3,964 research outputs found
Index Policies for Optimal Mean-Variance Trade-Off of Inter-delivery Times in Real-Time Sensor Networks
A problem of much current practical interest is the replacement of the wiring
infrastructure connecting approximately 200 sensor and actuator nodes in
automobiles by an access point. This is motivated by the considerable savings
in automobile weight, simplification of manufacturability, and future
upgradability.
A key issue is how to schedule the nodes on the shared access point so as to
provide regular packet delivery. In this and other similar applications, the
mean of the inter-delivery times of packets, i.e., throughput, is not
sufficient to guarantee service-regularity. The time-averaged variance of the
inter-delivery times of packets is also an important metric.
So motivated, we consider a wireless network where an Access Point schedules
real-time generated packets to nodes over a fading wireless channel. We are
interested in designing simple policies which achieve optimal mean-variance
tradeoff in interdelivery times of packets by minimizing the sum of
time-averaged means and variances over all clients. Our goal is to explore the
full range of the Pareto frontier of all weighted linear combinations of mean
and variance so that one can fully exploit the design possibilities. We
transform this problem into a Markov decision process and show that the problem
of choosing which node's packet to transmit in each slot can be formulated as a
bandit problem. We establish that this problem is indexable and explicitly
derive the Whittle indices. The resulting Index policy is optimal in certain
cases. We also provide upper and lower bounds on the cost for any policy.
Extensive simulations show that Index policies perform better than previously
proposed policies
Exploring many-body localization in quantum systems coupled to an environment via Wegner-Wilson flows
Inspired by recent experiments on many-body localized systems coupled to an
environment, we apply a Flow Equation method to study the problem of a disorder
chain of spinless fermions, coupled via density-density interactions to a
second clean chain of spinless fermions. In particular, we focus on the
conditions for the onset of a many-body localized phase in the clean sector of
our model by proximity to the dirty one. We find that a many-body localization
proximity effect in the clean component is established when the density of
dirty fermions exceeds a threshold value. From the flow equation method we find
that, similar to many-body localization in a single chain, the many-body
localization proximity effect is also described by an extensive set of local
integrals of motion. Furthermore, by tuning the geometry of the inter-chain
couplings, we show that the dynamics of the model is ruled, on intermediate
time scales, by an emergent set of quasi-conserved charges.Comment: 22 pages, 7 figure
Pathwise Performance of Debt Based Policies for Wireless Networks with Hard Delay Constraints
Hou et al have introduced a framework to serve clients over wireless channels
when there are hard deadline constraints along with a minimum delivery ratio
for each client's flow. Policies based on "debt," called maximum debt first
policies (MDF) were introduced, and shown to be throughput optimal. By
"throughput optimality" it is meant that if there exists a policy that fulfils
a set of clients with a given vector of delivery ratios and a vector of channel
reliabilities, then the MDF policy will also fulfill them. The debt of a user
is the difference between the number of packets that should have been delivered
so as to meet the delivery ratio and the number of packets that have been
delivered for that client. The maximum debt first (MDF) prioritizes the clients
in decreasing order of debts at the beginning of every period. Note that a
throughput optimal policy only guarantees that \begin{small} \liminf_{T \to
\infty} \frac{1}{T}\sum_{t=1}^{T} \mathbbm{1}\{\{client nt} \} \geq q_{i} \end{small}, where the right hand side
is the required delivery ratio for client . Thus, it only guarantees that
the debts of each user are , and can be otherwise arbitrarily large. This
raises the interesting question about what is the growth rate of the debts
under the MDF policy. We show the optimality of MDF policy in the case when the
channel reliabilities of all users are same, and obtain performance bounds for
the general case. For the performance bound we obtain the almost sure bounds on
for all , where
Dielectric properties of Li2O-3B2O3 glasses
The frequency and temperature dependence of the dielectric constant and the
electrical conductivity of the transparent glasses in the composition
Li2O-3B2O3 (LBO) were investigated in the 100 Hz- 10 MHz frequency range. The
dielectric constant and the loss in the low frequency regime were electrode
material dependent. Dielectric and electrical relaxations were respectively
analyzed using the Cole-Cole and electric modulus formalisms. The dielectric
relaxation mechanism was discussed in the framework of electrode and charge
carrier (hopping of the ions) related polarization using generalized Cole-Cole
expression. The frequency dependent electrical conductivity was rationalized
using Jonscher's power law. The activation energy associated with the dc
conductivity was 0.80 \pm 0.02 eV, which was ascribed to the motion of Li+ ions
in the glass matrix. The activation energy associated with dielectric
relaxation was almost equal to that of the dc conductivity, indicating that the
same species took part in both the processes. Temperature dependent behavior of
the frequency exponent (n) suggested that the correlated barrier hopping model
was the most apposite to rationalize the electrical transport phenomenon in
Li2O-3B2O3 glasses. These glasses on heating at 933 K/10h resulted in the known
non-linear optical phase LiB3O5.Comment: 32 pages, 13 figure
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