On a Distributed Cognitive MAC Protocol for IEEE 802.11s Wireless Mesh Networks

A distributed frequency agile medium access control (MAC) extension to the IEEE 802.11s for the next generation wireless mesh networks is proposed. The introduced protocol enhancements are capable of concurrent deployment of existing frequency opportunities in order to coordinate simultaneous data transmissions. The root concept is mainly based on the deployment of well-known ISM frequency bands, where the legacy 802.11-based wireless equipments operate, as the common control channel in order to establish contemporaneous transmissions. We apply the aforementioned key concept to the IEEE 802.11s common channel framework to attain two important goals: To improve the channel utilization using the concept of cognitive radio, and to lower the access delay. Through extensive event-driven simulations, taking into account primary user appearance in non-ISM frequency bands, performance of the proposed MAC enhancement is evaluated showing its higher efficiency compared to the existing solutions, in addition to its better wireless medium management.     

Miscibility and tack of blends of poly (vinylpyrrolidone)/acrylic pressure-sensitive adhesive

In this study, miscibility and tack of blends of poly (vinylpyrrolidone) (PVP)/acrylic pressure-sensitive adhesive (PSA) were evaluated. For this purpose, appropriate amounts of PVP (2–30% w/w) were added to an acrylic PSA to obtain visually homogeneous solution. The resulting solution was evenly applied on a polyethylene terephthalate (PET) film with final specific thicknesses of 10, 40, and 70 μm by using a film applicator and miscibility as well as tack values were evaluated. With the addition of 2% (w/w) PVP the tack value decreased and increased in 5% (w/w) PVP and then continuously decreased up to 30%(w/w). It was found that the tack value was related to miscibility as well as to viscosity and the free functional group such as hydroxyl group of the blend. By the morphological analysis performed through scanning electron microscopy (SEM) and also by the study of thermal analysis using the differential scanning calorimetry (DSC) behavior of blends, it was found that the two distinct phases constituted after adding 5% (w/w) of PVP. This resulted in the acrylic PSA forming the continuous phase, and by increasing the concentration of PVP a dispersed phase was developed. The dispersed phase has a higher viscosity than the continuous phase and therefore cannot wet the adherent and hence result in lowering the tack values.     

Level crossing rate in terms of the characteristic function: a new approach for calculating the fading rate in diversity systems

The level crossing rate (LCR) of a random process conveys useful information about the underlying process, and is of interest in diverse engineering fields. In wireless communications, it is related to the system characteristics such as handoff, outage probability, fading rate, average duration of fades, velocity (or maximum Doppler shift) of the mobile, and the effect of diversity on fading. The LCR formula was originally derived by Rice in terms of the joint probability density function (pdf) of the underlying process and its time derivative. In this letter, we express the LCR in terms of the joint characteristic function (cf). This new formula is useful for many cases where the joint cf is simpler to derive than the associated joint pdf. As an application and for a direct-sequence code-division multiple-access system, the fading rate at the output of a RAKE receiver with either maximal ratio combiner or postdetection equal gain combiner, operating over a frequency-selective fading channel with different path statistics, is easily calculated using the new cf-based LCR formula.     

A parametric model for the distribution of the angle of arrival andthe associated correlation function and power spectrum at the mobilestation

One of the main assumptions in Clarke's classic channel model is isotropic scattering, i.e., uniform distribution for the angle of arrival (AOA) of multipath components at the mobile station. However, in many mobile radio channels we encounter nonisotropic scattering, which strongly affects the correlation function and power spectrum of the complex envelope at the mobile receiver. We propose the use of the versatile von Mises (1918) angular distribution, which includes and/or closely approximates important distributions like uniform, impulse, cardioid, Gaussian, and wrapped Gaussian, for modeling the nonuniform AOAs at the mobile. Based on this distribution, the associated correlation function and. power spectrum of the complex envelope at the mobile receiver are derived. The utility of the new results is demonstrated by comparison with the correlation function estimates of measured data     

A space-time correlation model for multielement antenna systems inmobile fading channels

Analysis and design of multielement antenna systems in mobile fading channels require a model for the space-time cross correlation among the links of the underlying multiple-input multiple-output (MIMO) channel. In this paper, we propose a general space-time cross-correlation function for mobile frequency nonselective Rice fading MIMO channels, in which various parameters of interest such as the angle spreads at the base station and the user, the distance between the base station and the user, mean directions of the signal arrivals, array configurations, and Doppler spread are all taken into account. The new space-time cross-correlation function includes all the relevant parameters of the MIMO fading channel in a clean compact form, suitable for both mathematical analysis and numerical calculations/simulations. It also covers many known correlation models as special cases. We demonstrate the utility of the new space-time correlation model by clarifying the limitations of a widely accepted correlation model for MIMO fading channels. As another application, we quantify the impact of nonisotropic scattering around the user, on the capacity of a MIMO fading channel     

Fabrication of nanograined silicon by high-pressure torsion

This paper describes fabrication of Si nanograins through allotropic phase transformation by concurrent application of high pressure and intense straining using high-pressure torsion (HPT). Single-crystalline Si(100) wafers were processed by HPT under a pressure of 24 GPa at room temperature. X-ray diffraction and Raman analysis revealed that the HPT-processed samples were composed of metastable Si-III and Si-XII phases and amorphous phases in addition to the original diamond-cubic Si-I phase. It was found that nanograins formed because the Si-I diamond phase had transformed to high-pressure phases (Si-II, Si-XI, and Si-V) having metallic nature, and it then became easier to generate a high density of dislocations to form grain boundaries. The high-pressure phases were further transformed to the Si-XII and Si-III phases via the Si-II phase upon unloading and they existed as metastable phases at ambient pressure. Subsequent annealing at 873 K gave rise to reverse transformation to Si-I but with nanograin sizes. Although no appreciable photoluminescence (PL) peak was observed from the HPT-processed sample, a broad PL peak centered around 600 nm was detected from the annealed sample due to quantum confinement in the Si-I nanograins.     

Classification of acoustic emission signals collected during tensile tests on unidirectional ultra high molecular weight polypropylene fiber reinforced epoxy composites using principal component analysis

The objective of this study is to utilize a mathematical procedure for the evaluation of damage mechanism in ultra high molecular weight polypropylene/epoxy composite (UHMWPP/epoxy) specimen with different configuration based on the analysis of the AE signals of presented. Principal component analysis (PCA) is powerful tool, which utilized for the classification of the monitored AE transients. We worked on epoxy L160 resin, UHMWPP fiber bundle and UHMWPP/epoxy unidirectional specimens, subjected to tensile loading. Using model specimens exhibiting a predominant damage mechanism, correlations were established between the observed damage growth mechanisms and the AE parameter variances result in PCA. Outputs from this study revealed that the PCA is an effective tool for identifying damage modes such as matrix cracking, fibre/matrix debonding, fibre breakage and fibre pull-out in the UHMWPP/epoxy composites. The presence of damage modes in UHMWPP/epoxy composites was proven with scanning electron microscopy (SEM) images.     

Reducing temperature stratification using heated air recirculation for thermal energy saving

Using discrete heat sources such as a radiator or a gas heater, a large amount of heated air is accumulated under the ceiling surface where it is usually far enough from the occupancy zone. The idea of transferring this heated air into the occupancy zone has been the subject of the present study. A full length-scale model of a compartment was constructed and equipped with a radiator and an air circulating mechanism. Temperature distribution on the symmetry plane was specified for the two cases of usual heating and heating with air circulation. Heat loss from the surfaces of the enclosure was also determined using thermal maps of both the internal and the external surfaces of the enclosure. In order to examine the effect of circulating air on the thermal energy saving quantitatively, a numerical model was developed and validated using experimental results. The input heat rate of the enclosure was compared for the two cases of usual heating and heating with air circulation considering different heights for a typical heating space. Based on the results, having the same mean air temperature within the occupancy zone, the input heat rate of a compartment could be moderated using circulating air criterion.     

Propagation characteristics of coherent array beams from carbondioxide waveguide lasers

The characteristics of coherent array beams generated by waveguide carbon dioxide laser structures have been investigated, with particular emphasis on their propagation and transformation properties. Methods of array beam generation are presented, and measurements of array beam quality are given through the use of the M² parameter. Efficient transformation of an array beam from the antisymmetric to symmetric pattern is achieved through the use of a binary phase plate, and spatial filtering is shown to improve the beam quality at the expense of only a moderate loss of total beam power. Coherence effects in the near-field propagation of array beams are also explored