks(2022-05-16) Chamain, Lahiru D.; Ilter, Mehmet C.; Hamalainen, Jyri; Ding, Zhi; University of California Davis; School of Electrical Engineering; Department of Communications and NetworkingVertical sectorization introduces considerable gain to particular scenarios where user equipment (UE) are distributed in 3D domain, i.e. high-rise buildings, in terms of coverage and network capacity thanks to availability of active antenna systems. However, due to the huge varieties in distribution of UEs and different physical environments, presenting a comprehensive analytical framework is quite challenging. From this aspect, most available studies on vertical sectorization are limited to present only empirical results. In this paper, we introduce a novel methodology to forecast the performance of NB-IoT systems over urban scenarios. In particular, a logistic distribution-based analytical framework is exploited in order to calculate the group probabilities for each available UE. Based on these probabilities, we propose a scheduling framework with beamforming which improves physical resource block (PRB) utilization by over 50% compared to the case with no scheduling.Item Low-Overhead Joint Beam-Selection and Random-Access Schemes for Massive Internet-of-Things with Non-Uniform Channel and Load(2020-07) Zou, Yihan; Taik Kim, Kwang; Lin, Xiaojun; Chiang, Mung; Ding, Zhi; Wichman, Risto; Hämäläinen, Jyri; Purdue University; University of California Davis; Dept Signal Process and Acoust; Department of Communications and NetworkingWe study low-overhead uplink multi-access algorithms for massive Internet-of-Things (IoT) that can exploit the MIMO performance gain. Although MIMO improves system capacity, it usually requires high overhead due to Channel State Information (CSI) feedback, which is unsuitable for IoT. Recently, a Pseudo-Random Beam-Forming (PRBF) scheme was proposed to exploit the MIMO performance gain for uplink IoT access with uniform channel and load, without collecting CSI at the BS. For non-uniform channel and load, new adaptive beamselection and random-access algorithms are needed to efficiently utilize the system capacity with low overhead. Most existing algorithms for a related multi-channel scheduling problem require each node to at least know some information of the queue length of all contending nodes. In contrast, we propose a new Low-overhead Multi-Channel Joint Channel-Assignment and Random-Access (L-MC-JCARA) algorithm that reduces the overhead to be independent of the number of interfering nodes. A key noveltyis to let the BS estimate the total backlog in each contention group by only observing the random-access events, so that no queue-length feedback is needed from IoT devices. We prove that L-MC-JCARA can achieve at least '0.24'' of the capacity region of the optimal centralized scheduler for the corresponding multi-channel system.Item One Ring to Unite Them All(2015) Pon, Bryan; Seppälä, Timo; Kenney, Martin; University of California Davis; Department of Industrial Engineering and ManagementThis paper examines how recent trends in the smartphone industry may be expanding previous conceptions of the industry and its boundaries. The increasing importance of Internet and cloud-based services—which in many ways lie outside the control of the physical device, operating system, and even the cellular network—seems to be changing the roles and strategies of key firms in the ecosystem. Using industry architecture and platform theory, we examine how the key firms seem to be reacting to these new changes. Our analysis indicates that the platform “bottleneck,” or key control point, is moving away from the device and into the cloud, where a new meta-platform based on the Internet may be emerging.Item Power imbalance induced BER performance loss under limited-feedback CoMP techniques(2016-12-01) Haile, Beneyam B.; Hämäläinen, Jyri; Ding, Zhi; Department of Communications and Networking; University of California DavisCoordinated multipoint (CoMP) technology utilizes simultaneous transmission/reception from/to different access points, and it is considered as an important feature to exploit and/or mitigate intercell interference in fourth-generation mobile networks. Yet, channel power imbalance at the receiver is experienced in CoMP systems due to, e.g., spatially distributed transmissions. Traditional co-located multi-antenna systems may also experience power imbalance among antenna branches due to inaccurate antenna calibration. This paper presents a bit error rate (BER) analysis and derives asymptotic and approximate BER expressions for some practical CoMP transmission techniques under channel power imbalance. Besides the analytical results, numerical analysis is made to thoroughly capture the performance impact of channel power imbalance on the performance gain of the CoMP methods. The results demonstrate that power imbalance considerably affects BER performance and applying long-term amplitude information with fast phase feedback has insignificant benefit to effectively compensate the detrimental effect of large channel power imbalance when base stations use a single antenna. In this case, exploiting both short-term amplitude and phase information is a very good choice. On the contrary, for a large number of diversity antennas in base stations, using long-term amplitude information with a sparsely quantized phase shows BER performance close to the case where full channel state information is applied.Item Thermodynamics of emergent magnetic charge screening in artificial spin ice(2016-09-01) Farhan, Alan; Scholl, Andreas; Petersen, Charlotte F.; Anghinolfi, Luca; Wuth, Clemens; Dhuey, Scott; Chopdekar, Rajesh V.; Mellado, Paula; Alava, Mikko J.; Van Dijken, Sebastiaan; Department of Applied Physics; Lawrence Berkeley National Laboratory; University of Genoa; Daegu Gyeongbuk Institute of Science and Technology; University of California Davis; Universidad Adolfo IbáñezElectric charge screening is a fundamental principle governing the behaviour in a variety of systems in nature. Through reconfiguration of the local environment, the Coulomb attraction between electric charges is decreased, leading, for example, to the creation of polaron states in solids or hydration shells around proteins in water. Here, we directly visualize the real-time creation and decay of screened magnetic charge configurations in a two-dimensional artificial spin ice system, the dipolar dice lattice. By comparing the temperature dependent occurrence of screened and unscreened emergent magnetic charge defects, we determine that screened magnetic charges are indeed a result of local energy reduction and appear as a transient minimum energy state before the system relaxes towards the predicted ground state. These results highlight the important role of emergent magnetic charges in artificial spin ice, giving rise to screened charge excitations and the emergence of exotic low-temperature configurations.Item Uniqueness of nonnegative matrix factorizations by rigidity theory(SIAM PUBLICATIONS, 2021-02) Krone, Robert; Kubjas, Kaie; University of California Davis; Statistics and Mathematical Data Science; Department of Mathematics and Systems AnalysisNonnegative matrix factorizations are often encountered in data mining applications where they are used to explain datasets by a small number of parts. For many of these applications it is desirable that there exists a unique nonnegative matrix factorization up to trivial modifications given by scalings and permutations. This means that model parameters are uniquely identifiable from the data. Rigidity theory of bar and joint frameworks is a field that studies uniqueness of point configurations given some of the pairwise distances. The goal of this paper is to use ideas from rigidity theory to study uniqueness of nonnegative matrix factorizations in the case when nonnegative rank of a matrix is equal to its rank. We characterize infinitesimally rigid nonnegative factorizations, prove that a nonnegative factorization is infinitesimally rigid if and only if it is locally rigid and a certain matrix achieves its maximal possible Kruskal rank, and show that locally rigid nonnegative factorizations can be extended to globally rigid nonnegative factorizations. These results give so far the strongest necessary condition for the uniqueness of a nonnegative factorization. We also explore connections between rigidity of nonnegative factorizations and boundaries of the set of matrices of fixed nonnegative rank. Finally we extend these results from nonnegative factorizations to completely positive factorizations.