6,164 research outputs found
A Stochastic Geometric Analysis of Device-to-Device Communications Operating over Generalized Fading Channels
Device-to-device (D2D) communications are now considered as an integral part
of future 5G networks which will enable direct communication between user
equipment (UE) without unnecessary routing via the network infrastructure. This
architecture will result in higher throughputs than conventional cellular
networks, but with the increased potential for co-channel interference induced
by randomly located cellular and D2D UEs. The physical channels which
constitute D2D communications can be expected to be complex in nature,
experiencing both line-of-sight (LOS) and non-LOS (NLOS) conditions across
closely located D2D pairs. As well as this, given the diverse range of
operating environments, they may also be subject to clustering of the scattered
multipath contribution, i.e., propagation characteristics which are quite
dissimilar to conventional Rayeligh fading environments. To address these
challenges, we consider two recently proposed generalized fading models, namely
and , to characterize the fading behavior in D2D
communications. Together, these models encompass many of the most widely
encountered and utilized fading models in the literature such as Rayleigh, Rice
(Nakagami-), Nakagami-, Hoyt (Nakagami-) and One-Sided Gaussian. Using
stochastic geometry we evaluate the rate and bit error probability of D2D
networks under generalized fading conditions. Based on the analytical results,
we present new insights into the trade-offs between the reliability, rate, and
mode selection under realistic operating conditions. Our results suggest that
D2D mode achieves higher rates over cellular link at the expense of a higher
bit error probability. Through numerical evaluations, we also investigate the
performance gains of D2D networks and demonstrate their superiority over
traditional cellular networks.Comment: Submitted to IEEE Transactions on Wireless Communication
The Cepheid instability strip and the calibration of the primary distance scale
This study examines the possibility of galaxy-to-galaxy differences in the long-period Cepheid distributions of external galaxies. A simple theoretical framework is created and linear pulsation calculations are performed to model these distributions. The sturdy nature of the Cepheid period-luminosity (P-L) relation is affirmed, but both analytic arguments and the linear model grids point to potential systematic errors reaching up to a few tenths of a magnitude if the Cepheids in the calibrating and target galaxies have different distributions. We also point out some difficulties posed for stellar pulsation and evolution theory by the long-period Cepheids we have studied: the theoretical blue edge seems too hot and/or the inferred masses too large to account for the observed stars. Preliminary observational evidence is presented which marginally indicates the existence of two somewhat different types of distribution of long-period Cepheids in external galaxies, but further data are needed before this can be confirmed
A Comprehensive Analysis of 5G Heterogeneous Cellular Systems operating over - Shadowed Fading Channels
Emerging cellular technologies such as those proposed for use in 5G
communications will accommodate a wide range of usage scenarios with diverse
link requirements. This will include the necessity to operate over a versatile
set of wireless channels ranging from indoor to outdoor, from line-of-sight
(LOS) to non-LOS, and from circularly symmetric scattering to environments
which promote the clustering of scattered multipath waves. Unfortunately, many
of the conventional fading models adopted in the literature to develop network
models lack the flexibility to account for such disparate signal propagation
mechanisms. To bridge the gap between theory and practical channels, we
consider - shadowed fading, which contains as special cases, the
majority of the linear fading models proposed in the open literature, including
Rayleigh, Rician, Nakagami-m, Nakagami-q, One-sided Gaussian, -,
-, and Rician shadowed to name but a few. In particular, we apply an
orthogonal expansion to represent the - shadowed fading
distribution as a simplified series expression. Then using the series
expressions with stochastic geometry, we propose an analytic framework to
evaluate the average of an arbitrary function of the SINR over -
shadowed fading channels. Using the proposed method, we evaluate the spectral
efficiency, moments of the SINR, bit error probability and outage probability
of a -tier HetNet with classes of BSs, differing in terms of the
transmit power, BS density, shadowing characteristics and small-scale fading.
Building upon these results, we provide important new insights into the network
performance of these emerging wireless applications while considering a diverse
range of fading conditions and link qualities
Retelling racialized violence, remaking white innocence: the politics of interlocking oppressions in transgender day of remembrance
Transgender Day of Remembrance has become a significant political event among those resisting violence against gender-variant persons. Commemorated in more than 250 locations worldwide, this day honors individuals who were killed due to anti-transgender hatred or prejudice. However, by focusing on transphobia as the definitive cause of violence, this ritual potentially obscures the ways in which hierarchies of race, class, and sexuality constitute such acts. Taking the Transgender Day of Remembrance/Remembering Our Dead project as a case study for considering the politics of memorialization, as well as tracing the narrative history of the Fred F. C. Martinez murder case in Colorado, the author argues that deracialized accounts of violence produce seemingly innocent White witnesses who can consume these spectacles of domination without confronting their own complicity in such acts. The author suggests that remembrance practices require critical rethinking if we are to confront violence in more effective ways. Description from publisher's site: http://caliber.ucpress.net/doi/abs/10.1525/srsp.2008.5.1.2
Re-Evaluation of the UK’s HFC-134a Emissions Inventory Based on Atmospheric Observations
Independent
verification of national greenhouse gas inventories
is a vital measure for cross-checking the accuracy of emissions data
submitted to the United Nations Framework Convention on Climate Change
(UNFCCC). We infer annual UK emissions of HFC-134a from 1995 to 2012
using atmospheric observations and an inverse modeling technique,
and compare with the UK’s annual UNFCCC submission. By 2010,
the inventory is almost twice as large as our estimates, with an “emissions
gap” equating to 3.90 (3.20–4.30) Tg CO<sub>2</sub>e.
We evaluate the RAC (Refrigeration and Air-Conditioning) model, a
bottom up model used to quantify UK emissions from refrigeration and
air-conditioning sectors. Within mobile air-conditioning (MAC), the
largest RAC sector and most significant UK source (59%), we find a
number of assumptions that may be considered oversimplistic and conservative;
most notably the unit refill rate. Finally, a Bayesian approach is
used to estimate probable inventory inputs required for minimization
of the emissions discrepancy. Our top-down estimates provide only
a weak constraint on inventory model parameters and consequently,
we are unable to suggest discrete values. However, a significant revision
of the MAC servicing rate, coupled with a reassessment of non-RAC
aerosol emissions, are required if the discrepancy between methods
is to be reduced
Feedback-optimized parallel tempering Monte Carlo
We introduce an algorithm to systematically improve the efficiency of
parallel tempering Monte Carlo simulations by optimizing the simulated
temperature set. Our approach is closely related to a recently introduced
adaptive algorithm that optimizes the simulated statistical ensemble in
generalized broad-histogram Monte Carlo simulations. Conventionally, a
temperature set is chosen in such a way that the acceptance rates for replica
swaps between adjacent temperatures are independent of the temperature and
large enough to ensure frequent swaps. In this paper, we show that by choosing
the temperatures with a modified version of the optimized ensemble feedback
method we can minimize the round-trip times between the lowest and highest
temperatures which effectively increases the efficiency of the parallel
tempering algorithm. In particular, the density of temperatures in the
optimized temperature set increases at the "bottlenecks'' of the simulation,
such as phase transitions. In turn, the acceptance rates are now temperature
dependent in the optimized temperature ensemble. We illustrate the
feedback-optimized parallel tempering algorithm by studying the two-dimensional
Ising ferromagnet and the two-dimensional fully-frustrated Ising model, and
briefly discuss possible feedback schemes for systems that require
configurational averages, such as spin glasses.Comment: 12 pages, 14 figure
Galaxy Cluster Pressure Profiles as Determined by Sunyaev Zel'dovich Effect Observations with MUSTANG and Bolocam I: Joint Analysis Technique
We present a technique to constrain galaxy cluster pressure profiles by
jointly fitting Sunyaev-Zel'dovich effect (SZE) data obtained with MUSTANG and
Bolocam for the clusters Abell 1835 and MACS0647. Bolocam and MUSTANG probe
different angular scales and are thus highly complementary. We find that the
addition of the high resolution MUSTANG data can improve constraints on
pressure profile parameters relative to those derived solely from Bolocam. In
Abell 1835 and MACS0647, we find gNFW inner slopes of and , respectively when
and are constrained to 0.86 and 4.67 respectively. The fitted
SZE pressure profiles are in good agreement with X-ray derived pressure
profiles.Comment: 12 pages, 12 figures. Submitted to Ap
Dynamics of the Wang-Landau algorithm and complexity of rare events for the three-dimensional bimodal Ising spin glass
We investigate the performance of flat-histogram methods based on a
multicanonical ensemble and the Wang-Landau algorithm for the three-dimensional
+/- J spin glass by measuring round-trip times in the energy range between the
zero-temperature ground state and the state of highest energy. Strong
sample-to-sample variations are found for fixed system size and the
distribution of round-trip times follows a fat-tailed Frechet extremal value
distribution. Rare events in the fat tails of these distributions corresponding
to extremely slowly equilibrating spin glass realizations dominate the
calculations of statistical averages. While the typical round-trip time scales
exponential as expected for this NP-hard problem, we find that the average
round-trip time is no longer well-defined for systems with N >= 8^3 spins. We
relate the round-trip times for multicanonical sampling to intrinsic properties
of the energy landscape and compare with the numerical effort needed by the
genetic Cluster-Exact Approximation to calculate the exact ground state
energies. For systems with N >= 8^3 spins the simulation of these rare events
becomes increasingly hard. For N >= 14^3 there are samples where the
Wang-Landau algorithm fails to find the true ground state within reasonable
simulation times. We expect similar behavior for other algorithms based on
multicanonical sampling.Comment: 9 pages, 12 figure
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