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Nan Liu

Publications and source records attributed to Nan Liu.

At least 163 records · Page 9Linked to original sources
arXiv2018

New constraints on the major neutron source in low-mass AGB stars

We compare updated Torino postprocessing asymptotic giant branch (AGB) nucleosynthesis model calculations with isotopic compositions of mainstream SiC dust grains from low-mass AGB stars. Based on the data-model comparison, we provide new constraints on the major neutron source, 13C(α,n)16O in the He-intershell, for the s-process. We show that the literature Ni, Sr, and Ba grain data can only be consistently explained by the Torino model calculations that adopt the recently proposed magnetic-buoyancy-induced 13C-pocket. This observation provides strong support to the suggestion of deep mixing of H into the He-intershell at low 13C concentrations as a result of efficient transport of H through magnetic tubes.

astro-ph.SR↗
arXiv2018

The Economics of Video Websites with Membership-Advertising Mode in Wireless Networks

In this paper, we consider a novel business model of video websites via Membership-Advertising Mode in wireless network, where the video websites provide three video services for mobile users: \textit{VIP-Member} service, Regular-Member service and Non-Member service. The VIP-Member (Regular-Member) service provides the highest level (middle level) quality and non-advertising video service with high (low) price, while the Non-Member service provides the lowest level quality and advertising-containing video service for free. Meanwhile, the video websites sell their advertising spaces to the advertiser to create extra revenues. We formulate the interactions among the advertiser, video websites and mobile users as a three-stage Stackelberg game. Specifically, in Stage I, the advertiser decides the advertising budget; in Stage II, the video websites determine their advertising spaces selling strategies for advertiser and the membership pricing strategies for mobile users; in Stage III, the mobile users make their own decisions on video watching strategies for each video website. We analyze the equilibrium of each sub-game. Particularly, we derive the closed-form solutions of each mobile user's optimal video watching strategies, each video website's optimal membership price and the optimal advertising spaces selling number. In addition, we also investigate the piece-wise structure of the advertiser's utility function, and further propose an efficient algorithm to obtain the optimal advertising budget. Finally, numerical results show the impacts of different parameters' values on each entity's utility as well as the key indicators.

cs.NI↗
arXiv2018

Power-and Rate-Adaptation Improves the Effective Capacity of C-RAN for Nakagami-$m$ Fading Channels

We propose a power-and rate-adaptation scheme for cloud radio access networks (C-RANs), where each radio remote head (RRH) is connected to the baseband unit (BBU) pool through optical links. The RRHs jointly support the users by efficiently exploiting the enhanced spatial degrees of freedom. Our proposed scheme aims for maximizing the effective capacity (EC) of the user subject to both per-RRH average-and peak-power constraints, where the EC is defined as the maximum arrival rate that can be supported by the C-RAN under the statistical delay requirement. We first transform the EC maximization problem into an equivalent convex optimization problem. By using the Lagrange dual decomposition method and solving the Karush-Kuhn-Tucker (KKT) equations, the optimal transmission power of each RRH can be obtained in closed-form. Furthermore, an online tracking method is provided for approximating the average power of each RRH for the sake of updating the Lagrange dual variables. For the special case of two RRHs, the expression of the average power of each RRH can be calculated in explicit form. Hence, the Lagrange dual variables can be computed in advance in this special case. Furthermore, we derive the power allocation for two important extreme cases: 1) no delay constraint; 2) extremely stringent delay-requirements. Our simulation results show that the proposed scheme significantly outperforms the conventional algorithm without considering the delay requirements. Furthermore, when appropriately tuning the value of the delay exponent, our proposed algorithm is capable of guaranteeing a delay outage probability below $10^{-9}$ when the maximum tolerable delay is 1 ms. This is suitable for the future ultra-reliable low latency communications (URLLC).

cs.IT↗
arXiv2018

Extremely $^{54}$Cr- and $^{50}$Ti-rich presolar oxide grains in a primitive meteorite: Formation in rare types of supernovae and implications for the astrophysical context of solar system birth

We report the identification of 19 presolar oxide grains from the Orgueil CI meteorite with substantial enrichments in $^{54}$Cr, with $^{54}$Cr/$^{52}$Cr ratios ranging from 1.2 to 56 times the solar value. The most enriched grains also exhibit enrichments at mass 50, most likely due in part to $^{50}$Ti, but close-to-normal or depleted $^{53}$Cr/$^{52}$Cr ratios. There is a strong inverse relationship between $^{54}$Cr enrichment and grain size; the most extreme grains are all <80 nm in diameter. Comparison of the isotopic data with predictions of nucleosynthesis calculations indicate that these grains most likely originated in either rare, high-density Type Ia supernovae (SNIa), or in electron-capture supernovae (ECSN) which may occur as the end stage of evolution for stars of mass 8-10 M$_\odot$. This is the first evidence for preserved presolar grains from either type of supernova. An ECSN origin is attractive since these likely occur much more frequently than high-density SNIa, and their evolutionary timescales ($\sim$20 Myr) are comparable to those of molecular clouds. Self-pollution of the Sun's parent cloud from an ECSN may explain the heterogeneous distribution of n-rich isotopic anomalies in planetary materials, including a recently reported dichotomy in Mo isotopes in the solar system. The stellar origins of three grains with solar $^{54}$Cr/$^{52}$Cr, but anomalies in $^{50}$Cr or $^{53}$Cr, as well as of a grain enriched in $^{57}$Fe, are unclear.

astro-ph.EP↗
arXiv2018

Long-time asymptotics for the Hirota equation on the half-line

We consider the Hirota equation on the quarter plane with the initial and boundary values belonging to the Schwartz space. The goal of this paper is to study the long-time behavior of the solution of this initial-boundary value problem based on the asymptotic analysis of an associated matrix Riemann--Hilbert problem.

math.AP↗
arXiv2018

Common Occurrence of Explosive Hydrogen Burning in Type II Supernovae

We report Mo isotopic data for 16 15N-rich presolar SiC grains of type AB (14N/15N solar), thus providing further support to the division of the two AB subgroups recently proposed by Liu et al. (2017a), who showed that AB1 grains most likely originated from Type II supernovae (SNe) with explosive H burning. Comparison of the Sr, Mo, and Ba isotopic compositions of the AB1 grains with SN model predictions indicates that the s-process isotopic compositions of AB1 grains resulted from neutron-capture processes occurring during the progenitor massive stars' pre-SN evolution rather than from an explosive neutron-capture process. In addition, the observations of (1) explosive H burning occurring in the C-rich regions of the progenitor SNe of SN X grains as suggested by the isotopic compositions of X grains, and (2) explosive H burning occurring both at the bottom of the He/C zone and at the top of the He/N zone as suggested by model simulations, imply that explosive H burning is a common phenomenon in outer SN zones.

astro-ph.SR↗
arXiv2018

Late formation of silicon carbide in type II supernovae

We have found that individual presolar silicon carbide (SiC) dust grains from supernovae show a positive correlation between 49Ti and 28Si excesses, which is attributed to the radioactive decay of the short-lived (t1/2=330 day) 49V to 49Ti in the inner highly 28Si-rich Si/S zone. The 49V-49Ti chronometer constrains that these supernova SiC dust grains formed at least two years after their parent stars exploded. This result supports recent dust condensation calculations that predict a delayed formation of carbonaceous and SiC grains in supernovae. The astronomical observation of continuous buildup of dust in supernovae over several years can, therefore, be interpreted as a growing addition of C-rich dust to the dust reservoir in supernovae.

astro-ph.SR↗
arXiv2017

Managing Appointment Booking under Customer Choices

Motivated by the increasing use of online appointment booking platforms, we study how to offer appointment slots to customers in order to maximize the total number of slots booked. We develop two models, non-sequential offering and sequential offering, to capture different types of interactions between customers and the scheduling system. In these two models, the scheduler offers either a single set of appointment slots for the arriving customer to choose from, or multiple sets in sequence, respectively. For the non-sequential model, we identify a static randomized policy which is asymptotically optimal when the system demand and capacity increase simultaneously, and we further show that offering all available slots at all times has a constant factor of 2 performance guarantee. For the sequential model, we derive a closed-form optimal policy for a large class of instances and develop a simple, effective heuristic for those instances without an explicit optimal policy. By comparing these two models, our study generates useful operational insights for improving the current appointment booking processes. In particular, our analysis reveals an interesting equivalence between the sequential offering model and the non-sequential offering model with perfect customer preference information. This equivalence allows us to apply sequential offering in a wide range of interactive scheduling contexts. Our extensive numerical study shows that sequential offering can significantly improve the slot fill rate (6-8% on average and up to 18% in our testing cases) compared to non-sequential offering.

math.OC↗
arXiv2017

Initial-boundary value problem and long-time asymptotics for the Kundu--Eckhaus equation on the half-line

The initial-boundary value problem for the Kundu--Eckhaus equation on the half-line is considered in this paper by using the Fokas method. We will show that the solution $u(x,t)$ can be expressed in terms of the solution of a matrix Riemann--Hilbert problem formulated in the complex $k$-plane. Furthermore, based on a nonlinear steepest descent analysis of the associated Riemann--Hilbert problem, we can give the precise asymptotic formulas for the solution of the Kundu--Eckhaus equation on the half-line.

math.AP↗
arXiv2017

Determination of Effective Synaptic Conductances Using Somatic Voltage Clamp

The interplay between excitatory and inhibitory neurons imparts rich functions of the brain. To understand the underlying synaptic mechanisms, a fundamental approach is to study the dynamics of excitatory and inhibitory conductances of each neuron. The traditional method of determining conductance employs the synaptic current-voltage (I-V) relation obtained via voltage clamp. Using theoretical analysis, electrophysiological experiments, and realistic simulations, here we demonstrate that the traditional method conceptually fails to measure the conductance due to the neglect of a nonlinear interaction between the clamp current and the synaptic current. Consequently, it incurs substantial measurement error, even giving rise to unphysically negative conductance as observed in experiments. To elucidate synaptic impact on neuronal information processing, we introduce the concept of effective conductance and propose a framework to determine it accurately. Our work suggests re-examination of previous studies involving conductance measurement and provides a reliable approach to assess synaptic influence on neuronal computation.

q-bio.NC↗
arXiv2017

Random Caching in Backhaul-Limited Multi-Antenna Networks: Analysis and Area Spectrum Efficiency Optimization

Caching at base stations is a promising technology to satisfy the increasing capacity requirements and reduce the backhaul loads in future wireless networks. Careful design of random caching can fully exploit the file popularity and achieve good performance. However, previous works on random caching scheme usually assumed single antenna at BSs and users, which is not the case in practical multi-antenna networks. In this paper, we consider the analysis and optimization in the cache-enabled multi-antenna networks with limited backhaul. We first derive a closed-form expression and a simple tight upper bound of the successful transmission probability, using tools from stochastic geometry and a gamma approximation. Based on the analytic results, we then consider the area spectrum efficiency maximization by optimizing design parameters, which is a complicated mixed-integer optimization problem. After analyzing the optimal properties, we obtain a local optimal solution with lower complexity. To further simplify the optimization, we then solve an asymptotic optimization problem in the high user density region, using the upper bound as the objective function. Numerical simulations show that the asymptotic optimal caching scheme achieves better performance over existing caching schemes. The analysis and optimization results provide insightful design guidelines for random caching in practical networks.

cs.IT↗
arXiv2017

J-type carbon stars: A dominant source of 14N-rich presolar SiC grains of type AB

We report Mo isotopic data of 27 new presolar SiC grains, including 12 14N-rich AB (14N/15N>440, AB2) and 15 mainstream (MS) grains, and their correlated Sr and Ba isotope ratios when available. Direct comparison of the data for the MS grains, which came from low-mass asymptotic giant branch (AGB) stars with large s-process isotope enhancements, with the AB2 grain data demonstrates that AB2 grains show near-solar isotopic compositions and lack s-process enhancements. The near-normal Sr, Mo, and Ba isotopic compositions of AB2 grains clearly exclude born-again AGB stars, where the intermediate neutron-capture process (i-process) takes place, as their stellar source. On the other hand, low-mass CO novae, and early R- and J-type carbon stars show 13C and 14N excesses but no s-process enhancements and are thus potential stellar sources of AB2 grains. Since both early R-type carbon stars and CO novae are rare objects, the abundant J-type carbon stars (10-15% of all carbon stars) are thus likely to be a dominant source of AB2 grains.

astro-ph.SR↗
arXiv2017

Learning without Prejudice: Avoiding Bias in Webly-Supervised Action Recognition

Webly-supervised learning has recently emerged as an alternative paradigm to traditional supervised learning based on large-scale datasets with manual annotations. The key idea is that models such as CNNs can be learned from the noisy visual data available on the web. In this work we aim to exploit web data for video understanding tasks such as action recognition and detection. One of the main problems in webly-supervised learning is cleaning the noisy labeled data from the web. The state-of-the-art paradigm relies on training a first classifier on noisy data that is then used to clean the remaining dataset. Our key insight is that this procedure biases the second classifier towards samples that the first one understands. Here we train two independent CNNs, a RGB network on web images and video frames and a second network using temporal information from optical flow. We show that training the networks independently is vastly superior to selecting the frames for the flow classifier by using our RGB network. Moreover, we show benefits in enriching the training set with different data sources from heterogeneous public web databases. We demonstrate that our framework outperforms all other webly-supervised methods on two public benchmarks, UCF-101 and Thumos'14.

cs.CV↗
arXiv2017

Stellar origin of 15N-rich presolar SiC grains of type AB: supernovae with explosive hydrogen burning

We report C, N, and Si isotopic data for 59 highly 13C-enriched presolar submicron- to micron-sized SiC grains from the Murchison meteorite, including eight putative nova grains (PNGs) and 29 15N-rich (14N/15N<=solar) AB grains, and their Mg-Al, S, and Ca-Ti isotope data when available. These 37 grains are enriched in 13C, 15N and 26Al with the PNGs showing more extreme enhancements. The 15N-rich AB grains show systematically higher 26Al and 30Si excesses than the 14N-rich AB grains. Thus, we propose to divide the AB grains into groups 1 (14N/15N =solar). For the first time, we have obtained both S and Ti isotopic data for five AB1 grains and one PNG, and found 32S and/or 50Ti enhancements. Interestingly, one AB1 grain had the largest 32S and 50Ti excesses, strongly suggesting a neutron-capture nucleosynthetic origin of the 32S excess and thus the initial presence of radiogenic 32Si (t1/2=153 yr). More importantly, we found that the 15N and 26Al excesses of AB1 grains form a trend that extends to the region in the N-Al isotope plot occupied by C2 grains, strongly indicating a common stellar origin for both AB1 and C2 grains. Comparison of supernova models with the AB1 and C2 grain data indicates that these grains came from SNe that experienced H ingestion into the He/C zones of their progenitors.

astro-ph.SR↗
arXiv2016

An Upper Bound on the Sum Capacity of the Downlink Multicell Processing with Finite Backhaul Capacity

In this paper, we study upper bounds on the sum capacity of the downlink multicell processing model with finite backhaul capacity for the simple case of 2 base stations and 2 mobile users. It is modelled as a two-user multiple access diamond channel. It consists of a first hop from the central processor to the base stations via orthogonal links of finite capacity, and the second hop from the base stations to the mobile users via a Gaussian interference channel. The converse is derived using the converse tools of the multiple access diamond channel and that of the Gaussian MIMO broadcast channel. Through numerical results, it is shown that our upper bound improves upon the existing upper bound greatly in the medium backhaul capacity range, and as a result, the gap between the upper bounds and the sum rate of the time-sharing of the known achievable schemes is significantly reduced.

cs.IT↗
arXiv2016

Stellar Origins of Extremely $^{\text{13}}C$- and $^{15}N$-enriched Presolar SiC Grains: Novae or Supernovae?

Extreme excesses of $^{13}C$ ($^{12}C$/$^{13}C$<10) and $^{15}N$ ($^{14}N$/$^{15}N$<20) in rare presolar SiC grains have been considered diagnostic of an origin in classical novae, though an origin in core collapse supernovae (CCSNe) has also been proposed. We report C, N, and Si isotope data for 14 submicron- to micron-sized $^{13}C$- and $^{15}N$-enriched presolar SiC grains ($^{12}C$/$^{13}C$<16 and $^{14}N$/$^{15}N$<~100) from Murchison, and their correlated Mg-Al, S, and Ca-Ti isotope data when available. These grains are enriched in $^{13}C$ and $^{15}N$, but with quite diverse Si isotopic signatures. Four grains with $^{29,30}Si$ excesses similar to those of type C SiC grains likely came from CCSNe, which experienced explosive H burning occurred during explosions. The independent coexistence of proton- and neutron-capture isotopic signatures in these grains strongly supports heterogeneous H ingestion into the He shell in pre-supernovae. Two of the seven putative nova grains with $^{30}Si$ excesses and $^{29}Si$ depletions show lower-than-solar $^{34}S$/$^{32}S$ ratios that cannot be explained by classical nova nucleosynthetic models. We discuss these signatures within the CCSN scenario. For the remaining five putative nova grains, both nova and supernova origins are viable because explosive H burning in the two stellar sites could result in quite similar proton-capture isotopic signatures. Three of the grains are sub-type AB grains that are also $^{13}C$ enriched, but have a range of higher $^{14}N$/$^{15}N$. We found that $^{15}N$-enriched AB grains (~50<$^{14}N$/$^{15}N$<~100) have distinctive isotopic signatures compared to putative nova grains, such as higher $^{14}N$/$^{15}N$, lower $^{26}Al$/$^{27}Al$, and lack of $^{30}Si$ excess, indicating weaker proton-capture nucleosynthetic environments.

astro-ph.SR↗
arXiv2016

Energy Efficient Transmission for Multicast Services in MISO Distributed Antenna Systems

This paper aims to solve the energy efficiency (EE) maximization problem for multicast services in a multiple-input single-output (MISO) distributed antenna system (DAS). A novel iterative algorithm is proposed, which consists of solving two subproblems iteratively: the power allocation problem and the beam direction updating problem. The former subproblem can be equivalently transformed into a one-dimension quasi-concave problem that is solved by the golden search method. The latter problem can be efficiently solved by the existing method. Simulation results show that the proposed algorithm achieves significant EE performance gains over the existing rate maximization method. In addition, when the backhaul power consumption is low, the EE performance of the DAS is better than that of the centralized antenna system (CAS).

cs.IT↗