Main Source Of Performance Degradation Research Articles

Analysis of the impact of prototyping‐related parasitic effects in a hybrid GaN power amplifier for 5G FR1 applications

AbstractThis article analyses the impact of parasitic effects encountered in the manufacturing of a Gallium Nitride power amplifier optimized for a specific trade‐off of efficiency and linearity at 5G FR1 frequencies. The analysis focuses on an example based on a packaged transistor and implemented in hybrid technology, adopting a two‐layer printed circuit board for passive distributed elements and surface‐mount lumped components. The design is summarized, and the focus is then posed on the comparison of simulated and measured results, the main causes of inaccuracy in their agreement, and a simple way of reproducing these in simulation. The mechanical stability of the assembly and the sensitivity of the second harmonic control are identified as the main sources of performance degradation. These effects are correctly modeled at the simulation level thanks to the insertion of specific parasitic lumped elements but could not be fully eliminated at the experimental level in this specific prototype. However, post‐tuning performed on the prototype, guided by the proposed analysis, still allows to reach satisfactory performance although somewhat sub‐optimal compared to the one expected from simulations. At 3.5 GHz, the re‐tuned PA demonstrates a measured output power in excess of 37.5 dBm at saturation, with associated gain higher than 14 dB and PAE of 60%. In a 2‐tone test with 20 MHz spacing, the IMD3 remains lower than −30 dBc up to 29 dBm of output power with a corresponding PAE of the order of 32%.

Assessing performance degradation induced by thermal cycling in solid oxide cells

Commercialization of solid oxide cells (SOCs) is hindered by their poor durability. In particular, electrode–electrolyte delamination due to the mismatch in thermal expansion coefficients under thermal cycling is one of the main sources of performance degradation during the long-term SOCs operation. We establish a macroscopic mathematical model describing the delamination propagation inside SOCs under thermal cycling. The extent to which excessive stresses at different sides affect delamination is assessed. Then the change in delamination length with different thermal cycles and the influence factors on delamination is quantified by using a cohesion zone model. Accordingly, control strategies that can effectively inhibit delamination are further proposed. In addition, a separate computational fluid dynamics model based on the results of the cohesion zone model is developed to evaluate the mechanism and extent of delamination on the electrochemical performance degradation. Based on the results of the above model, after 10 thermal cycles, the electrode–electrolyte delamination can reach 40.7%, resulting in a 37.3% loss of electrochemical performance. Furthermore, electrochemical analysis demonstrates that, regardless of which side the delamination occurs on, it affects the electrochemical reaction within the electrode at the same location on the opposite side by hindering the electron and ion transport paths.

The Case for Domain-Specialized Branch Predictors for Graph-Processing

Branch prediction is believed by many to be a solved problem, with state-of-the-art predictors achieving near-perfect prediction for many programs. In this article, we conduct a detailed simulation of graph-processing workloads in the GAPBS benchmark suite and show that branch mispredictions occur frequently and are still a large limitation on performance in key graph-processing applications. We provide a detailed analysis of which branches are mispredicting and show that a few key branches are the main source of performance degradation across the graph-processing benchmarks we looked at. We also propose a few ideas for future work to improve branch prediction accuracy on graph workloads.

On the reliability of structures equipped with a class of friction-based devices under stochastic excitation

This work presents a general framework for performing reliability analyses of involved structural models equipped with friction-based devices under stochastic excitation. An experimentally validated model that takes into account main sources of performance degradation that these devices experience during seismic events is considered. To deal with the variability of future excitations, a stochastic excitation model is adopted in the present formulation. First excursion probabilities are used as measures of system reliability. The associated reliability analysis is carried out by combining an advanced simulation technique with an adaptive parametric meta-model. The proposed framework is suitable for direct implementation in realistic applications. The effectiveness of the methodology is demonstrated by means of two application problems involving relatively large finite element models.

Optimizing symmetry-based recoupling sequences in solid-state NMR by pulse-transient compensation and asynchronous implementation.

Pulse imperfections like pulse transients and radio-frequency field maladjustment or inhomogeneity are the main sources of performance degradation and limited reproducibility in solid-state nuclear magnetic resonance experiments. We quantitatively analyze the influence of such imperfections on the performance of symmetry-based pulse sequences and describe how they can be compensated. Based on a triple-mode Floquet analysis, we develop a theoretical description of symmetry-based dipolar recoupling sequences, in particular, R26411, calculating first- and second-order effective Hamiltonians using real pulse shapes. We discuss the various origins of effective fields, namely, pulse transients, deviation from the ideal flip angle, and fictitious fields, and develop strategies to counteract them for the restoration of full transfer efficiency. We compare experimental applications of transient-compensated pulses and an asynchronous implementation of the sequence to a supercycle, SR26, which is known to be efficient in compensating higher-order error terms. We are able to show the superiority of R26 compared to the supercycle, SR26, given the ability to reduce experimental error on the pulse sequence by pulse-transient compensation and a complete theoretical understanding of the sequence.

Asymmetric-shell ignition capsule design to tune the low-mode asymmetry during the peak drive

The low-mode radiation flux asymmetry in the hohlraum is a main source of performance degradation in the National Ignition Facility (NIF) implosion experiments. To counteract the deleterious effects of the large positive P2 flux asymmetry during the peak drive, this paper develops a new tuning method called asymmetric-shell ignition capsule design which adopts the intentionally asymmetric CH ablator layer or deuterium-tritium (DT) ice layer. A series of two-dimensional implosion simulations have been performed, and the results show that the intentionally asymmetric DT ice layer can significantly improve the fuel ρR symmetry, hot spot shape, hot spot internal energy, and the final neutron yield compared to the spherical capsule. This indicates that the DT asymmetric-shell capsule design is an effective tuning method, while the CH ablator asymmetric-shell capsule could not correct the fuel ρR asymmetry, and it is not as effective as the DT asymmetric-shell capsule design.

Friction Model for Sliding Bearings under Seismic Excitation

In this article, an experimentally validated model is proposed in order to take into account main sources of performance degradation that could be experienced by friction-based devices during a seismic event. Particular attention is dedicated to the degradation of friction characteristics due to repetition of cycles and consequent temperature rise. This effect can be responsible for overestimate of the dissipation capacity of the device. The proposed model of frictional behavior is suitable for immediate implementation in generalized structural analysis codes and provides an important design tool for realistic assessment of the seismic response of structures equipped with friction-based isolators.

Hole Surface Trapping in CdSe Nanocrystals: Dynamics, Rate Fluctuations, and Implications for Blinking

Carrier trapping is one of the main sources of performance degradation in nanocrystal-based devices. Yet the dynamics of this process is still unclear. We present a comprehensive investigation into the efficiency of hole transfer to a variety of trap sites located on the surface of the core or the shell or at the core/shell interface in CdSe nanocrystals with both organic and inorganic passivation, using the atomistic semiempirical pseudopotential approach. We separate the contribution of coupling strength and energetics in different systems and trap configurations, obtaining useful general guidelines for trapping rate engineering. We find that trapping can be extremely efficient in core-only systems, with trapping times orders of magnitude faster than radiative recombination. The presence of an inorganic shell can instead bring the trapping rates well below the typical radiative recombination rates observed in these systems.

Soft‐decision a priori knowledge interpolation for robust telephone speaker identification

Handsets which are not seen in the training phase (a.k.a unseen handsets) are main sources of performance degradation for speaker identification (SID) applications in telecommunication environments. To alleviate the problem, a soft‐decision a priori knowledge interpolation (SD‐AKI) method of handset characteristic estimation for handset mismatch‐compensated SID is proposed in this paper. The idea of the SD‐AKI method is to first collect a set of characteristics of seen handsets in the training phase, and to then estimate the characteristic of the unknown testing handset by interpolating the set of seen handset characteristics in the test phase. The estimated handset characteristic is then used to compensate for handset mismatch for robust SID. The SD‐AKI method can be realized in both feature and model spaces. Experimental results on the handset TIMIT (HTIMIT) database showed that both the proposed feature‐ and model‐space SD‐AKI schemes were more robust than the blind cepstral mean subtraction (CMS), feature warping (FW) methods and their hard‐decision counterpart (HD‐AKI) for both cases of all‐handset and unseen‐handset SID tests. It is therefore a promising robust SID method.

Proper nouns in English–Arabic cross language information retrieval

AbstractOut of vocabulary words, mostly proper nouns and technical terms, are one main source of performance degradation in Cross Language Information Retrieval (CLIR) systems. Those are words not found in the dictionary. Bilingual dictionaries in general do not cover most proper nouns, which are usually primary keys in the query. As they are spelling variants of each other in most languages, using an approximate string matching technique against the target database index is the common approach taken to find the target language correspondents of the original query key. N‐gram technique proved to be the most effective among other string matching techniques. The issue arises when the languages dealt with have different alphabets. Transliteration is then applied based on phonetic similarities between the languages involved. In this study, both transliteration and the n‐gram technique are combined to generate possible transliterations in an English–Arabic CLIR system. We refer to this technique as Transliteration N‐Gram (TNG). We further enhance TNG by applying Part Of Speech disambiguation on the set of transliterations so that words with a similar spelling, but a different meaning, are excluded. Experimental results show that TNG gives promising results, and enhanced TNG further improves performance.

Isometric Data Sequences and Data-Modulation Schemes in Fading Channels

In multiplicative fading channels, joint channel estimation and data detection (CE/DD) schemes cannot differentiate among certain sequences of amplitude- and/or phase-modulated (AM/PM) symbols drawn from rotationally invariant signal constellations. This paper identifies these so-called isometric sequences as the main source of performance degradation, and introduces a unifying framework that effectively solves the problem by using asymmetric signal constellations (ASC) and a normalized innovations-based detector. The encompassing nature of the solution is clearly demonstrated by showing that seemingly unrelated previous results, such as training-based solutions, can be viewed as special cases of the modulation-based solution discussed here. A comprehensive analysis, supported by simulation studies, of the relationships among modulation schemes, isometry, and detection performance is provided. Results indicate that the proposed ASC solution offers excellent performance without incurring significant complexity or reducing the transmission rate. Furthermore, it is shown to be robust in various fading rates, and for different signal constellations.

Analysis of a Multiphase Induction Machine Under Fault Condition in a Phase-Redundant A.C. Drive System

Phase-redundancy is a unique technique for improving the reliability of large variable speed AC drives. This technique, derived from the concept of parallel-redundancy, is based on the capability of a multiphase machine to start and continue running with one or more of its many stator phases open or short circuited with lower performance. Structural unbalance in the machine following loss of excitation is the main source of performance degradation. This paper, therefore, presents a simple generalized approach to analyze the dynamic as well as steady-state behavior of a multiphase induction machine during fault condition in a phase-redundant AC drive system. The key analytical results have been experimentally verified by tests conducted on a laboratory prototype.