21,895 research outputs found
5G 3GPP-like Channel Models for Outdoor Urban Microcellular and Macrocellular Environments
For the development of new 5G systems to operate in bands up to 100 GHz,
there is a need for accurate radio propagation models at these bands that
currently are not addressed by existing channel models developed for bands
below 6 GHz. This document presents a preliminary overview of 5G channel models
for bands up to 100 GHz. These have been derived based on extensive measurement
and ray tracing results across a multitude of frequencies from 6 GHz to 100
GHz, and this document describes an initial 3D channel model which includes: 1)
typical deployment scenarios for urban microcells (UMi) and urban macrocells
(UMa), and 2) a baseline model for incorporating path loss, shadow fading, line
of sight probability, penetration and blockage models for the typical
scenarios. Various processing methodologies such as clustering and antenna
decoupling algorithms are also presented.Comment: To be published in 2016 IEEE 83rd Vehicular Technology Conference
Spring (VTC 2016-Spring), Nanjing, China, May 201
The Modified Weighted Slab Technique: Models and Results
In an attempt to understand the source and propagation of galactic cosmic
rays we have employed the Modified Weighted Slab technique along with recent
values of the relevant cross sections to compute primary to secondary ratios
including B/C and Sub-Fe/Fe for different galactic propagation models. The
models that we have considered are the disk-halo diffusion model, the dynamical
halo wind model, the turbulent diffusion model and a model with minimal
reacceleration. The modified weighted slab technique will be briefly discussed
and a more detailed description of the models will be given. We will also
discuss the impact that the various models have on the problem of anisotropy at
high energy and discuss what properties of a particular model bear on this
issue.Comment: LaTeX - AASTEX format, Submitted to ApJ, 8 figures, 20 page
Time-dependent particle acceleration in supernova remnants in different environments
We simulate time-dependent particle acceleration in the blast wave of a young
supernova remnant (SNR), using a Monte Carlo approach for the diffusion and
acceleration of the particles, coupled to an MHD code. We calculate the
distribution function of the cosmic rays concurrently with the hydrodynamic
evolution of the SNR, and compare the results with those obtained using simple
steady-state models. The surrounding medium into which the supernova remnant
evolves turns out to be of great influence on the maximum energy to which
particles are accelerated. In particular, a shock going through a density profile causes acceleration to typically much higher energies
than a shock going through a medium with a homogeneous density profile. We find
systematic differences between steady-state analytical models and our
time-dependent calculation in terms of spectral slope, maximum energy, and the
shape of the cut-off of the particle spectrum at the highest energies. We also
find that, provided that the magnetic field at the reverse shock is
sufficiently strong to confine particles, cosmic rays can be easily
re-accelerated at the reverse shock.Comment: 19 pages, 20 figures, accepted for publication in MNRA
High resolution X-ray spectroscopy of bright O type stars
Archival X-ray spectra of the four prominent single, non-magnetic O stars
Zeta Pup, Zeta Ori, Ksi Per and Zeta Oph, obtained in high resolution with
Chandra HETGS/MEG have been studied. The resolved X-ray emission line profiles
provide information about the shocked, hot gas which emits the X-radiation, and
about the bulk of comparably cool stellar wind material which partly absorbs
this radiation. In this paper, we synthesize X-ray line profiles with a model
of a clumpy stellar wind. We find that the geometrical shape of the wind
inhomogeneities is important: better agreement with the observations can be
achieved with radially compressed clumps than with spherical clumps. The
parameters of the model, i.e. chemical abundances, stellar radius, mass-loss
rate and terminal wind velocity, are taken from existing analyses of UV and
optical spectra of the programme stars. On this basis, we also calculate the
continuum-absorption coefficient of the cool-wind material, using the Potsdam
Wolf-Rayet (PoWR) model atmosphere code. The radial location of X-ray emitting
gas is restricted from analysing the fir line ratios of helium-like ions. The
only remaining free parameter of our model is the typical distance between the
clumps; here, we assume that at any point in the wind there is one clump
passing by per one dynamical time-scale of the wind. The total emission in a
model line is scaled to the observation. There is a good agreement between
synthetic and observed line profiles. We conclude that the X-ray emission line
profiles in O stars can be explained by hot plasma embedded in a cool wind
which is highly clumped in the form of radially compressed shell fragments.Comment: a typo corrected, 14 pages, MNRAS, in pres
- …