Signal-to-noise Ratio Loss Research Articles

Truncation length for Viterbi decoding

A bound is derived and analyzed for the bit error rate (BER) of a Viterbi decoder with survivor truncation. Estimates of the SNR (signal-to-noise ratio) loss on the AWGN (additive white Gaussian noise) channel due to truncation are obtained for convolutional codes. Larger truncation lengths are required than the smallest value that does not effectively decrease the code's free distance, especially at low E/sub b//N/sub 0/. >

NAR estimators of spatial covariance matrices for adaptive array detection

The theory of noise-alone-reference (NAR) power estimation is extended to the estimation of spatial covariance matrices. A NAR covariance estimator insensitive to signal presence is derived. The SNR (signal-to-noise ratio) loss incurred by using this estimator is independent of the input SNR and is less than that encountered with the maximum likelihood covariance estimator given that the same number of uncorrelated snapshots is available to both estimators. The analysis assumes first a deterministic signal. The results are extended and generalized to signals with unknown parameters or random signals. For the random signal case, generalized and quasi-NAR covariance estimators are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance analysis of a noncoherently combined large aperture optical heterodyne receiver

The performance of a noncoherently combined, multiple-mirror heterodyne receiver is analyzed. In the absence of atmospheric turbulence, the performance of the noncoherently combined receiver is shown to be inferior to that of a monolithic, diffraction-limited receiver with equivalent aperture area. when atmospheric turbulence is taken into consideration, however, the efficiency of a monolithic aperture heterodyne receiver, is limited by the phase coherence length of the atmosphere, and generally does not improve with increasing aperture size. In contrast, the performance of a noncoherently combined system improves with an increasing number of receivers. Consequently, given a fixed collecting area, the noncoherently combined system can offer a superior performance. The performance of the noncoherently combined heterodyne receiver is studied by analyzing the combining loss of the receiver SNR (signal-to-noise ratio). It is shown that, given a constant collecting area, the performance of the combined receiver is optimized when the diameter of each of the individual receivers is on the order of the phase coherence length r/sub 0/ of the atmospheric turbulence.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Signal‐to‐noise efficiency in magnetic resonance imaging

The dependence of signal-to-noise ratio (SNR) on the analog filter, the sampling rate and the number and dimensions of voxels is derived for magnetic resonance imaging (MRI). It is shown that the object signal-to-noise ratio scales directly with the voxel volume and the square root of the number of voxels. Defining an efficiency figure of merit as the SNR divided by the square root of the imaging time, it is shown that efficiency is always improved when imaging with the lowest possible resolution (largest voxel dimensions) consistent with viewing the desired anatomical detail. The results directly imply the relative efficiency of 3-D (volume), 2-D (plane), 1-D (line) and 0-D (point) imaging techniques. It is shown that spatial averaging is an inefficient method of noise reduction in MRI. As long as voxel size is maintained constant, one can image as many pixels in the readout direction as desired with no loss in SNR; that is, the number of pixels in the readout direction has no effect on the image SNR. Further, multiple sampling of each phase encoding value (to improve SNR) has no advantage over increasing the number of pixels in the phase encoding direction while leaving the voxel size constant. Some experimental observations are given.

Signal to noise ratio loss in correlators using real filters

Simple expressions are derived for the loss in signal-tonoise ratio when a real filter is used instead of a complex filter in an optical correlator.

Degradation of signal-to-noise ratio due to amplitude distortion

The effect of filtering on the signal-to-noise ratio (SNR) of a coherently demodulated band-limited signal is determined in the presence of worst-case amplitude ripple. The problem is formulated as an optimization in the Hilbert space L/sub 2/. The form of the worst-case amplitude ripple is specified, and the degradation in the SNR is derived in closed form. It is shown that, when the maximum passband amplitude ripple is 2 delta (peak-to-peak), the SNR is degraded by at most (1- delta /sup 2/), even when the ripple is unknown or uncompensated. For example, an SNR loss of less than 0.01 dB due to amplitude ripple can be assured by keeping the amplitude ripple under 0.42 dB. >

On suboptimal detection of 3-dimensional moving targets

The author designates matched filters that are completely characterized by the velocity of the target as assumed velocity filters (AVFs). Like most matched filtering techniques where the signal parameters range in a continuum, the AVF must be implemented suboptimally by partitioning the velocity space. The author investigates the possibility of using a signal-to-noise ratio (SNR) loss factor as the criterion for the partition. The loss factor is a measurement of the loss of SNR at the output of the matched filter due to mismatch of filter parameters. In the scenario of detecting a moving satellite from a ground-based sensor, because of the vast sky the sensor has to search, it is important to keep the number of filters minimal. The author shows that, with a fixed loss factor, the number of filters required for coverage increases linearly as the span of the two-dimensional velocity space increases quadratically. The rate of increase is further reduced when the loss factor is made proportional to expected target angular speed. >

Results on 'controlled polarity' modulation and coding

The authors present results on a controlled-polarity modulation/coding scheme. They review this scheme and describe how to choose certain code parameters for the purpose of comparing the controlled polarity scheme to conventional (d,k) coding with NRZI modulation. A computer simulation is then utilized to compare the different schemes in terms of effective signal-to-noise ratio (SNR) and peak shift. It is shown that the controlled polarity schemes have less peak shift than the conventional schemes, whereas the conventional schemes have a higher effective SNR. It appears that in systems with a high enough SNR, the loss in effective SNR could be outweighed by the decrease in average peak shift. This could be of particular interest in future systems using magnetoresistive heads which have the potential to yield substantially higher SNRs. >

Dual-energy x-ray projection imaging: two sampling schemes for the correction of scattered radiation.

In addition to the familiar problems of reduced contrast and signal-to-noise ratio (SNR) in the single energy case, dual-energy subtractions in the presence of scattered radiation suffer further degradations from: (1) artifacts due to nonuniform subtraction of scatter, and (2) a serious deterioration of the signal of interest. To determine the expected performance of scatter correcting schemes, we simulated energy subtractions performed in the presence of scatter. We discuss scatter's detrimental effects on contrast and SNR in these simulations and the expected improvements from scatter corrections to within 5% to 10%. We introduce two sampling schemes for the correction of scatter. Each scheme requires two measurements, and each involves placing an x-ray opaque sampling grid between the source and the object. In the first method, the grid is an array of lead disks present only during one measurement. Using these samples we generate an estimate of the scatter field and then subtract it from the second measurement yielding a scatter corrected image. In the second method, the grid is an array of lead strips present during both measurements but displaced between measurements by one-half of a strip spacing to completely sample the image. From the two measurements we generate an image to be corrected, an estimate of the scatter field, and a scatter corrected image. In phantom studies implemented on a digital fluoroscopy system, we observed for single energy images of blood vessel phantoms improved contrast and field uniformity. For scatter corrected selective material cancellations in human phantoms we observed improved contrast and significant reduction in artifacts. In both cases we observed no significant loss in SNR. These results facilitate the implementation of efficient large area detectors for dual-energy imaging.

The effect of a finite-distance signal source on a far-field steering Applebaum array-two dimensional array case

Approximate expressions are obtained for the output signal-to-noise ratio (SNR) expressed in terms of the finite signal distance and signal direction for a finite-distance signal source's effect on the performance of a far-field steering two-dimensional Applebaum-type adaptive array. The expression is shown to be consistent with the actual simulated value. Using that expression, a simplified rule is obtained to determine the distance between the signal source and the array center at which the output SNR loss is given by a specific value. The SNR value so obtained varied with two-dimensional signal direction. The analysis for the case of an arbitrarily located array is presented, followed by the cases for rectangular, circular and elliptical arrays. It was found that this distance associated with the given SNR value is always less for a rectangular array than that of a linear array when the total number of array elements for both are equal and the performances of an elliptical array are similar to those of a circular array.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Resolution and Noise Considerations in MRI Systems with Time-Varying Gradients

The design and performance of spatial localization filters in an MR imaging system with time-varying gradients is presented. The system is based on recording measurements of the imaged volume-done in the presence of cosinusoidal gradients. A postprocessing scheme is shown which enables every point to be imaged from a single FID signal. Methods are shown which achieve a three-demensional localization with arbitrarily small sidelobes. Those localization filters are implemented without significant computational penalty. The signal-to-noise ratio (SNR) in the suggested imaging system is shown to be space-invariant (same SNR for each imaged point). We find a closed-form relation between the localization filter and the resulting noise. It is shown that the gain achieved by the localization filters highly exceeds the relatively small loss in SNR.

Beam Splitters for Optical Heterodyne Receivers

The signal-to-noise ratio (SNR) of an optical heterodyne receiver depends on the transmissivity of the beam splitter used. With a proper choice of the transmissivity, some of the loss in the SNR may be alleviated.