247,070 research outputs found
Correlation Functions of Multisite Interaction Spin-S models on the Bethe-like Lattices
Multisite interaction spin-S models in an external magnetic field are studied
recursively on the Bethe-like lattices. The transfer-matrix method is extended
to calculate exactly the two-spin correlation functions. The exact expressions
for the correlation length and magnetic susceptibility are derived for spin-1/2
models. The singularity of the correlation length with critical index
and the proportionality of magnetic susceptibility to correlation length in the
second order phase transition region of spin-1/2 ferromagnetic models on the
Bethe-like lattices are established analytically.Comment: 13 pages, In Press Int. J. Mod. Phys.
Decentralized Fair Scheduling in Two-Hop Relay-Assisted Cognitive OFDMA Systems
In this paper, we consider a two-hop relay-assisted cognitive downlink OFDMA
system (named as secondary system) dynamically accessing a spectrum licensed to
a primary network, thereby improving the efficiency of spectrum usage. A
cluster-based relay-assisted architecture is proposed for the secondary system,
where relay stations are employed for minimizing the interference to the users
in the primary network and achieving fairness for cell-edge users. Based on
this architecture, an asymptotically optimal solution is derived for jointly
controlling data rates, transmission power, and subchannel allocation to
optimize the average weighted sum goodput where the proportional fair
scheduling (PFS) is included as a special case. This solution supports
decentralized implementation, requires small communication overhead, and is
robust against imperfect channel state information at the transmitter (CSIT)
and sensing measurement. The proposed solution achieves significant throughput
gains and better user-fairness compared with the existing designs. Finally, we
derived a simple and asymptotically optimal scheduling solution as well as the
associated closed-form performance under the proportional fair scheduling for a
large number of users. The system throughput is shown to be
, where is the
number of users in one cluster, is the number of subchannels and is
the active probability of primary users.Comment: 29 pages, 9 figures, IEEE JOURNAL OF SELECTED TOPICS IN SIGNAL
PROCESSIN
Queue-Aware Distributive Resource Control for Delay-Sensitive Two-Hop MIMO Cooperative Systems
In this paper, we consider a queue-aware distributive resource control
algorithm for two-hop MIMO cooperative systems. We shall illustrate that relay
buffering is an effective way to reduce the intrinsic half-duplex penalty in
cooperative systems. The complex interactions of the queues at the source node
and the relays are modeled as an average-cost infinite horizon Markov Decision
Process (MDP). The traditional approach solving this MDP problem involves
centralized control with huge complexity. To obtain a distributive and low
complexity solution, we introduce a linear structure which approximates the
value function of the associated Bellman equation by the sum of per-node value
functions. We derive a distributive two-stage two-winner auction-based control
policy which is a function of the local CSI and local QSI only. Furthermore, to
estimate the best fit approximation parameter, we propose a distributive online
stochastic learning algorithm using stochastic approximation theory. Finally,
we establish technical conditions for almost-sure convergence and show that
under heavy traffic, the proposed low complexity distributive control is global
optimal.Comment: 30 pages, 7 figure
Calculating Dilepton Rates from Monte Carlo Simulations of Parton Production
To calculate dilepton rates in a Monte Carlo simulation of ultrarelativistic
heavy ion collisions, one usually scales the number of similar QCD processes by
a ratio of the corresponding differential probabilities. We derive the formula
for such a ratio especially for dilepton bremsstrahlung processes. We also
discuss the non-triviality of including higher order corrections to direct
Drell-Yan process. The resultant mass spectra from our Monte Carlo simulation
are consistent with the semi-analytical calculation using dilepton
fragmentation functions.Comment: 14 pages in RevTex, 3 figures in uuencoded files, LBL-3466
Studying minijets via the dependence of two-particle correlation in azimuthal angle
Following my previous proposal that two-particle correlation functions can be
used to resolve the minijet contribution to particle production in minimum
biased events of high energy hadronic interactions, I study the and
energy dependence of the correlation. Using HIJING Monte Carlo model, it is
found that the correlation in azimuthal angle between
two particles with resembles much like two back-to-back jets as
increases at high colliding energies due to minijet production. It
is shown that , which is related to the relative fraction of
particles from minijets, increases with energy. The background of the
correlation for fixed also grows with energy due to the increase of
multiple minijet production. Application of this analysis to the study of jet
quenching in ultrarelativistic heavy ion collisions is also discussed.Comment: 11 pages Latex text and 8 ps figures, LBL-3349
Quantising Higher-spin String Theories
In this paper, we examine the conditions under which a higher-spin string
theory can be quantised. The quantisability is crucially dependent on the way
in which the matter currents are realised at the classical level. In
particular, we construct classical realisations for the algebra,
which is generated by a primary spin- current in addition to the
energy-momentum tensor, and discuss the quantisation for . From these
examples we see that quantum BRST operators can exist even when there is no
quantum generalisation of the classical algebra. Moreover, we find
that there can be several inequivalent ways of quantising a given classical
theory, leading to different BRST operators with inequivalent cohomologies. We
discuss their relation to certain minimal models. We also consider the
hierarchical embeddings of string theories proposed recently by Berkovits and
Vafa, and show how the already-known strings provide examples of this
phenomenon. Attempts to find higher-spin fermionic generalisations lead us to
examine the whether classical BRST operators for ( odd)
algebras can exist. We find that even though such fermionic algebras close up
to null fields, one cannot build nilpotent BRST operators, at least of the
standard form.Comment: CTP TAMU-24/94, KUL-TF-94/11, SISSA-135/94/E
- …