1,431,431 research outputs found
Heterogeneity in Short Gamma-ray Bursts
We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective
Bayesian Block procedure to extract temporal descriptors of the bursts' initial
pulse complexes (IPCs). The sample comprises 12 and 41 bursts with and without
extended emission (EE) components, respectively. IPCs of non-EE bursts are
dominated by single pulse structures, while EE bursts tend to have two or more
pulse structures. The medians of characteristic timescales - durations, pulse
structure widths, and peak intervals - for EE bursts are factors of ~ 2-3
longer than for non-EE bursts. A trend previously reported by Hakkila and
colleagues unifying long and short bursts - the anti-correlation of pulse
intensity and width - continues in the two short burst groups, with non-EE
bursts extending to more intense, narrower pulses. In addition we find that
preceding and succeeding pulse intensities are anti-correlated with pulse
interval. We also examine the short burst X-ray afterglows as observed by the
Swift/XRT. The median flux of the initial XRT detections for EE bursts (~ 6 x
10^-10 erg cm^-2 s^-1) is ~> 20 x brighter than for non-EE bursts, and the
median X-ray afterglow duration for EE bursts (~ 60,000 s) is ~ 30 x longer
than for non-EE bursts.
The tendency for EE bursts toward longer prompt-emission timescales and
higher initial X-ray afterglow fluxes implies larger energy injections powering
the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest
that a significant fraction explode into more dense environments than non-EE
bursts, or that the sometimes-dominant EE component efficiently powers the
afterglow. Combined, these results favor different progenitors for EE and
non-EE short bursts.Comment: 30 pages, 11 figures, 3 tables; accepted to The Astrophysical Journa
Flexible resource allocation for joint optimization of energy and spectral efficiency in OFDMA multi-cell networks
The radio resource allocation problem is studied, aiming to jointly optimize the energy efficiency (EE) and spectral efficiency (SE) of downlink OFDMA multi-cell networks. Different from existing works on either EE or SE optimization, a novel EE-SE tradeoff (EST) metric, which can capture both the EST relation and the individual cells’ preferences for EE or SE performance, is introduced as the utility function for each base station (BS). Then the joint EE-SE optimization problem is formulated, and an iterative subchannel allocation and power allocation algorithm is proposed. Numerical results show that the proposed algorithm can exploit the EST relation flexibly and optimize the EE and SE simultaneously to meet diverse EE and SE preferences of individual cells.<br/
Spectral Efficiency and Energy Efficiency of OFDM Systems: Impact of Power Amplifiers and Countermeasures
In wireless communication systems, the nonlinear effect and inefficiency of
power amplifier (PA) have posed practical challenges for system designs to
achieve high spectral efficiency (SE) and energy efficiency (EE). In this
paper, we analyze the impact of PA on the SE-EE tradeoff of orthogonal
frequency division multiplex (OFDM) systems. An ideal PA that is always linear
and incurs no additional power consumption can be shown to yield a decreasing
convex function in the SE-EE tradeoff. In contrast, we show that a practical PA
has an SE-EE tradeoff that has a turning point and decreases sharply after its
maximum EE point. In other words, the Pareto-optimal tradeoff boundary of the
SE-EE curve is very narrow. A wide range of SE-EE tradeoff, however, is desired
for future wireless communications that have dynamic demand depending on the
traffic loads, channel conditions, and system applications, e.g.,
high-SE-with-low-EE for rate-limited systems and high-EE-with-low-SE for
energy-limited systems. For the SE-EE tradeoff improvement, we propose a PA
switching (PAS) technique. In a PAS transmitter, one or more PAs are switched
on intermittently to maximize the EE and deliver an overall required SE. As a
consequence, a high EE over a wide range SE can be achieved, which is verified
by numerical evaluations: with 15% SE reduction for low SE demand, the PAS
between a low power PA and a high power PA can improve EE by 323%, while a
single high power PA transmitter improves EE by only 68%.Comment: to be published, IEEE J. Sel. Areas Commu
Energy-Efficiency Based Resource Allocation for the Scalar Broadcast Channel
Until recently, link adaptation and resource allocation for communication system relied extensively on the spectral efficiency as an optimization criterion. With the emergence of the energy efficiency (EE) as a key system design criterion, resource allocation based on EE is becoming of great interest. In this paper, we propose an optimal EE-based resource allocation method for the scalar broadcast channel (BC-S). We introduce our EE framework, which includes an EE metric as well as a realistic power consumption model for the base station, and utilize this framework for formulating our EE-based optimization problem subject to a power as well as fairness constraints. We then prove the convexity of this problem and compare our EE-based resource allocation method against two other methods, i.e. one based on sum-rate and one based on fairness optimization. Results indicate that our method provides large EE improvement in comparison with the two other methods by significantly reducing the total consumed power. Moreover, they show that near-optimal EE and average fairness can be simultaneously achieved over the BC-S channel
On the relation between energy efficiency and spectral efficiency of multiple-antenna systems
Motivated by the increasing interest in energy-efficient communication systems, the relation between energy efficiency (EE) and spectral efficiency (SE) for multiple-input-multiple-output (MIMO) systems is investigated in this paper. To provide insights into the design of practical MIMO systems, we adopt a realistic power model and consider both independent Rayleigh fading and semicorrelated fading channels. We derived a novel and closed-form upper bound (UB) for the system EE as a function of SE. This UB exhibits great accuracy for a wide range of SE values and, thus, can be utilized for explicit assessment of the influence of SE on EE and for analytically addressing the EE optimization problems. Using this tight EE UB, our analysis unfolds two EE optimization issues: Given the number of transmit and receive antennas, an optimum value of SE is derived, such that the overall EE can be maximized, and given a specific value of SE, the optimal number of antennas is derived for maximizing the system EE
Cubic symmetroids and vector bundles on a quadric surface
We investigate the jumping conics of stable vector bundles \Ee of rank 2 on
a smooth quadric surface with the Chern classes c_1=\Oo_Q(-1,-1) and
with respect to the ample line bundle \Oo_Q(1,1). We describe the set
of jumping conics of \Ee, a cubic symmetroid in \PP_3, in terms of the
cohomological properties of \Ee. As a consequence, we prove that the set of
jumping conics, S(\Ee), uniquely determines \Ee. Moreover we prove that the
moduli space of such vector bundles is rational.Comment: 6 pages; Comments welcom
Neutrino mass spectrum and neutrinoless double beta decay
The relations between the effective Majorana mass of the electron neutrino,
, responsible for neutrinoless double beta decay, and the neutrino
oscillation parameters are considered. We show that for any specific
oscillation pattern can take any value (from zero to the existing
upper bound) for normal mass hierarchy and it can have a minimum for inverse
hierarchy. This means that oscillation experiments cannot fix in general
. Mass ranges for can be predicted in terms of oscillation
parameters with additional assumptions about the level of degeneracy and the
type of hierarchy of the neutrino mass spectrum. These predictions for
are systematically studied in the specific schemes of neutrino mass and flavor
which explain the solar and atmospheric neutrino data. The contributions from
individual mass eigenstates in terms of oscillation parameters have been
quantified. We study the dependence of on the non-oscillation
parameters: the overall scale of the neutrino mass and the relative mass
phases. We analyze how forthcoming oscillation experiments will improve the
predictions for . On the basis of these studies we evaluate the
discovery potential of future \znbb decay searches. The role \znbb decay
searches will play in the reconstruction of the neutrino mass spectrum is
clarified. The key scales of , which will lead to the discrimination
among various schemes are: eV and eV.Comment: 47 pages, 35 figure
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