Use Of Prefilters Research Articles

Pre-Filtering Techniques for Spectrum Narrowing Caused by Optical Node Traversal in Ultra-Dense WDM Networks

To improve the spectral efficiency and increase network capacity, wavelength signals must be multiplexed densely, i.e., ultra-dense wavelength-division-multiplexing (WDM) networks. However, the spectrum of each WDM signal is narrowed by the wavelength-selective switches placed in optical nodes, so the transmissible distance and node hop count of the signal are strictly limited. To counter this spectrum narrowing problem caused by optical node traversal in ultra-dense WDM networks, pre-filtering techniques have been proposed. This paper comprehensively investigates the performance of four pre-filtering techniques: conventional Nyquist filtering, pre-emphasis filtering, partial-response filtering, and the combination of pre-emphasis and partial-response filtering. We conduct extensive computer simulations and discuss the optimality of pre-filtering techniques. The simulation results show that the adaptive use of pre-filtering can substantially extend the maximum attainable transmission distance and hop counts of 4/16/64-QAM signals.

Meeting Report--Workshop on Virus Removal by Filtration: Trends and New Developments

The workshop was held on 27 June 2011 in Barcelona, in conjunction with the PDA Virus & TSE (transmissible spongiform encephalopathy) Safety Forum 2011. Virus-retentive filters are important tools to assure a high virus safety level of biological medicinal products. Important parameters such as properties of virus spike preparations, mechanism of virus retention by different filter brands, use of prefilters to improve the filtration performance, and, finally, strategies to select the most appropriate filter for a specific product were discussed on the workshop. The panel discussion at the end of the workshop that involved speakers and regulators from different global areas came to following conclusions: The major mechanism of virus retention is size exclusion; filtration, however, is complex and protein and virus can interact with the membrane in multiple ways. Pressure interruption during filtration resulted in enhanced virus passage. It has never been reported that murine leukemia virus (MuLV) passes a parvovirus filter. It makes sense that a small virus can be used to provide a claim for a large virus like MuLV. This relies on the assumption that there is no aggregation or interaction of the model parvovirus with proteins leading to aggregates larger than retroviruses. Several prefilters are under investigation to improve flow rate and throughput of filtration in large-scale manufacture. It was discussed whether the prefilter and the virus-retentive filter can be viewed as one unit operation so that virus retention by both can be claimed as the viral clearance capacity of this manufacturing step. This question engendered some controversy: whereas some saw the combination as a correct reflection of manufacturing conditions, others discussed the different mechanisms of virus retention, which need to be studied separately. All together, the workshop was seen as a valuable forum for the discussion between regulators and industry; it was proposed that such forum should be provided again if possible in connection with one of the next PDA Virus & TSE Safety Conferences. The workshop was held on 27 June 2011 in Barcelona, in conjunction with the PDA Virus & TSE (transmissible spongiform encephalopathy) Safety Forum 2011. Virus-retentive filters are important tools to assure a high virus safety level of biological medicinal products. Important parameters such as properties of virus spike preparations, mechanism of virus retention by different filter brands, use of prefilters to improve the filtration performance and, finally, strategies to select the most appropriate filter for a specific product were discussed on the workshop. At the end of the workshop, aspects of the discussion were summarized by the following: The major mechanism of virus retention is size exclusion, but interactions are complex. Pressure interruption during filtration resulted in enhanced virus passage. It has never been reported that murine leukemia virus (MuLV) passes a parvovirus filter, and thus the parvovirus may provide a claim for a large virus like MuLV. Combination of prefilter and the virus-retentive filter are seen by some panelists as a correct reflection of manufacturing conditions; others discussed the different mechanisms of virus retention, which need to be studied separately. All together, the workshop was seen as a valuable forum for the discussion between regulators and industry.

Prefiltered Beamforming for Early-Stage Breast Cancer Detection

Ultrawideband radar imaging has been extensively investigated as a potential alternative to X-ray mammography and has shown promising results in the detection of small tumors within dielectrically homogeneous breast tissue. A range of confocal microwave imaging algorithms has been reported in the literature, showing good tumor detection capabilities, but most of these algorithms suffer significant deterioration in performance with increased amounts of fibro-glandular tissue in the breast. This letter proposes an improved beamformer based on a prefiltering approach to compensate for dispersion and phase effects in a dielectrically heterogeneous breast. The use of prefiltering is shown to improve on conventional beamformer performance and enables detection of multiple scatterers in dense breast tissue. The proposed improved beamformer is tested on 3-D anatomically and dielectrically accurate finite-difference time-domain (FDTD) breast models and the corresponding performance of the beamformer is evaluated across a range of appropriate metrics.

MULTIFILTERS WITH AND WITHOUT PREFILTERS

To explore the full approximation order and thus compression power of a multifilter, it is usually necessary to incorporate prefilters. Using matrix factorization techniques, we describe an explicit construction of such prefilters. Although in the case of approximation order 1 these prefilters are simply bi-infinite block diagonal matrices, they can become very intricate as soon as one aims for higher approximation order. For this reason, we introduce a particular class of multifilters which we call full rank multifilters. These filters have a peculiar structure which allows us to obtain approximation order without the use of prefilters. The construction of such filters via the lifting scheme is pointed out and examples of the performance of these filters for image compression are given.

Balanced multiwavelet bases based on symmetric FIR filters

This paper describes a basic difference between multiwavelets and scalar wavelets that explains, without using zero moment properties, why certain complications arise in the implementation of discrete multiwavelet transforms. Assuming we wish to avoid the use of prefilters in implementing the discrete multiwavelet transform, it is suggested that the behavior of the iterated filter bank associated with a multiwavelet basis of multiplicity r is more fully revealed by an expanded set of r/sup 2/ scaling functions /spl phi//sub i,j/. This paper also introduces new K-balanced orthogonal multiwavelet bases based on symmetric FIR filters. The nonlinear design equations arising in this work are solved using the Grobner basis. The minimal-length K-balanced multiwavelet bases based on even-length symmetric FIR filters are better behaved than those based on odd-length symmetric FIR filters, as illustrated by special relations they satisfy and by examples constructed.

DOA estimation of wide-band signals based on time-frequency analysis

A numerical study on an approach based on the time-frequency analysis is performed for the high-resolution direction finding applications of wide-band signals. In this method, the direction of arrival (DOA) is estimated using a subband of the incoming signal instead of the whole signal spectrum. This treatment forces the DOA of the wide-band signals to be similar to that of the DOA of the narrow-band signals. The reason behind this can be thought of as the after effects of reducing the signal bandwidth, which constrains the signal subspace. An empirical analysis of the performance of the DOA estimator Is also investigated. This is done to compare the correct and estimated angles of separation between the targets, and it is demonstrated that there is a good agreement which implies that the use of prefiltering does not affect the resolution of the DOA estimator.

On the use of pre-filters in quadrupole mass spectrometers

Computer simulation, based on real, circular-rod quadrupole geometry, is used for studying the effect of RF-only pre-filters on the performance of the quadrupole mass analyser. Although they were originally introduced to avoid undesirable fringing field effects, pre-filters have proven to be useful even if these effects are not taken into account. It is also shown, through the example of the He ion, that the fringe fields do not really play a significant role in the case of fast ions. Consequently, mass spectrometers supplied with pre-filters produce better performance than the analyser itself not only for higher masses, where the fringing field effects are considerable, but also for lower ones where they can be neglected. The intensity-pre-filter length curve has a definite maximum. The optimum pre-filter length belonging to this maximum value is found to be about 12% that of the main analyser used and yields nearly 100% higher transmission at a better resolution. It is shown that pre-filters of optimum size remarkably enlarge the effective inlet aperture of the main filter.

Optimal deconvolution based on polynomial methods

The problem of estimating the input to a known linear system is treated in a shift operator polynomial formulation. The mean-square estimation error is minimized. The input and a colored measurement noise are described by independent ARMA (autoregressive moving average) processes. The filter is calculated by performing a spectral factorization and solving a polynomial equation. The approach can be applied to input prediction, filtering, and smoothing problems as well as to the use of prefilters in the quadratic criterion. It applies to nonminimum-phase as well as unstable systems, as illustrated by two examples. >