Showing papers on "Signal-to-noise ratio published in 1996"

Speech enhancement based on a priori signal to noise estimation

Abstract: This paper addresses the problem of single microphone frequency domain speech enhancement in noisy environments. The main characteristics of available frequency domain noise reduction algorithms are presented. We have confirmed that the a priori SNR estimation leads to the best subjective results. According to these conclusions, a new approach is then developed which achieves a trade-off between effective noise reduction and low computational load for real-time operations. The obtained solutions demonstrate that the subjective and objective results are much better than existing methods.

A 100-MHz ultrasound imaging system for dermatologic and ophthalmologic diagnostics

Abstract: A major design problem concerning high-frequency broad-band ultrasound imaging systems is caused by the strong dispersive attenuation of the tissue, which gives rise to images with inhomogeneous resolution and poor signal to noise ratio (SNR). To address the noise problem, strongly focused transducers with high energy density in a narrow focal region are utilized, which also provide more isotropic images due to improved lateral resolution. To account for the short depth of the focal area two suitable imaging conceptions are used: 1) synthetic aperture concept and 2) B/D-scan concept. To avoid the inhomogeneity of the images, different transmitter signals for each depth are applied, which are pseudoinversely prefiltered according to the transfer function of the tissue. To gain signal energy required for inverse filtering, a pulse compression technique with nonlinearly frequency modulated chirp signals is utilized. These procedures have been implemented in an ultrasound imaging system, which has been developed in the authors' laboratory for eye and skin examinations, It can be used with transducers in a frequency range from 20 to 250 MHz.

SNR-based multipath error correction for GPS differential phase

Abstract: Carrier phase multipath is currently the limiting error source for high precision Global Positioning System (GPS) applications such as attitude determination and short baseline surveying. Multipath is the corruption of the direct GPS signal by one or more signals reflected from the local surroundings. Multipath reflections affect both the carrier phase measured by the receiver and signal-to-noise ratio (SNR). A technique is described which uses the SNR information to correct multipath errors in differential phase observations. The potential of the technique to reduce multipath to almost the level of receiver noise was demonstrated in simulations. The effectiveness on real data was demonstrated with controlled static experiments. Small errors remained, predominantly from high frequency multipath. The low frequency multipath was virtually eliminated. The remaining high frequency receiver noise can be easily removed by smoothing or Kalman filtering.

Effects of nonlinear distortion on CDMA communication systems

Abstract: We report a rigorous approach to analyze the effects of nonlinear distortion on code division multiple access (CDMA) wireless communication systems based on time-domain analysis and band-pass nonlinearity theory. Given AM-AM and AM-PM characteristics of a nonlinear device, this technique is capable of predicting adjacent channel power rejection (ACPR), noise power ratio (NPR), two-tone intermodulation products, CDMA waveform quality, and baseband signal vector constellation at the output of the nonlinear device. To demonstrate and verify the capability of this technique, an L-band power amplifier was designed, built, tested with CDMA waveforms, and compared with the simulated results. Excellent agreement between the measured and predicted results has been achieved.

Fast timing estimation for CDMA waveforms in a near-far environment

Abstract: A new algorithm is proposed for extracting timing information for all users in a CDMA system operating in a near-far environment without a-priori knowledge of individual user's power. With unique design of the training sequences, the algorithm is capable of rejecting multiuser interference and thus is not near-far limited. It is shown that the total acquisition time for all system users is considerably reduced compared to previously studied CDMA timing acquisition algorithms without increasing the computation complexity. In fact, the computation complexity of the proposed algorithm is also reduced to MN/sub b//sup 2/+(Mlog M)N/sub b/, where M is the number of users in the system and N/sub b/ is the number of samples taken per bit interval. The most significant advantage of this algorithm is that the estimation can be obtained with high accuracy even at a very low signal to noise ratio (SNR). For instance, with M=20 and N/sub b/=254, the probability of acquisition error for a user with SNR at -15 dB is on the order of 10/sup -5/.

Analysis of adaptive multistage interference cancellation for CDMA using an improved Gaussian approximation

Abstract: We consider a simple model for adaptive multistage interference cancellation within a CDMA system, and seek to develop an accurate analytical expression for the performance of this system. Previous work on interference cancellation has relied heavily on simulation techniques or a simple Gaussian approximation (GA). The standard GA ran lead to bit-error rate (BER) results which are optimistic for the conventional receiver, and this also occurs when the approximation is applied to the interference cancellation problem. Additionally, this approximation does not allow the second order effects of the multiple access interference (MAI) to be included in the performance estimates. Several improvements on the standard GA have been suggested which result in accurate performance results for a standard CDMA receiver. This paper presents an analytical expression for the probability of bit error for an adaptive multistage interference canceller, using an improved Gaussian approximation (IGA) for MAI. The BER at any stage of interference cancellation can be recursively computed from the signal-to-noise ratio (SNR), the statistics of the random powers of users, and the processing gain of the CDMA system. The performance of the resulting EER expression is compared with simulation results. Since the second order effects of MAI can be included, the analytical framework presented here can also be used to evaluate the performance of multistage interference cancellation in arbitrary fading environments, and we present results for the performance of interference cancellation in lognormal fading environments.

Method and apparatus for detecting and compensating for undesired phase shift in a radio transceiver

Abstract: The present invention provides a method and apparatus for detecting and compensating for transmitter phase shift in a radio transmitter. A local oscillator generates a local oscillator signal. A modulator such as an I/Q modulator modulates input signals onto the local oscillator signal to generate a forward signal. That forward signal is amplified and otherwise prepared for transmission via an antenna. The transmitted signal is detected, and a phase adjuster substantially continuously detects and cancels undesired phase shifts in the detected signal caused by elements in the radio chain. A specific, example embodiment of the present invention is provided in a Cartesian feedback system wherein the phase of the feedback signal is detected and aligned with the phase of the original modulated signal, providing uncorrupted I and Q baseband feedback signals. Another specific example embodiment of the present invention is to a diversity radio receiver. Received signals from the diversity antennas are processed to determine the phase difference between the two signals. The phase of one of the received signals is rotated to match the phase of the other, and thereafter, the diversity received signals are added together before demodulating the originally transmitted signal. Such additive combination of diversity received signals under same or similar received signal level conditions when two signals are aligned in phase considerably improves the signal to noise ratio of the received signal.

Monte Carlo and experimental evaluation of accuracy and noise properties of two scatter correction methods for SPECT

Abstract: Scatter correction is a prerequisite for quantitative SPECT, but potentially increases noise. Monte Carlo simulations (EGS4) and physical phantom measurements were used to compare accuracy and noise properties of two scatter correction techniques: the triple-energy window (TEW), and the transmission dependent convolution subtraction (TDCS) techniques. Two scatter functions were investigated for TDCS: (i) the originally proposed mono-exponential function and (ii) an exponential plus Gaussian scatter function demonstrated to be superior from our Monte Carlo simulations. Signal to noise ratio (S/N) and accuracy were investigated in cylindrical phantoms and a chest phantom. Results from each method were compared to the true primary counts (simulations), or known activity concentrations (phantom studies). was used in all cases. The optimized method overall performed best, with an accuracy of better than 4% for all simulations and physical phantom studies. Maximum errors for TEW and of -30 and -22%, respectively, were observed in the heart chamber of the simulated chest phantom. TEW had the worst S/N ratio of the three techniques. The S/N ratios of the two TDCS methods were similar and only slightly lower than those of simulated true primary data. Thus, accurate quantitation can be obtained with , with a relatively small reduction in S/N ratio.

Analysis of noise in phase contrast MR imaging.

Abstract: In this work we analyze the effects of inherent random noise on the detectability of low‐contrast vessel structures that possess slow flow. When flow is encoded in more than one direction, the number of independent noise contributions increases in addition to the scan time. In a fast‐flow scenario, only the noise contribution from sampling along the direction of flow is of any significance. At slow flow rates, however, it becomes necessary to account for the noise in each encoded Cartesian direction. The degree to which noise affects low‐contrast detectability also depends on the method of phase contrastimage processing employed. A theoreticalanalysis of the statistical properties of signal and noise in processed phase contrast magnitude images is presented and verified from experimental MR image data. Results show a progressively increased bias in the processed phase contrastimage magnitude at slow flow rates due to contributions from inherent random noise. The amount of this bias increases with the number of physical directions in which flow is encoded and is larger for complex difference processed images than for phase difference processing. Correspondingly, the output signal‐to‐noise ratio associated with flow is compromised.

Performance evaluation of experimental 50-Mb/s diffuse infrared wireless link using on-off keying with decision-feedback equalization

Abstract: We report an experimental nondirected optical link for short-range, indoor data transmission at 50 Mb/s. The system uses on-off keying (OOK) and achieves low bit-error rates (BERs) in the presence of intersymbol interference, background light noise, and shadowing, with a range of 2.9 m in a skylit room. The transmitter produces an eye-safe Lambertian pattern at 806 nm with an average power of 474 mW. The receiver utilizes a hemispherical concentrator with a hemispherical bandpass optical filter, a 1-cm/sup 2/ silicon p-i-n photodiode, and a high-impedance hybrid preamplifier to achieve a high signal-to-noise ratio (SNR). A high-pass filter is used to mitigate fluorescent light noise, with quantized feedback removing the resulting baseline wander. A decision-feedback equalizer provides resistance to intersymbol interference due to multipath. The system and its components are characterized, and compared to theory. We observe that decision-feedback equalization yields a reduction of multipath power penalties that is in good agreement with theory.

100 Gbit/s optical signal eye-diagram measurement with optical sampling using organic nonlinear optical crystal

Abstract: The eye diagram of a 100 Gbit/s optical signal has been successfully measured using sum-frequency-generation optical sampling, which uses an organic crystal (AANP: 2-adamantylamino-5-nitropyridine) with high optical nonlinearity to improve the signal-to-noise ratio of the measured waveform. Using this method, a signal-to-noise ratio > 17 dB is obtained at a signal peak power of 270 mW.

Adaptive bimodal sensor fusion for automatic speechreading

Abstract: We present work on improving the performance of automated speech recognizers by using additional visual information: (lip-/speechreading); achieving error reduction of up to 50%. This paper focuses on different methods of combining the visual and acoustic data to improve the recognition performance. We show this on an extension of an existing state-of-the-art speech recognition system, a modular MS-TDNN. We have developed adaptive combination methods at several levels of the recognition network. Additional information such as estimated signal-to-noise ratio (SNR) is used in some cases. The results of the different combination methods are shown for clean speech and data with artificial noise (white, music, motor). The new combination methods adapt automatically to varying noise conditions making hand-tuned parameters unnecessary.

Extraction of turbulent wall-pressure time-series using an optimal filtering scheme

Abstract: Accurate measurements of pressure beneath turbulent wall-bounded flows are generally difficult to achieve due to signal contamination resulting from facility-induced noise, sensor vibration, etc. This is particularly true at low Reynolds numbers where the noise signature overwhelms the low-level turbulent fluctuations. In the current work a noise-cancellation technique based on an optimal filtering approach is developed. This technique is particularly useful for conditions of low signal-to-noise ratio and therefore it is well suited for low to moderate Reynolds number measurements. Unlike the conventional, subtraction-based, noise-cancellation methods, the utility of the optimal-filter scheme is not limited to the extraction of the turbulent statistics but it can be used to obtain the noise-canceledtime-series. Furthermore, the energy of the low-frequency turbulent motion lost due to the application of the noise-cancellation scheme is an order of magnitude smaller in the case of the optimal filter as compared to the subtraction scheme. Employment of the technique developed here is not confined to two-dimensional flows and therefore it is also useful for measurements in applications involving non-equilibrium flows.

On-board phase and modulus calibration of large aperture synthesis radiometers: study applied to MIRAS

Abstract: On-board calibration of bidimensional aperture synthesis radiometers with a large number of antennas by the standard correlated noise injection method is technologically very critical because of the stringent requirements on mass, volume, and phase equalization of the noise distribution network. A novel approach, which makes use of a set of uncorrelated noise sources uniformly distributed in the array, is proposed. Each noise source drives correlated noise only to a small set of adjacent antennas. These sets of antennas are overlapped in order to maintain phase and modulus track along the array. This approach reduces drastically mass and volume of the noise distribution network. Moreover, its phase matching requirement is strongly relaxed because it is only necessary within small sets of adjacent antennas. Power stability of the uncorrelated noise sources is also not a stringent requirement. This procedure allows independent phase and modulus calibration by making use of a reduced number of redundant correlations.

Divergent Signal-to-Noise Ratio and Stochastic Resonance in Monostable Systems.

Abstract: We present a class of systems for which the signal-to-noise ratio always increases when increasing the noise and diverges at infinite noise level. This new phenomenon is a direct consequence of the existence of a scaling law for the signal-to-noise ratio and implies the appearance of stochastic resonance in some monostable systems. We outline applications of our results to a wide variety of systems pertaining to different scientific areas. Two particular examples are discussed in detail. [S0031-9007(96)01331-2]

Anisotropic spectral magnitude estimation filters for noise reduction and image enhancement

Abstract: Describes an algorithm for noise reduction and enhancement of images which is able to take into account anisotropies of signal as well as of noise. Processing is based on subjecting each image to a block DFT, followed by comparing each observed magnitude coefficient to the expected noise standard deviation for it. Depending on this comparison, each coefficient is attenuated the more, the more likely it is that it contains only noise. In addition, the attenuation is made dependent on whether or not the observed coefficient contributes to an oriented prominent structure within the processed image block. Orientation as well as the distinctness with which it occurs are detected in the spectral domain by an inertia-like matrix. Orientation information is additionally exploited to selectively enhance oriented structures, thus only marginally increasing noise as compared to isotropic enhancement.

Method and system for adaptive filtering of speech signals using signal-to-noise ratio to choose subband filter bank

Abstract: A method and system for adaptively filtering a speech signal. The method includes decomposing the signal into subbands, which may include performing a discrete Fourier transform on the signal to provide approximately orthogonal components. The method also includes determining a speech quality indicator for each subband, which may include estimating a signal-to-noise ratio for each subband. The method also includes selecting a filter for filtering each subband depending on the speech quality indicator, which may include estimating parameters for the filter based on a clean speech signal. The method further includes determining an overall average error for the filtered subbands, which may include calculating a mean-squared error. The method still further includes identifying at least one filtered subband which, if excluded from the filtered speech signal, would reduce the overall average error determined, and combining, with exception of the filtered subbands identified, the filtered subbands to provide an estimated filtered speech signal. The system includes filters and software for performing the method.

A remote supervisory system based on subcarrier overmodulation for submarine optical amplifier systems

Abstract: A supervisory (SV) signal transmission scheme for long haul optical amplifier systems is described. The scheme is based on subcarrier overmodulation of the line signal and features simple configuration; due to the use of EDFA gain modulation, no additive optical components are required in a submarine repeater to transmit a response signal. This paper discusses the subcarrier modulation index and frequency used for the SV signaling, the most significant design parameters. The dependence of the signal-to-noise ratio (SNR) on the modulation index in SV signal transmission is analyzed to clarify the modulation index value required for reliable SV signaling. In addition, the line signal impairment caused by subcarrier overmodulation is also analyzed to show the system's in-service feasibility. The subcarrier frequencies used for command and response signal transmission are discussed from the experimental results on the frequency response of an optical amplifier system and EDFA gain modulation efficiency.

Efficient method for multiscale small target detection from a natural scene

Abstract: According to the principle of human discrimination of small targets from a natural scene that there is a signature of discontinuity between the object and its neighboring regions, we develop an efficient method for multiscale small target detection using template matching based on a dissimilarity measure, which is called an average gray absolute difference maximum map (AGADMM), and infer the criterion of recognizing multiscale small objects from the properties of the AGADMM of the natural scene, which is a spatially independent and stable Gaussian random field. We explain how the AGADMM increases the ratio of the signal of object-to-background perturbations, improves the detectable probability, and keeps the false alarm probability very low. We analyze the complexity of computing an AGADMM and justify the validity and efficiency. Experiments with images of a natural scene such as a sky and sea surface have shown the great potential of the proposed method for distinguishing multiscale small objects from a natural scene.

An extended Kalman filter frequency tracker for high-noise environments

Abstract: The problem of constructing a frequency tracker for weak, narrowband signals with slowly varying frequency is considered. An extended Kalman filter is proposed that uses prior knowledge of the nature of the signal to overcome the difficulties presented by the inherent nonlinearity of the problem and the very low signal-to-noise ratios.

Optimum array detector for a weak signal in unknown noise

Abstract: We study the design of constant false-alarm rate (CFAR) tests for detecting a rank-one signal in the presence of background Gaussian noise with unknown spatial covariance. We look at invariant tests, i.e., those tests whose performance is independent of the nuisance parameters, like the background noise covariance. Such tests are shown to have the desirable CFAR property. We characterize the class of all such tests by showing that any invariant decision statistic can be written as a function of two basic statistics which are in fact the adaptive matched filter (AMF) statistic and Kelly's generalized likelihood ratio statistic. Further, we establish an optimum test in the limit of low signal-to-noise ratio (SNR), the locally most powerful invariant (LMPI) test. We also derive the bound for the probability of detection of any invariant detector, at a fixed false-alarm rate, and compare the LMPI and the published detectors (Kelly and AMF) to it.

System and method for suppressing noise in digitally represented voice signals

Abstract: A noise suppressor that increases a signal to noise ratio of time domain audio data and a method of increasing such signal to noise ratio. The noise suppressor includes: (1) frequency domain transformation circuitry that transforms a frame of the time domain audio data into a frequency domain, (2) noise background modeling circuitry, coupled to the domain transformation circuitry, that spectrally analyzes the frame to model an estimated noise background spectrum thereof, (3) a frequency domain suppression filter, coupled to the noise background modeling circuitry, that filters at least some of the noise background spectrum from the frame and (4) time domain transformation circuitry, coupled to the frequency domain suppression filter, that transforms the frame back into a time domain, the transformed frame having an increased signal to noise ratio.

Dynamic range of an electro‐optic field sensor and its imaging applications

Abstract: We report the measurement of the dynamics of free‐space electro‐optic field sensors for pulsed electromagnetic wave radiation. With an optical probe power spanning six decades of linearity and excellent signal‐to‐noise ratio (SNR), it is feasible to convert a far‐infrared 2D image into an optical 2D image. A simple estimation indicates that 100 mW of optical probe power can achieve an image of 256×256 pixels with a 50 pA signal current per pixel and a SNR ≳200. We also present a comparison measurement of an ultrafast photoconductive antenna and an electro‐optic sensor crystal.

Fast and accurate ADC testing via an enhanced sine wave fitting algorithm

Abstract: A new sine-wave fitting algorithm for A/D converters (ADC's) testing is presented and discussed. The key feature of the algorithm is the possibility of using test signals with amplitude greater than the full-scale range of the device under test. The new technique, combined with an improved evaluation of the conversion noise via weighted mean square error, gives very accurate and repeatable estimates of the number of effective bits, which cannot be obtained by traditional methodologies. The performance of the new method is proved by means of computer simulations and experimental results as well.

Analysis of an adaptive decorrelating detector for synchronous CDMA channels

Abstract: Multiuser detection allows for the efficient use of bandwidth in code-division multiple-access (CDMA) channels through mitigation of near-far effects and multiple-access noise limitations. The decorrelating detector, developed by Lupas and Verdu (1989), is a linear multiuser detector that is asymptotically optimal in terms of near-far resistance when certain communication parameters are completely known to the detector. In this paper, a simple adaptive decorrelating detector is developed by placing constraints on the set of spreading codes to be used by the active users. This adaptive detector has two modules: it first decorrelates the existing users, and then it determines the spreading code of a new user entering the network with or without the use of a training sequence. Maximum likelihood detection is proposed for determining the new user's spreading code. The performance of this algorithm is studied by investigating the probability of making an error in determining the new user's spreading code as a function of the number of samples used to make the determination, the number of users transmitting, and the signal to noise ratio of the new user with respect to the ambient Gaussian noise.

A robust and recursive identification method for MISO Hammerstein model

Abstract: The problem of identification of MISO Hammerstein model in case of correlated measurement noise is addressed. Because of the special structure of this kind of model, global convergence of the proposed estimation algorithm is proved while the model is nonlinear in the parameters. The analysis is in fact a generalisation of the work by M. Boutayeb et al. (1996) and consists first in transforming the nonlinear model into an input-output one linear in parameters. Afterwards, four successive stages based on the pseudo-inverse technique, are derived and lead us to a consistent estimator of the initial realisation as well as the model of the noise. Accuracy and performances of the proposed technique are shown through numerical examples with different signal to noise ratio values.

Receiving method and receiver

Abstract: The invention relates to a receiving method in a digital cellular radio system. The invention further relates to a receiver of the cellular radio network. In computing the signal to noise ratio, a reference signal is utilized which is the convolution of the estimated impulse response of a channel and the predetermined sequence. The signal to noise ratio is obtained as a ratio between the variance of the reference signal and the predetermined sequence received from the channel. The signal to noise ratio is used for weighing the diversity branches of the diversity receiver.