Showing papers on "Signal-to-noise ratio published in 2003"

How much training is needed in multiple-antenna wireless links?

Abstract: Multiple-antenna wireless communication links promise very high data rates with low error probabilities, especially when the wireless channel response is known at the receiver. In practice, knowledge of the channel is often obtained by sending known training symbols to the receiver. We show how training affects the capacity of a fading channel-too little training and the channel is improperly learned, too much training and there is no time left for data transmission before the channel changes. We compute a lower bound on the capacity of a channel that is learned by training, and maximize the bound as a function of the received signal-to-noise ratio (SNR), fading coherence time, and number of transmitter antennas. When the training and data powers are allowed to vary, we show that the optimal number of training symbols is equal to the number of transmit antennas-this number is also the smallest training interval length that guarantees meaningful estimates of the channel matrix. When the training and data powers are instead required to be equal, the optimal number of symbols may be larger than the number of antennas. We show that training-based schemes can be optimal at high SNR, but suboptimal at low SNR.

Optimal training design for MIMO OFDM systems in mobile wireless channels

Abstract: This paper describes a least squares (LS) channel estimation scheme for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems based on pilot tones. We first compute the mean square error (MSE) of the LS channel estimate. We then derive optimal pilot sequences and optimal placement of the pilot tones with respect to this MSE. It is shown that the optimal pilot sequences are equipowered, equispaced, and phase shift orthogonal. To reduce the training overhead, an LS channel estimation scheme over multiple OFDM symbols is also discussed. Moreover, to enhance channel estimation, a recursive LS (RLS) algorithm is proposed, for which we derive the optimal forgetting or tracking factor. This factor is found to be a function of both the noise variance and the channel Doppler spread. Through simulations, it is shown that the optimal pilot sequences derived in this paper outperform both the orthogonal and random pilot sequences. It is also shown that a considerable gain in signal-to-noise ratio (SNR) can be obtained by using the RLS algorithm, especially in slowly time-varying channels.

Analysis of transmit-receive diversity in Rayleigh fading

Abstract: We analyze the error performance of a wireless communication system employing transmit-receive diversity in Rayleigh fading. By focusing on the complex Gaussian statistics of the independent and identically distributed entries of the channel matrix, we derive a formula for the characteristic function (c.f.) of the maximum output signal-to-noise ratio. We use this c.f. to obtain a closed-form expression of the symbol error probability (SEP) for coherent binary keying. The method is easily extended to obtain the SEP for the coherent reception of M-ary modulation schemes.

Grassmannian beamforming for multiple-input multiple-output wireless systems

Abstract: Multiple-input multiple-output (MIMO) wireless systems provides capacity much larger than that provided by traditional single-input single-output (SISO) wireless systems. Beamforming is a low complexity technique that increases the receive signal-to-noise ratio (SNR), however, it requires channel knowledge. Since in practice channel knowledge at the transmitter is difficult to realize, we propose a technique where the receiver designs the beamforming vector and sends it to the transmitter by transmitting a label in a finite set, or codebook, of beamforming vectors. A codebook design method for quantized versions of maximum ratio transmission, equal gain transmission, and generalized selection diversity with maximum ratio combining at the receiver is presented. The codebook design criterion exploits the quantization problem's relationship with Grassmannian line packing. Systems using the beamforming codebooks are shown to have a diversity order of the product of the number of transmit and the number of receive antennas. Monte Carlo simulations compare the performance of systems using this new codebook method with the performance of systems using previously proposed quantized and unquantized systems.

Reconstruction and enhancement of active thermographic image sequences

Abstract: Active thermography has gained broad acceptance as a non- destructive evaluation method for numerous in-service and manufactur- ing applications in the aerospace industry. However, because of the dif- fusive nature of the process, it is subject to blurring and degradation of the signal as one attempts to image deeper subsurface features. Despite this constraint, active thermographic response is deterministic, to the extent that the postexcitation time evolution for a defect-free sample can be accurately predicted using a simple one-dimensional model. In the patented thermal signal reconstruction method, the time history of every pixel in the field of view is compared to such a model in the logarithmic domain, where deviations from ideal behavior are readily identifiable. The process separates temporal and spatial nonuniformity noise compo- nents in the image sequence and significantly reduces temporal noise. Time-derivative images derived from the reconstructed data allow detec- tion of subsurface defects at earlier times in the sequence than conven- tional contrast images, significantly reducing undesirable blurring effects and facilitating detection of low-thermal-contrast features that may not be detectable in the original data sequence. © 2003 Society of Photo-Optical In-

Photonic time-stretched analog-to-digital converter: fundamental concepts and practical considerations

Abstract: Ultra-wide-band analog-to-digital (A/D) conversion is one of the most critical problems faced in communication, instrumentation, and radar systems. This paper presents a comprehensive analysis of the recently proposed time-stretched A/D converter. By reducing the signal bandwidth prior to digitization, this technique offers revolutionary enhancements in the performance of electronic converters. The paper starts with a fundamental-physics analysis of the time-wavelength transformation and the implication of time dilation on the signal-to-noise ratio. A detailed mathematical description of the time-stretch process is then constructed. It elucidates the influence of linear and nonlinear optical dispersion on the fidelity of the electrical signal. Design issues of a single-sideband time-stretch system, as they relate to broad-band operation, are examined. Problems arising from the nonuniform optical power spectral density are explained, and two methods for overcoming them are described. As proof of the concept, 120 GSa/s real-time digitization of a 20-GHz signal is demonstrated. Finally, design issues and performance features of a continuous-time time-stretch system are discussed.

A channelized digital ultrawideband receiver

Abstract: A channelized digital ultrawideband (UWB) receiver that efficiently samples the UWB signal at a fraction of the chip frequency is proposed. The received signal is channelized in the frequency domain by employing a bank of mixers and low-pass filters. After sampling at a much reduced frequency, digital synthesis filters optimally estimate the transmitted signals. The signal-to-noise ratio (SNR) of the proposed receiver has been solved and compared against an ideal conventional receiver, which is defined as a receiver that samples at the signal Nyquist rate. When finite resolution analog-to-digital converters (ADC) are employed in the presence of a large narrowband interferer, the proposed receiver significantly outperforms the ideal conventional receiver. For example, the SNR of the proposed receiver is as much as 20 dB higher than the ideal conventional receiver when a 4-bit ADC is used in the presence of a 50 dB (relative to the noise floor) brickwall narrowband interferer with a bandwidth of 15% of the chip frequency.

A wavelet-based method for action potential detection from extracellular neural signal recording with low signal-to-noise ratio

Abstract: We present a method for the detection of action potentials, an essential first step in the analysis of extracellular neural signals. The low signal-to-noise ratio (SNR) and similarity of spectral characteristic between the target signal and background noise are obstacles to solving this problem and, thus, in previous studies on experimental neurophysiology, only action potentials with sufficiently large amplitude have been detected and analyzed. In order to lower the level of SNR required for successful detection, we propose an action potential detector based on a prudent combination of wavelet coefficients of multiple scales and demonstrate its performance for neural signal recording with varying degrees of similarity between signal and noise. The experimental data include recordings from the rat somatosensory cortex, the giant medial nerve of crayfish, and the cutaneous nerve of bullfrog. The proposed method was tested for various SNR values and degrees of spectral similarity. The method was superior to the Teager energy operator and even comparable to or better than the optimal linear detector. A detection ratio higher than 80% at a false alarm ratio lower than 10% was achieved, under an SNR of 2.35 for the rat cortex data where the spectral similarity was very high.

On the power efficiency of sensory and ad-hoc wireless networks

Abstract: We consider the power efficiency of a communications channel, i.e., the maximum bit rate that can be achieved per unit power (energy rate). For additive white Gaussian noise (AWGN) channels, it is well known that power efficiency is attained in the low signal-to-noise ratio (SNR) regime where capacity is proportional to the transmit power. In this paper, we first show that for a random sensory wireless network with n users (nodes) placed in a domain of fixed area, with probability converging to one as n grows, the power efficiency scales at least by a factor of /spl radic/n. In other words, each user in a wireless channel with n nodes can support the same communication rate as a single-user system, but by expending only 1//spl radic/n times the energy. Then we look at a random ad hoc network with n relay nodes and r simultaneous transmitter/receiver pairs located in a domain of fixed area. We show that as long as r/spl les//spl radic/n, we can achieve a power efficiency that scales by a factor of /spl radic/n. We also give a description of how to achieve these gains.

Theoretical investigation of the signal-to-noise ratio in fluorescence lifetime imaging.

Abstract: We deduce the signal-to-noise ratio for fluorescence lifetime imaging when using frequency-domain methods. We assume mono-exponential decay and quantum-noise-limited performance. The results are compared with Monte Carlo simulations with good agreement. We also compare our results with previous investigations of time-domain methods for fluorescence lifetime imaging. For a given number of detected photons, we find that frequency-domain and time-domain methods are equally good. The correct choice of detection technique and its parameters is important for obtaining good results.

SNR estimation based on amplitude modulation analysis with applications to noise suppression

Abstract: A single-microphone noise suppression algorithm is described that is based on a novel approach for the estimation of the signal-to-noise ratio (SNR) in different frequency channels: The input signal is transformed into neurophysiologically-motivated spectro-temporal input features. These patterns are called amplitude modulation spectrograms (AMS), as they contain information of both center frequencies and modulation frequencies within each 32 ms-analysis frame. The different representations of speech and noise in AMS patterns are detected by a neural network, which estimates the present SNR in each frequency channel. Quantitative experiments show a reliable estimation of the SNR for most types of nonspeech background noise. For noise suppression, the frequency bands are attenuated according to the estimated present SNR using a Wiener filter approach. Objective speech quality measures, informal listening tests, and the results of automatic speech recognition experiments indicate a substantial benefit from AMS-based noise suppression, in comparison to unprocessed noisy speech.

A Fast Algorithm for Cosmic Rays Removal from Single Images

Abstract: We present a method for detecting cosmic rays in single images. The algorithm is based on simple analysis of the histogram of the image data and does not use any modeling of the picture of the object. It does not require a good signal to noise ratio in the image data. Identification of multiple-pixel cosmic-ray hits is realized by running the procedure for detection and replacement iteratively. The tests performed by us, show that the method is very effective, when applied to the images with the spectroscopic data. It is also very fast in comparison with other single image algorithms found in astronomical data processing packages. Practical implementation and examples of application are presented.

Effects of spectrometer band pass, sampling, and signal-to-noise ratio on spectral identification using the Tetracorder algorithm

Abstract: [1] Estimates of spectrometer band pass, sampling interval, and signal-to-noise ratio required for identification of pure minerals and plants were derived using reflectance spectra convolved to AVIRIS, HYDICE, MIVIS, VIMS, and other imaging spectrometers. For each spectral simulation, various levels of random noise were added to the reflectance spectra after convolution, and then each was analyzed with the Tetracorder spectral identification algorithm [Clark et al., 2003]. The outcome of each identification attempt was tabulated to provide an estimate of the signal-to-noise ratio at which a given percentage of the noisy spectra were identified correctly. Results show that spectral identification is most sensitive to the signal-to-noise ratio at narrow sampling interval values but is more sensitive to the sampling interval itself at broad sampling interval values because of spectral aliasing, a condition when absorption features of different materials can resemble one another. The band pass is less critical to spectral identification than the sampling interval or signal-to-noise ratio because broadening the band pass does not induce spectral aliasing. These conclusions are empirically corroborated by analysis of mineral maps of AVIRIS data collected at Cuprite, Nevada, between 1990 and 1995, a period during which the sensor signal-to-noise ratio increased up to sixfold. There are values of spectrometer sampling and band pass beyond which spectral identification of materials will require an abrupt increase in sensor signal-to-noise ratio due to the effects of spectral aliasing. Factors that control this threshold are the uniqueness of a material's diagnostic absorptions in terms of shape and wavelength isolation, and the spectral diversity of the materials found in nature and in the spectral library used for comparison. Array spectrometers provide the best data for identification when they critically sample spectra. The sampling interval should not be broadened to increase the signal-to-noise ratio in a photon-noise-limited system when high levels of accuracy are desired. It is possible, using this simulation method, to select optimum combinations of band-pass, sampling interval, and signal-to-noise ratio values for a particular application that maximize identification accuracy and minimize the volume of imaging data.

Fundamental amplitude noise limitations to supercontinuum spectra generated in a microstructured fiber

Abstract: Broadband supercontinuum spectra are generated in a microstructured fiber using femtosecond laser pulses. Noise properties of these spectra are studied through experiments and numerical simulations based on a generalized stochastic nonlinear Schrodinger equation. In particular, the relative intensity noise as a function of wavelength across the supercontinuum is measured over a wide range of input pulse parameters, and experimental results and simulations are shown to be in good quantitative agreement. For certain input pulse parameters, amplitude fluctuations as large as 50% are observed. The simulations clarify that the intensity noise on the supercontinuum arises from the amplification of two noise inputs during propagation – quantum-limited shot noise on the input pulse, and spontaneous Raman scattering in the fiber. The amplification factor is a sensitive function of the input pulse parameters. Short input pulses are critical for the generation of very broad supercontinua with low noise.

Dynamic X-ray computed tomography

Abstract: Dynamic computed tomography (CT) imaging aims at reconstructing image sequences where the dynamic nature of the living human body is of primary interest. The main applications concerned are image-guided interventional procedures, functional studies and cardiac imaging. The introduction of ultra-fast rotating gantries along with multi-row detectors and in near future area detectors allows huge progress toward the imaging of moving organs with low-contrast resolution. This paper gives an overview of the different concepts used in dynamic CT. A new reconstruction algorithm based on a voxel-specific dynamic evolution compensation is also presented. It provides four-dimensional image sequences with accurate spatio-temporal information, where each frame is reconstructed using a long-scan acquisition mode on several half-turns. In the same time, this technique permits to reduce the dose delivered per rotation while keeping the same signal to noise ratio for every frame using an adaptive motion-compensated temporal averaging. Results are illustrated on simulated data.

Multispectral or hyperspectral imaging system and method for tactical reconnaissance

Abstract: A two-dimensional focal plane array (FPA) is divided into sub-arrays of rows and columns of pixels, each sub-array being responsive to light energy from a target object which has been separated by a spectral filter or other spectrum dividing element into a predetermined number of spectral bands There is preferably one sub-array on the FPA for each predetermined spectral band Each sub-array has its own read out channel to allow parallel and simultaneous readout of all sub-arrays of the array The scene is scanned onto the array for simultaneous imaging of the terrain in many spectral bands Time Delay and Integrate (TDI) techniques are used as a clocking mechanism within the sub-arrays to increase the signal to noise ratio (SNR) of the detected image Additionally, the TDI length (ie, number of rows of integration during the exposure) within each sub-array is adjustable to optimize and normalize the response of the photosensitive substrate to each spectral band The array provides for parallel and simultaneous readout of each sub-array to increase the collection rate of the spectral imagery All of these features serve to provide a substantial improvement in the area coverage of a hyperspectral imaging system while at the same time increasing the SNR of the detected spectral image

Buffer delay change in the presence of power and ground noise

Abstract: Variations of power and ground levels affect very large scale integration circuit performance. Trends in device technology and in packaging have necessitated a revision in conventional delay models. In particular, simple scalable models are needed to predict delays in the presence of uncorrelated power and ground noise. In this paper, we analyze the effect of such noise-on-signal propagation through a buffer and present simple, closed-form formulas to estimate the corresponding change of delay. The model captures both positive (slowdown) and negative (speedup) delay changes. It is consistent with short-channel MOSFET behavior, including carrier velocity saturation effects. An application shows that repeater chains using buffers instead of inherently faster inverters tend to have superior supply-level-induced jitter characteristics. The expressions can be used in any existing circuit performance optimization design flow or can be combined into any delay calculations as a correction factor.

Error-rate analysis of switched diversity receivers in Weibull fading

Abstract: A novel approach to the performance analysis of switched-and-stay combining diversity receivers over independent Weibull fading channels is presented. Closed-form expressions are extracted for important performance parameters, such as the average output signal-to-noise ratio, the amount of fading, the outage probability, and the switching rate.

Training sequence assisted channel estimation for MIMO OFDM

Abstract: In this paper we present several results of our study on training sequence assisted channel estimation for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. After developing a linear matrix algebraic model for the cyclic prefix based MIMO OFDM systems, we define a generalised preamble structure which is a simple extension of the preambles used in single-input single-output (SISO) OFDM so that its good properties, such as low peak to average power ratio, can be maintained. We then derive the least squares (LS) and linear minimum mean squared error (LMMSE) channel estimation algorithms based on the proposed preamble design. In order to reduce the preamble length, we further propose a switched subcarrier preambles scheme in which the transmit antennas are divided into groups, and preambles are transmitted in alternative subset of subcarriers in each group. A LMMSE filter-based interpolation scheme and a DFT-based LS interpolation scheme are then used to obtain the channel estimates for all the subcarriers of interest. In all the proposed schemes in this paper, the filter parameters can be fixed and robust performance is obtained even when mismatched SNR and channel statistics are used in the filter parameter calculations.

Method and system for interference reduction in a wireless communication network using a joint detector

Abstract: A method and system for reducing interference in a wireless communication network is disclosed. The wireless communication network has at least one base station using an antenna array and one or more code channels to receive or transmit one or more communication signals from or to a plurality of terminals used by one or more users. A signal received by the antenna array carries one or more training sequences and a traffic signal in a frame. After estimating a spatial signature and joint channel response model per user based on the training sequences, one or more spatial weights are found based on the estimated spatial signature and joint channel response model to maximize a signal to noise ratio. A joint detection matrix is then formed based on the estimated spatial weights, the joint channel response model, and a user code channel assignment. After code correlating a traffic signal to obtain one or more user specific multi-antenna signals, a spatial combining is performed on one or more multi-antenna signals associated with each user to generate scalar symbol estimates. Thereafter, a joint detection is done based on the scalar symbol estimates using the joint detection matrix. Similar techniques can be used for downlink communications.

Spectral kurtosis: from definition to application

Abstract: The objective of this paper is to present the Spectral Kurtosis (SK) as a spectral analysis tool complementary to the classical power spectrum density (PSD). Theoretical definition of the SK is firstly justified, followed by a study of its behavior with respect to the spectral components of signals. An unbiased estimator is thus proposed, with its exact variance, and its signal to noise ratio sensivity. Finally, results are illustrated by applying SK and PSD on synthetic and real signals.

Low-sampling rate UWB channel characterization and synchronization

Abstract: We consider the problem of low-sampling rate high-resolution channel estimation and timing for digital ultra-wideband (UWB) receivers. We extend some of our recent results in sampling of certain classes of parametric non-bandlimited signals and develop a frequency domain method for channel estimation and synchronization in ultra-wideband systems, which uses sub-Nyquist uniform sampling and well-studied computational procedures. In particular, the proposed method can be used for identification of more realistic channel models, where different propagation paths undergo different frequency-selective fading. Moreover, we show that it is possible to obtain high-resolution estimates of all relevant channel parameters by sampling a received signal below the traditional Nyquist rate. Our approach leads to faster acquisition compared to current digital solutions, allows for slower A/D converters, and potentially reduces power consumption of digital UWB receivers significantly.

Method and apparatus for transmission power control

Abstract: The invention relates to a system for power control in a cellular communication system. The power control may be operated ( 201 ) in a first mode of operation wherein power control data are determined in response to a quality parameter, such as a desired signal to noise ratio. The method further comprises entering ( 205 ) a reduced power mode of operation by communicating power down power control data between a base station and a communication unit. The base station and/or communication unit then proceeds to operate ( 207 ) in the reduced power mode by communicating power control data corresponding to a reduced transmit power level. The reduced transmit power level may specifically be zero. After a given duration, the base station and/or communication unit proceeds to exit ( 211 ) the reduced power mode by communicating power up power control data between the base station and the communication unit.

Noise canceling cardiac electrodes

Abstract: Implementing a subcutaneous medical electrode system involves positioning a number of electrode subsystems in relation to a heart so that noise cancellation provides an improved signal to noise ratio of the cardiac signal and/or to provide one electrode arrangement preferential for cardiac signals and another arrangement preferential for noise signals. One of the electrode subsystems so positioned may include one or more can electrodes located on a housing enclosing a medical device. The medical device may be configured to provide therapeutic, diagnostic, or monitoring functions, including, for example, cardiac arrhythmia therapy.

Adaptive noise estimation algorithm for speech enhancement

Abstract: An efficient noise estimation algorithm for speech enhancement is proposed. The noisy speech is decomposed into subband signals and the subband noise estimate is updated by adaptively smoothing the noisy signal power. The smoothing parameter is chosen as a function of the estimated signal-to-noise ratio (SNR). This noise estimation technique gives reliable results even at very low SNRs.

Performance of ultra-wideband correlator receiver using Gaussian monocycles

Abstract: This paper investigates the performance of ultra-wideband (UWB) correlator receivers for Gaussian monocycles under the condition of equal mean power and provides constructive reference to the selection of pulses. Several channel situations are examined including ideal single user AWGN channel, non-ideal synchronous, multipath fading and multiple access interference. Both numerical and analytical techniques show that the shapes of pulses have notable impact on the performance of correlator receivers, especially on the interference resistance ability and signal-to-noise ratio (SNR) of the output. he results are also extended to the field of fractional bandwidth to better understand the possible relationship between fractional bandwidth and correlator receivers.

System and method for adaptive phase compensation of OFDM signals

Abstract: An OFDM receiver applies phase compensation to subcarriers of data symbols of an OFDM packet. A phase compensation estimate is generated from pilot subcarriers within the data symbol and applied to the subcarriers of the data symbol prior to demapping. The pilot subcarriers of the data symbol are combined and weighted to generate an observation vector. Recursive filtering is performed on the observation vector to generate the phase compensation estimate. The recursive filtering may include performing an extended Kalman-type filtering (EKF) operation on the observation vector using a channel estimate, an additive noise power estimate, a signal to noise ratio (SNR) estimate and a priori information about a dynamic model of the phase noise spectrum of transceiver oscillators. The channel estimate may be generated from a long training symbol of the OFDM packet, and the additive noise power estimate and the SNR estimate may be generated from short training symbols of the OFDM packet.