Controlling Intercell Interference in CDMA-based Fixed Wireless Networks Through Multirate Techniques

A number of research works have suggested a possible capacity advantage when employing direct sequence-code division multiple access (DS-CDMA) instead of the more common time division multiple access option for fixed broadband wireless access networks operating at millimeter-waves. The choice of this multiple access scheme, which, up to now, has not been foreseen by recent WiMax (worldwide interoperability for microwave access) standards, exhibits an improved performance in terms of cochannel interference in the upstream, even though it experiences higher interference in the downstream. The present paper focuses on enhancing the bottleneck downstream performance of fixed wireless networks by proposing a methodology for controlling the signal-to-interference ratio throughout the cell, so that all subscribers meet the given interference quality-of-service specifications. This is accomplished by exploiting the properties of a combination of multirate CDMA techniques. The paper describes a simple policy for code assignment, which is proved to be optimal, incorporating the effect of rain fading over multiple paths. The performance of the algorithm is tested against single code DS-CDMA both under long term channel statistics (static code assignment) and under dynamic channel effects (dynamic code assignment). The first test verifies the compliance of all sector subscribers with more demanding SIR specifications, while the second yields a significant improvement in terms of resource utilization.     

Statistical model for the compressive response of anisotropic polymeric and metallic foams

Cellular materials, based either on polymers or metals, exhibit similar compressive stress–strain response. In a previous work, this behavior has been described in terms of a statistical micromechanics model, which deals with the deformation of individual struts of the foam material, assuming that it is an isotropic one. However, due to the manufacturing process, cellular materials are substantially anisotropic, having different properties in the (foam) rise and transverse direction. In this study, the degree of anisotropy, has been taken into account by an orientation distribution function (ODF). The ODF has been treated as a probability density function and was used to calculate the rate of struts transition from elastic to buckling (yield) region. It has been found, that with this statistical model, the particular features of compressive response of polymeric and metallic foams in the rise and transverse direction, could be successfully described.     

Creep failure time prediction of polymers and polymer composites

A theoretical approach for the prediction of creep rupture time of polymers and polymer composites is analyzed in the present work. This analysis takes into account the viscoelastic path at small strains and the viscoplastic path at higher stresses. The calculation of the rate of creep strain is based on a thermally activated rate process, while the emergence and growth of plastic strain, with increasing creep time, is also taken into account. When the accumulated strain attains values, high enough to lead to failure, its slope versus time exhibits an abrupt change. At this specific time, the creep rate function in respect to time appears a minimum. The creep failure time is defined as the time where the creep rate takes its minimum value. The model has been tested for various types of polymeric materials, as well as for polymer composites. Once the model parameters are estimated from short time creep strain data, then it was proved to successfully predict the creep failure time at a variety of stress levels, for all material types examined.     

Cation Valence in VOx

By comparing the position of the K absorption edge of VO_x to that of V metal, V₂O₄, and V₂O₅, it is concluded that the cation in VO_x has the classical valence of 2 + only for oxygen-rich compositions.     

Dynamic resource allocation for DVB‐RCS networks

To provide high‐speed multimedia services using the digital video broadcasting‐return channel satellite (DVB‐RCS) standard, it is imperative to efficiently assign timeslots according to user demands and dynamically take into account the variations of the propagation conditions. In satellite networks operating above 10 GHz, rain fading constitutes the dominant factor deteriorating the quality of service. In this paper, a novel two‐phase resource allocation scheme for a DVB‐RCS compliant satellite network is proposed. The scheme operates in two phases, the resource calculation and the resource assignment. In the resource calculation phase, based on a dynamic channel model, an efficient algorithm is presented to determine the superframe design that maximizes system throughput. In the resource assignment phase, a novel Hybrid Bin‐packing algorithm is introduced maximizing the utilization of the multi‐frequency time division multiple access frame. The proposed algorithm has been compared with existing schemes exhibiting significantly better results with regard to overall system throughput. Copyright © 2008 John Wiley & Sons, Ltd.     

Elastic behavior of natural rubber filled vulcanizates

The reinforcement effect of carbon black and, the effect of accelerator-to-sulfur ratio variation on the elastic behavior of natural rubber vulcanizates have been studied. The Mooney–Rivlin relation was used to describe the behavior of the rubber matrix, and values of constants c₁ and c₂ have been evaluated with the use of the strain-amplification factor. The stress softening of the vulcanizates tested has also been examined.     

Comparison and evaluation of empirical zone settling velocity parameters based on sludge volume index using a unified settling characteristics database

Over the past few decades classic and contemporary research on the process of secondary clarification in activated sludge plants has illuminated several aspects of the solids-flux theory which actually require a closer examination. Inter alia the most challenging and controversial part in the field has been the development of reliable models for the settling properties of the activated sludge in the settler. Numerous studies have been performed aiming at the evolvement of reliable mathematical formulas that would satisfactorily describe this behaviour, but no universally accepted solution seems to exist to date. That is mostly because different experimental conditions, sludge types and instrumentation have been employed, thus complicating the process of reaching a conclusive result. In order to bring theoretical and practical developments of secondary settling tank design and simulation closer together, a number of related tasks are addressed in this study by the use of an integrated and unified settling characteristics database. Several drawbacks and advantages of the methodologies published hitherto are examined on a universal basis and under the same assumptions in order to reveal artifacts that complicate the procedure of settling velocity estimation. It is suggested that universally accepted solutions may be feasible especially for design purposes. For simulation analysis real-time data of settling velocity should be tested rather than values derived from laboratory experiments which are shown to produce different results depending on the applied approach. In conclusion, an integrated database is proposed as a means for a more robust and universally accepted design procedure.