6,871 research outputs found
Analysis and Optimization of Caching and Multicasting in Large-Scale Cache-Enabled Heterogeneous Wireless Networks
Heterogeneous wireless networks (HetNets) provide a powerful approach to meet
the dramatic mobile traffic growth, but also impose a significant challenge on
backhaul. Caching and multicasting at macro and pico base stations (BSs) are
two promising methods to support massive content delivery and reduce backhaul
load in HetNets. In this paper, we jointly consider caching and multicasting in
a large-scale cache-enabled HetNet with backhaul constraints. We propose a
hybrid caching design consisting of identical caching in the macro-tier and
random caching in the pico-tier, and a corresponding multicasting design. By
carefully handling different types of interferers and adopting appropriate
approximations, we derive tractable expressions for the successful transmission
probability in the general region as well as the high signal-to-noise ratio
(SNR) and user density region, utilizing tools from stochastic geometry. Then,
we consider the successful transmission probability maximization by optimizing
the design parameters, which is a very challenging mixed discrete-continuous
optimization problem due to the sophisticated structure of the successful
transmission probability. By using optimization techniques and exploring the
structural properties, we obtain a near optimal solution with superior
performance and manageable complexity. This solution achieves better
performance in the general region than any asymptotically optimal solution,
under a mild condition. The analysis and optimization results provide valuable
design insights for practical cache-enabled HetNets.Comment: 37 pages, 7 figures. arXiv admin note: text overlap with
arXiv:1512.0617
Perspectives on stability and mobility of transit passenger's travel behaviour through smart card data
Existing studies have extensively used spatiotemporal data to discover the
mobility patterns of various types of travellers. Smart card data (SCD)
collected by the automated fare collection systems can reflect a general view
of the mobility pattern of public transit riders. Mobility patterns of transit
riders are temporally and spatially dynamic, and therefore difficult to
measure. However, few existing studies measure both the mobility and stability
of transit riders' travel patterns over a long period of time. To analyse the
long-term changes of transit riders' travel behaviour, the authors define a
metric for measuring the similarity between SCD, in this study. Also an
improved density-based clustering algorithm, simplified smoothed ordering
points to identify the clustering structure (SS-OPTICS), to identify transit
rider clusters is proposed. Compared to the original OPTICS, SS-OPTICS needs
fewer parameters and has better generalisation ability. Further, the generated
clusters are categorized according to their features of regularity and
occasionality. Based on the generated clusters and categories, fine- and
coarse-grained travel pattern transitions of transit riders over four years
from 2010 to 2014 are measured. By combining socioeconomic data of Beijing in
the year of 2010 and 2014, the interdependence between stability and mobility
of transit riders' travel behaviour is also discussed.Comment: 9 pages, 10 figure
Effect of energy conservation on in-medium \mbox{NN}\to \mbox{N}\Delta cross section in isospin-asymmetric nuclear matter
In this paper, the in-medium cross section is
calculated in the framework of the one-boson exchange model by including the
isovector mesons, i.e. and mesons. Due to the isospin exchange
in the process, the vector self-energies of the
outgoing particles are modified relative to the incoming particles in isospin
asymmetric nuclear matter, and it leads to the effective energies of the
incoming pair being different from the outgoing pair. This
effect is investigated in the calculation of the in-medium cross section. With the corrected energy conservation, the cross
sections of the and channels are suppressed, and the
cross sections of the and channels are enhanced relative
to the results obtained without properly considering the potential energy
changes. Our results further confirm the dependence of medium correction
factor, , on the charge state of
especially around the threshold energy, but the isospin splitting of medium
correction factor becomes weak at high beam energies.Comment: 8 pages, 5 figures, accepted by Phys.Rev.
User-Centric Interference Nulling in Downlink Multi-Antenna Heterogeneous Networks
Heterogeneous networks (HetNets) have strong interference due to spectrum
reuse. This affects the signal-to-interference ratio (SIR) of each user, and
hence is one of the limiting factors of network performance. However, in
previous works, interference management approaches in HetNets are mainly based
on interference level, and thus cannot effectively utilize the limited resource
to improve network performance. In this paper, we propose a user-centric
interference nulling (IN) scheme in downlink two-tier HetNets to improve
network performance by improving each user's SIR. This scheme has three design
parameters: the maximum degree of freedom for IN (IN DoF), and the IN
thresholds for the macro and pico users, respectively. Using tools from
stochastic geometry, we first obtain a tractable expression of the coverage
(equivalently outage) probability. Then, we characterize the asymptotic
behavior of the outage probability in the high reliability regime. The
asymptotic results show that the maximum IN DoF can affect the order gain of
the asymptotic outage probability, while the IN thresholds only affect the
coefficient of the asymptotic outage probability. Moreover, we show that the IN
scheme can linearly improve the outage performance, and characterize the
optimal maximum IN DoF which minimizes the asymptotic outage probability.Comment: Shorter version to appear in ISIT 201
Stochastic Throughput Optimization for Two-hop Systems with Finite Relay Buffers
Optimal queueing control of multi-hop networks remains a challenging problem
even in the simplest scenarios. In this paper, we consider a two-hop
half-duplex relaying system with random channel connectivity. The relay is
equipped with a finite buffer. We focus on stochastic link selection and
transmission rate control to maximize the average system throughput subject to
a half-duplex constraint. We formulate this stochastic optimization problem as
an infinite horizon average cost Markov decision process (MDP), which is
well-known to be a difficult problem. By using sample-path analysis and
exploiting the specific problem structure, we first obtain an \emph{equivalent
Bellman equation} with reduced state and action spaces. By using \emph{relative
value iteration algorithm}, we analyze the properties of the value function of
the MDP. Then, we show that the optimal policy has a threshold-based structure
by characterizing the \emph{supermodularity} in the optimal control. Based on
the threshold-based structure and Markov chain theory, we further simplify the
original complex stochastic optimization problem to a static optimization
problem over a small discrete feasible set and propose a low-complexity
algorithm to solve the simplified static optimization problem by making use of
its special structure. Furthermore, we obtain the closed-form optimal threshold
for the symmetric case. The analytical results obtained in this paper also
provide design insights for two-hop relaying systems with multiple relays
equipped with finite relay buffers.Comment: 15 pages, double-column, 9 figures, 3 tables. Accepted by IEEE
Transaction on Signal Processin
Stochastic Content-Centric Multicast Scheduling for Cache-Enabled Heterogeneous Cellular Networks
Caching at small base stations (SBSs) has demonstrated significant benefits
in alleviating the backhaul requirement in heterogeneous cellular networks
(HetNets). While many existing works focus on what contents to cache at each
SBS, an equally important problem is what contents to deliver so as to satisfy
dynamic user demands given the cache status. In this paper, we study optimal
content delivery in cache-enabled HetNets by taking into account the inherent
multicast capability of wireless medium. We consider stochastic content
multicast scheduling to jointly minimize the average network delay and power
costs under a multiple access constraint. We establish a content-centric
request queue model and formulate this stochastic optimization problem as an
infinite horizon average cost Markov decision process (MDP). By using
\emph{relative value iteration} and special properties of the request queue
dynamics, we characterize some properties of the value function of the MDP.
Based on these properties, we show that the optimal multicast scheduling policy
is of threshold type. Then, we propose a structure-aware optimal algorithm to
obtain the optimal policy. We also propose a low-complexity suboptimal policy,
which possesses similar structural properties to the optimal policy, and
develop a low-complexity algorithm to obtain this policy.Comment: Accepted to IEEE Trans. on Wireless Communications (June 6, 2016).
Conference version appears in ACM CoNEXT 2015 Workshop on Content Caching and
Delivery in Wireless Networks (CCDWN
Analysis and Optimization of Caching and Multicasting in Large-Scale Cache-Enabled Wireless Networks
Caching and multicasting at base stations are two promising approaches to
support massive content delivery over wireless networks. However, existing
analysis and designs do not fully explore and exploit the potential advantages
of the two approaches. In this paper, we consider the analysis and optimization
of caching and multicasting in a large-scale cache-enabled wireless network. We
propose a random caching and multicasting scheme with a design parameter. By
carefully handling different types of interferers and adopting appropriate
approximations, we derive a tractable expression for the successful
transmission probability in the general region, utilizing tools from stochastic
geometry. We also obtain a closed-form expression for the successful
transmission probability in the high signal-to-noise ratio (SNR) and user
density region. Then, we consider the successful transmission probability
maximization, which is a very complex non-convex problem in general. Using
optimization techniques, we develop an iterative numerical algorithm to obtain
a local optimal caching and multicasting design in the general region. To
reduce complexity and maintain superior performance, we also derive an
asymptotically optimal caching and multicasting design in the asymptotic
region, based on a two-step optimization framework. Finally, numerical
simulations show that the asymptotically optimal design achieves a significant
gain in successful transmission probability over some baseline schemes in the
general region.Comment: 31 pages, 6 figures, 1 table. Transactions on Wireless Communication
(submitted in July 2015, now under 2nd revision
Analysis and Optimization of Interference Nulling in Downlink Multi-Antenna HetNets with Offloading
Heterogeneous networks (HetNets) with offloading is considered as an
effective way to meet the high data rate demand of future wireless service.
However, the offloaded users suffer from strong inter-tier interference, which
reduces the benefits of offloading and is one of the main limiting factors of
the system performance. In this paper, we investigate the use of an
interference nulling (IN) beamforming scheme to improve the system performance
by carefully managing the inter-tier interference to the offloaded users in
downlink two-tier HetNets with multi-antenna base stations. Utilizing tools
from stochastic geometry, we derive a tractable expression for the rate
coverage probability of the IN scheme. Then, we optimize the design parameter,
i.e., the degrees of freedom that can be used for IN, to maximize the rate
coverage probability. Specifically, in the asymptotic scenario where the rate
threshold is small, by studying the order behavior of the rate coverage
probability, we characterize the optimal design parameter. For the general
scenario, we show some properties of the optimal design parameter. Finally, by
numerical simulations, we show the IN scheme can outperform both the simple
offloading scheme without interference management and the almost blank
subframes scheme in 3GPP LTE, especially in large antenna regime.Comment: to appear in ICC 2015. arXiv admin note: text overlap with
arXiv:1411.327
Optimal Dynamic Multicast Scheduling for Cache-Enabled Content-Centric Wireless Networks
Caching and multicasting at base stations are two promising approaches to
support massive content delivery over wireless networks. However, existing
scheduling designs do not make full use of the advantages of the two
approaches. In this paper, we consider the optimal dynamic multicast scheduling
to jointly minimize the average delay, power, and fetching costs for
cache-enabled content-centric wireless networks. We formulate this stochastic
optimization problem as an infinite horizon average cost Markov decision
process (MDP). It is well-known to be a difficult problem due to the curse of
dimensionality, and there generally only exist numerical solutions. By using
relative value iteration algorithm and the special structures of the request
queue dynamics, we analyze the properties of the value function and the
state-action cost function of the MDP for both the uniform and nonuniform
channel cases. Based on these properties, we show that the optimal policy,
which is adaptive to the request queue state, has a switch structure in the
uniform case and a partial switch structure in the nonuniform case. Moreover,
in the uniform case with two contents, we show that the switch curve is
monotonically non-decreasing. Then, by exploiting these structural properties
of the optimal policy, we propose two low-complexity optimal algorithms.
Motivated by the switch structures of the optimal policy, to further reduce the
complexity, we also propose a low-complexity suboptimal policy, which possesses
similar structural properties to the optimal policy, and develop a
low-complexity algorithm to compute this policy.Comment: 17 double-column pages; Shorter version appears in ISIT 201
In-medium \mbox{NN}\to \mbox{N}\Delta cross section and its dependence on effective Lagrange parameters in isospin-asymmetric nuclear matter
The in-medium cross section and its differential
cross section in isospin asymmetric nuclear medium are investigated in the
framework of the one-boson exchange model by including the isovector mesons,
i.e., and mesons. Our results show that the in-medium
cross sections are suppressed with density increasing,
and the differential cross sections become isotropic with the density
increasing at the beam energy around the threshold energy. The isospin
splitting on the medium correction factor, is observed for
different channels of , especially around the threshold energy
for all the effective Lagrangian parameters. By analyzing the selected
effective Lagrangian parameters, our results show that the larger effective
mass is, the weaker medium correction is.Comment: 9 pages, 8 figures, accepted by Chin. Phys.
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