542 research outputs found
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Leveraging Backscatter for Ultra-low Power Wireless Sensing Systems
The past few years have seen a dramatic growth in wireless sensing systems, with millions of wirelessly connected sensors becoming first-class citizens of the Internet. The number of wireless sensing devices is expected to surpass 6.75 billion by 2017, more than the world\u27s population as well as the combined market of smartphones, tablets, and PCs. However, its growth faces two pressing challenges: battery energy density and wireless radio power consumption. Battery energy density looms as a fundamental limiting factor due to slow improvements over the past several decades (3x over 22 years). Wireless radio power consumption is another key challenge because high-speed wireless communication is often far more expensive energy-wise than computation, storage and sensing. To make matters worse, wireless sensing devices are generating an increasing amount of data. These challenges raise a fundamental question --- how should we power and communicate with wireless sensing devices. More specifically, instead of using batteries, can we leverage other energy sources to reduce, if not eliminate, the dependence on batteries? Similarly, instead of optimizing existing wireless radios, can we fundamentally change how radios transmit wireless signals to achieve lower power consumption? A promising technique to address these questions is backscatter --- a primitive that enables RF energy harvesting and ultra-low-power wireless communication. Backscatter has the potential to reduce dependence on batteries because it can obtain energy by rectifying the wireless signals transmitted by a backscatter reader. Backscatter can also work by reflecting existing wireless signals (WiFi, BLE) when these are available nearby. Because signal reflection only consumes uWs of power, backscatter can enable ultra-low-power wireless communication. However, the use of backscatter for communicating with wireless sensing devices presents several challenges. First, decreasing RF power across distance limits the operational range of micro-powered backscatter devices. This raises the question of how to maintain a communication link with a backscatter device despite tiny amount of harvested power. Second, even though the backscatter RF front-end is extremely power-efficient, the computational and sensing overhead on backscatter sensors limit its ability to operate with a few micro-Watts of power. Such overhead is a negligible factor of overall power consumption for platforms where radio power consumption is high (e.g. WiFi or Bluetooth based devices). However, it becomes the bottleneck for backscatter based platforms. Third, backscatter readers are not currently deployed in existing indoor environments to provide a continuous carrier for carrying backscattered information. As a result, backscatter deployment is not yet widespread. This thesis addresses these challenges by making the following contributions. First, we design a network stack that enables continuous operation despite decreasing harvested power across distance by employing an OS abstraction --- task fragmentation. We show that such a network stack enables packet transfer even when the whole system is powered by a 3cmx3cm solar panel under natural indoor light condition. Second, we design a hardware architecture that minimizes the computational overhead of backscatter to enable over 1Mbps backscatter transmission while consuming less than 100uWs of power, a two order of magnitude improvement over the state-of-the-art. Finally, we design a system that can leverage both ambient WiFi and BLE signals for backscatter. Our empirical evaluation shows that we can backscatter 500bps data on top of a WiFi stream and 50kbps data on top of a Bluetooth stream when the backscatter device is 3m away from the commercial WiFi and Bluetooth receivers
Understanding Mobile Traffic Patterns of Large Scale Cellular Towers in Urban Environment
Understanding mobile traffic patterns of large scale cellular towers in urban
environment is extremely valuable for Internet service providers, mobile users,
and government managers of modern metropolis. This paper aims at extracting and
modeling the traffic patterns of large scale towers deployed in a metropolitan
city. To achieve this goal, we need to address several challenges, including
lack of appropriate tools for processing large scale traffic measurement data,
unknown traffic patterns, as well as handling complicated factors of urban
ecology and human behaviors that affect traffic patterns. Our core contribution
is a powerful model which combines three dimensional information (time,
locations of towers, and traffic frequency spectrum) to extract and model the
traffic patterns of thousands of cellular towers. Our empirical analysis
reveals the following important observations. First, only five basic
time-domain traffic patterns exist among the 9,600 cellular towers. Second,
each of the extracted traffic pattern maps to one type of geographical
locations related to urban ecology, including residential area, business
district, transport, entertainment, and comprehensive area. Third, our
frequency-domain traffic spectrum analysis suggests that the traffic of any
tower among the 9,600 can be constructed using a linear combination of four
primary components corresponding to human activity behaviors. We believe that
the proposed traffic patterns extraction and modeling methodology, combined
with the empirical analysis on the mobile traffic, pave the way toward a deep
understanding of the traffic patterns of large scale cellular towers in modern
metropolis.Comment: To appear at IMC 201
Quantum Transport Simulation of III-V TFETs with Reduced-Order K.P Method
III-V tunneling field-effect transistors (TFETs) offer great potentials in
future low-power electronics application due to their steep subthreshold slope
and large "on" current. Their 3D quantum transport study using non-equilibrium
Green's function method is computationally very intensive, in particular when
combined with multiband approaches such as the eight-band K.P method. To reduce
the numerical cost, an efficient reduced-order method is developed in this
article and applied to study homojunction InAs and heterojunction GaSb-InAs
nanowire TFETs. Device performances are obtained for various channel widths,
channel lengths, crystal orientations, doping densities, source pocket lengths,
and strain conditions
Hydration Mechanism of Portland Cement Prepared from Stonecoal Vanadium Slag
AbstractThis paper presents an objective study on the utilization of stone coal vanadium slag in preparing cement clinker. The hydrates and hydration mechanism of this cement were analyzed and studied by means of the hydration heat analysis, X-ray diffraction (XRD) and the differential thermal gravity (DTG) analysis. The results of experiments show that the hydration mechanism is similar to ordinary Portland cement. The hydration process can be divided into five stages: (I) initial period; (II) induction period; (III) acceleration period; (IV) deceleration period; (V) final period And the hydrates are basically the same as Portland cement, mainly containing the calcium silicate hydrates (C-S-H), ettringite (AFt), portlandite (CH). It is proved that stone coal vanadium slag can be used as siliceous materials to prepare cement clinker Furthermore, the addition of fine materials such as the waste and fly ash can accelerate cement hydration, which is the result of giving rise to water-to-cementitious ratio. On the other hand, the fine materials may provide the crystal nucleus for hydrates such as portlandite. Using the waste and fly ash to replace part of clinker can prepare series of cement, whose compositions and physical properties are fully complied with the requirements of national standard, and bring huge ecological and economic benefits
Evaporation-triggered microdroplet nucleation and the four life phases of an evaporating Ouzo drop
Evaporating liquid droplets are omnipresent in nature and technology, such as
in inkjet printing, coating, deposition of materials, medical diagnostics,
agriculture, food industry, cosmetics, or spills of liquids. While the
evaporation of pure liquids, liquids with dispersed particles, or even liquid
mixtures has intensively been studied over the last two decades, the
evaporation of ternary mixtures of liquids with different volatilities and
mutual solubilities has not yet been explored. Here we show that the
evaporation of such ternary mixtures can trigger a phase transition and the
nucleation of microdroplets of one of the components of the mixture. As model
system we pick a sessile Ouzo droplet (as known from daily life - a transparent
mixture of water, ethanol, and anise oil) and reveal and theoretically explain
its four life phases: In phase I, the spherical cap-shaped droplet remains
transparent, while the more volatile ethanol is evaporating, preferentially at
the rim of the drop due to the singularity there. This leads to a local ethanol
concentration reduction and correspondingly to oil droplet nucleation there.
This is the beginning of phase II, in which oil microdroplets quickly nucleate
in the whole drop, leading to its milky color which typifies the so-called
'Ouzo-effect'. Once all ethanol has evaporated, the drop, which now has a
characteristic non-spherical-cap shape, has become clear again, with a water
drop sitting on an oil-ring (phase III), finalizing the phase inversion.
Finally, in phase IV, also all water has evaporated, leaving behind a tiny
spherical cap-shaped oil drop.Comment: 40 pages, 12 figure
Evaporating pure, binary and ternary droplets: thermal effects and axial symmetry breaking
The Greek aperitif Ouzo is not only famous for its specific anise-flavored
taste, but also for its ability to turn from a transparent miscible liquid to a
milky-white colored emulsion when water is added. Recently, it has been shown
that this so-called Ouzo effect, i.e. the spontaneous emulsification of oil
microdroplets, can also be triggered by the preferential evaporation of ethanol
in an evaporating sessile Ouzo drop, leading to an amazingly rich drying
process with multiple phase transitions [H. Tan et al., Proc. Natl. Acad. Sci.
USA 113(31) (2016) 8642]. Due to the enhanced evaporation near the contact
line, the nucleation of oil droplets starts at the rim which results in an oil
ring encircling the drop. Furthermore, the oil droplets are advected through
the Ouzo drop by a fast solutal Marangoni flow. In this article, we investigate
the evaporation of mixture droplets in more detail, by successively increasing
the mixture complexity from pure water over a binary water-ethanol mixture to
the ternary Ouzo mixture (water, ethanol and anise oil). In particular,
axisymmetric and full three-dimensional finite element method simulations have
been performed on these droplets to discuss thermal effects and the complicated
flow in the droplet driven by an interplay of preferential evaporation,
evaporative cooling and solutal and thermal Marangoni flow. By using image
analysis techniques and micro-PIV measurements, we are able to compare the
numerically predicted volume evolutions and velocity fields with experimental
data. The Ouzo droplet is furthermore investigated by confocal microscopy. It
is shown that the oil ring predominantly emerges due to coalescence
Pairwise Instance Relation Augmentation for Long-tailed Multi-label Text Classification
Multi-label text classification (MLTC) is one of the key tasks in natural
language processing. It aims to assign multiple target labels to one document.
Due to the uneven popularity of labels, the number of documents per label
follows a long-tailed distribution in most cases. It is much more challenging
to learn classifiers for data-scarce tail labels than for data-rich head
labels. The main reason is that head labels usually have sufficient
information, e.g., a large intra-class diversity, while tail labels do not. In
response, we propose a Pairwise Instance Relation Augmentation Network (PIRAN)
to augment tailed-label documents for balancing tail labels and head labels.
PIRAN consists of a relation collector and an instance generator. The former
aims to extract the document pairwise relations from head labels. Taking these
relations as perturbations, the latter tries to generate new document instances
in high-level feature space around the limited given tailed-label instances.
Meanwhile, two regularizers (diversity and consistency) are designed to
constrain the generation process. The consistency-regularizer encourages the
variance of tail labels to be close to head labels and further balances the
whole datasets. And diversity-regularizer makes sure the generated instances
have diversity and avoids generating redundant instances. Extensive
experimental results on three benchmark datasets demonstrate that PIRAN
consistently outperforms the SOTA methods, and dramatically improves the
performance of tail labels
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