Asymptotic Capacity of Large Relay Networks with Conferencing Links

In this correspondence, we consider a half-duplex large relay network, which consists of one source-destination pair and $N$ relay nodes, each of which is connected with a subset of the other relays via signal-to-noise ratio (SNR)-limited out-of-band conferencing links. The asymptotic achievable rates of two basic relaying schemes with the "$p$-portion" conferencing strategy are studied: For the decode-and-forward (DF) scheme, we prove that the DF rate scales as $\mathcal{O} (\log (N))$; for the amplify-and-forward (AF) scheme, we prove that it asymptotically achieves the capacity upper bound in some interesting scenarios as $N$ goes to infinity.Comment: submitted to IEEE Transactions on Communication

Asymptotic Capacity of Large Fading Relay Networks with Random Node Failures

To understand the network response to large-scale physical attacks, we investigate the asymptotic capacity of a half-duplex fading relay network with random node failures when the number of relays $N$ is infinitely large. In this paper, a simplified independent attack model is assumed where each relay node fails with a certain probability. The noncoherent relaying scheme is considered, which corresponds to the case of zero forward-link channel state information (CSI) at the relays. Accordingly, the whole relay network can be shown equivalent to a Rayleigh fading channel, where we derive the $\epsilon$-outage capacity upper bound according to the multiple access (MAC) cut-set, and the $\epsilon$-outage achievable rates for both the amplify-and-forward (AF) and decode-and-forward (DF) strategies. Furthermore, we show that the DF strategy is asymptotically optimal as the outage probability $\epsilon$ goes to zero, with the AF strategy strictly suboptimal over all signal to noise ratio (SNR) regimes. Regarding the rate loss due to random attacks, the AF strategy suffers a less portion of rate loss than the DF strategy in the high SNR regime, while the DF strategy demonstrates more robust performance in the low SNR regime.Comment: 24 pages, 5 figures, submitted to IEEE Transactions on Communication

Efficient satisfiability solver

The past few decades saw great improvements in the performance of satisfiability (SAT) solvers. In this thesis, we discuss the state-of-the-art techniques used in building an efficient SAT solver. Modern SAT solvers are mainly constituted by the following components: decision heuristics, Boolean constraint propagation, conflict analysis, restart, clause deletion and preprocessing. Various algorithms and implementations in each component will be discussed and analyzed. Then we propose a new backtracking strategy, partial backtracking, which can be easily implemented in SAT solvers. It is essentially an extension of the backtracking strategy used in most SAT solvers. With partial backtracking, the solver consecutively amends the variable assignments instead of discarding them completely so that it does not backtrack as many levels as the classic strategy does after analyzing a conflict. We implemented this strategy in our solver Nigma and the experiments show that the solver benefits from this adjustment

Witness generation in existential CTL model checking

Hardware and software systems are widely used in applications where failure is prohibitively costly or even unacceptable. The main obstacle to make such systems more reliable and capable of more complex and sensitive tasks is our limited ability to design and implement them with sufficiently high degree of confidence in their correctness under all circumstances. As an automated technique that verifies the system early in the design phase, model checking explores the state space of the system exhaustively and rigorously to determine if the system satisfies the specifications and detect fatal errors that may be missed by simulation and testing. One essential advantage of model checking is the capability to generate witnesses and counterexamples. They are simple and straightforward forms to prove an existential specification or falsify a universal specification. Beside enhancing the credibility of the model checker\u27s conclusion, they either strengthen engineers\u27 confidence in the system or provide hints to reveal potential defects. In this dissertation, we focus on symbolic model checking with specifications expressed in computation tree logic (CTL), which describes branching-time behaviors of the system, and investigate the witness generation techniques for the existential fragment of CTL, i.e., ECTL, covering both decision-diagram-based and SAT-based. Since witnesses provide important debugging information and may be inspected by engineers, smaller ones are always preferable to ease their interpretation and understanding. To the best of our knowledge, no existing witness generation technique guarantees the minimality for a general ECTL formula with nested existential CTL operators. One contribution of this dissertation is to fill this gap with the minimality guarantee. With the help of the saturation algorithm, our approach computes the minimum witness size for the given ECTL formula in every state, stored as an additive edge-valued multiway decision diagrams (EV+MDD), a variant of the well-known binary decision diagram (BDD), and then builds a minimum witness. Though computationally intensive, this has promising applications in reducing engineers\u27 workload. SAT-based model checking, in particular, bounded model checking, reduces a model checking problem problem into a satisfiability problem and leverages a SAT solver to solve it. Another contribution of this dissertation is to improve the translation of bounded semantics of ECTL into propositional formulas. By realizing the possibility of path reuse, i.e., a state may build its own witness by reusing its successor\u27s, we may generate a significantly smaller formula, which is often easier for a SAT solver to answer, and thus boost the performance of bounded model checking

The Top Quark Production Asymmetries AFBtA_{FB}^t and AFBℓA_{FB}^{\ell}

A large forward-backward asymmetry is seen in both the top quark rapidity distribution $A_{FB}^t$ and in the rapidity distribution of charged leptons $A_{FB}^\ell$ from top quarks produced at the Tevatron. We study the kinematic and dynamic aspects of the relationship of the two observables arising from the spin correlation between the charged lepton and the top quark with different polarization states. We emphasize the value of both measurements, and we conclude that a new physics model which produces more right-handed than left-handed top quarks is favored by the present data.Comment: accepted for publication in Physical Review Letter

Progressive amorphization of GeSbTe phase-change material under electron beam irradiation

Fast and reversible phase transitions in chalcogenide phase-change materials (PCMs), in particular, Ge-Sb-Te compounds, are not only of fundamental interests, but also make PCMs based random access memory (PRAM) a leading candidate for non-volatile memory and neuromorphic computing devices. To RESET the memory cell, crystalline Ge-Sb-Te has to undergo phase transitions firstly to a liquid state and then to an amorphous state, corresponding to an abrupt change in electrical resistance. In this work, we demonstrate a progressive amorphization process in GeSb2Te4 thin films under electron beam irradiation on transmission electron microscope (TEM). Melting is shown to be completely absent by the in situ TEM experiments. The progressive amorphization process resembles closely the cumulative crystallization process that accompanies a continuous change in electrical resistance. Our work suggests that if displacement forces can be implemented properly, it should be possible to emulate symmetric neuronal dynamics by using PCMs

Empirical Study About the Motivations for Using Multifunctional Social Media: Based upon the Uses and Gratifications Theory

Social media is a kind of online platform which used by people to share contents and contact each other via virtual communities and networks. With the development of information technology, it becomes neoteric and multifunctional. In order to examine what factors affecting individuals\u27 usage intentions for the various functions of social media, we proposed a model based on the uses and gratifications theory, this theory indicates there are three types of motivations, named as social motivation, utilitarian motivation and hedonic motivation, then six factors derived from the three motivations are presented. We collected 404 samples from the QQ users from 77 cities in China, with statistical analysis, the result shows that four of them can significantly affect individuals\u27 usage intentions of QQ\u27 various functions beyond as an instant messenger. This study extends current understanding about the usage motivations of social media and it can be a reference for future research in this field