Wide-band packet radio for multipath environments

A direct-sequence spread-spectrum packet radio is described that has versatile signal-processing and local-control capabilities designed to support the functions required of a robust mobile communications network. Noteworthy capabilities include eleven selectable data rates with accurate range measurements in a fading multipath channel. The radio uses a hybrid analog/digital signal processor and nonrepeating spreading codes for suppression of intersymbol interference and jamming. It incorporates two sets of monolithic surface-acoustic-wave convolvers as programmable matched filters with time-bandwidth products of 64 and 2000. The analog matched filters are coupled with binary postprocessing for the functions of detection, RAKE demodulations and ranging measurements over a wide multipath spread. The data rate can be varied in response to channel conditions from 1.45 Mb/s down to 44 b/s with an almost ideal tradeoff in signal-processing gain from 18 dB up to 61 dB prior to multipath combining     

A mesoscale study on the thermodynamic effect of stress on martensitic transformation

The effect of stress on martensitic transformation (MT) is addressed with special emphasis on the mechanical driving force (MDF) for triaxial stress states. The mechanical driving force appears to be additional to the chemical driving force in thermodynamic transformation con-ditions derived from a Gibbs free energy formulation for stressed solids undergoing MT. The thermodynamic criterion of Patel and Cohen predicts the change in martensite start temperature if MT occurs in a stress field. This criterion is extended from uniaxial to triaxial stress states and is discussed in the light of emerging microstresses. As a source of microstress, the elastic anisotropy of single crystals is taken into account. Its influence on the martensite start temper-ature is investigated by mesomechanical finite-element modeling. The critical stress for the start of transformation occurring in a stress field is calculated from a transformation condition and compared with results based on statistical theories for stress-assisted nucleation. In the context of martensitic transformation and mechanical effects, the MDF accounts for the Magee effect. The range of temperature for which the Magee effect has an influence on the macroscopic deformation behavior of a specimen is determined in dependence of the level of uniaxial applied stress. Finally, a constitutive equation in incremental form for transformation-induced plasticity (TRIP) derived within the continuum-thermodynamics framework is suggested.     

Annealing effects in epoxy resins examined by a refined density-gradient technique

An aqueous density-gradient column was used to study annealing effects in a series of epoxy resins. The crosslinking densities were varied via the monomer composition. The height of the specimens floating in the column was observed to change linearly with the square root of time during several days. The corresponding total density change typically amounts to 6 kg m^–3, or 0.6%. By back-extrapolating the sample heights to time zero, accurate density values, uninfluenced by liquid absorption and other confounding effects, may be determined. Analysis of variance shows a residual error of 0.1 kg m^–3, or 0.01 %, for the means of three specimens. Consistent density increases, ranging from 0.11 to 0.17%, were shown to occur upon annealing of the cured resins. The differences between samples of varying crosslinking densities were found to be significant.     

Absorption spectra of excited Kr 84 states between 1.5 and 1.58μm and their use for absolute frequency locking

We have shown to our knowledge for the first time the absorption spectrum of Kr between 1500 and 1580 nm via the opto-galvanic effect and we measured the stability of a laser locked to the absorption line of Kr 1s²-2p⁸ at 1547.825 nm. The noise of the Galvatron signal as a function of the current through the bulb was carried out. Additionally it was shown to our knowledge for the first time that is it possible to lock more than one laser simultaneously at different wavelengths to one Galvatron tube     

Integrating code generation and peephole optimization

Summary Peephole optimization when integrated with automatic code generation into a uniform framework has significant advantages in the specification and implementation of efficient compiler back-ends. Attribute grammars provide a framework for expression of machine-specific code optimizations. We present a grammar-driven peephole optimization algorithm that is particularly well suited to attributed-parsing code generators. Integration via semantic attributes corrects interrelated phase-ordering problems and produces a faster and smaller compiler back-end.