1,347 research outputs found
Correct synthesis and integration of compiler-generated function units
PhD ThesisComputer architectures can use custom logic in addition to general pur-
pose processors to improve performance for a variety of applications. The
use of custom logic allows greater parallelism for some algorithms. While
conventional CPUs typically operate on words, ne-grained custom logic
can improve e ciency for many bit level operations. The commodi ca-
tion of eld programmable devices, particularly FPGAs, has improved
the viability of using custom logic in an architecture.
This thesis introduces an approach to reasoning about the correctness of
compilers that generate custom logic that can be synthesized to provide
hardware acceleration for a given application. Compiler intermediate
representations (IRs) and transformations that are relevant to genera-
tion of custom logic are presented. Architectures may vary in the way
that custom logic is incorporated, and suitable abstractions are used in
order that the results apply to compilation for a variety of the design
parameters that are introduced by the use of custom logic
Constraints from Accelerator Experiments on the Elastic Scattering of CMSSM Dark Matter
We explore the allowed ranges of cross sections for the elastic scattering of
neutralinos \chi on nucleons in the constrained minimal supersymmetric
extension of the Standard Model (CMSSM), in which scalar and gaugino masses are
each assumed to be universal at some input grand unification scale. We extend
previous calculations to larger \tan \beta and investigate the limits imposed
by the recent LEP lower limit on the mass of the Higgs boson and by b \to s
\gamma, and those suggested by g_\mu - 2. The Higgs limit and b \to s \gamma
provide upper limits on the cross section, particularly at small and large \tan
\beta, respectively, and the value of g_\mu - 2 suggests a lower limit on the
cross section for \mu > 0. The spin-independent nucleon cross section is
restricted to the range 6 \times 10^{-8} pb > \sigma_{SI} > 2 \times 10^{-10}
pb for \mu > 0, and the spin-dependent nucleon cross section to the range
10^{-5} pb > \sigma_{SD} > 2 \times 10^{-7} pb. Lower values are allowed if \mu
<0.Comment: 15 pages, latex, 18 eps figure
Simulating the mesoscale impacts of sea wall defences on coastal morphology
Solid coastal defences are deployed in many countries to halt or slow coastal erosion. Although the impacts
on local sediment fluxes have been studied in detail, the non-local impact of a modified sediment flux regime
on mesoscale coastal morphology has received less attention. Morphological changes imparted by defensive
structures at these scales (decadal processes over tens of kilometres) can be difficult to quantify or even identify
with field data. Difficulties in assessing the impact of these structures arise in the separation of natural and
anthropogenic influences, both of which can be highly dynamic and non-linear. Numerical modelling allows these
influences to be separated and the impacts of coastal defensive structures to be assessed. We extend previous work
(Barkwith et al., 2013) to explore the influences of sea walls on the evolution and morphological sensitivity of a
pinned, soft-cliff, sandy coastline under a changing wave climate.
The Holderness coast of East Yorkshire, UK, is one of the fastest eroding coastlines in Europe and is used
as a case study for this research. Using a mesoscale numerical coastal evolution model, stochastic wave climate
data are perturbed gradually to assess the sensitivity of the coastal morphology to changing wave climate for both
the defended and natural scenarios. Comparative analysis of the simulated output suggests that sea walls in the
south of the region have a greater impact on sediment flux due to the increased sediment availability along this
part of the coast. Multiple defended structures, including those separated by several kilometres, were found to
interact with each other, producing a complex imprint on coastal morphology under a changing wave climate.
Although spatially and temporally heterogeneous, sea walls generally slowed coastal recession and accumulated
sediment on their up-drift side
The effectiveness of beach mega-nourishment, assessed over three management epochs
Resilient coastal protection requires adaptive management strategies that build with nature to maintain long-term sustainability. With increasing pressures on shorelines from urbanisation, industrial growth, sea-level rise and changing storm climates soft approaches to coastal management are implemented to support natural habitats and maintain healthy coastal ecosystems. The impact of a beach mega-nourishment along a frontage of interactive natural and engineered systems that incorporate soft and hard defences is explored. A coastal evolution model is applied to simulate the impact of different hypothetical mega-nourishment interventions to assess their impacts’ over 3 shoreline management planning epochs: present-day (0–20 years), medium-term (20–50 years) and long-term (50–100 years). The impacts of the smaller interventions when appropriately positioned are found to be as effective as larger schemes, thus making them more cost-effective for present-day management. Over time the benefit from larger interventions becomes more noticeable, with multi-location schemes requiring a smaller initial nourishment to achieve at least the same benefit as that of a single-location scheme. While the longer-term impact of larger schemes reduces erosion across a frontage the short-term impact down drift of the scheme can lead to an increase in erosion as the natural sediment drift becomes interrupted. This research presents a transferable modelling tool to assess the impact of nourishment schemes for a variety of sedimentary shorelines and highlights both the positive and negative impact of beach mega-nourishment
The adsorption of helium atoms on coronene cations
We report the first experimental study of the attachment of multiple foreign
atoms to a cationic polycyclic aromatic hydrocarbon (PAH). The chosen PAH was
coronene, C24H12, which was added to liquid helium nanodroplets and then
subjected to electron bombardment. Using mass spectrometry, coronene cations
decorated with helium atoms were clearly seen and the spectrum shows peaks
with anomalously high intensities (“magic number” peaks), which represent ion-
helium complexes with added stability. The data suggest the formation of a
rigid helium layer consisting of 38 helium atoms that completely cover both
faces of the coronene ion. Additional magic numbers can be seen for the
further addition of 3 and 6 helium atoms, which are thought to attach to the
edge of the coronene. The observation of magic numbers for the addition of 38
and 44 helium atoms is in good agreement with a recent path integral Monte
Carlo prediction for helium atoms on neutral coronene. An understanding of how
atoms and molecules attach to PAH ions is important for a number of reasons
including the potential role such complexes might play in the chemistry of the
interstellar medium
Exploring the sensitivities of crenulate-bay shorelines to wave climates using a new vector-based one-line model
We use a new exploratory model that simulates the evolution of sandy coastlines over decadal to centennial timescales to examine the behavior of crenulate-shaped bays forced by differing directional wave climates. The model represents the coastline as a vector in a Cartesian reference frame, and the shoreface evolves relative to its local orientation, allowing simulation of coasts with high planform-curvature. Shoreline change is driven by gradients in alongshore transport following newly developed algorithms that facilitate dealing with high planform-curvature coastlines. We simulated the evolution of bays from a straight coast between two fixed headlands with no external sediment inputs to an equilibrium condition (zero net alongshore sediment flux) under an ensemble of directional wave climate conditions. We find that planform bay relief increases with obliquity of the mean wave direction, and decreases with the spread of wave directions. Varying bay size over 2 orders of magnitude (0.1–16 km), the model predicts bay shape to be independent of bay size. The time taken for modeled bays to attain equilibrium was found to scale with the square of the distance between headlands, so that, all else being equal, small bays are likely to respond to and recover from perturbations more rapidly (over just a few years) compared to large bays (hundreds of years). Empirical expressions predicting bay shape may be misleading if used to predict their behavior over planning timescales
Complex coastlines responding to climate change: do shoreline shapes reflect present forcing or “remember” the distant past?
A range of planform morphologies emerge along sandy coastlines as a function of offshore wave climate. It has been implicitly assumed that the morphological response time is rapid compared to the timescales of wave climate change, meaning that coastal morphologies simply reflect the extant wave climate. This assumption has been explored by focussing on the response of two distinctive morphological coastlines – flying spits and cuspate capes – to changing wave climates, using a coastline evolution model. Results indicate that antecedent conditions are important in determining the evolution of morphologies, and that sandy coastlines can demonstrate hysteresis behaviour. In particular, antecedent morphology is particularly important in the evolution of flying spits, with characteristic timescales of morphological adjustment on the order of centuries for large spits. Characteristic timescales vary with the square of aspect ratios of capes and spits; for spits, these timescales are an order of magnitude longer than for capes (centuries vs. decades). When wave climates change more slowly than the relevant characteristic timescales, coastlines are able to adjust in a quasi-equilibrium manner. Our results have important implications for the management of sandy coastlines where decisions may be implicitly and incorrectly based on the assumption that present-day coastlines are in equilibrium with current conditions
Calculation of receiver sensitivities in (orthogonal) subcarrier multiplexing microwave-optical links
Microwave-based all-analogue (orthogonal) subcarrier multiplexing (SCM) permits a direct processing of baseband data at Gbit/s while achieving low power consumption, low latency, low cost, and tolerance to dispersion. A key figure of merit in any SCM link is the sensitivity in the receiver, which depends on the transmitter, the link and the receiver. By analysing the impact of the nonlinearities of an optical IQ modulator in the presence of optical noise, sensitivities are mathematically estimated as a function of the optical modulation index (OMI) at the transmitter. The results are verified with simulations achieving a good agreement with the mathematical model. The theoretical model provided can be employed as a tool to predict the best achievable sensitivities and the optimum OMI in broadband SCM and orthogonal SCM links
All-analogue real-time broadband filter bank multicarrier optical communications system
This paper studies the key aspects of an optical link which transmits a broadband microwave filter bank multicarrier (FBMC) signal. The study is presented in the context of creating an all-analogue real-time multigigabit orthogonal frequency division multiplexing electro-optical transceiver for short range and high-capacity data center networks. Passive microwave filters are used to perform the pulse shaping of the bit streams, allowing an orthogonal transmission without the necessity of digital signal processing (DSP). Accordingly, a cyclic prefix that would cause a reduction in the net data rate is not required. An experiment consisting of three orthogonally spaced 2.7 Gbaud quadrature phase shift keyed subchannels demonstrates that the spectral efficiency of traditional DSP-less subcarrier multiplexed links can be potentially doubled. A sensitivity of -29.5 dBm is achieved in a 1-km link
- …