Noise Whitening Research Articles

Coded Single-Sideband QPSK and Its Turbo Detection for Mobile Communication Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper proposes a novel single-sideband quadrature phase shift keying (SSB QPSK) modulation scheme that utilizes a single-reference carrier frequency and transforms both baseband I and Q components into the same radio frequency band, whose width is exactly half that of QPSK. The modulation method is first derived and followed by the analysis of the matched filter demodulation. The analysis shows that there are performance degradations due to intersymbol interference (ISI) in the received signal and correlation in the received noise. To suppress the degradations, this paper employs a turbo equalizer for demodulation, in which the effects of ISI and noise correlation can be suppressed by a soft canceler and a noise whitening filter in the minimum mean square error (MMSE) linear equalizer, respectively. The computer simulations are then used to verify the proposed modulation scheme. At the transmitter, the root-raised cosine rolloff filter is employed for pulse shaping and Hilbert-transformed pulse generation. The truncation of the signal pulse is performed by the window function. The results show that the proposed modulation scheme outperforms 16-quadratic-amplitude modulation (16-QAM) with the turbo code in the Rayleigh-fading environment. </para>

An oversampled channelized UWB receiver with transmitted reference modulation

To digitize the ultra-wideband (UWB) signal at its Nyquist rate, a frequency channelized receiver for UWB radio based on hybrid filter banks (HFB) is presented. Among the challenges of such receivers are the uncertainties of the analog analysis filters and the slow convergence speed. To overcome these problems, a channelized receiver operating at slightly above the critically sampling rate is presented. The proposed receiver, which is designed for use in transmitted reference (TR) systems, combines the synthesis filters and the matched filter so that the joint response of the analysis filter and the propagation channel can be estimated independently in each subband. When the input noise is colored or a narrowband interference (NBI) is present, the weighting in each subband can be adaptively adjusted so to approximate the noise whitening and matched filtering operation for near optimal detection. The adaptive performance of the proposed receiver is slightly better than an ideal full-band receiver when the input noise is white and significantly better when a NBI is present. The effect of the automatic gain controller (AGC) and the analog-to-digital converter (ADC) are also considered. The proposed receiver with 3-bit ADC seems sufficient to achieve performance comparable to an infinite bit channelized receiver even in the presence of large NBI

Interference robustness aspects of space-time block code-based transmit diversity

Transmit diversity, which was initially developed for noise-limited environments, has been promoted as a viable candidate for improving the link quality in both existing and future systems for wireless communication. However, to ensure efficient spectrum utilization, receivers operating within wireless multiuser networks must be robust not only to fading and noise but to interference from other system users as well. This work considers interference robustness aspects when transmit diversity, in the form of space-time block coding, is used in multiuser systems. Properties of the space-time block encoded signals such as code rate, block structure, diversity order, etc., and their implications on detection and interference rejection by means of noise whitening are discussed. To handle the presence of space-time block encoded interference, a space-time processing-based extension of an interference rejection combining algorithm is proposed. Results are presented indicating that transmit diversity based on space-time block codes (STBCs) of the linear dispersion type improve robustness against interference in terms of an increased diversity advantage. This can be achieved either by increasing the number of transmit antennas or by reducing the rate of the code. It is also shown, by analysis and by simulation examples, that the performance improvements obtained by using transmit diversity in multiuser systems may rapidly subside as the signal-to-interference ratio decreases. However, by using the proposed interference rejection scheme tailored to the space-time encoded structure, performance improvements of transmit diversity are also obtained in a multiuser environment.

Maximum-Likelihood Estimation of Low-Rank Signals for Multiepoch MEG/EEG Analysis

A maximum-likelihood-based algorithm is presented for reducing the effects of spatially colored noise in evoked response magneto- and electro-encephalography data. The repeated component of the data, or signal of interest, is modeled as the mean, while the noise is modeled as the Kronecker product of a spatial and a temporal covariance matrix. The temporal covariance matrix is assumed known or estimated prior to the application of the algorithm. The spatial covariance structure is estimated as part of the maximum-likelihood procedure. The mean matrix representing the signal of interest is assumed to be low-rank due to the temporal and spatial structure of the data. The maximum-likelihood estimates of the components of the low-rank signal structure are derived in order to estimate the signal component. The relationship between this approach and principal component analysis (PCA) is explored. In contrast to prestimulus-based whitening followed by PCA, the maximum-likelihood approach does not require signal-free data for noise whitening. Consequently, the maximum-likelihood approach is much more effective with nonstationary noise and produces better quality whitening for a given data record length. The efficacy of this approach is demonstrated using simulated and real MEG data.

Optimal Minimum Distance-Based Precoder for MIMO Spatial Multiplexing Systems

We describe a new precoder based on optimization of the minimum Euclidean distance d/sub min/ between signal points at the receiver side and for use in multiple-input multiple-output (MIMO) spatial multiplexing systems. Assuming that channel state information (CSI) can be made available at the transmitter, the three steps ( noise whitening, channel diagonalization and dimension reduction), which are currently used in investigations on MIMO systems, are performed. Thanks to this representation, an optimal d/sub min/ precoder is derived in the case of two different transmitted data streams. For quadrature phase-shift keying (QPSK) modulation, a numerical approach shows that the precoder design depends on the channel characteristics. Comparisons with maximum signal-to-noise ratio (SNR) strategy and other precoders based on criteria, such as water-filling (WF), minimum mean square error (MMSE), and maximization of the minimum singular value of the global channel matrix, are performed to illustrate the significant bit-error-rate (BER) improvement of the proposed precoder.

Whitening of non-stationary noise from gravitational wave detectors

We investigate the possibility of improving the detection efficiency for deterministic gravitational wave signals in the presence of non-stationary noise. The proposed method is based on an adaptive whitening procedure to be applied to the experimental data, and depends on a ‘forgetting factor’ parameter λ. We test the method on simulated data, and show that by choosing λ appropriately it is possible to reduce the non-stationarity noise of the time series and to implement a detector which approaches the performances of the optimal Wiener filter for a deterministic signal in the presence of non-stationary noise of known Gaussian statistic.

An approach based on simplified KLT and wavelet transform for enhancing speech degraded by non-stationary wideband noise

It is well known that the non-stationary wideband noise is the most difficult to be removed in speech enhancement. In this paper a novel speech enhancement algorithm based on the dyadic wavelet transform and the simplified Karhunen–Loeve transform (KLT) is proposed to suppress the non-stationary wideband noise. The noisy speech is decomposed into components by the wavelet space and KLT-based vector space, and the components are processed and reconstructed, respectively, by distinguishing between voiced speech and unvoiced speech. There are no requirements of noise whitening and SNR pre-calculating. In order to evaluate the performance of this algorithm in more detail, a three-dimensional spectral distortion measure is introduced. Experiments and comparison between different speech enhancement systems by means of the distortion measure show that the proposed method has no drawbacks existing in the previous methods and performs better shaping and suppressing of the non-stationary wideband noise for speech enhancement.

Comparison of filters for detecting gravitational wave bursts in interferometric detectors

Filters developed in order to detect short bursts of gravitational waves in interferometric detector outputs are compared according to three main points. Conventional Receiver Operating Characteristics (ROC) are first built for all the considered filters and for three typical burst signals. Optimized ROC are shown for a simple pulse signal in order to estimate the best detection efficiency of the filters in the ideal case, while realistic ones obtained with filters working with several ``templates'' show how detection efficiencies can be degraded in a practical implementation. Secondly, estimations of biases and statistical errors on the reconstruction of the time of arrival of pulse-like signals are then given for each filter. Such results are crucial for future coincidence studies between Gravitational Wave detectors but also with neutrino or optical detectors. As most of the filters require a pre-whitening of the detector noise, the sensitivity to a non perfect noise whitening procedure is finally analysed. For this purpose lines of various frequencies and amplitudes are added to a Gaussian white noise and the outputs of the filters are studied in order to monitor the excess of false alarms induced by the lines. The comparison of the performances of the different filters finally show that they are complementary rather than competitive.

Noise Whitening-Based Pitch Detection for Speech Highly Corrupted by Colored Noise

Pitch estimation is important in various speech research areas, but when the speech is noisy, accurate pitch estimation with conventional pitch detectors is almost impossible. To solve this problem, we propose a new pitch detection algorithm for noisy speech using a noise whitening technique on the background noise and obtain successful results.

The mean-square generalized delayed decision feedback sequence detector

The generalized delayed decision feedback sequence detector is a very general equalization scheme. The receiver consists of a matched filter, a prefilter, and a Viterbi algorithm with reduced number of states and multiple survivors per state, where a decision feedback equalizer is attached to each survivor. Two prefilters are considered: the noise whitening prefilter and the mean-square prefilter. Simulation results show that the mean-square prefilter gives substantial benefits when a severely frequency-selective channel is considered. Copyright © 2003 AEI.

Joint multi‐user detection and intersymbol interference cancellation for WCDMA satellite UMTS

AbstractIn this work we present two different intersymbol interference (ISI) cancellation systems, namely the commutation signaling (CS) and orthogonal code‐division transmit diversity (O‐CDTD) in combination with satellite path diversity, which leads to a high diversity order. The ISI is due to the use of satellite path diversity (in order to introduce multipath diversity artificially). This work combines the proposed ISI cancellation systems with subtractive multi‐user detectors—a variation of the conventional parallel interference cancellation (PIC) and successive interference cancellation (SIC) for the downlink channel of S‐UMTS. The blind detection is also considered without knowledge of spreading sequences or the channel state information (CSI) of the interfering users with a noise whitening matched filter (NWMF) that combats the MAI through the whitening of the interfering power spectrum, also with and without the ISI cancellation systems—CS and O‐CDTD. It is shown that the proposed PIC is a very promising detector to be used for the downlink of S‐UMTS, especially when combined with CS or O‐CDTD, the NWMF being efficient only when we consider jointly any of the proposed ISI cancellation systems for low number of interfering users. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Subspace-based multiuser detection for DS-CDMA systems with correlated noise

Two subspace-based multiuser detectors are presented for a synchronous DS-CDMA system operating over a multipath fading channel with correlated noise. Based on the training sequence transmitted by the desired user and employing a noise whitening process, a linear MMSE detector is obtained which does not require knowledge of the code sequences and the propagation channel response. When the training sequence is not available, a matrix decomposition method is adopted to estimate the channel response. Through signature waveform estimation, a linear MMSE detector can be obtained blindly. Simulation results show that these two methods are robust against the near–far problem and the correlated noise.

Effect of tap spacing on the performance of direct-sequence spread-spectrum RAKE receiver

The effect of tap spacing on the performance of a RAKE receiver is analyzed analytically in a frequency-selective fading channel. A continuous time multipath fading channel model is used for the analysis, and the expression of the correlation between the desired signals, interference signals, and noise signals at the output of each branch of the RAKE receiver is derived for various chip waveforms. Since the noise components of each branch signal are correlated to each other, an optimum combining rule based on the maximum-likelihood criterion is derived to gain utmost performance. It is shown that the performance of the system can be improved by setting the tap spacing of the RARE receiver below the chip duration when the bandwidth of the transmitted signal is larger than the inverse of the chip duration. Also, it is shown that the normalized capacity of the system can be increased by using a chip waveform occupying wider bandwidth, which takes advantage of the increased diversity gain merits of a wide-band code-division multiple-access system at the same chip rate. It is noted that the derived combining rule gives diversity gain against the fading process as well as noise whitening processing gain against multiple-access interference at the same time.

Signal/noise KLT based approach for enhancing speech degraded by colored noise

A signal/noise Karhunen-Loeve transform (KLT) based approach for enhancing speech degraded by colored noise is proposed. The noisy speech frames are classified into speech-dominated frames and noise-dominated frames. In the speech-dominated frames, the signal KLT matrix is used and in the noise dominated frames, the noise KLT matrix is used. The approach does not require noise whitening and hence works well even with narrowband noise. A two-dimensional objective measure which captures both the speech distortion and the noise shaping characteristics of the algorithm is proposed. This measure indicates that the proposed method performs better noise shaping than a modified form of the signal subspace approach proposed by Ephraim and Van Trees (1995) and the standard spectral subtraction method. Informal listening tests show that the proposed algorithm does not suffer from the problem of residual musical noise and performs better noise masking than the signal subspace approach.

生き残りパスを利用した雑音予測による雑音白色化機能を備えたビタビ復号法の検討

This paper describes a method of eliminating the auto correlation of equalized noises to have the optimum performance of a Viterbi decoder designed for white noise. This elimination is performed by subtracting predicted noises from an arriving equalized noise. This predicted noise is generated by using both coefficients calculated from auto correlation of the equalized noises and noises corresponding to survived paths. A correctly predicted noise reduces an arriving equalized noise on average, so the error rate becomes lower. A Viterbi decoder with a noise whitening function reduced the number of errors to about 1/6 of those of a conventional Viterbi decoder's for an experimental digital video cassette recorder. The conditions of the experiments were : relative tap e-to-head speed of 7.5 m/s, recording rate of 40.5 Mb/s, and 8/12 code whose minimum run length was limited to 2 (in NRZL notation) combined with EPR 4, and four coefficients for noise prediction.

Near optimum tree-search detection schemes for bit-synchronous multiuser CDMA systems over Gaussian and two-path Rayleigh-fading channels

Simple reduced tree-search detection schemes of the breadth-first type are applied to suboptimal joint multiuser detection in bit-synchronous code-division multiple access (CDMA) systems over both Gaussian and two-path Rayleigh-fading channels. It is pointed out that in contrast to the case of the optimal multiuser detector, the choice of the receiver filter severely influences the performance of suboptimal multiuser detectors. Simulation results supported by analysis show that breadth-first tree-search algorithms using a decorrelating noise whitening receiver filter perform better than similar receivers, which solely use a matched filter (MF) for virtually all nonsingular spreading code sets studied. Most of the code sets are randomly generated. The M- and T-algorithm detectors based on decorrelating noise whitening filter (WF) outputs can achieve near optimum performance at a very low complexity compared to the optimal detector, although the proposed detectors are more complex than some known suboptimum detectors. Furthermore, the use of combining techniques is considered for a two-path Rayleigh-fading channel, and a semi-synchronous CDMA structure is proposed. It is shown that if maximum ratio combining (MRC) is employed, the decorrelating noise WF still exists. The corresponding suboptimal combining detector with a decorrelating noise WF outperforms a similar noncombining detector.

A minimum misadjustment adaptive FIR filter

The performance of an adaptive filter is limited by the misadjustment resulting from the variance of adapting parameters. This paper develops a method to reduce the misadjustment when the additive noise in the desired signal is correlated. Given a static linear model, the linear estimator that can achieve the minimum parameter variance estimate is known as the best linear unbiased estimator (BLUE). Starting from classical estimation theory and a Gaussian autoregressive (AR) noise model, a maximum likelihood (ML) estimator that jointly estimates the filter parameters and the noise statistics is established. The estimator is shown to approach the best linear unbiased estimator asymptotically. The proposed adaptive filtering method follows by modifying the commonly used mean-square error (MSE) criterion in accordance with the ML cost function. The new configuration consists of two adaptive components: a modeling filter and a noise whitening filter. Convergence study reveals that there is only one minimum in the error surface, and global convergence is guaranteed. Analysis of the adaptive system when optimized by LMS or RLS is made, together with the tracking capability investigation. The proposed adaptive method performs significantly better than a usual adaptive filter with correlated additive noise and tracks a time-varying system more effectively.

A noise whitening approach to multiple-access noise rejection .II. Implementation issues

Several practical methods are proposed for the implementation of linear filters that reject multiple-access noise in direct sequence code-division multiple-access (DS-CDMA) receivers. These methods rely on taking advantage of variations in the received chip spectrum, and do not require locking and despreading multiple signals. The resulting structures are simple, and some can operate at a high chip rate. Adaptive structures are also proposed. It is shown that a close relationship exists between noise whitening and chip-equalization for a DS-CDMA receiver. System issues affecting the choice of the chip spectrum, and thus the applicability of these methods, are described.

Speech pattern matching in non-white noise

A system for matching an input signal, comprising non-white noise and a patterned signal corrupted by said non-white noise, to a plurality of reference signals, the system including an estimator estimating noise features of said non-white noise and producing from the noise features at least one noise whitening filter; a filter generally simultaneously filtering the input signal and the plurality of reference signals using the noise whitening filter and producing a filtered input signal, having a white noise component, and a plurality of filtered reference signals; and a pattern matcher generally robust to white noise for matching the filtered input signal to one of the filtered reference signals.

A near ideal noise whitening filter for an asynchronous time-varying CDMA system

A near ideal noise whitening filter for a time-varying code-division multiple-access (CDMA) system is examined. First, the structure of the ideal noise whitening filter is determined. The ideal noise whitening filter for a time-varying CDMA system is dependent on unknown future system parameters and is, therefore, impractical. A near ideal, realizable whitening filter is introduced as a solution. The convergence of the factorization method for a time-varying CDMA system is considered. The truncation of the number of taps of the ideal noise whitening filter is studied and a metric function based on the near ideal noise whitening filter for a tree search detection algorithm is formulated. Simulation results are obtained for five and ten-user time-varying CDMA systems with binary random signature sequences of length 10 and a rectangular chip waveform. The results show that the near ideal noise whitening filter can accurately approximate the ideal noise whitening filter at a low complexity level. The performance degradation of a suboptimal tree search detector for a time-varying, asynchronous CDMA system using a typical near ideal noise whitening filter is minimal compared to an identical system using the ideal noise whitening filter.