485 research outputs found
Upper bounds on the capacity of deletion channels using channel fragmentation
Cataloged from PDF version of article.We study memoryless channels with synchronization
errors as defined by a stochastic channel matrix allowing for
symbol drop-outs or symbol insertions with particular emphasis
on the binary and non-binary deletion channels. We offer
a different look at these channels by considering equivalent
models by fragmenting the input sequence where different
subsequences travel through different channels. The resulting
output symbols are combined appropriately to come up with an
equivalent input–output representation of the original channel
which allows for derivation of new upper bounds on the channel
capacity. We consider both random and deterministic types
of fragmentation processes applied to binary and nonbinary
deletion channels. With two specific applications of this idea,
a random fragmentation applied to a binary deletion channel
and a deterministic fragmentation process applied to a nonbinary
deletion channel, we prove certain inequality relations among the
capacities of the original channels and those of the introduced
subchannels. The resulting inequalities prove useful in deriving
tighter capacity upper bounds for: 1) independent identically
distributed (i.i.d.) deletion channels when the deletion probability
exceeds 0.65 and 2) nonbinary deletion channels. Some extensions
of these results, for instance, to the case of deletion/substitution
channels are also explored
Spectrally Effiecient Alamouti Code Structure in Asynchronous Cooperative Systems
Cataloged from PDF version of article.A relay communication system with two amplify and
forward (AF) relays under flat fading channel conditions is considered
where the signals received from the relays are not necessarily
time aligned. We propose a new time-reversal (TR)-based scheme
providing an Alamouti code structure which needs a smaller overhead
in transmitting every pair of data blocks in comparison with
the existing schemes and, as a result, increases the transmission
rate significantly (as much as 20%) in exchange for a small performance
loss. The scheme is particularly useful when the delay
between the two relay signals is large, e.g., in typical underwater
acoustic (UWA) channels
Achievable Rates for Noisy Channels with Synchronization Errors
Cataloged from PDF version of article.We develop several lower bounds on the capacity of binary input symmetric output channels with synchronization errors, which also suffer from other types of impairments such as substitutions, erasures, additive white Gaussian noise (AWGN), etc. More precisely, we show that if a channel suffering from synchronization errors as well as other type of impairments can be decomposed into a cascade of two component channels where the first one is another channel with synchronization errors and the second one is a memoryless channel (with no synchronization errors), a lower bound on the capacity of the original channel in terms of the capacity of the component synchronization error channel can be derived. A primary application of our results is that we can employ any lower bound derived on the capacity of the component synchronization error channel to find lower bounds on the capacity of the (original) noisy channel with synchronization errors. We apply the general ideas to several specific classes of channels such as synchronization error channels with erasures and substitutions, with symmetric q-ary outputs and with AWGN explicitly, and obtain easy-to-compute bounds. We illustrate that, with our approach, it is possible to derive tighter capacity lower bounds compared to the currently available bounds in the literature for certain classes of channels, e.g., deletion/substitution channels and deletion/AWGN channels (for certain signal-to-noise ratio (SNR) ranges). © 2014 IEEE
Achieving Delay Diversity in Asynchronous Underwater Acoustic (UWA) Cooperative Communication Systems
Cataloged from PDF version of article.In cooperative UWA systems, due to the low speed
of sound, a node can experience significant time delays among
the signals received from geographically separated nodes. One
way to combat the asynchronism issues is to employ orthogonal
frequency division multiplexing (OFDM)-based transmissions at
the source node by preceding every OFDM block with an
extremely long cyclic prefix (CP) which reduces the transmission
rates dramatically. One may increase the OFDM block length
accordingly to compensate for the rate loss which also degrades
the performance due to the significantly time-varying nature of
UWA channels. In this paper, we develop a new OFDM-based
scheme to combat the asynchronism problem in cooperative
UWA systems without adding a long CP (in the order of the
long relative delays) at the transmitter. By adding a much
more manageable (short) CP at the source, we obtain a delay
diversity structure at the destination for effective processing and
exploitation of spatial diversity by utilizing a low complexity
Viterbi decoder at the destination, e.g., for a binary phase shift
keying (BPSK) modulated system, we need a two-state Viterbi
decoder. We provide pairwise error probability (PEP) analysis
of the system for both time-invariant and block fading channels
showing that the system achieves full spatial diversity. We find
through extensive simulations that the proposed scheme offers a
significantly improved error rate performance for time-varying
channels (typical in UWA communications) compared to the
existing approaches
Validating and assessing the reaction of medical students toward team-based learning
The aim of this study was to evaluate the psychometric properties of tools �Team-Based Learning Student Assessment Tool�, Classroom Engagement Survey (CES) and to assess the reaction of learners toward TBL sessions at Tehran University of Medical Sciences. This descriptive study was done in 2013. The first step was to assess the reliability and validity of the tools. TBL-SAI questionnaire include 39 items, and CES consists 8 items. The validity was assessed through Delphi rounds by experts and reliability, through internal consistency and Test-Retest approach. Then, the reaction of medical students (N=78) was assessed concerning the aspects of team-based learning sessions through TBL-SAI and CES. The data were analyzed through descriptive tests. Our results have study confirmed the TBL-SAI and CEA validity. The tools �reliability was approved through: TBL-SAI Cronbach's alpha=0.79, CES Cronbach�s alpha=0.71 and TBLSAI ICC=0.82, CES ICC=0.75. The result of the second phase showed the TBLSAI scores of participation were appropriate concerning TBL session (159.60�12.89) According to confirmed validity of tools, these can be used in researches related to team-based learning in Iran. It could facilitate assessing the learners� reaction of team-based learning studies at Iranian medical science universities. In the present study, the reaction of students who participate in TBL sessions had been positive and their participation, satisfaction, and accountability had been improved. � 2016 Tehran University of Medical Sciences. All rights reserved
Suspended nanocrystalline diamond ridge waveguides designed for the mid-infrared
A comprehensive study and design of air-clad suspended ridge diamond waveguides for operation across the 2.5–16 µm spectral range is presented, specifically targeting nanocrystalline diamond (NCD) thin films directly grown on silicon substrates. Three film thicknesses of 520, 1000 and 2000 nm are considered, to cover overlapping sub-bands of 2.5–5, 4–9 and 8–16 µm, respectively. Within each sub-band, the waveguide dimensions for single mode quasi-TE operation are found and the waveguide material losses and bending losses are estimated at each design point. In addition, in each case the minimum required undercut depth and etch hole placement for optical isolation of the waveguide mode from the silicon substrate is also quantified. We also estimate the losses associated with scattering from surface roughness, which is an unavoidable byproduct of the NCD thin film growth process. Our results indicate that despite the relatively low film thickness-to-wavelength ratio, mechanically stable waveguides with good optical confinement and low material and bending losses can be realised to cover the full 2.5–16 µm range. In addition, scattering loss estimations predict a drastic drop in roughness-induced scattering losses above 6 µm, even for relatively rough films. In addition to highlighting the utility of suspended NCD as a versatile platform for mid-infrared integrated photonics, the approaches and results presented here can be used to inform the design of suspended air-clad waveguides in other material platforms
Pressure Gradients Driving Ion Transport in the Topside Martian Atmosphere
An edited version of this paper was published by AGU. Copyright 2019 American Geophysical Union.Magnetic and thermal pressure gradient forces drive plasma flow in the topside ionosphere of Mars. Some of this flow can contribute to ion loss from the planet and thus affect atmospheric evolution. MAVEN measurements of the magnetic field, electron density, and electron temperature, taken over a 3‐year time period, are used to obtain averaged magnetic and thermal pressures in the topside ionosphere versus altitude, solar zenith angle, and latitude. Magnetic pressures are several times greater than thermal pressures for altitudes greater than about 300 km; that is, the plasma beta is less than one. The total pressure increases with altitude in the ionosphere and decreases with increasing solar zenith angle. Using these pressure patterns in the dayside ionosphere to estimate the pressure gradient force in the fluid momentum equation, we estimate horizontal day‐to‐night plasma flow speeds of a few kilometers per second near 400 km
The localization of amyloid precursor protein to ependymal cilia in vertebrates and its role in ciliogenesis and brain development in zebrafish
Amyloid precursor protein (APP) is expressed in many tissues in human, mice and in zebrafish. In zebrafish, there are two orthologues, Appa and Appb. Interestingly, some cellular processes associated with APP overlap with cilia-mediated functions. Whereas the localization of APP to primary cilia of in vitro-cultured cells has been reported, we addressed the presence of APP in motile and in non-motile sensory cilia and its potential implication for ciliogenesis using zebrafish, mouse, and human samples. We report that Appa and Appb are expressed by ciliated cells and become localized at the membrane of cilia in the olfactory epithelium, otic vesicle and in the brain ventricles of zebrafish embryos. App in ependymal cilia persisted in adult zebrafish and was also detected in mouse and human brain. Finally, we found morphologically abnormal ependymal cilia and smaller brain ventricles in appa-/-appb-/- mutant zebrafish. Our findings demonstrate an evolutionary conserved localisation of APP to cilia and suggest a role of App in ciliogenesis and cilia-related functions
Assessment of Bond Strength between Metal Brackets and Non-Glazed Ceramic in Different Surface Treatment Methods
Objective: The aim of this study was to evaluate the bond strength between metal brackets and non-glazed ceramic with three different surface treatment methods.Materials and Methods: Forty-two non-glazed ceramic disks were assigned into three groups. Group I and II specimens were etched with 9.5% hydrofluoric acid. Subsequently in group I, silane and adhesive were applied and in group II, bonding agent was used only.In group III, specimens were treated with 35% phosphoric acid and then silane and adhesive were applied. Brackets were bonded with light-cured composites. The specimens were stored in water in room temperature for 24 hours and then thermocycled 500 times between 5°C and 55°C.Results: The difference of tensile bond strength between groups I and III was not significant(P=0.999). However, the tensile bond strength of group II was significantly lower than groups I, and III (P<0.001). The adhesive remnant index scores between the threegroups had statistically significant differences (P<0.001).Conclusion: With the application of scotch bond multi-purpose plus adhesive, we can use phosphoric acid instead of hydrofluoric acid for bonding brackets to non-glazed ceramic restorations
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