Showing papers on "Impulse noise published in 2001"

Adaptive impulse detection using center-weighted median filters

Abstract: Previous median-based impulse detection strategies tend to work well for fixed-valued impulses but poorly for random-valued impulse noise, or vice versa. This letter devises a novel adaptive operator, which forms estimates based on the differences between the current pixel and the outputs of center-weighted median (CWM) filters with varied center weights. Extensive simulations show that the proposed scheme consistently works well in suppressing both types of impulses with different noise ratios.

Noise adaptive soft-switching median filter

Abstract: Existing state-of-the-art switching-based median filters are commonly found to be nonadaptive to noise density variations and prone to misclassifying pixel characteristics at high noise density interference. This reveals the critical need of having a sophisticated switching scheme and an adaptive weighted median filter. We propose a novel switching-based median filter with incorporation of fuzzy-set concept, called the noise adaptive soft-switching median (NASM) filter, to achieve much improved filtering performance in terms of effectiveness in removing impulse noise while preserving signal details and robustness in combating noise density variations. The proposed NASM filter consists of two stages. A soft-switching noise-detection scheme is developed to classify each pixel to be uncorrupted pixel, isolated impulse noise, nonisolated impulse noise or image object's edge pixel. "No filtering" (or identity filter), standard median (SM) filter or our developed fuzzy weighted median (FWM) filter will then be employed according to the respective characteristic type identified. Experimental results show that our NASM filter impressively outperforms other techniques by achieving fairly close performance to that of ideal-switching median filter across a wide range of noise densities, ranging from 10% to 70%.

Understanding the effects of phase noise in orthogonal frequency division multiplexing (OFDM)

Abstract: Phase noise must be carefully considered when designing an OFDM-based communication system since an accurate prediction of the tolerable phase noise can allow the system and RF engineers to relax specifications. This paper analyzes the performance of OFDM systems under phase noise and its dependence on the number of sub-carriers both in the presence and absence of a phase correction mechanism. Besides some practical results are provided so as to give some insight into the phase noise spectral specifications that should be required of the local oscillator.

Application of partition-based median type filters for suppressing noise in images

Abstract: An adaptive median based filter is proposed for removing noise from images. Specifically, the observed sample vector at each pixel location is classified into one of M mutually exclusive partitions, each of which has a particular filtering operation. The observation signal space is partitioned based an the differences defined between the current pixel value and the outputs of CWM (center weighted median) filters with variable center weights. The estimate at each location is formed as a linear combination of the outputs of those CWM filters and the current pixel value. To control the dynamic range of filter outputs, a location-invariance constraint is imposed upon each weighting vector. The weights are optimized using the constrained LMS (least mean square) algorithm. Recursive implementation of the new filter is then addressed. The new technique consistently outperforms other median based filters in suppressing both random-valued and fixed-valued impulses, and it also works satisfactorily in reducing Gaussian noise as well as mixed Gaussian and impulse noise.

Method and system for detecting, timing, and correcting impulse noise

Abstract: A system for detecting and correcting impulse noise present on an input data signal includes an impulse detector module receiving an input data signal and producing as output an correction enable signal indicating when an impulse correction is required. An impulse corrector module receives the input data signal and a correction enable signal and produces a corrected data signal, e.g., having the impulse canceled or blanked, as output. A reliability estimator and selector module receives the corrected data signal and the input data signal and selects as output the input signal which is more reliable. In one embodiment, the impulse detector includes first and second complementary impulse detectors, the outputs of which are analyzed by an enable and correction module to produce an impulse detection signal with improved accuracy. Preferably, the enable and correction module also indicates the most appropriate type of impulse correction in accordance with the detection signals from the complementary detectors. A novel system and method of detecting impulses based on Gram Schmidt techniques is also presented. In this method, one or more channels of a multi-channel data signal are kept free of data. When a whitening filter is applied, impulses on these quiet channels are emphasized. The Gram Schmidt technique exploits this fact to provide for improved impulse detection. The system can be modified to detect other types of low dimensionality noise.

Robust Wigner distribution with application to the instantaneous frequency estimation

Abstract: The Wigner distribution (WD) produces highly concentrated time-frequency (TF) representation of nonstationary signals. It may be used as an efficient signal analysis tool, including the cases of frequency modulated signals corrupted with the Gaussian noise. In some applications, a significant amount of impulse noise is present. Then, the WD fails to produce satisfactory results. The robust periodogram has been introduced for spectral estimation of this kind of noisy signals. It can produce good concentration for pure harmonic signals. However, it is not so efficient in the cases of signals with rapidly varying frequency. This is the motivation for introducing the robust WD. It is a reliable TF representation tool for wide class of nonstationary signals corrupted with impulse noise. This distribution produces good accuracy of the instantaneous frequency (IF) estimation. Using the Huber (1981) loss function, a generalization of the WD is presented. It includes both the standard and the robust WD as special cases. This distribution can be used for TF analysis of signals corrupted with a mixture of impulse and Gaussian noise. The presented theory is illustrated on examples, including applications on the IF estimation and time-varying filtering of signals corrupted with a mixture of the Gaussian and impulse noise. The case study analysis of the IF estimators' accuracy, based on the standard and the robust WD forms, is performed. In order to improve the IF estimation, a median filter is applied on the obtained IF estimate.

Robust M-estimate adaptive filtering

Abstract: An M-estimate adaptive filter for robust adaptive filtering in impulse noise is proposed. Instead of using the conventional least-square cost function, a new cost function based on an M-estimator is used to suppress the effect of impulse noise on the filter weights. The resulting optimal weight vector is governed by an M-estimate normal equation. A recursive least M-estimate (RLM) adaptive algorithm and a robust threshold estimation method are derived for solving this equation. The mean convergence performance of the proposed algorithm is also analysed using the modified Huber function (a simple but good approximation to the Hampel's three-parts-redescending M-estimate function) and the contaminated gaussian noise model. Simulation results show that the proposed RLM algorithm has better performance than other recursive least squares (RLS) like algorithms under either contaminated gaussian or alpha-stable noise environment. The initial convergence, steady-state error, robustness to system change and computational complexity are also found to be comparable to the conventional RLS algorithm under gaussian noise alone.

LUM smoother with smooth control for noisy image sequences

Abstract: This paper focuses on adaptive structure of LUM (lower-upper-middle) smoothers for noisy image sequences. For the balance between noise suppression and signal-detail preservation, the LUM smoothers are widely used in smoothing applications. The amount of smoothing done by LUM smoothers is controlled by tuning parameter. However, the smoothing level is fixed for whole image. Thus, the excessive or insufficient smoothing can be performed. This problem is solved by a new method based on the adaptive controlled level of smoothing. A new method has excellent performance of the noise reduction in the environments corrupted by the impulse noise. In addition, minimal signal-detail and motion blurring can be observed. The performance of proposed method is evaluated through objective criteria and compared with traditional temporal, spatial, and spatiotemporal LUM smoothers.

Adaptive 3-D median filtering for restoration of an image sequence corrupted by impulse noise

Abstract: An adaptive 3-D median filtering, which achieves optimal image quality as well as fast computing time, is proposed to remove the impulse noise from a highly corrupted image sequence. The proposed algorithm is compared with the widely used impulse noise removal algorithms with respect to the peak signal-to-noise ratio and the number of computations. The proposed algorithm preserves the image details which are not expected to be corrupted by impulse noise so that the number of computations can be minimized. It has good restoration performance whether the number of pixels corrupted by impulse noise is large or small. In the proposed algorithm, the impulse noise ratio, which is the ratio of the number of pixels corrupted by impulse noise to the total number of pixels, is estimated, and the restoration filtering is adaptively applied based on the estimated impulse noise ratio.

Statistics of biological noise and performance of generalized energy detectors for passive detection

Abstract: Underwater noise due to snapping shrimp is highly impulsive, and often dominates the ambient noise environment of warm, shallow waters at frequencies above 1 kHz. We report here on the statistics of bandpass snapping shrimp noise data, and on the modeling of the joint distribution of the in-phase and quadrature components using bivariate versions of the generalized Gaussian (GG), generalized Cauchy, and Gaussian-Gaussian mixture models. We evaluate the performance of several generalized energy detectors for passive bandpass detection, by inserting stochastic signals into the noise data. Detection thresholds were measured for an integration time of 0.5 s and false alarm probabilities down to 1%. The locally optimum detector based on the mixture model gave the best weak signal detection performance, with an 8 dB reduction in detection threshold over conventional energy detection. A significance test detector based on the GG model performed 1-2 dB worse, but exhibited better strong signal performance.

COFDM tuner with impulse noise reduction

Abstract: A turner for digital terrestrial television (DVB-T, employing COFDM modulation), has an input section (2-15) which supplies a sample intermediate signal, for example at zero intermediate frequency and digitised by an ADC (12), corresponding to a desired reception channel corrupted by interference such as impulsive noise interference A threshold generator (16) generates a threshold which is larger than a moving average of the amplitudes of consecutive samples and a comparator (17) compares the amplitudes of the samples with the threshold If a sample amplitude exceeds the threshold, a corrector (18) sets to zero the sample before processing by a fast Fourier transform (19) The threshold generator (16) excludes samples which have been set to zero from the moving average

Effects of peak levels and number of impulses to hearing among forge hammering workers.

Abstract: The purpose of the study was to (1) compare measured and estimated hearing loss among forge hammering workers by applying models for risk assessment based on measurements of impulse noise, and (2) model the hazardous effects of impulse noise on hearing. Noise exposure and hearing loss among forge hammering workers were studied at two forge workshops of an automobile company, where the equivalent sound pressure levels (104 and 105 dB) were the same, but the peak levels and degree of impulsiveness were significantly different. The hearing threshold levels of selected groups of workers (97 and 235 workers)were determined. Comparison between the measured and expected hearing losses defined according to the ISO standard revealed 2 dB difference in excessive hearing loss (1 dB and 3 dB for the workers of workshop 1 and 2, respectively). The excessive hearing loss equals an increase of 3.5 years of exposure. The hearing loss of workers exposed to low impulsive noise could be predicted well using ISO 1999-1990. T...

A direction-finding technique for wide-band impulsive noise source

Abstract: A direction-finding technique for wide-band impulsive electromagnetic interference is presented. An experimental investigation was performed using a spark generator as the impulsive noise source. The signals were captured using a two-antenna array directly sampled at 1 Gs/s. A digital signal processing based correlation technique allowed assessment of the time delay between the antennas. The results in a nonideal radio propagation environment show the bearing can be estimated to an accuracy of 20.

Robust adaptive spread-spectrum receiver with neural net preprocessing in non-Gaussian noise

Abstract: Multiuser communications channels based on code division multiple access (CDMA) technique exhibit non-Gaussian statistics due to the presence of highly structured multiple access interference (MAI) and impulsive ambient noise. Linear adaptive interference suppression techniques are attractive for mitigating MAI under Gaussian noise. However, the Gaussian noise hypothesis has been found inadequate in many wireless channels characterized by impulsive disturbance. Linear finite impulse response (FIR) filters adapted with linear algorithms are limited by their structural formulation as a simple linear combiner with a hyperplanar decision boundary, which are extremely vulnerable to impulsive interference. This raises the issues of devising robust reception algorithms accounting at the design stage the non-Gaussian behavior of the interference. We propose a multiuser receiver that involves an adaptive nonlinear preprocessing front-end based on a multilayer perceptron neural network, which acts as a mechanism to reduce the influence of impulsive noise followed by a postprocessing stage using linear adaptive filters for MAI suppression. Theoretical arguments supported by promising simulation results suggest that the proposed receiver, which combines the relative merits of both nonlinear and linear signal processing, presents an effective approach for joint suppression of MAI and non-Gaussian ambient noise.

Fast Modied Vector Median Filter

Abstract: A new ltering approach designed to eliminate impulsive noise in color images, while preserving ne image details is presented in this paper. The computational complexity of the new lter is signif- icantly lower than that of the Vector Median Filter. The comparison shows that the new lter outperforms the VMF, as well as other standard procedures used in color image processing, when the impulse noise is to be eliminated.

Interleaved generalized convolutional encoder

Abstract: Several embodiments of an interleaved generalized convolutional encoder system and method are provided for communicating data over a transmission path in order to inhibit distortion caused by impulse noise or other correlated noise and enhance transmission rate of data communications. The encoder system is designed to introduce a variable delay to the convolutional encoder. The variable delay effectively spreads correlated noise over a variable number of symbols in time. Correlated noise that is spread over time allows for better error correction encoding.

Impulse noise cancellation in multicarrier transmission

Abstract: A parallel between Reed Solomon codes in the complex field and multicarrier transmission using OFDM is first presented. This shows that when the signal is sent over some channel composed of Gaussian plus impulse noise, the impulse noise can be removed by a procedure similar to channel decoding, using information carried by the "syndrome". These results are first derived in a simple situation (oversampled DMT, additive channel), which is merely of theoretical interest. Several extensions are then provided in order to increase the practical usefulness of the method. Simulations combining classical convolutive codes with the above mentioned approach are provided.

On the use of pilot tones for impulse noise cancellation in Hiperlan2

Abstract: This paper presents an elegant solution to impulse noise cancellation in OFDM systems. A parallel between Reed Solomon codes in the complex field and multicarrier transmission using OFDM is first presented. This shows that when the signal is sent over some channel composed of Gaussian plus impulse noise, the impulse noise can be removed by a procedure similar to channel decoding, using information carried by the "syndrome". These results are first derived in a simple situation (oversampled DMT, additive channel), which is merely of theoretical interest. In any case consecutive zeroes, in the output of the OFDM modulator, do not correspond to real subcarriers. However in many cases, pilot tones are emitted for synchronization or channel estimation purposes. These pilot tones are generally scattered among the information ones. Our approach is to use these pilot tones as "syndromes", in order to correct impulse noise. We show that the correction capacity is conditioned by the position of these pilot tones in the emitted sequence. Efficiency of this technique is corroborated with simulation in the practical context of the Hiperlan2.

Crosstalk mitigation in DMT VDSL with impulse noise

Abstract: Crosstalk and impulse noise are two principal sources of degradation in very-high-rate digital subscriber line (VDSL) transmission systems. The traditional single-user data detector for such systems merges crosstalk into the background noise, which is assumed to be white and Gaussian. Recent research has explored the nature of crosstalk signals and shown the potential benefits of multiuser detection (MUD) for VDSL signals with strong crosstalkers. Impulse noise is one of the most difficult transmission impairments to suppress, and is poorly characterized and understood as well. In DSL transmission, impulse noise is typically combated with interleaved forward-error correction (FEC). However, recent data indicate that a significant minority of impulse-noise events are longer than the maximum error-correcting capacities of the default interleaved FEC provided within current ANSI standards. Thus, it is of interest to consider signal processing methods that can jointly mitigate crosstalk and impulse noise. In this paper, we explore such a technique based on a recently developed robust M-detector structure for MUD in non-Gaussian ambient noise.

Robust time-frequency distributions

Abstract: The time-frequency (TF) distributions robust with respect to the heavy-tailed impulse noise are introduced. The robust spectrogram (SPEC) and robust Wigner distribution (WD) are considered. The calculation procedure and properties of these representations are given.

Iterative multiuser detection for turbo-coded synchronous CDMA in Gaussian and non-Gaussian impulsive noise

Abstract: We develop an iterative multiuser receiver for decoding turbo-coded synchronous code-division multiple-access signals in both Gaussian and non-Gaussian noise. A soft-input soft-output nonlinear multiuser detector is combined with a set of single-user channel decoders in an iterative detection/decoding structure. The nonlinear multiuser detector utilizes the prior probabilities of each user's bits to form soft estimates used for multiple-access interference cancellation. The channel decoders perform turbo-code decoding and produce posterior probabilities which are fed back to the multiuser detector for use as prior probabilities. Simulation results show that the proposed multiuser receiver performs well in both Gaussian and non-Gaussian noise. In particular, single-user turbo-code performance can be approached within a few iterations with medium to low cross correlation (/spl rho//spl les/0.5).

Fast Modified Vector Median Filter

Abstract: A new filtering approach designed to eliminate impulsive noise in color images, while preserving fine image details is presented in this paper. The computational complexity of the new filter is significantly lower than that of the Vector Median Filter. The comparison shows that the new filter outperforms the VMF, as well as other standard procedures used in color image processing, when the impulse noise is to be eliminated.

Rank Image Processing Using Spatially Adaptive Neighborhoods 1

Abstract: A new approach to the design of rank-order filters based on the effective use of spatial relations between image elements is proposed. Many rank-order processing techniques can benefit from this approach, such as noise suppression, local contrast enhancement, and local detail extraction. An extension of the approach to rank-order filtering of three-dimensional signals is also discussed. The performance of the proposed rank- order filters in suppressing mixed additive and impulse noise in a test image is compared to that of conventional rank-order algorithms. The comparisons are made using criteria of a mean square error, a mean absolute error, and a subjective human visual error.

Nonlinear group-blind multiuser detection

Abstract: A nonlinear group-blind technique is developed for joint detection of some given users' data in a CDMA uplink environment with the presence of unknown interference. This method performs the so-called "slowest-descent search" over a likelihood function of the desired users, starting from the estimate closest to the unconstrained maximizer of the likelihood function, and along mutually orthogonal directions where this likelihood function drops to the slowest. Simulation results show that this new nonlinear technique offers substantial performance improvement over the previously proposed linear group-blind multiuser detectors with little attendant increase in computational complexity. The problem of group-blind multiuser detection in the presence of both unknown interference and impulsive ambient noise is also treated under the framework of slowest-descent search, with the aid of a novel subspace-based robust interference cancellation scheme. It is seen that this robust group-blind method significantly outperforms the robust blind multiuser detection scheme proposed previously.

Impulse noise detection algorithms for multicarrier communication systems -performance analysis

Abstract: Impulse noise is known to be highly damaging to the performance of a multicarrier system. To be able to deal with impulsive disturbances, the receiver has to recognize which symbol was contaminated by the impulse noise. We analyze performance of a selection of impulse noise detection algorithms. Impulse noise can be detected in the time domain, in the frequency domain, or by combining the time and frequency domain analysis. The choice of the impulse noise detection algorithm depends on the characteristics of an impulse.

Robustness of OFDM signal against temporally localized impulsive noise

Abstract: We present the theoretical analysis of the bit error rate (BER) performance of single-carrier modulation (SCM) and orthogonal frequency division multiplexing (OFDM) systems under two types of temporally localized impulsive noise environments, namely stationary and bursty impulsive noise. The robustness of OFDM system against impulsive noise is then discussed and compared with that of the SCM system at the same transmission rate. In addition to the common knowledge on OFDM, we show that OFDM system is sometime less robust to impulsive noise than SCM system.

Multiuser detection in impulsive noise

Abstract: A new technique is proposed for robust multiuser detection in channels with impulsive noise. The method is a modification of traditional nonlinear multiuser detection techniques, whereby the nonlinearity is now positioned between the multiplier and the summation, within the correlator, instead of preceding it. This is then extended to provide a modification to the two-stage nonlinear detector proposed in Weng et al., (1997). Simulation results show that the use of this technique increases multiple access interference (MAI) rejection. Near-far effects are also investigated.