6,340 research outputs found
Analysis of Device-to-Device Communications in Uplink Cellular Networks with Lognormal Fading
In this paper, using the stochastic geometry theory, we present a framework
for analyzing the performance of device-to-device (D2D) communications
underlaid uplink (UL) cellular networks. In our analysis, we consider a D2D
mode selection criterion based on an energy threshold for each user equipment
(UE). Specifically, a UE will operate in a cellular mode, if its received
signal strength from the strongest base station (BS) is large than a threshold
\beta. Otherwise, it will operate in a D2D mode. Furthermore, we consider a
generalized log-normal shadowing in our analysis. The coverage probability and
the area spectral efficiency (ASE) are derived for both the cellular network
and the D2D one. Through our theoretical and numerical analyses, we quantify
the performance gains brought by D2D communications and provide guidelines of
selecting the parameters for network operations.Comment: conference submissio
Optimum Interference Management in Underlay Inband D2D-Enhanced Cellular Networks
For device-to-device (D2D) communications underlaying a cellular network with
uplink resource sharing, both cellular and D2D links cause significant
co-channel interference. In this paper, we address the critical issue of
interference management in the network considering a practical path loss model
incorporating both line-of-sight (LoS) and non-line-of-sight (NLoS)
transmissions. To reduce the severe interference caused by active D2D links, we
consider a mode selection scheme based on the maximum received signal strength
(MRSS) for each user equipment (UE) to control the D2D-to-cellular
interference. Specifically, a UE will operate in a cellular mode, only if its
received signal strength from the strongest base station (BS) is larger than a
threshold beta; otherwise, the UE will operate in a D2D mode. Furthermore, we
analyze the performance in terms of the coverage probability and the area
spectral efficiency (ASE) for both the cellular network and the D2D one.
Analytical results are obtained and the accuracy of the proposed analytical
framework is validated through Monte Carol simulations. Through our theoretical
and numerical analyses, we quantify the performance gains brought by D2D
communications in cellular networks and we find an optimum mode selection
threshold beta to maximize the total ASE in the network
What determines the observational differences of blazars?
We examine the scenario that the Doppler factor determines the observational
differences of blazars in this paper. Significantly negative correlations are
found between the observational synchrotron peak frequency and the Doppler
factor. After correcting the Doppler boosting, the intrinsic peak frequency
further has a tightly linear relation with the Doppler factor. It is more
interesting that this relation is consistent with the scenario that the black
hole mass governs both the bulk Lorentz factor and the synchrotron peak
frequency. In addition, the distinction of the kinetic jet powers between BL
Lacs and FSRQs disappears after the boosting factor is considered.
The negative correlation between the peak frequency and the observational
isotropic luminosity, known as the blazar sequence, also disappears after the
Doppler boosting is corrected. We also find that the correlation between the
Compton dominance and the Doppler factor exists for all types of blazars.
Therefore, this correlation is unsuitable to examine the external Compton
emission dominance.Comment: 15 pages, 6 figures, 1 tabl
Effect of spectral index distribution on estimating the AGN radio luminosity function
In this paper, we scrutinize the effect of spectral index distribution on
estimating the AGN (active galactic nucleus) radio luminosity function (RLF) by
a Monte Carlo method. We find that the traditional bivariate RLF estimators can
cause bias in varying degree. The bias is especially pronounced for the
flat-spectrum radio sources whose spectral index distribution is more
scattered. We believe that the bias is caused because the -corrections
complicate the truncation boundary on the plane of the sample, but the
traditional bivariate RLF estimators have difficulty in dealing with this
boundary condition properly. We suggest that the spectral index distribution
should be incorporated into the RLF analysis process to obtain a robust
estimation. This drives the need for a trivariate function of the form
which we show provides an accurate basis for measuring the
RLF.Comment: 8 pages, 4 figures, Accepted for publication in Ap
Construction of three classes of Strictly Optimal Frequency-Hopping Sequence Sets
In this paper, we construct three classes of strictly optimal
frequency-hopping sequence (FHS) sets with respect to partial Hamming
correlation and family size. The first class is based on a generic
construction, the second and third classes are based from the trace map
A mixture evolution scenario of the AGN radio luminosity function
We propose a mixture evolution scenario to model the evolution of the radio
luminosity function (RLF) of steep spectrum AGNs (active galactic nuclei) based
on a Bayesian method. In this scenario, the shape of the RLF is determined by
both the density and luminosity evolution. Our models indicate that the density
evolution is positive until a redshift of , at which point it
becomes negative, while the luminosity evolution is positive to a higher
redshift ( for model B and for model C), where
it becomes negative. Our mixture evolution model works well, and the modeled
RLFs are in good agreement with previous determinations. The mixture evolution
scenario can naturally explain the luminosity-dependent evolution of the RLFs.Comment: 7 pages, 5 figures, accepted for publication in Ap
Transportation dynamics on coupled networks with limited bandwidth
The communication networks in real world often couple with each other to save
costs, which results in any network does not have a stand-alone function and
efficiency. To investigate this, in this paper we propose a transportation
model on two coupled networks with bandwidth sharing. We find that the
free-flow state and the congestion state can coexist in the two coupled
networks, and the free-flow path and congestion path can coexist in each
network. Considering three bandwidth-sharing mechanisms, random, assortative
and disassortative couplings, we also find that the transportation capacity of
the network only depends on the coupling mechanism, and the fraction of coupled
links only affects the performance of the system in the congestion state, such
as the traveling time. In addition, with assortative coupling, the
transportation capacity of the system will decrease significantly. However, the
disassortative coupling has little influence on the transportation capacity of
the system, which provides a good strategy to save bandwidth. Furthermore, a
theoretical method is developed to obtain the bandwidth usage of each link,
based on which we can obtain the congestion transition point exactly.Comment: 9 pages, 5 figure
A Simple Adaptive p-persistent MAC Scheme for Service Differentiation and Maximum Channel Utilization
Abstract:- In this paper, we propose an adaptive p-persistent MAC scheme, named QDA-MAC (QoS differentiation based adaptive MAC scheme), for WLAN to maximize the channel utilization and provide the service differentiation among different traffic stations. Specifically, different from the previous works, the proposed scheme does not need to estimate the number of active stations for each priority class and still achieves the channel utilization close to its optimal value by exploiting a new parameter, persistent factor, whose optimal value can dynamically follow the change of the load based on a simple estimation of the network status. At the same time, the transmission probability of each priority class can be updated by optimal persistent factor. Simulation and numerical results show that QDA-MAC can achieve much higher channel utilization and shorter delay than standard IEEE 802.11 DCF and IEEE 802.11e EDCA in all different WLAN environments. Key-Words:- QoS differentiation; p-persistent; persistent factor; adaptive scheme; transmission attempt
Unextendible maximally entangled bases in dxd
We investigate the unextendible maximally entangled bases in
and present a -number UMEB
construction in . For higher
dimensional case, we show that for a given -number UMEB in
, there is a -number,
, UMEB in
for any . As an
example, for systems, we show that
there are at least two sets of UMEBs which are not equivalent.Comment: Errors correcte
Statistical Study of the Swift X-ray Flash and X-ray Rich Gamma-Ray Bursts
We build a comprehensive sample to statistically describe the properties of
X-ray flashes (XRFs) and X-ray riches (XRRs) from the latest third Swift Burst
Alert Telescope (BAT3) catalog of Gamma-ray bursts (GRBs). We obtain 81 XRFs,
540 XRRs, and 394 classical GRBs (C-GRBs). We statistically explore the
different properties of the -ray prompt emission, the X-ray emission,
the X-ray lightcurve type, the association with supernovae (SNe), and the host
galaxy properties for these sources. We confirm that most XRFs/XRRs are long
GRBs with low values of peak energy and they are
low-luminosity GRBs. XRFs, XRRs, and C-GRBs follow the same
-- correlations. Compared to the
classical GRBs, XRFs are favorable to have the association with SN explosions.
We do not find any significant differences of redshift distribution and host
galaxy properties among XRFs, XRRs, and C-GRBs. We also discuss some
observational biases and selection effects that may affect on our statistical
results. The GRB detectors with wide energy range and low energy threshold are
expected for the XRF/XRR research in the future.Comment: ApJ accepte
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