Rotational molecular motion in solid H2 and D2 under pressure

The rotational state of a H₂ or D₂ molecule in solid hydrogen or deuterium is calculated over the range of densities for which the molecular phases are expected to be stable. For para-H₂ and ortho-D₂ we predict sudden transitions as the pressure is increased from states where the molecules are rotating freely to states where they are librating about equilibrium orientations. These transitions are predicted to occur in the region of several hundred megabars of pressure. Although in principle observable, these first-order transitions are expected to lead to only small changes in the equation-of-state curves. These transitions from rotating to librating molecular motion are expected to be accompanied by changes in the crystal structures from hcp to fcc.Supported by NSF Grant No. 22553.     

Invertase activity in fresh and processed honeys

The enzymatic activities of 147 samples of commercially produced Spanish polyfloral and monofloral honeys are reported. Important variations in invertase activity and peroxide accumulation were determined to obtain objective information related to honey quality. Variations from 4.04 (Robinia pseudoacacia) to 25.61 g sucrose hydrolysed per 100 g h⁻¹ (SN) (Castanea sativa) and from 11.31 (Erica vagans) to 45.25 µg H₂O₂ g⁻¹ h⁻¹ (Hedysarum coronarium) were detected, with a range from 10.57 to 46.2 SN in polyfloral honeys. Honeys from Erica spp had different invertase activities—high for E cinerea (18.3 SN) and low for E vagans (8.36 SN). The different heating processes of honey have been evaluated to determine the best treatment for preserving invertase activity. The hydroxymethylfurfural content is not a sufficiently good indicator of the degree of deterioration of honey. Another objective was to assess the use of invertase activity in honey quality legislation. © 2000 Society of Chemical Industry     

Orthogonal polynomials for power amplifier modeling and predistorter design

The polynomial model is commonly used in power amplifier (PA) modeling and predistorter design. However, the conventional polynomial model exhibits numerical instabilities when higher order terms are included. In this paper, we introduce a novel set of orthogonal polynomials, which can be used for PA as well as predistorter modeling. Theoretically, the conventional and orthogonal polynomial models are "equivalent" and, thus, should behave similarly. In practice, however, the two approaches can perform quite differently in the presence of finite precision processing. Simulation results show that the orthogonal polynomials can alleviate the numerical instability problem associated with the conventional polynomials and generally yield better PA modeling accuracy as well as predistortion linearization performance.     

Subspace based approaches for Wiener system identification

We consider the problem of Wiener system identification in this note. A Wiener system consists of a linear time invariant block followed by a memoryless nonlinearity. By modeling the inverse of the memoryless nonlinearity as a linear combination of known nonlinear basis functions, we develop two subspace based approaches, namely an alternating projection algorithm and a minimum norm method, to solve for the Wiener system parameters. Based on computer simulations, the algorithms are shown to be robust in the presence of modeling error and noise.     

Bearing estimation for a distributed source: modeling, inherentaccuracy limitations and algorithms

The problem of using sensor array measurements to estimate the bearing of a radiating source surrounded by local scatterers is considered. The concept of “partial coherence” is introduced to account for temporal as well as spatial correlation effects often encountered in a Rayleigh fading-type propagation channel formed between a source and sensor array elements. A simple parametric model for temporal channel correlation is presented, yielding an overall spatio-temporal channel model that is more realistic than formerly proposed models (which assume either full or zero temporal channel correlation). Thus, previously proposed “distributed source” models for bearing estimation problems are generalized to a parametric spatio-temporal model for what is called “partially coherently distributed (PCD) sources”. A study of the associated Cramer-Rao Bound (CRB) is undertaken for a simple but illustrative problem formulation. The inherent limitations in bearing estimation accuracy for this spatio-temporal problem are seen to lie between the cases of zero and full temporal correlation, becoming more severe as temporal channel correlation increases. In addition, the associated maximum likelihood estimators for source bearing are proposed, and their performance is compared with that predicted by the CRB