138,373 research outputs found
Space Alignment Based on Regularized Inversion Precoding in Cognitive Transmission
For a two-tier Multiple-Input Multiple-Output (MIMO) cognitive network with common receiver, the precoding matrix has a compact relationship with the capacity performance in the unlicensed secondary system. To increase the capacity of secondary system, an improved precoder based on the idea of regularized inversion for secondary transmitter is proposed. An iterative space alignment algorithm is also presented to ensure the Quality of Service (QoS) for primary system. The simulations reveal that, on the premise of achieving QoS for primary system, our proposed algorithm can get larger capacity in secondary system at low Signal-to-Noise Ratio (SNR), which proves the effectiveness of the algorithm
A study of physical processes for space radiation protection
The determination of stopping power for monatomic molecules and water vapor is addressed. Intermediate and low energy protons are considered
What if pulsars are born as strange stars?
The possibility and the implications of the idea, that pulsars are born as
strange stars, are explored. Strange stars are very likely to have atmospheres
with typical mass of but bare polar caps almost
throughout their lifetimes, if they are produced during supernova explosions. A
direct consequence of the bare polar cap is that the binding energies of both
positively and negatively charged particles at the bare quark surface are
nearly infinity, so that the vacuum polar gap sparking scenario as proposed by
Ruderman & Sutherland should operate above the cap, regardless of the sense of
the magnetic pole with respect to the rotational pole. Heat can not accumulate
on the polar cap region due to the large thermal conductivity on the bare quark
surface. We test this ``bare polar cap strange star'' (BPCSS) idea with the
present broad band emission data of pulsars, and propose several possible
criteria to distinguish BPCSSs from neutron stars.Comment: 31 pages in Latex. Accepted by AstroParticle Physic
Solar flare hard X-ray spikes observed by RHESSI: a case study
In this paper, we analyze hard X-ray spikes observed by RHESSI to understand
their temporal, spectral, and spatial properties. A recently developed
demodulation code was applied to hard X-ray light curves in several energy
bands observed by RHESSI. Hard X-ray spikes were selected from the demodulated
flare light curves. We measured the spike duration, the energy-dependent time
delay, and count spectral index of these spikes. We also located the hard X-ray
source emitting these spikes from RHESSI mapping that was coordinated with
imaging observations in visible and UV wavelengths. We identify quickly varying
structures of <1 s during the rise of hard X-rays in five flares. These hard
X-ray spikes can be observed at photon energies over 100 keV. They exhibit
sharp rise and decay with a duration (FWHM) of less than 1 s. Energy-dependent
time lags are present in some spikes. It is seen that the spikes exhibit harder
spectra than underlying components, typically by 0.5 in the spectral index when
they are fitted to power-law distributions. RHESSI clean maps at 25-100 keV
with an integration of 2 s centered on the peak of the spikes suggest that hard
X-ray spikes are primarily emitted by double foot-point sources in magnetic
fields of opposite polarities. With the RHESSI mapping resolution of ~ 4 arsec,
the hard X-ray spike maps do not exhibit detectable difference in the spatial
structure from sources emitting underlying components. Coordinated
high-resolution imaging UV and infrared observations confirm that hard X-ray
spikes are produced in magnetic structures embedded in the same magnetic
environment of the underlying components. The coordinated high-cadence TRACE UV
observations of one event possibly reveal new structures on spatial scales <1-2
arsec at the time of the spike superposed on the underlying component. They are
probably sources of hard X-ray spikes.Comment: 20 pages, 11 figure
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