8,412 research outputs found
On the Capacity of the Finite Field Counterparts of Wireless Interference Networks
This work explores how degrees of freedom (DoF) results from wireless
networks can be translated into capacity results for their finite field
counterparts that arise in network coding applications. The main insight is
that scalar (SISO) finite field channels over are analogous
to n x n vector (MIMO) channels in the wireless setting, but with an important
distinction -- there is additional structure due to finite field arithmetic
which enforces commutativity of matrix multiplication and limits the channel
diversity to n, making these channels similar to diagonal channels in the
wireless setting. Within the limits imposed by the channel structure, the DoF
optimal precoding solutions for wireless networks can be translated into
capacity optimal solutions for their finite field counterparts. This is shown
through the study of the 2-user X channel and the 3-user interference channel.
Besides bringing the insights from wireless networks into network coding
applications, the study of finite field networks over also
touches upon important open problems in wireless networks (finite SNR, finite
diversity scenarios) through interesting parallels between p and SNR, and n and
diversity.Comment: Full version of paper accepted for presentation at ISIT 201
Dynamic exponent in Extremal models of Pinning
The depinning transition of a front moving in a time-independent random
potential is studied. The temporal development of the overall roughness w(L,t)
of an initially flat front, , is the classical means to
have access to the dynamic exponent. However, in the case of front propagation
in quenched disorder via extremal dynamics, we show that the initial increase
in front roughness implies an extra dependence over the system size which comes
from the fact that the activity is essentially localized in a narrow region of
space. We propose an analytic expression for the exponent and confirm
this for different models (crack front propagation, Edwards-Wilkinson model in
a quenched noise, ...).Comment: RevTex, 3 figures .ep
Musical chairs: a comment on the credit crisis.
Uncertainty –that is, a rise in unknown and immeasurable risk rather than the measurable risk that the financial sector specializes in managing– is at the heart of the recent liquidity crisis. The financial instruments and derivative structures underpinning the recent growth in credit markets are complex. Because of the rapid proliferation of these instruments, market participants cannot refer to a historical record to measure how these financial structures will behave during a time of stress. These two factors, complexity and lack of history, are the preconditions for rampant uncertainty. We explain how a rise in uncertainty can cause a liquidity crisis and discuss central bank policies in this context.
Strong light fields coax intramolecular reactions on femtosecond time scales
Energetic H ions are formed as a result of intra-molecular
rearrangement during fragmentation of linear alcohols (methanol, ethanol,
propanol, hexanol, and dodecanol) induced by intense optical fields produced by
100 fs long, infrared, laser pulses of peak intensity 8 W
cm. Polarization dependent measurements show, counterintuitively, that
rearrangement is induced by the strong optical field within a single laser
pulse, and that it occurs before Coulomb explosion of the field-ionized
multiply charged alcohols
Dimming cellular networks
We propose a novel technique called dimming to improve the energy efficiency of cellular networks by reducing the capacity, services, and energy consumption of cells without turning off the cells. We define three basic methods to dim the network: coverage, frequency, and service dimming. We construct a multi-time period optimization problem to implement frequency dimming and extend it to implement both frequency and service dimming together. We illustrate the ability of dimming techniques to adapt the capacity and network services in proportion to the dynamic spatial and temporal load resulting in significant energy savings through numerical results for a sample network. ©2010 IEEE
Psychoacoustic Test to Determine Sound Quality Metric Indicators of Rotorcraft Noise Annoyance
Noise certification metrics such as Effective Perceived Noise Level and Sound Exposure Level are used to ensure that helicopters meet regulations, but these metrics may not be good indicators of annoyance since noise complaints against helicopters persist. Sound quality (SQ) metrics, specifically fluctuation strength, tonality, impulsiveness, roughness, and sharpness, are explored to determine their relationship with annoyance. A psychoacoustic test was conducted at the NASA Langley Research Center Exterior Effects Room to assess annoyance to helicopter-like sounds over a range of SQ metric values. The amplitude, phase, and frequency of the AS350 helicopter main and tail rotor blade passage signal harmonics were manipulated to produce 105 unique helicopter-like sounds with prescribed values of SQ metrics. All sounds were set to roughly the same loudness level. These sounds were played to 40 subjects who rated each sound for annoyance. Analyses given in this paper point to which SQ metrics are important to the helicopter noise annoyance response
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