Visual acuity improvement following fading and feedback training—II. Relationship to changes in refractive error
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Rose bengal
Reactive dye
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This report describes an experimental study of fading at 9.6 GHz on a fixed slanted line-of-sight path simulating aircraft-to-ground propagation conditions. Recordings of signal fading were made over about one year. The data were analyzed to provide a statistical description of the fading, and to compare some of these statistics with those obtained elsewhere under different climatic and terrain conditions. The results include the probabilities of occurrence of fading rates and fade durations at several signal levels below the hourly median level. The extent to which fading behavior depends on time-of-day and time-of-year was investigated, along with the correlation of hourly average fade duration and hourly fading rate.
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This paper analyses the UAV(Unmanned Aerial Vehicle) TTC channel model,the generation mechanism and characteristics of three kinds of small-scale-fading including frequency selective fading,frequency selective fading and time selective fading,discusses anti-fading methods,and finally demonstrates OFDM,interleaving and coded modulation,and spatial diversity are potential anti-fading techniques in UAV TTC.
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Prolonged fixation leads to fading of a peripheral target (Troxler, 1804). Moreover, fading can be induced more rapidly in other contexts: yellow disks disappear when presented against dynamic background elements (Bonneh et al., 2001), colored circles disappear as a function of attentional modulation (Lou, 1999), and display transients near a target induce fading (Kanai & Kamitani, 2003). We have found that full-color, low-pass filtered photographs of natural scenes fade to a uniform hue and luminance in less than 30 seconds when observers maintain fixation. Moreover, fading is more complete and more rapid when transient high-contrast splotches flashed repeatedly on the display. Here we show that perceived fading of scenes is induced by the offset of high-contrast splotches superimposed on the scene but not by the onset of such splotches. Given that the appearance of splotches does not induce fading, our results are not readily explained by transient-induced fading. Because the fading can be induced nearly instantaneously in some conditions, it does not appear to reflect a slow adaptation process such as Troxler fading. Finally, we find that the presence of static splotches does not facilitate fading, suggesting that the results are not readily attributable to contrast gain-control settings. Instead, building on May et al (2003), we argue that the fading of entire scenes can be induced by a contrast decrement. Most of our induced scene fading effects are sufficiently robust that they can be experienced in a single trial.
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The function of the fading machine is to simulate the frequency-selective fading which is characteristic of long-distance short-wave radio channels. Fading of this type is usually due to interference between waves which have traversed multiple paths between sender and receiver, and its effect is often to produce distortion of the modulation of radio waves. Many radio transmission systems have been devised with the object of minimizing the effects of selective fading; the fading machine enables different systems to be compared in the laboratory under precisely controlled fading conditions. The equipment described in the paper incorporates three transmission paths, the group time-delay differences between which may be varied in steps from 0 to 2 milliseconds. The phase differences between the paths may be varied manually (“static” fading) or continuously (“dynamic” fading), with fading rates ranging from 0.1 to 10 fades per second. Random noise, either fading or non-fading, may be included so as to synthesize a complete short-wave radio channel. The fading machine may also be used to simulate diversity reception obtained by spaced aerials. Examples are given of the use of the equipment to assess the merits of double-sideband, single-sideband and frequency-modulated transmission systems with telephony or telegraphy modulation, under conditions of severe selective fading and high noise level.
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In this paper we present amount of fading for composite fading channel. For this purpose two probability density functions have been taken. One is Weibull function and another one is log-normal function. Both functions have been combined to find the new composite fading channel called as Weibull/Log-Normal fading channel. Further to find amount of fading first we need equation in simplest form of composite fading channel which is really a tedious work to find. So Holtzman calculation method has been considered to find the equation in simplest form of composite fading channel. The achieved simplest-form equation is precise for undesirable light shadowed fading as well as multipath fading channels which further enable to develop the amount of fading parameter. Finally amount of fading has been derived from obtained Weibull/Log-Normal composite function. Further probability density function and Cumulative distribution function of simplest form equation have been plotted and analysed. At the end amount of fading parameter for composite Weibull/Log-Normal function has been plotted and analysed.
Fading distribution
Weibull fading
Channel state information
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Fading distribution
Diversity scheme
Channel state information
Diversity combining
Maximal-ratio combining
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Several fading models, including the general fading model, the Rayleigh fading model, and the simple three-path fading model are simulated to study the effect of fading in radio communication links. These models are implemented in hardware using computer control. The effect of fading is demonstrated using speech as the test signal. For the Rayleigh fading model, it is found that, when the automatic gain control (AGC) of the receiver is off, the received signal fades as in real-life transmission. When the AGC is on, the effect of fading is not noticeable most of the times but the noise level goes up when fading occurs, as expected. When the fading is severe, it is found that AGC cannot totally compensate for the fading; this is also expected. Other models are tested in a similar manner and yield expected results.< >
Fading distribution
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Rician fading
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Our investigations in understanding the physical mechanism of the LOS microcellular channel reveal that the principle source of slow fading is the random interference pattern of a few strong contributing paths which causes what we term as deceptive shadowing. It is shown that in a comparative curve fitting of slow fading, Nakagami distribution also fits the fading data. Also in non-zero mean complex Gaussian channels, the segregation of slow and fast fading using moving mean normalization is illusionary as the fading parameters in both cases remain the same. We argue that radio link performance calculation based on the segregation of slow and fast fading are therefore unnecessary for Rice/lognormal or Nakagami/lognormal fading models and may be avoided in order to simplify complex analyses.
Log-normal distribution
Fading distribution
Nakagami distribution
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Weibull fading
Shadow mapping
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A clinical pilot study comparing refractive error and preferential looking (PL) visual acuity in infants 2 to 12 months of age is described. The PL visual acuity of 30 normal infants without significant visual disorders was assessed using the Acuity Card Procedure. Near retinoscopy was used to determine refractive error. Infants of this sample had monocular PL visual acuities similar to those established by McDonald et al. in a laboratory setting. Statistical analysis of the data for this sample of infants showed that refractive error did not change systematically from 2 to 12 months of age. We have found that results obtained with the Acuity Card Procedure in a clinical setting agree with infant visual acuity as described in the research literature. Refractive error did not correlate with changes in PL visual acuity in infants 2 to 12 months of age.
Retinoscopy
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