Signal-to-noise Ratio Variation Research Articles

Symbol level equalizer-based receiver for downlink MAI suppression in CDMA systems

In this paper, a type of symbol-level adaptive equalizer-based receiver is proposed for downlink multiple access interference (MAI) suppression in WCDMA cellular systems. This type of receiver does not need channel estimation, which is necessary it? most other proposals, and hence has relative low complexity. After introducing the loop structure of the basic MAI suppression receiver, the paper further proposes a practical modification to improve performance in time-varying and low signal-to-noise ratio (SNR) environments. This modified receiver is called pre-combining adaptive-order receiver. The performance enhancement owes to multipath pre-combining which increases and smoothes SNR variation of input signal and the adaptive order adjustment which optimizes the equalizer length in terms of channel conditions. The receiver structures are described and represented mathematically. The performances are evaluated by computer simulation and compared to each other as well as to currently used Rake receiver.

Dependence of signal-to-noise ratio on excitation current and signal frequency in spin valve sensors

Spin valves are widely used as magnetic microsensors. Their performance is limited by the presence of noise. In this article, signal and noise responses of microfabricated spin valve sensors are investigated in the linear region and in the nonlinear region close to saturation where peaks in noise output have been reported. The anisotropy of the free layer and the pinned layer are in a cross configuration. The noise of the sensor is of 1/f type at low frequency. The signal-to-noise ratio (SNR) as a function of signal frequency was measured near saturation. It is found that below 300 Hz the SNR increases almost linearly as the signal frequency increases. The variation of SNR with excitation current, measured at different frequencies, is shown. The SNR shows a nonlinear characteristic. At low frequency the increase in signal power is matched by that in noise. The SNR curve at 35 Hz shows a maximum as the current increases. The slope of other curves increases with increasing frequency. We explain these curves in a model with thermal excitation.

Minimal gradient encoding for robust estimation of diffusion anisotropy☆

This study has investigated the relationship between the noise sensitivity of measurement by magnetic resonance imaging (MRI) of the diffusion tensor ( D) of water and the number N of diffusion-weighting (DW) gradient directions, using computer simulations of strongly anisotropic fibers with variable orientation. The DW directions uniformly sampled the diffusion ellipsoid surface. It is shown that the variation of the signal-to-noise ratio (SNR) of three ideally rotationally invariant scalars of D due to variable fiber orientation provides an objective quantitative measure for the diffusion ellipsoid sampling efficiency, which is independent of the SNR value of the baseline signal obtained without DW; the SNR variation decreased asymptotically with increasing N. The minimum number N 0 of DW directions, which minimized the SNR variation of the three scalars of D was determined, thereby achieving the most efficient ellipsoid sampling. The resulting time efficient diffusion tensor imaging (DTI) protocols provide robust estimation of diffusion anisotropy in the presence of noise and can improve the repeatability/reliability of DTI experiments when there is high variability in the orientation of similar anisotropic structures, as for example, in studies which require repeated measurement of one individual, intersubject comparisons or multicenter studies.

Impact of spreading bandwidth on RAKE reception in dense multipath channels

Spread spectrum (SS) multiple access techniques have been proposed for third generation broadband wireless access. We develop an analytical framework to quantify the effects of spreading bandwidth on SS systems operating in dense multipath environments in terms of the receiver performance, receiver complexity, and multipath channel parameters. In particular, we consider wide-sense stationary uncorrelated scattering (WSSUS) Gaussian channels with frequency-selective fading. The focus of the paper is to characterize the combined signal of the RAKE receiver fingers tracking the strongest multipath components. Closed form expressions for the mean and the variance of the total RAKE receiver output signal-to-noise ratio (SNR) are derived in terms of the number of RAKE fingers, spreading bandwidth, and multipath spread of the channel. The proposed problem is made analytically tractable by transforming the physical RAKE paths into the virtual path domain. A representative result indicates that for SS systems with 5 MHz signal bandwidth operating in a channel with constant power delay profile having 5 /spl mu/s spread, the average SNR gain from increasing the number of RAKE fingers from one to three is 3.8 dB and from three to five is 1.5 dB. Furthermore, the reduction in the variation of SNR is 1.1 dB and 0.4 dB for the same increments in the number of fingers.

Spatial variation of SNR in two- and three-dimensional neuro-PET

A method for region of interest (ROI) evaluation for three-dimensional (3-D) positron emission tomography (PET) in the sinogram space was implemented, according to the fully 3-D filtered back-projection algorithm. With this method, the statistical error in the image that propagates from the Poisson noise in the raw data was computed. The signal-to-noise ratio (SNR) for ROIs at various locations inside a cylindrical phantom was computed from both scanner data and simulation data and was verified via the standard deviation method through multiple measurements. As a comparison, two-dimensional (2-D) scans were also collected and similar computations carried out, Results show that the SNR increases with radius due to decreased attenuation at the edge of the phantom. For 3-D scans, the SNR drops gradually for ROIs outside the central 8 cm of the field of view (FOV). Also, it was found that the random events must be recorded and considered in the error computation.

Modeling of four-wave mixing and gain peaking in amplified WDM optical communication systems and networks

A theoretical model is presented for analyzing the propagation of densely spaced WDM optical signals through a cascade of erbium-doped fiber amplifiers and single-mode optical fibers with nonuniform chromatic dispersion. By combining a numerical solution for the EDFA and an analytical expression for FWM components generated through the cascade, the model allows a realistic system analysis which includes gain peaking effect, amplified spontaneous emission accumulation and the effect of dispersion management on the four-wave mixing efficiency. The FWM power distribution at the end of the multi-amplifier transmission link is computed taking into account the phase relation between FWM light amplitudes generated within different sections of the link. The transmission of many WDM channels, evenly spaced around 1547.5 nm, has been analyzed for various dispersion management techniques and propagation distances. Numerical results point out the importance of such a model for a realistic design of WDM optical communication systems and networks. A proper choice of chromatic dispersion, amplifier characteristics, span length, input signal powers and wavelengths, combined with the use of gain equalizing filters, allows to maximize the transmission distance ensuring acceptable signal-to-noise ratio (SNR) and limited SNR variation among channels.

Multipath propagation effects on a CDMA cellular system

The performance of the reverse link of a code division multiple access cellular system is evaluated. At the base station, the signal from each user is demodulated by a coherent BPSK RAKE receiver. Parameters for the model of the impulse response of the channel were taken from measurements of the digital cellular channel in Toronto. The signal-to-noise ratio (SNR) for a received signal is used to measure the performance of the reverse link. The variation in SNR of received signals at the base station should be as small as possible to reduce interference in the network. A power control scheme to lower the variation in SNR of the received signals is analyzed. The effects of lowering the bandwidth of the transmitted signal were also investigated. >

A Multidigit Adaptive Delta Modulation (ADM) System

A multidigit adaptive delta modulation (ADM) system has been proposed where the error signal, between the input and the approximated signal produced by ADM coder, is coded in an auxiliary encoder. The error in the auxiliary coder is processed by another ADM and so on. The bit rate of each of these coders is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_{r}/N</tex> where f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> is the overall transmission rate and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</tex> is the number of coders used. The bit streams are interleaved for transmission and at the receiver they are separated and decoded, and these signals are added and filtered. It is shown that for a given transmission rate, each coder operates at a basic sampling rate of f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rB</inf> such that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N_{opt} = f_{r}/f_{rB}</tex> gives the optimum number of coders to be used for maximum signal-to-noise ratio (SNR). A bound is derived for the maximum SNR of such a system and is compared with the bounds derived for other predictive coders. The experimental results of a two-digit ADM are presented. An average SNR of 30 dB is obtained with a dynamic range of 32 dB at f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</inf> = 32 kbits/s for band-limited noise signals. The SNR increases with the sampling rate at 15 dB/octave, as against 9 dB for a single-digit ADM. The frequency response is good and the variation of SNR with the message frequency of the delta coding system has been improved. The effect of channel errors has also been studied and the performance of the system is found satisfactory.