Fast impulsive noise removal

A generic n-dimensional filter with the primary purpose of eliminating impulsive-like noise is presented. This recursive nonlinear filter is composed of two conditional rules, which are applied independently, in any order, one after the other. It identifies noisy items by inspection of their surrounding neighborhood, and afterwards it replaces their values with the most "conservative" ones out of their neighbors' values. In this way, no new values are introduced and the histogram distribution range is conserved. This n-dimensional filter can be decomposed recursively to a lower dimensional space, each time generating two sets of n(n-1)-dimensional filters. This study, which focuses on the case of two-dimensional signals (gray scale images), explores one possible implementation of this new filter and orients the evaluation of its performance toward the median filter, as this filter is the basis of many more sophisticated filters for impulsive noise reduction. Tests were carried out using both real and artificial images. We found this new filter to be much faster than the median filter while performing comparably in terms of both image information conservation and noise reduction, which suggests that it could replace the median filter for the preliminary processing included in state-of-the-art noise removal filters. This new filter should either eliminate or attenuate most noisy pixels in synthetic and natural images not excessively contaminated. It has a slight smoothing effect on nonnoisy image regions. In addition, it is scalable, easily implemented, and adaptable to specific applications.     

Detail preserving impulsive noise removal

Most image processing applications require noise elimination. For example, in applications where derivative operators are applied, any noise in the image can result in serious errors. Impulsive noise appears as a sprinkle of dark and bright spots. Transmission errors, corrupted pixel elements in the camera sensors, or faulty memory locations can cause impulsive noise. Linear filters fail to suppress impulsive noise. Thus, non-linear filters have been proposed. Windyga's peak-and-valley filter, introduced to remove impulsive noise, identifies noisy pixels and then replaces their values with the minimum or maximum value of their neighbors depending on the noise (dark or bright). Its main disadvantage is that it removes fine image details. In this work, a variation of the peak-and-valley filter is proposed to overcome this problem. It is based on a recursive minimum–maximum method, which replaces the noisy pixel with a value based on neighborhood information. This method preserves constant and edge areas even under high impulsive noise probability. Finally, a comparison study of the peak-and-valley filter, the median filter, and the proposed filter is carried-out using different types of images. The proposed filter outperforms other filters in the noise reduction and the image details preservation. However, it operates slightly slower than the peak-and-valley filter.     

Turbo equalisation in non-Gaussian impulsive noise

Turbo equalisation is a state-of-the-art receiving scheme for coded data transmission over channels introducing intersymbol interference (ISI). The author investigates turbo equalisation performance in the presence of ISI and impulsive noise. The design imperfections contributing to the non-robustness of the standard turbo equaliser to outliers are identified, and a novel turbo equaliser, at almost no additional increase in complexity, is proposed for joint mitigation of ISI and impulsive noise. The proposed turbo equaliser incorporates a Talwar penalty function into the maximum a posteriori (MAP) component equaliser to serve two purposes. First, it improves the estimation of the transition probabilities for all transitions through the trellis and for subsequent determination of the a posteriori log-likelihood ratio. Secondly, it absorbs the outliers and prevents them from spreading into the MAP constituent decoder. Simulation results based on Proakis's channel models show that the proposed turbo equaliser achieves a dramatic improvement over the standard turbo equaliser in impulsive noise. At a bit error rate (BER) of 10/sup -2/, the performance gain is as large as 3.5 to 5 dB, and as large as 7 to 8 dB at a BER of 10/sup -3/.     

Space-time blind equalisation in impulsive noise

The presence of non-Gaussian impulsive noise in wireless system can degrade the performance of existing equalisers and signal detectors. The problem of blind source separation and equalisation for multipleinput/ multiple-output (MIMO) channels under heavy-tailed impulsive noise environment is studied. A generalised multi-user constant modulus (CM) cost function is proposed by employing the fractional lower order CM property of the equaliser input signals as well as the fractional lower order cross-correlations between them. The associated adaptive blind equalisation algorithm based on a stochastic gradient descent method is defined as fractional lower order multi-user constant modulus algorithm (FMU_CMA), which is able to mitigate impulsive channel noise while recovering all input signals simultaneously. The steady-state meansquare error (MSE) performance of the FMU_CMA is studied in a noise-free environment; the approximate expression is derived based on the energy-preserving relation and the Taylor series expansion. Simulation studies are undertaken to support the analysis with CM and non-CM signal.     

Blind turbo equalization in Gaussian and impulsive noise

We consider the problem of simultaneous parameter estimation and restoration of finite-alphabet symbols that are blurred by an unknown linear intersymbol interference (ISI) channel and contaminated by additive Gaussian or non-Gaussian white noise with unknown parameters. Non-Gaussian noise is found in many wireless channels due to the impulsive phenomena of radio-frequency interference. Bayesian inference of all unknown quantities is made from the blurred and noisy observations. The Gibbs sampler, a Markov chain Monte Carlo procedure, is employed to calculate the Bayesian estimates. The basic idea is to generate ergodic random samples from the joint posterior distribution of all unknowns and then to average the appropriate samples to obtain the estimates of the unknown quantities. Blind Bayesian equalizers based on the Gibbs sampler are derived for both Gaussian ISI channel and impulsive ISI channel. A salient feature of the proposed blind Bayesian equalizers is that they can incorporate the a priori symbol probabilities, and they produce as output the a posteriori symbol probabilities. (That is, they are “soft-input soft-output” algorithms.) Hence, these methods are well suited for iterative processing in a coded system, which allows the blind Bayesian equalizer to refine its processing based on the information from the decoding stage and vice versa-a receiver structure termed as blind turbo equalizer     

Acoustic noise and echo cancelling with microphone array

A novel method of performing acoustic echo cancelling using microphone arrays is presented. The method employs a digital self-calibrating microphone system. The calibration process is a simple indirect on-site calibration that adapts to the particulars of the acoustic environment and the electronic equipment in use. Primarily intended for handsfree telephones in automobiles, the method simultaneously suppresses the handsfree loudspeaker and car noise. The system also continuously takes into account disturbances such as fan noise. Examples from an extensive evaluation in a car are also included. Typical performance results demonstrate 20-dB echo cancellation and 10-dB noise reduction simultaneously     

Image detail-preserving filter for impulsive noise attenuation

A new nonlinear filter is proposed for attenuating impulsive noise while preserving image details. The filter truncates the grey value of a pixel to the maximal or minimal value of its enclosed surrounding band. Impulsive noise inside the band is thus attenuated while image details are preserved as long as they stretch to the band. The recursive form of the proposed filter leads to a simple architecture for fast implementation. Theoretical analysis and experimental results demonstrate the effectiveness of this new filter for both noise attenuation and detail preservation. For moderately contaminated images, as shown in the experiments, the proposed filter outperforms the standard median filter, the centre-weighted median filter and the unidirectional multistage median filter in terms of mean absolute error and filtering speed.     

Detection in correlated impulsive noise using fourth-ordercumulants

We consider detection and estimation in correlated impulsive noise. The non-Gaussian impulsive noise is modeled as the sum of two linear processes: a nominal part and an impulsive part. This model admits correlated impulsive bursts lasting many data samples. Identifiability of the noise model is established using fourth- and second-order cumulants. Under this model, the correlated time series can be whitened and an appropriate memoryless nonlinearity applied to attenuate the impulsive events. A detection statistic is then formed from the output of the nonlinearity. In the threshold detection case, the use of cumulants allows identification of the noise in the presence of the signal to be detected, obviating the need for noise-only training records. Simulation results with a sample size of 512 show small loss in detector performance versus an ideal detector with no impulsive part present     

Detection-estimation based approach for impulsive noise removal

A new algorithm for effectively removing impulsive noise in digital images is proposed. The computationally efficient algorithm presented first classifies corrupted pixels and then performs median filtering for them only. New efficiency factors are introduced to compare the performance of the noise detection algorithms. Simulation results show that the proposed algorithm performs better than existing methods in terms of both objective and subjective evaluations     

Adaptive TDOA estimation in presence of impulsive noise

A fractional lower-order, moment-based adaptive algorithm for estimating the time difference of arrival between signals received at two spatially separated sensors in the presence of impulsive noise modelled as a stable process is proposed. The method is computationally efficient and is a generalisation of the simplified explicit time delay estimator. Convergence of the algorithm is analysed and simulation results are included to evaluate its estimation performance for both static and nonstationary delays     

Acoustic echo cancellor with sub-band noise cancelling

An acoustic echo canceller with sub-band noise cancelling that employs a cascade configuration is proposed. The adaptation control adopted to match the occurrence of intermittent speech/echo and continuous room noise using the NLMS algorithm is very effective in echo and noise cancellation. Hardware is implemented and its performance evaluated through experiments. The noise cancellation significantly enhances overall echo-cancellation performance.<>     

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

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     

Novel procedure to determine statistical functions of impulsive noise

One of the problems found when measuring impulsive noise is to distinguish this kind of noise from Gaussian noise. Usually, a threshold level is used to make the difference. The problem is that a high threshold level will miss low amplitude pulses, while a low level will include Gaussian noise samples as being impulsive noise. In this paper, results of a novel radio UHF impulsive noise measurement procedure are presented. This work exhibits the peculiarity that data was taken in both horizontal and vertical polarizations simultaneously. One polarization is used to determine the presence of impulsive noise so analysis can be performed in the other polarization considering this circumstance. Measurements were made in four different locations on UHF TV channels around 800 MHz with a bandwidth of 10 MHz, demodulating the in-phase and quadrature phase components in each polarization. When environments are constituted by varied and scattered impulsive sources, horizontal and vertical polarizations show similar statistical behavior. However, horizontal and vertical emissions may be different when a single source is individually measured. The conditional amplitude probability density function (pdf) and conditional amplitude cumulative distribution function (cdf) found for the amplitude in all the locations lead to the conclusion that the variable that best fits the results is a lognormal one.     

Spread-spectrum multiple-access performance of orthogonal codes:impulsive noise

A direct-sequence spread-spectrum multiple-access (SSMA) communication system that assigned a set of M-orthogonal sequences to each user is analyzed. An accurate model is incorporated for the impulsive noise that characterizes the LF and MF bands, so that the SSMA receiver operates in a combination of multiple-access interference and impulsive (atmospheric) noise. The performance of a linear receiver operating in such an environment is analyzed, and probability-of-error curves are presented. The presence of impulsive noise motivates the derivation and analysis of a nonlinear receiver that use a variable-gain stage to suppress noise impulses. This receiver is effectively optimum when the signal amplitudes are below a certain bound and when the noise and interference samples are independent, or nearly so. However, the gain stage of this nearly optimum receiver depends on the noise model parameters including the various user delays. Consequently, a nonparametric receiver that incorporates a simple clipper is also analyzed. The asymptotic relative efficiency of both receivers is determined     

Deblurring of color images corrupted by impulsive noise.

We consider the problem of restoring a multichannel image corrupted by blur and impulsive noise (e.g., salt-and-pepper noise). Using the variational framework, we consider the L1 fidelity term and several possible regularizers. In particular, we use generalizations of the Mumford-Shah (MS) functional to color images and gamma-convergence approximations to unify deblurring and denoising. Experimental comparisons show that the MS stabilizer yields better results with respect to Beltrami and total variation regularizers. Color edge detection is a beneficial by-product of our methods.     

Blind separation of impulsive noise from corrupted audio signal

The authors present a new method for separating the impulsive noise from a corrupted audio signal. It is shown that if the audio signal is assumed to be Gaussian distributed, the third-order cumulants of the impulsive noise can be separated, based upon the corrupted audio signal. The impulsive noise can then be reconstructed via the relationship between the bispectrum and the Fourier spectrum. Finally, the audio signal is restored by simply subtracting the reconstructed impulsive noise from the signal     

Optimization of a cascaded structure for noise and echo cancellation using a noise filtering preprocessing

This paper deals with speech enhancement in hands-free telecommunication systems. The two major problems encountered in such contexts - noise reduction and acoustic echo cancellation - are addressed. After the presentation of the two basic cascaded structures found in the literature, we propose a new promising technique which takes advantage of the previous ones. The main objective is to get a near-end speech signal with low levels of distortion, echo and noise. The structure is tested in a real mobile environment and compared with the two others. An objective evaluation shows the improvement provided by this structure.     

Measurements and models of radio frequency impulsive noise forindoor wireless communications

The authors present the results of average and impulsive noise measurements inside several office buildings and retail stores. The noise measurement system operated at 918 MHz, 2.44 GHz, and 4 GHz with a nominal 40-MHz, 3-dB RF bandwidth. Omnidirectional and directional antennas were used to investigate the characteristics and sources of RF noise in indoor channels. Statistical analyses of the measurements are presented in the form of peak amplitude probability distributions, pulse duration distributions, and interarrival time distributions. Simple first-order mathematical models for these statistical characterizations are also presented. These analyses indicate that photocopiers, printers (both line printers and cash register receipt printers), elevators, and microwave ovens are significant sources of impulse noise in office and retail environments     

冲击噪声环境中的联合对角化波达方向矩阵法

针对对称α平稳(SαS)噪声环境中信号源的高分辨二维波达方向(DOA)估计问题,提出了一种新的联合对角化(JD)分数低阶矩(FLOM)-DOA矩阵方法.构建了一种特殊的阵列结构,把其分为三个子阵,然后通过联合对角化获得信号的二维角估计.新方法不仅保持原协变异DOA矩阵方法无需二维谱峰搜索和参数配对等优点,还较好地解决了一维角度兼并问题.仿真结果表明,新方法优于协变异DOA矩阵法.