Slotted ALOHA multihop packet radio networks with directional antennas

The use of directional antennas in multihop packet radio networks using slotted ALOHA access is considered. Tractable approximations to the network performance in terms of expected forward progress are derived. Results show that packet collisions can be strongly reduced already with moderate gain antennas to allow for a substantial increase in network performance.<>     

Neuere Entwicklungen und Perspektiven auf dem Gebiet des technischen Kohlenstoffs

New developments and outlook for technical carbon. The present world production of technical carbon amounts to 20 Mio. t/a (metallurgical coke as well as petroleum and pitch coke not included). More recent developments in the field of technical carbon are discussed in detail. Topics covered include: production and utilization of technical carbon and graphite products in metallurgy; recent advances in the use of carbon black as reinforcing material for car tyres; the significance of activated carbon for waste gas purification including the recovery of valuable materials; high-performance materials from carbon such as carbon fibres, carbon composites, and full-matrix carbons; synthesis of diamond for technical applications; preparation of diamond or diamond-like films by carbon vapor deposition; structure, preparation, and properties of fullerite, a new form of carbon, and conceivable future applications.     

Gradual removals in cellular PCS with constrained power control and noise

In this paper we study themobile removal problem in a cellular PCS network where transmitter powers are constrained and controlled by a Distributed Constrained Power Control (DCPC) algorithm. Receivers are subject to non-negligible noise, and the DCPC attempts to bring each receiver's CIR above a given target. To evaluate feasibility and computational complexity, we assume a paradigm where radio bandwidth is scarce and inter-base station connection is fast. We show that finding the optimal removal set is an NP-Complete problem, giving rise for heuristic algorithms. We study and compare among three classes of transmitter removal algorithms. Two classes consist of algorithms which are invoked only when reaching a stable power vector under DCPC. The third class consist of algorithms which combine transmitter removals with power control. These areOne-by-one Removals, Multiple Removals, andPower Control with Removals Combined. In the class of power control with removals combined, we also consider a distributed algorithm which uses the same local information as DCPC does. All removal algorithms are compared with respect to their outage probabilities and their time to converge to a stable state. Comparisons are made in a hexagonal macro-cellular system, and in two metropolitan micro-cellular systems. ThePower Control with Removals Combined algorithm emerges as practically the best approach with respect to both criteria.This research has been done while the 2nd author was visiting at the Royal Inst. of Tech. under a grant from ISS '90 foundation, Sweden.     

Algorithms for the Optimum Estimation of the Parameters of a Radio Signal in a Measurement Time of Less Than a Period and a Nonmultiple of a Period in Which the Result Is Tied to the Beginning of the Measurement Interval

Optimum estimates of the parameters of a noisy (white noise) sinusoidal radio signal of known frequency are considered, based on an investigation of the likelihood function, when the measurement time is less than a period and a nonmultiple of a period. Estimates are presented for the phase shift and the amplitude in the presence (absence) of a constant component and nonlinear distortions when the result is tied to the beginning of the measurement interval.     

One parameter model for strength properties of hardenable aluminium alloys

Models for strength properties are proposed for commercially aluminium alloys. The alloy group investigated are the hardenable alloys from the 2000 (Al–Cu and Al–Cu–Mg), 6000 (Al–Mg–Si) and 7000 (Al–Zn–Mg) series. The same model for solid solution hardening that has successfully been applied to non-hardenable alloys has been used. For precipitation hardening, particle cutting and the Orowan mechanism have been considered. The same basic model is used for all strength properties. It is demonstrated that with one fitting parameter for each property, a representation with reasonable accuracy can be obtained that is applicable to a wide range of alloys. Such models are useful in materials optimisation and selection.     

Our changing atmosphere: evidence based on long-term infrared solar observations at the Jungfraujoch since 1950

The Institute of Astrophysics of the University of Liège has been present at the High Altitude Research Station Jungfraujoch, Switzerland, since the late 1940s, to perform spectrometric solar observations under dry and weakly polluted high-mountain conditions. Several solar atlases of photometric quality, extending altogether from the near-ultra-violet to the middle-infrared, were produced between 1956 and 1994, first with grating spectrometers then with Fourier transform instruments. During the early 1970s, scientific concerns emerged about atmospheric composition changes likely to set in as a consequence of the growing usage of nitrogen-containing agricultural fertilisers and the industrial production of chlorine-bearing compounds such as the chlorofluorocarbons and hydro-chlorofluorocarbons. Resulting releases to the atmosphere with ensuing photolysis in the stratosphere and catalytic depletion of the protective ozone layer prompted a worldwide consortium of chemical manufacturing companies to solicit the Liège group to help in clarifying these concerns by undertaking specific observations with its existing Jungfraujoch instrumentation. The following pages evoke the main steps that led from quasi full sun-oriented studies to priority investigations of the Earth's atmosphere, in support of both the Montreal and the Kyoto Protocols.     

Observations of long-lived anthropogenic halocarbons at the high-Alpine site of Jungfraujoch (Switzerland) for assessment of trends and European sources

Anthropogenic halocarbons, such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), bromocarbons (halons) and long-lived chlorinated solvents have been measured continuously at the high-Alpine site of Jungfraujoch (Switzerland) since January 2000. Chloro- and bromo-containing halocarbons are responsible for the stratospheric ozone depletion and will be globally banned from usage within the next years. With the exception of the stable CFC-12 (CF2 Cl2), all major CFCs and chlorinated solvents show a negative trend in recent years in their background concentrations at Jungfraujoch. HCFCs, as their first-generation substitute, are still increasing with a few percent per year. However, the frequency and the strength of HCFCs pollution events, which are caused by regional European emissions, are already declining. This can be seen as a sign of the impending ban of these gases within the next years in Europe. On the other hand, HFCs as the second-generation substitutes, are increasing with relative rates of at least 10% per year (e.g. almost 5 ppt per year for HFC-134a). An allocation of European sources was attempted by combining measured concentrations with trajectories of air masses reaching the Jungfraujoch during pollution events. Potential source regions could be detected in Italy, France, Spain and Germany.     

Precursor-directed syntheses and biological evaluation of new elansolid derivatives

The antibiotic elansolid C1 (8) was isolated from Chitinophaga sancti strain FxGBF13 after fermentation in the presence of anthranilic acid. Remarkably, 8 was also obtained by addition of anthranilic acid to a crude fermentation extract containing the macrolide elansolid A2 (1*). This Michael-type conjugate addition allowed us to generate 21 new derivatives of elansolid C1 (9-29) by using various nucleophiles. Biological activities of all derivatives were evaluated against Staphylococcus aureus, Micrococcus luteus, and the mouse cell line L929.