2,024 research outputs found
Relay Assisted Cooperative OSTBC Communication with SNR Imbalance and Channel Estimation Errors
In this paper, a two-hop relay assisted cooperative Orthogonal Space-Time
Block Codes (OSTBC) transmission scheme is considered for the downlink
communication of a cellular system, where the base station (BS) and the relay
station (RS) cooperate and transmit data to the user equipment (UE) in a
distributed fashion. We analyze the impact of the SNR imbalance between the
BS-UE and RS-UE links, as well as the imperfect channel estimation at the UE
receiver. The performance is analyzed in the presence of Rayleigh flat fading
and our results show that the SNR imbalance does not impact the spatial
diversity order. On the other hand, channel estimation errors have a larger
impact on the system performance. Simulation results are then provided to
confirm the analysis.Comment: 5 pages, 3 figures, IEEE 69th Vehicular Technology Conferenc
Correcting for the solar wind in pulsar timing observations: the role of simultaneous a nd l ow-frequency observations
The primary goal of the pulsar timing array projects is to detect
ultra-low-frequency gravitational waves. The pulsar data sets are affected by
numerous noise processes including varying dispersive delays in the
interstellar medium and from the solar wind. The solar wind can lead to rapidly
changing variations that, with existing telescopes, can be hard to measure and
then remove. In this paper we study the possibility of using a low frequency
telescope to aid in such correction for the Parkes Pulsar Timing Array (PPTA)
and also discuss whether the ultra-wide-bandwidth receiver for the FAST
telescope is sufficient to model the solar wind variations. Our key result is
that a single wide-bandwidth receiver can be used to model and remove the
effect of the solar wind. However, for pulsars that pass close to the Sun such
as PSR J1022+1022, the solar wind is so variable that observations at two
telescopes separated by a day are insufficient to correct the solar wind
effect.Comment: accepted by RA
Energy Spectrum Extraction and Optimal Imaging via Dual-Energy Material Decomposition
Inferior soft-tissue contrast resolution is a major limitation of current CT
scanners. The aim of the study is to improve the contrast resolution of CT
scanners using dual-energy acquisition. Based on dual-energy material
decomposition, the proposed method starts with extracting the outgoing energy
spectrum by polychromatic forward projecting the material-selective images. The
extracted spectrum is then reweighted to boost the soft-tissue contrast. A
simulated water cylinder phantom with inserts that contain a series of six
solutions of varying iodine concentration (range, 0-20 mg/mL) is used to
evaluate the proposed method. Results show the root mean square error (RMSE)
and mean energy difference between the extracted energy spectrum and the
spectrum acquired using an energy-resolved photon counting detector(PCD), are
0.044 and 0.01 keV, respectively. Compared to the method using the standard
energy-integrating detectors, dose normalized contrast-to-noise ratio (CNRD)
for the proposed method are improved from 1 to 2.15 and from 1 to 1.88 for the
8 mg/mL and 16 mg/mL iodine concentration inserts, respectively. The results
show CT image reconstructed using the proposed method is superior to the image
reconstructed using the standard method that using an energy-integrating
detector.Comment: 4 pages, 4 figures in The 2015 IEEE Nuclear Science Symposium and
Medical Imaging Conference Recor
Resource Allocation for Device-to-Device Communications Underlaying Heterogeneous Cellular Networks Using Coalitional Games
Heterogeneous cellular networks (HCNs) with millimeter wave (mmWave)
communications included are emerging as a promising candidate for the fifth
generation mobile network. With highly directional antenna arrays, mmWave links
are able to provide several-Gbps transmission rate. However, mmWave links are
easily blocked without line of sight. On the other hand, D2D communications
have been proposed to support many content based applications, and need to
share resources with users in HCNs to improve spectral reuse and enhance system
capacity. Consequently, an efficient resource allocation scheme for D2D pairs
among both mmWave and the cellular carrier band is needed. In this paper, we
first formulate the problem of the resource allocation among mmWave and the
cellular band for multiple D2D pairs from the view point of game theory. Then,
with the characteristics of cellular and mmWave communications considered, we
propose a coalition formation game to maximize the system sum rate in
statistical average sense. We also theoretically prove that our proposed game
converges to a Nash-stable equilibrium and further reaches the near-optimal
solution with fast convergence rate. Through extensive simulations under
various system parameters, we demonstrate the superior performance of our
scheme in terms of the system sum rate compared with several other practical
schemes.Comment: 13 pages, 12 figure
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