3,696 research outputs found
Coordinated Multicasting with Opportunistic User Selection in Multicell Wireless Systems
Physical layer multicasting with opportunistic user selection (OUS) is
examined for multicell multi-antenna wireless systems. By adopting a two-layer
encoding scheme, a rate-adaptive channel code is applied in each fading block
to enable successful decoding by a chosen subset of users (which varies over
different blocks) and an application layer erasure code is employed across
multiple blocks to ensure that every user is able to recover the message after
decoding successfully in a sufficient number of blocks. The transmit signal and
code-rate in each block determine opportunistically the subset of users that
are able to successfully decode and can be chosen to maximize the long-term
multicast efficiency. The employment of OUS not only helps avoid
rate-limitations caused by the user with the worst channel, but also helps
coordinate interference among different cells and multicast groups. In this
work, efficient algorithms are proposed for the design of the transmit
covariance matrices, the physical layer code-rates, and the target user subsets
in each block. In the single group scenario, the system parameters are
determined by maximizing the group-rate, defined as the physical layer
code-rate times the fraction of users that can successfully decode in each
block. In the multi-group scenario, the system parameters are determined by
considering a group-rate balancing optimization problem, which is solved by a
successive convex approximation (SCA) approach. To further reduce the feedback
overhead, we also consider the case where only part of the users feed back
their channel vectors in each block and propose a design based on the balancing
of the expected group-rates. In addition to SCA, a sample average approximation
technique is also introduced to handle the probabilistic terms arising in this
problem. The effectiveness of the proposed schemes is demonstrated by computer
simulations.Comment: Accepted by IEEE Transactions on Signal Processin
Fluctuations of Entropy Production in Partially Masked Electric Circuits: Theoretical Analysis
In this work we perform theoretical analysis about a coupled RC circuit with
constant driven currents. Starting from stochastic differential equations,
where voltages are subject to thermal noises, we derive time-correlation
functions, steady-state distributions and transition probabilities of the
system. The validity of the fluctuation theorem (FT) is examined for scenarios
with complete and incomplete descriptions.Comment: 4 pages, 1 figur
A Balanced Budget View on Forming Giant Planets by Pebble Accretion
Pebble accretion refers to the assembly of rocky planet cores from particles
whose velocity dispersions are damped by drag from circumstellar disc gas.
Accretion cross-sections can approach maximal Hill-sphere scales for particles
whose Stokes numbers approach unity. While fast, pebble accretion is also
lossy. Gas drag brings pebbles to protocores but also sweeps them past; those
particles with the largest accretion cross-sections also have the fastest
radial drift speeds and are the most easily drained out of discs. We present a
global model of planet formation by pebble accretion that keeps track of the
disc's mass budget. Cores, each initialized with a lunar mass, grow from discs
whose finite stores of mm-cm sized pebbles drift inward across all radii in
viscously accreting gas. For every 1 netted by a core, at least 10
and possibly much more are lost to radial drift. Core growth rates
are typically exponentially sensitive to particle Stokes number, turbulent Mach
number, and solid surface density. This exponential sensitivity, when combined
with disc migration, tends to generate binary outcomes from 0.1-30 AU: either
sub-Earth cores remain sub-Earth, or explode into Jupiters, with the latter
migrating inward to varying degrees. When Jupiter-breeding cores assemble from
mm-cm sized pebbles, they do so in discs where such particles drain out in
10 yr or less; such fast-draining discs do not fit mm-wave
observations.Comment: Accepted to MNRA
Resolved Depletion Zones and Spatial Differentiation of N2H+ and N2D+
We present a study on the spatial distribution of N2D+ and N2H+ in thirteen
protostellar systems. Eight of thirteen objects observed with the IRAM 30m
telescope show relative offsets between the peak N2D+ (J=2-1) and N2H+ (J=1-0)
emission. We highlight the case of L1157 using interferometric observations
from the Submillimeter Array and Plateau de Bure Interferometer of the N2D+
(J=3-2) and N2H+ (J=1-0) transitions respectively. Depletion of N2D+ in L1157
is clearly observed inside a radius of ~2000 AU (7") and the N2H+ emission is
resolved into two peaks at radii of ~1000 AU (3.5"), inside the depletion
region of N2D+. Chemical models predict a depletion zone in N2H+ and N2D+ due
to destruction of H2D+ at T ~ 20 K and the evaporation of CO off dust grains at
the same temperature. However, the abundance offsets of 1000 AU between the two
species are not reproduced by chemical models, including a model that follows
the infall of the protostellar envelope. The average abundance ratios of N2D+
to N2H+ have been shown to decrease as protostars evolve by Emprechtinger et
al., but this is the first time depletion zones of N2D+ have been spatially
resolved. We suggest that the difference in depletion zone radii for N2H+ and
N2D+ is caused by either the CO evaporation temperature being above 20 K or an
H2 ortho-to-para ratio gradient in the inner envelope.Comment: Accepted to ApJ. 44 pages 13 Figure
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