46 research outputs found
NB-IoT via LEO satellites: An efficient resource allocation strategy for uplink data transmission
In this paper, we focus on the use of Low-Eart Orbit (LEO) satellites
providing the Narrowband Internet of Things (NB-IoT) connectivity to the
on-ground user equipment (UEs). Conventional resource allocation algorithms for
the NBIoT systems are particularly designed for terrestrial infrastructures,
where devices are under the coverage of a specific base station and the whole
system varies very slowly in time. The existing methods in the literature
cannot be applied over LEO satellite-based NB-IoT systems for several reasons.
First, with the movement of the LEO satellite, the corresponding channel
parameters for each user will quickly change over time. Delaying the scheduling
of a certain user would result in a resource allocation based on outdated
parameters. Second, the differential Doppler shift, which is a typical
impairment in communications over LEO, directly depends on the relative
distance among users. Scheduling at the same radio frame users that overcome a
certain distance would violate the differential Doppler limit supported by the
NB-IoT standard. Third, the propagation delay over a LEO satellite channel is
around 4-16 times higher compared to a terrestrial system, imposing the need
for message exchange minimization between the users and the base station. In
this work, we propose a novel uplink resource allocation strategy that jointly
incorporates the new design considerations previously mentioned together with
the distinct channel conditions, satellite coverage times and data demands of
various users on Earth. The novel methodology proposed in this paper can act as
a framework for future works in the field.Comment: Tis work has been submitted to the IEEE IoT Journal for possible
publication. Copyright may be transferred without notice, after which this
version may no longer be accessibl
The ECAPS Experiment for Solar Cell Characterization in the Stratosphere
The ECAPS project (Experimental Characterization of Advanced Photovoltaics in the Stratosphere) aims at the characterization of performance of a number of different solar cells in the stratospheric environment. ECAPS has been selected to fly as a zero-pressure balloon payload in the frame of the HEMERA H2020 project. Flight is scheduled for August 2022 from CNES’ base in Timmins, Canada.
Testing solar cells in the stratosphere is of great interest for the development of High-Altitude Pseudo Satellite (HAPS) platforms, which will be equipped with high efficiency, flexible solar cells capable to operate at 20-30 km altitude for weeks or months, as well as to perform high-quality calibration of spacecraft solar cells in a near-air mass zero environment. The experiment includes a panel with up to 4 solar cells of different kinds (multi-junction GaAs, CIGS, perovskite, etc.), a dedicated I/V curve recording circuit, temperature and irradiance sensors, and an inertial measurement unit to monitor the instantaneous attitude of the gondola.
During the ascent part of the flight, the I/V characteristic curves of the cells will be continuously recorded so to allow for comparison of performance of the different photovoltaic technologies in identical, real stratospheric flight conditions, as well as to detect performance changes with external temperature, irradiance and altitude. Upon recovery of the experiment, post-flight inspection will also yield useful information on the solar cell compatibility with the high altitude environment
Architectures and Key Technical Challenges for 5G Systems Incorporating Satellites
Satellite Communication systems are a promising solution to extend and
complement terrestrial networks in unserved or under-served areas. This aspect
is reflected by recent commercial and standardisation endeavours. In
particular, 3GPP recently initiated a Study Item for New Radio-based, i.e., 5G,
Non-Terrestrial Networks aimed at deploying satellite systems either as a
stand-alone solution or as an integration to terrestrial networks in mobile
broadband and machine-type communication scenarios. However, typical satellite
channel impairments, as large path losses, delays, and Doppler shifts, pose
severe challenges to the realisation of a satellite-based NR network. In this
paper, based on the architecture options currently being discussed in the
standardisation fora, we discuss and assess the impact of the satellite channel
characteristics on the physical and Medium Access Control layers, both in terms
of transmitted waveforms and procedures for enhanced Mobile BroadBand (eMBB)
and NarrowBand-Internet of Things (NB-IoT) applications. The proposed analysis
shows that the main technical challenges are related to the PHY/MAC procedures,
in particular Random Access (RA), Timing Advance (TA), and Hybrid Automatic
Repeat reQuest (HARQ) and, depending on the considered service and
architecture, different solutions are proposed.Comment: Submitted to Transactions on Vehicular Technologies, April 201
Critical Temperature and Energy Gap for the BCS Equation
We derive upper and lower bounds on the critical temperature and the
energy gap (at zero temperature) for the BCS gap equation, describing
spin 1/2 fermions interacting via a local two-body interaction potential
. At weak coupling and under appropriate
assumptions on , our bounds show that and
for some explicit coefficients , and
depending on the interaction and the chemical potential . The ratio
turns out to be a universal constant, independent of both and
. Our analysis is valid for any ; for small , or low density,
our formulas reduce to well-known expressions involving the scattering length
of .Comment: RevTeX4, 23 pages. Revised version, to appear in Phys. Rev.
Ward identity and optical-conductivity sum rule in the d-density wave state
We consider the role of the Ward identity in dealing with the transport
properties of an interacting system forming a d-wave modulated charge-density
wave or staggered flux phase. In particular, we address this issue from the
point of view of the restricted optical-conductivity sum rule. Our aim is to
provide a controlled approximation for the current-current correlation function
which allows us also to determine analytically the corresponding sum rule. By
analyzing the role of the vertex functions in both the microscopic interacting
model and in the effective mean-field Hamiltonian, we propose a non-standard
low-energy sum-rule for this system. We also discuss the possible applicability
of these results for the description of cuprate superconductors in the
pseudogap regime.Comment: Revised version, accepted for publication in Phys. Rev.
Superconductivity with hard-core repulsion: BCS-Bose crossover and s-/d-wave competition
We consider fermions on a 2D lattice interacting repulsively on the same site
and attractively on the nearest neighbor sites. The model is relevant, for
instance, to study the competition between antiferromagnetism and
superconductivity in a Kondo lattice. We first solve the two-body problem to
show that in the dilute and strong coupling limit the s-wave Bose condensed
state is always the ground state. We then consider the many-body problem and
treat it at mean-field level by solving exactly the usual gap equation. This
guarantees that the superconducting wave-function correctly vanishes when the
two fermions (with antiparallel spin) sit on the same site. This fact has
important consequences on the superconducting state that are somewhat unusual.
In particular this implies a radial node-line for the gap function. When a next
neighbor hopping t' is present we find that the s-wave state may develop nodes
on the Fermi surface.Comment: 10 pages, 9 fig
Pseudogap and spectral function from superconducting fluctuations to the bosonic limit
The crossover from weak to strong coupling for a three dimensional continuum
model of fermions interacting via an attractive contact potential is studied
above the superconducting critical temperature. The pair-fluctuation
propagator, the one-loop self-energy, and the spectral function are
investigated in a systematic way from the superconducting fluctuation regime
(weak coupling) to the bosonic regime (strong coupling). Analytic and numerical
results are reported. In the strong-coupling regime, where the pair fluctuation
propagator has bosonic character, two quite different peaks appear in the
spectral function, a broad one at negative frequencies and a narrow one at
positive frequencies. By decreasing coupling, the two-peak structure evolves
smoothly. In the weak-coupling regime, where the fluctuation propagator has
diffusive Ginzburg-Landau character, the overall line-shape of the spectral
function is more symmetric. The systematic analysis of the spectral function
identifies specific features which allow one to distinguish by ARPES whether a
system is in the weak- or strong-coupling regime. Connection of the results of
our analysis with the phenomenology of cuprate superconductors is also
attempted and rests on the recently introduced two-gap model.Comment: 19 pages, 18 figure
Temperature-doping phase diagram of layered superconductors
The superconducting properties of a layered system are analyzed for the cases
of zero- and non-zero angular momentum of the pairs. The effective
thermodynamic potential for the quasi-2D XY-model for the gradients of the
phase of the order parameter is derived from the microscopic superconducting
Hamiltonian. The dependence of the superconducting critical temperature T_c on
doping, or carrier density, is studied at different values of coupling and
inter-layer hopping. It is shown that the critical temperature T_c of the
layered system can be lower than the critical temperature of the
two-dimensional Berezinskii-Kosterlitz-Thouless transition T_BKT at some values
of the model parameters, contrary to the case when the parameters of the
XY-model do not depend on the microscopic Hamiltonian parameters.Comment: To be published in Phys. Rev.
Density-induced BCS to Bose-Einstein crossover
We investigate the zero-temperature BCS to Bose-Einstein crossover at the
mean-field level, by driving it with the attractive potential and the particle
density.We emphasize specifically the role played by the particle density in
this crossover.Three different interparticle potentials are considered for the
continuum model in three spatial dimensions, while both s- and d-wave solutions
are analyzed for the attractive (extended) Hubbard model on a two-dimensional
square lattice. For this model the peculiar behavior of the crossover for the
d-wave solution is discussed.In particular, in the strong-coupling limit when
approaching half filling we evidence the occurrence of strong correlations
among antiparallel-spin fermions belonging to different composite bosons, which
give rise to a quasi-long-range antiferromagnetic order in this limit.Comment: 10 pages, 5 enclosed figure
Pairing symmetry of superconducting graphene
The possibility of intrinsic superconductivity in alkali-coated graphene
monolayers has been recently suggested theoretically. Here, we derive the
possible pairing symmetries of a carbon honeycomb lattice and discuss their
phase diagram. We also evaluate the superconducting local density of states
(LDOS) around an isolated impurity. This is directly related to scanning
tunneling microscopy experiments, and may evidence the occurrence of
unconventional superconductivity in graphene.Comment: Eur. Phys. J. B, to appea