Average Fade Research Articles

Dose reconstruction using mobile phones

Electronic components inside mobile phones are regarded as useful tools for accident and retrospective dosimetry using optically stimulated luminescence (OSL) and thermoluminescence. Components inside the devices with suitable properties for luminescence dosimetry include, amongst others, ceramic substrates in resistors, capacitors, transistors and antenna switches. Checking the performance of such devices in dosimetric experiments is a crucial step towards developing a reliable dosimetry system for emergency situations using personal belongings. Here, the results of dose assessment experiments using irradiated mobile phones are reported. It will be shown that simple regenerative dose estimates, derived from various types of components removed from different mobile phone models, are consistent with the given dose, after applying an average fading correction factor.

High capacity and excellent cycling stability of branched cobalt oxide nanowires as Li-insertion materials

In this paper, the authors report the high-performance lithium storage properties of branched cobalt oxide nanowires synthesized through a topotactic conversion route. Compared with other nanosized materials, such branched nanostructures not only inherit the advantages of high surface-to-volume ratio and good dispersion but also show better electronic contact and conduction between nanobuilding units. Both features facilitate charge transport and insertion/extraction of Li ions and lead to higher capacity and better cycling stability compared with other transition metal oxide nanomaterials. Under the condition of high current density of 100 mA/g, the reversible capacity after 50 cycles is up to 1043 mAhg−1 with an average fading rate of 0.15% per cycle.

Second-order statistics of SC macrodiversity system operating over Gamma shadowed Nakagami- m fading channels

This paper derives the second-order statistics of SC (selection combining) macrodiversity operating over the Gamma shadowed Nakagami- m fading channels. Macrodiversity system of SC type consists of two microdiversity systems and selection (switching) is based on their output signal power values. Each microdiversity system is of MRC (maximal ratio combining) type with arbitrary number of branches in the presence of correlative Nakagami- m fading. We have derived the infinite-series expressions for LCR (level crossing rate) and AFD (average fading duration) at the output of this system. Numerical results are also presented in order to show the influence of various parameters such as number of the diversity branches at the microcmbiners, fading severity and level of correlation between those branches on the system's statistics, and then compared to the previously published results from this area.

Burst-Error Analysis of Dual-Hop Fading Channels Based on the Second-Order Channel Statistics

The burst-error (BE) rate of dual-hop fading channels under a fixed fade threshold is estimated based on the level crossing rate (LCR) and average fade duration (AFD). The LCR and AFD of the equivalent signal-to-noise ratio (SNR) are first derived for dual-hop Nakagami-m and Weibull fading channels with a fixed-gain amplify-and-forward (AF) relay, where closed-form lower and upper bounds are derived for the LCR and AFD of the Nakagami-m fading channels. Numerical results from theoretical evaluations and Monte Carlo simulations are illustrated to validate the analysis and to compare the performance of the two fading channels.

On the Scatterers' Mobility and Second Order Statistics of Narrowband Fixed Outdoor Wireless Channels

In this paper, we study the temporal behavior of narrowband fixed outdoor wireless channels by modeling the impact of scatterers' mobility on the second order statistics of such channels. We show that the Nakagami-m, gamma, Weibull and lognormal probability density functions (PDFs) can adequately approximate the scatterers' mobility at outdoor environments by comparing the theoretically derived autocorrelation functions (ACFs) with measured ACFs. These theoretical ACFs arise after considering several candidate PDFs for the impact of scatterers mobility. We select that PDF whose ACF provides the best fitting to measurements. The modeling of scatterers' mobility lead us to present analytical expressions for the level crossing rate (LCR) and average fade duration (AFD) together with an exact expression for the power spectral density (PSD).

Mobile-to-mobile fading channels in amplify-and-forward relay systems under line-of-sight conditions: statistical modeling and analysis

This paper deals with the modeling and analysis of narrowband mobile-to-mobile (M2M) fading channels for amplify-and-forward relay links under line-of-sight (LOS) conditions. It is assumed that a LOS component exists in the direct link between the source mobile station (SMS) and the destination mobile station (DMS), as well as in the links via the mobile relay (MR). The proposed channel model is referred to as the multiple-LOS second-order scattering (MLSS) channel model. The MLSS channel model is derived from a second-order scattering process, where the received signal is modeled in the complex baseband as the sum of a single and a double scattered component. Analytical expressions are derived for the mean value, variance, probability density function (PDF), cumulative distribution function (CDF), level-crossing rate (LCR), and average duration of fades (ADF) of the received envelope of MLSS channels. The PDF of the channel phase is also investigated. It is observed that the LOS components and the relay gain have a significant influence on the statistics of MLSS channels. It is also shown that MLSS channels include various other channel models as special cases, e.g., double Rayleigh channels, double Rice channels, single-LOS double-scattering (SLDS) channels, non-line-of-sight (NLOS) second-order scattering (NLSS) channels, and single-LOS second-order scattering (SLSS) channels. The correctness of all analytical results is confirmed by simulations using a high performance channel simulator. Our novel MLSS channel model is of significant importance for the system level performance evaluation of M2M communication systems in different M2M propagation scenarios. Furthermore, our studies pertaining to the fading behavior of MLSS channels are useful for the design and development of relay-based cooperative wireless networks.

Second-Order Statistics for the Envelope of α - κ - μ Fading Channels

Exact closed-form expressions for the level crossing rate (LCR) and the average duration of fades (ADF) of α - κ - μ fading channels are presented. Confirmation of the derived analytical formulations is attained by reducing the general results to some widely known cases (Nakagami-m, Rice, and Weibull). Extra validation is performed by comparison with measurement data and by means of a sample of numerical results obtained by the Monte Carlo simulation.

Analytical second‐order statistics of maximal‐ratio diversity over spatially correlated nakagami fading channels

To evaluate the performance of antenna diversity with maximal‐ratio combining (MRC) over spatially correlated Nakagami fading channels, the average level crossing rate (LCR) and the average fade duration (AFD) of the signal envelope are derived. To characterize the dependence among antenna branches, both the exponential and equal correlation models are considered. Numerical results of the average LCR and AFD are presented for various channel parameters and different numbers of branches.

An Extended Generalized Rice Model for Wireless Communications

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> The multipath component arriving at the mobile receiver cannot explicitly be of homogeneous nature due to nonuniform scattering caused by objects with irregular surfaces having various scattering properties. Another reason, which may contribute to this inhomogeneity, is the nonlinearity of the propagation medium. In this paper, a new general statistical small-scale model for wireless channels, which is affected by nonfrequency-selective fading, is proposed to describe the received signal level by taking into account the aforementioned effects. The advantageous physical interpretation of the model is introduced, and solutions for the probability density functions (PDFs) of the envelope and the phase as well as the cumulative distribution function (CDF) of the envelope are derived. In addition, the second-order statistics have been studied, and solutions for the level-crossing rate (LCR) and the average duration of fades (ADF) are obtained. The usefulness of the model is demonstrated by providing the best fit, compared with widely known models, to certain measurement data corresponding to a variety of real-world channel conditions. Furthermore, its validation is confirmed by simulation. Consequently, the extended generalized Rice model is applicable in wireless channels subjected to various conditions, and it can be preferable because of its outperformance. </para>

Characterisation of narrowband communication channels on the human body at 2.45 GHz

A number of propagation measurements of belt-to-head and belt-to-wrist channels have been performed with inverted F-antennas, planar inverted F-antennas (PIFAs), monopole, loop and dipole antennas, and propagation models were derived. It is found that the channel can be separated into short-term and long-term fading components. The short-term fading can generally be modelled with a Rician distribution, whereas in many cases, long-term fading is lognormal, the best distribution is dependent on the channel, antenna type and orientation. Variation of channel parameters with time is studied and an alternative approach to modelling long-term fading deterministically is suggested. Power spectral densities, autocorrelation functions, fade probabilities, level crossing rates and average fade durations are also investigated.

Three-dimensional multipath shape factors for spatial modeling of wireless channels

This paper develops a generalized theory of multipath shape factors used to study the small-scale fading statistics in three-dimensional non-omnidirectional multipath channels. Previous results for horizontal propagation channels are special cases of the present framework when the multipath angular power density (APD) and the travel direction of the mobile unit are confined to a plane. The three-dimensional multipath shape factors are derived in terms of the first three degrees of unnormalized spherical harmonics of the APD. Based on the so derived shape factors, analytical expressions for the level crossing rate, average fade duration and approximate envelope correlation are derived for a Rayleigh fading three-dimensional channel.

An Extended Loo Model with Inhomogeneous Scattering and its Statistical Properties

In this paper, an analytical model for a land mobile satellite channel is introduced that takes into consideration the Generalized Rice model for characterizing the small scale (fast) fading. The proposed model is enriched by the property of non-homogeneous scattering allowing higher flexibility and a more comprehensive description of the channel. In addition, a lognormal process affects the line of sight component, in order to characterize the large scale (slow) fading due to the shadowing effect caused by physical obstacles such as trees and small buildings. The main statistical properties of the model are investigated. Exact solutions for the power spectral density, the probability density function of the envelope and phase, and the cumulative distribution function of the envelope are derived. Approximate solutions for the level crossing rate and the average duration of fades are obtained. The validation of the model is confirmed by simulation and demonstrated by providing the best fit to certain measurement data corresponding to conditions of light and heavy shadowing. As a conclusion, the extended Loo model is applicable in realistic land mobile satellite channels subjected to various conditions and can be preferable because of its outperformance.

The Weibull–Lognormal Fading Channel: Analysis, Simulation, and Validation

In frequency-nonselective fading channels, the partial waves arriving at the mobile receiver cannot explicitly be of homogeneous nature due to nonuniform scattering caused by objects of specific reflective nature. Moreover, shadowing influences the received signal level by causing slow variations to its local mean. In this paper, we investigate a mixture stochastic process accounting for both inhomogeneous scattering and shadow fading by multiplying a Weibull process with a lognormal process. The first process models the possible scattering nonuniformities of the channel, whereas the second process accounts for the slow-term variations of the local mean due to shadowing. An exact solution for the composite probability density function (pdf) will be given, together with approximate solutions for the second-order statistics, i.e., the level crossing rate (LCR) and the average duration of fades (ADF). The approximate solutions come from the assumption of a slowly time-varying lognormal process compared with the Weibull process, the validity of which will be tested via an efficient deterministic simulation scheme that implements the analytical model on a digital computer. Finally, a curve fitting of the LCR to real-world data drawn from channel measurements will demonstrate the flexibility and usefulness of the proposed model.

Capacity fades analysis of MIMO Rician channels in mobile ad hoc networks

Multiple-input multiple-output (MIMO) mobile ad hoc networks have been receiving increased attention in both commercial and military applications. Just as in cellular networks, MIMO technologies can benefit ad hoc networks by providing the diversity and capacity advantages as well as the spatial degree of freedom in designing the media access control (MAC) protocol. However, one fundamental issue of MIMO mobile ad hoc networks is how to accurately model the impact of spatial/temporal channel correlation in the mobile-to-mobile communications environment. In such a channel, line-of-sight (LOS) components and different scattering environments will affect both ergodic capacity and average capacity fading duration of the MIMO system. In this paper, based on the double-ring scattering model with LOS components we suggest a sum-of-sinusoids MIMO mobile-to-mobile channel simulation method, which can characterize the spatial/temporal channel correlation and Rician fading effect. Our numerical results show the ergodicity of our proposed channel model, correctness of the analytical channel correlation, effect of spatial correlation on channel capacity, impact of the number of antennas and scatterers on capacity, capacity distribution, and level crossing rate (LCR) and average fading duration (AFD) of MIMO Rician channel capacity. We also show the impacts of Rician factor K on channel capacity, probability density function (PDF), LCR, and AFD.

End-to-end performance of dual-hop semi-blind relaying systems with partial relay selection

The end-to-end performance of dual-hop cooperative links using semi-blind (fixed gain) relays and partial relay selection is investigated. The selection scheme considers that the source monitors the connectivity among the nodes (relays) of the first hop only. This scheme is interesting in practical ad-hoc and sensor networks. In our analysis, compact closed-form expressions are obtained for the outage probability, probability density function, moment generating functions, and generalized moments of the end-to-end signal-to-noise ratio(SNR), from which other relevant statistics that well-describe the distribution of the end-to-end SNR, such as mean, variance, kurtosis, skewness, and amount of fading, can also be deduced. Furthermore, the dynamic behavior of the end-to-end envelope is investigated, and the corresponding level crossing rate and average fade duration are obtained in an exact manner. Also, tight lower and upper bounds for these second-order statistics are presented in closed-form. Numerical results illustrating the system's performance in terms of the above metrics are provided, and the influence of the relay selection on performance is analyzed and discussed. For instance, it is shown that the power imbalance between the hops may have positive or negative effects on the overall system performance irrespective of the number of selected relays.

New performance results for multiuser optimum combining in the presence of rician fading

This paper analyzes the performance of multiuser optimum combining systems. We consider the practical scenario where the desired user undergoes Rician fading, and the received signals are corrupted by Rayleigh-faded interference and noise. We derive new closed-form expressions for the symbol error rate, ergodic capacity, level crossing rate, and average fade duration. These results are given as either accurate approximations or asymptotic (eg. high signal-to-noise ratio (SNR)) expressions, and are based on new closed-form statistical properties which we derive for the signal-to-interference noise ratio (SINR). Specifically, we derive exact first order expansions and accurate gamma approximations for the SINR distribution, as well as simplified exact expressions for the moments. We also present closed-form expressions for key performance parameters such as the diversity order and array gain, wideband slope, and high SNR power offset, and analyze the impact of the Rician K-factor, which is shown in all cases to yield a performance improvement.

Characterization of time variation on 1.9 GHz fixed wireless channels in suburban macrocell environments

We observed temporal fading on 1.9 GHz fixed wireless channels during short-term measurements at 107 different locations in a suburban macrocell environment characterized by flat terrain and heavy foliage in order to determine how the rate of fading varies with average wind speed and distance. For each location, we estimated: (1) the Ricean K-factor using a moment-based estimator and (2) an equivalent Doppler frequency which is related to the maximum Doppler frequency by a factor that depends upon the shape of the Doppler spectrum. We did so by fitting the measured level crossing rate (LCR) and average fade duration (AFD) distributions to expressions normally justified for mobile wireless links using a method recently proposed by Feick, Valenzuela and Ahumada (2007). As has been observed at other sites, the Ricean K-factor decreased with both average wind speed and distance. However, we found that the equivalent Doppler frequency was effectively uncorrelated with wind speed and noticeably increased with distance. Similar measurements at other sites will be required to determine the extent to which these trends are affected by foliage density and tower height.

MRCS -- selecting maximal ratio combined signals: a practical hybrid diversity combining scheme

This paper presents and investigates a general diversity combining scheme, here named MRCS, in which maximal-ratio combined signals are chosen on a selection combining basis. This combining method has a simple implementation and a tractable analytical formulation that can be directly applied to situations in which site selection exists. A general analysis of the probability distribution (reliability), level crossing rate, and average fade duration at the output of the combiner is provided, along with examples for a Nakagami-m fading environment. The main result of the present work, however, is the derivation of an exact, easy-to-evaluate closed-form expression for the mean signal-to-noise ratio at the output of the combiner. Such an expression is applicable for conditions in which the product of the number of maximal-ratio combining branches and the Nakagami-m parameter is an integer and it generalizes a result presented elsewhere in the literature. The formulations derived here find a direct applicability in the dimensioning of practical wireless networks.

On the second order statistics of the multihop rayleigh fading channel

Second order statistics provides a dynamic representation of a fading channel and plays an important role in the evaluation and design of the wireless communication systems. In this paper, we present a novel analytical framework for the evaluation of important second order statistical parameters, as the level crossing rate (LCR) and the average fade duration (AFD) of the amplify-and-forward multihop Rayleigh fading channel. More specifically, motivated by the fact that this channel is a cascaded one and can be modeled as the product of N fading amplitudes, we derive novel analytical expressions for the average LCR and the AFD of the product of N Rayleigh fading envelopes (or of the recently so-called NRayleigh channel). Furthermore, we derive simple and efficient closed-form approximations to the aforementioned parameters, using the multivariate Laplace approximation theorem. It is shown that our general results reduce to the corresponding ones of the specific dual-hop case, previously published. Numerical and computer simulation examples verify the accuracy of the presented mathematical analysis and show the tightness of the proposed approximations.

Preparation and electrochemical performance of micro-nanostructured nickel

Abstract Micro-nanostructured nickel has been prepared as anode materials for Li ion batteries, via a rheological phase reaction method. Ni 2 C 2 O 4 · x H 2 O ( x = 2 or 2.5) as precursors are obtained from the solid–liquid rheological mixture of (NH4) 2 C 2 O 4 ·H 2 O and Ni(NO 3 ) 2 . The nickel powders are prepared by thermal decomposition of the precursors. The structural, morphological and electrochemical performance are investigated by means of thermogravimetry (TG), differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscope (TEM) and typical electrochemical tests. The micro-nanostructured nickel displays an initial discharge capacity of 457 mAh g −1 . It also has a remarkable cycling stability with an average capacity fade of 0.17% per cycle from 13th to 50th cycle in 0.01–3.00 V versus Li at a constant current density of 100 mA g −1 .