Analyse théorique d’un canal de propagation en présence de trajets multiples

Frequency representation of multipath fading on line-of-sight microwave paths is revisited together with a general point of view and with the aim of giving sound foundations of medium bandwidth modelling. Moreover some typical laws of variation of the selectivity according to the frequency and the hop length are derived.     

Statistiques de la fonction de transfert par trajets multiples pour des largeurs de bande moyennes. Application à la prévision de la qualité

It is shown, from data gathered on three different test links, that multipath transfer functions on a medium bandwidth can be modelled with three parameters. Moreover, the model parameters follow similar probability laws on the three links, thus leading to a universalstatistical model. This statistical model depends on two independent parameters. When the two statistical parameters are known, the outage time of the link can be deduced from the model. A first estimation of these parameters can be inferred from the distribution law of the levels at fixed frequency. The application of the method to two particular cases is presented.     

La réception hétérodyne en microondes et en optique: analogies et différences

The introduction of the heterodyne reception in radio technique many years ago, allows a better sensitivity of the receiver and then an increase of the range of the transmission system, when compared to the direct detection. The same improvement can be expected for optical systems and this is the reason why this technique is now being investigated in several laboratories and could be applied to transmission over single mode fibres. This paper tries to exhibit the analogies which can exist between the microwaves and optical techniques and are sometimes not very evident because the words are different, although describing identical or at least comparable concepts. Some key functions of a system are compared (amplification mixing, oscillation).     

Rayonnement en ondes métriques et décimétriques des orages

This article presents the result of the current knowledge on the very high frequency electromagnetic radiation of lightning. A phenomenology analysis of the new results obtained from the study of triggered lightning in New Mexico (TRIP 82) and at Saint-Privat-d’ Allier in Haute-Loire, France (1983) is first described. The authors then present the spectral and temporal characteristics of the VHF/UHF radiation which can be applied to the electromagnetic compatibility of systems. From a synthesis of the experimental and theoretical contributions, they propose a reflection on the lightning initiation process and on the mechanisms generating lightning radiation.     

Les télécommunications par câbles sous-marins: l’avènement des fibres optiques

This paper explains why the new generation of the submarine transmission systems by optical fibres has come into existence, ensuing from the telegraphic cables and coaxial cable systems. The author expounds how the main characteristics of this new system have been selected and describes its different elements such as its optical fibre, repeaters, branching units and the equipment of the terminal stations. The French program of study and development of the system is indicated and some thoughts are given to what the future in this particular field could be.     

Influence of Si:Al ratio on the microstructural and mechanical properties of a fine-limestone aggregate alkali-activated slag concrete

Three series of fine limestone aggregate, alkali-activated blast furnace slag (AAS) concretes were fabricated and tested; two through activation with waterglass/NaOH solution, of which one included NaCl as a retarding agent, and one activated by Na₂CO₃. Each of these series was made up of three formulae containing different amounts of Al₂O₃. The compressive strengths of the series activated by waterglass/NaOH after 28 days were ≈65 ± 5.3 MPa, a 22% increase compared to previously reported formulae containing no additional Al₂O₃. Increasing the amount of Al₂O₃ did not further increase strength, however. The Na₂CO₃-activated formulae had strengths of ≈35 ± 3 MPa after 28 days, representing no increase in strength over formulae not containing Al₂O₃ previously reported. X-ray diffraction showed the main binding phase to be calcium silicate hydrate (C–S–H) gel, as is commonly found in ordinary Portland cement (OPC). Fourier transform infrared spectroscopy showed little difference from the previously reported results for formulae not containing Al₂O₃ and strongly resemble the spectra reported elsewhere for C–S–H. Electron microscopy, coupled with energy dispersive spectroscopy, showed the cementing phase to be a single homogenous phase—not a mixed system of geopolymer and C–S–H gel—with a lower volume fraction of unreacted slag than formulae without Al₂O₃. The reason for the increase in strength of Al₂O₃-containing formulae is unclear, but is unlikely to be ascribed to the formation of large amounts of ‘geopolymers’ and may be related to a possible increase in reaction temperature of between 2 and 5°C, depending on amount of additive.     

Ge-on-Si optoelectronics

Electronic-photonic synergy has become an increasingly clear solution to enhance the bandwidth and improve the energy efficiency of information systems. Monolithic integration of optoelectronic devices is the ideal solution for large-scale electronic-photonic synergy. Due to its pseudo-direct gap behavior in optoelectronic properties and compatibility with Si electronics, epitaxial Ge-on-Si has become an attractive solution for monolithic optoelectronics. In this paper we will review recent progress in Ge-on-Si optoelectronics, including photodetectors, electroabsorption modulators, and lasers. The performance of these devices has been enhanced by band-engineering such as tensile strain and n-type doping, which transforms Ge towards a direct gap material. Selective growth reduces defect density and facilitates monolithic integration at the same time. Ge-on-Si photodetectors have approached or exceeded the performance of their III-V counterparts, with bandwidth-efficiency product > 30 GHz for p-i-n photodiodes and bandwidth-gain product > 340 GHz for avalanche photodiodes. Enhanced Franz-Keldysh effect in tensile-strained Ge offers ultralow energy photonic modulation with < 30 fJ/bit energy consumption and > 100 GHz intrinsic bandwidth. Room temperature optically-pumped lasing as well as electroluminescence has also been achieved from the direct gap transition of band-engineered Ge-on-Si waveguides. These results indicate that band-engineered Ge-on-Si is promising to achieve monolithic active optoelectronic devices on a Si platform.     

In vitro gastrointestinal digestion of liquid and semi-liquid dairy matrixes

Protein digestion in two liquid dairy matrixes with different heat treatments (pasteurized and sterilized milks) and in one semi-liquid dairy matrix (stirred-yogurt) was investigated using an in vitro gastrointestinal digestion model. After buccal digestion, significantly lower amount of soluble proteins were measured in yogurt than in both milks. This difference between dairy matrixes decreased during gastric digestion and disappeared at the end of the duodenal digestion upon the proteolytic action of pepsin and pancreatin. Electrophoresis pattern of digested mixtures showed that casein digestion began at the gastric phase and was slower for pasteurized milk than sterilized milk and yogurt. At the end of duodenal digestion, no more intact caseins were present in all the dairy matrixes while faint bands of whey proteins were still visible for pasteurized milk and yogurt. The release of free amino acids during the duodenal phase varied according to their nature (acid, basic, neutral or hydrophobic) and seems to be governed by the specificity of the enzymes. These results suggest that the severity of milk's heat treatment influences the kinetics of protein digestion, mainly during the gastric phase, and that the impact of processing has to be considered to study protein digestion in dairy products.