Amplitude Of Received Signal Research Articles

Bit-error bounds for trellis-coded MPSK in mixed fading channels

Bit-error probability (BEP) bounds of trellis-coded MPSK systems over two classes of mixed fading channels are studied. These two classes of channels have been proposed as candidate models for mobile satellite communications. The first class consists of slow and frequency-nonselective fading channels whose output field strengths follow a probability law characterized by a convex combination of Rician and Rayleigh/lognormal distributions. For the other class of fading channels, the received signal amplitude has a convex combination of Rician and Rician/lognormal distributions. We analyze performance bounds for trellis codes that belong to the class of either geometrically uniform codes (GUCs) or quasi-regular codes (QRCs). Receivers with either ideal channel state information (CSI) or no CSI at all are considered. We examine asymptotic behaviors of these codes and identify key design parameters. Numerical results are provided to illustrate and compare the BEP performances of various codes and to validate the usefulness of the asymptotic analysis.

Responses of bowhead whales to sonobuoy impacts

Air-dropped sonobuoys have been used to record sounds near bowhead whales since 1979. They have not been observed to react to sonobuoys dropped 0.5–1 km from them during numerous studies conducted during the summer; however, on a few occasions they have reacted to sonobuoys air dropped near them during their spring and fall migrations. The sound from an AN/SSQ-57A sonobuoy impact was recorded at distance 800 m in water 90 m deep in the western Beaufort Sea. Hydrophone depths were 3 and 18 m. Analysis of the impact signature and sound transmission paths revealed two reflected paths: one with a single bottom reflection and the second with two bottom reflections. A third arrival, occurring later and very weak, was also noted. The presence of a strong, low-frequency sound source nearby led us to high-pass filter the signal at 1 kHz. Analysis of the received signal amplitude, corrected for spreading loss, revealed a peak source pressure level of 211 dB re: 1 μPa/m. The positive acoustic impulse at a distance of 1 m, assuming no positive pulse spreading and scaling the pressure for spherical spreading, is estimated to be 5.55 Pa/s. The overall sound duration of the single bottom bounce arrival was 0.9 ms. [Work supported by MMS.]

Rapidly converging multiuser CDMA detector for an asynchronous AWGN channel

A two-stage multiuser CDMA detector that utilises the decorrelator's tentative decisions and a recursive least squares-type algorithm to form the multiple access interference estimates is proposed. The detector is near-far resistant, and it requires neither prior estimation of the received signal amplitudes, nor the use of training sequences. It offers a fast transient response, and better error performance than a similar detector that utilises a gradient descent algorithm.

Antenna sidelobes in the presence of flat reflectors

Reflecting surfaces near the antenna may produce surprisingly large sidelobes even if the designed free-space sidelobes in the directions of these surfaces are very small. Calculations of such sidelobes are presented for an example of a linear aperture with a parabolic weighting function for the received signal amplitude.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dynamic behaviour of radio channels due to trans-horizon propagation mechanisms

To study the propagation mechanisms of trans-horizon radio links, the Letter proposes an event-based approach. Data from one year captured on a trans-horizon link of 156 km are divided into different groups according to the signal amplitude signature of the chart recordings. The propagation mechanisms for each group are investigated using the cross-correlation between received signal amplitudes along the same path. These amplitudes are obtained using two different transmitter antenna heights.

Geophysical diffraction tomography at a dinosaur site

In 1979 two hikers discovered the partially exposed bones from the tail of a dinosaur in the high desert of New Mexico. Excavation in 1985 led to the recognition that the dinosaur, informally named “Seismosaurus,” was new to science. It is by far the longest dinosaur on record. Excavations continued in 1986 with the discovery that additional bones leading forward from the middle region of the tail, potentially toward the remainder of the skeleton, remained entombed in the 145 million year old sandstone. The site has been the subject of intensive experimentation to assess the feasibility of geophysical remote sensing in exploration paleontology. The efforts of numerous investigators at the site may serve to change the century‐old methods of hammer and chisel, in part, to those based on more modern technology. One of the methods tested is geophysical diffraction tomography (GDT). Unlike the more common straightray algorithms of geophysical tomography that map subsurface properties by projecting received signal amplitude or time of the first signal arrival back to the source, GDT treats the problem as one of wave propagation by inverting the linearized wave equation to compute the spatial distribution of refractive index. Excavation of the “Seismosaurus” skeleton continues. The purpose of our most recent GDT study is to further test the method and to guide paleontologists in their future excavations by locating and identifying the buried skeletal remains. A total of eleven imaged vertical cross‐sections displayed features of sufficient acoustic contrast to be tentatively identified as dinosaur bone. Although the GDT resolution is insufficient to characterize bone shape, bone identification was subjectively accomplished on the basis of size, aspect ratio, and position. Based on this interpretation, it is believed that much of the remaining skeleton exists and is in near articulation.

Coding for meteor burst communications

The use of error correction coding for meteor burst communications is discussed. The cutoff rate is derived for a meteor burst channel model which assumes that the received signal amplitude decays exponentially. The message error rate of a coded system, obtained by means of computer simulation, is compared to that of an uncoded system. A tight upper bound on the performance of the coded system is presented. It is shown that coding can reduce the signal-to-noise ratio (SNR) required for reliable communications via meteor bursts by many decibels. The coding gain is larger at shorter communication ranges where meteor trails, decay faster. It is also shown that the additional improvement achieved by providing the decoder with side information on the instantaneous level of the received signal is small.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Quantitative measurement capabilities of the scanning laser acoustic microscope

The scanning laser acoustic microscope (SLAM) has the capability of determining the spatial distributions of attenuation coefficient and speed in materials under evaluation. The attenuation coefficient is determined by the insertion loss procedure [K. M. U. Tervola et al., IEEE Trans. Sonics Ultrason. SU-32, 259–265 (1985)], which compares the received signal amplitude of a specimen of known thickness in the sound path with that of the reference medium. The entire image is broken into 64 subimage areas, each of approximately 400 uμm × 250 uμm in size. This subarea image is used to determine the insertion loss. The slope of the linear least-squares fit line for an insertion loss versus specimen thickness plot yields the attenuation coefficient. Using known materials, the attenuation coefficient accuracy and precision are −+ 12% and −+ 5%, respectively. Ultrasonic speed is determined from the interference mode image by way of the spatial frequency domain technique [K. M. U. Tervola and W. D. O'Brien, Jr., IEEE Trans. Sonics Ultrason. SU-32, 544–554 (1985)]. The image's field of view (3mm × 2 mm) contains approximately 39 vertical interference lines equally spaced about 85 uμm apart. The speed of sound is determined by the horizontal shift of the interference lines between the reference medium and the specimen. Again, using a known material, the speed accuracy, and precision are −+ 2.9% and −+ 0.4%, respectively. Smaller regional differences can be distinguished with the speed analysis than the attenuation coefficient analysis. Each speed pixel is approximately 4 uμm × 8 uμm and a speed profile (80 uμm × 2 mm) along the vertical direction of the image is generated. In contrast, in each insertion loss pixel (400 uμm × 250 uμm), an average insertion loss value per section is generated over a region approximately 1 mm × 1 mm in extent. Further, at least three separate sections are required to calculate the attenuation coefficient. The application of SLAM towards an understanding of ultrasonic interaction action with biological materials is being studied by a number of different animal and tissue models. These models consider both normal and pathologic, homogeneous and heterogeneous, and soft and hard tissues. [Work supported by NIH Grants AM21557, CA30629, and AR36794.]

Oscillations in Ultrasonic Attenuation: Real or Spurious?

Occasionally, in pulsed ultrasonic attenuation measurements, oscillations in the amplitude of the received sound signal are observed, when a certain parameter (temperature or magnetic field) is varied. Sometimes these oscillations in the received signal amplitude are of spurious origin, (that is, they have nothing to do with any attenuation mechanism within the sample under investigation). In this paper we discuss two examples of oscillatory behavior in ultrasonic attenuation that we have seen when the magnetic field was varied; one real, the other spurious.

Radar CFAR Thresholding in Clutter and Multiple Target Situations

Radar detection procedures involve the comparison of the received signal amplitude to a threshold. In order to obtain a constant false-alarm rate (CFAR), an adaptive threshold must be applied reflecting the local clutter situation. The cell averaging approach, for example, is an adaptive procedure. A CFAR method is discussed using as the CFAR threshold one single value selected from the so-called ordered statistic (this method is fundamentally different from a rank statistic). This procedure has some advantages over cell averaging CFAR, especially in cases where more than one target is present within the reference window on which estimation of the local clutter situation is based, or where this reference window is crossing clutter edges.

Correlation of phase and amplitude fluctuations at hydrophones spaced one-half to two wavelengths apart

An experiment was performed at sea to study the temporal properties of amplitude and phase coherence of cw signals received on a linear array of four deep bottomed hydrophones with consecutive spacings of 287, 22, and 890 ft. The source was suspended at each of several depths from a drifting ship in the second convergence zone, and activated at frequencies of 115, 200, and 400 Hz for an hour at a time each. The average received signal amplitude and the average cosine of the phase difference between hydrophones 22 ft apart for 8.73-min samples were found to be related. When average amplitude was large, the average cosine was 1.0; when amplitude was small (e.g., 20 dB less), the cosine was near zero. Within the 8.73-min period also, large rapid phase variations often occurred accompanied by large variations in signal amplitude. The signal gain of the array of four receivers was found to be similarly related to the average hydrophone power. These results support previous theoretical predictions. Phase and amplitude oscillations which occurred at frequencies around 0.1 Hz are attributed to interaction of the signal with the sea surface.

Temporal and spatial fluctuations in single-path underwater acoustic wave fronts, II: Transmission over 270-nmi range

The fluctuations in phase and amplitude of 400-Hz pulses 10 ms long transmitted over a single 270-nmi path were measured at each receiver of an experimental installation of eleven bottomed at about 5000 ft on a slope near Bermuda. Seventh-order and eighth-order arrivals were employed. From the data obtained, computations were made of arrival angles and acoustic bearings of the source, delay fluctuations between receivers, autocorrelations and cross correlations of relative delays, and cross correlations of relative delays, and cross correlations and coefficients of variation of received signal amplitudes. Plots of cosines of phase (delay) fluctuations between two receivers versus receiver spacing show an exponential decline, with decay lengths of 450 to 800 ft for downslope separations and 1200 to 1700 ft for cross-slope pairs, as compared with 2600 and 4000 ft, respectively, in a 43-nmi experiment.

On detecting reflections in presence of scattering from amplitude statistics with application to D region partial reflections

In many radar experiments which use transmission or scattering of radio waves through a random medium, the received signal is modeled as a reflected sine wave of amplitude A in a scattered narrow‐band Gaussian random process of variance 2σ2. In this model the received signal amplitude has a Rice family of distribution characterized by a parameter α = A/σ. It is shown that amplitude statistics cannot be used to discriminate weak reflections from scattering (α &lt; 0.5). Behavior of mean, skewness, and kurtosis of the normalized signal amplitude is examined for a constant and an intermittent reflected component. It is shown that intermittency has a strong effect on these. Applications to a D region partial‐reflection experiment at Tromsø are briefly discussed. It is shown that, at least in some simple cases, enhanced reflections arise from the top and bottom parts of turbulent layers.

Tomographic imaging of attenuation with simulation correction for refraction

Beam refraction in soft tissue and phase cancellation at the receiver can cause large errors and prevent tomographic reconstruction of attenuation. The method developed in this study incorporates simulation of ultrasound propagation into tomographic imaging to correct these effects. The method is based on input scan data of time-of-flight and received signal amplitude. The speed of propagation is estimated from the time-of-flight scan data. Based on this estimate, the simulation generates scan data of signal amplitude. This scan data represents the loss from refraction and phase cancellation with zero attenuation, and is used to correct the input amplitude scans. We evaluate the accuracy of the method, and investigate the trade-off between various scanning and filtering parameters. The evaluation is based on scan data generated by a comprehensive simulation including refraction, attenuation, phase cancellation, finite beam width, several target shapes and sizes with sound parameters consistent with biologic values, and discrete translational and rotational transducer positions. In several cases with large refraction distortions, attenuation images are accurately reconstructed which are not feasible using standard tomographic methods.

Microprocessors in collection and analysis of Tursiops truncatus echolocation data

A 6502 microprocessor‐based system (Apple II) was used to collect and analyze parameters of the echolocation signals emitted by a bottlenosed dolphin in various target detection tasks. The parameters measured included: the number of transmitted clicks, peak source level of each click, time of occurrence of each click (milliseconds), the pulse train duration, and interclick intervals. Switch closures from response paddles, corresponding to target present and/or target absent decisions by the animal, were interfaced with the microprocessor for collection of response category and latency data. The results of these analyses were displayed on a CRT monitor in alphanumeric and graphic formats. All hardware was microprocessor controlled via either machine or BASIC language. Echolocation clicks were received by an EDO Western 6166 hydrophone, amplified, and passed through a 60–160 kHz bandpass filter and peak detected. When the received signal amplitude exceeded a pre‐set level, a flag was set in the microcompute...

Radio wave loss deviation and shadow loss at 900 MHz

Radio wave propagation between base and mobile stations is normally described as being Rayleigh distributed due to multipath radio wave combining. When the number of radio wave paths are limited the variation in received signal amplitude frequently follows a more general case given by a Weibull distribution. A significant portion of the Weibull distribution is defined as the transmission loss deviation. Other definitions have been included with the object of standardizing methods of measuring and reporting propagation data. Data will be presented showing loss deviation between 3 and 30 dB. Shadow loss over hills and around buildings are usually assumed to be knife-edge or rounded knife-edge. Shadow loss based on these assumptions is generally found to be less than the true measured value. Data will be presented comparing calculated shadow loss with measured value.

Applications of digital color display techniques for sodar

The monostatic—bistatic Doppler Sodar at Penn State University has been equipped with a digital color display unit for presenting signal amplitude and Doppler information. Effective system performance for measurements of planetary boundary layer structure and dynamics has been markedly improved. The color technique greatly enhances effective grey-scale resolution in vertical-time section displays of received signal amplitude. Color presentation of signal Doppler information facilitates analysis of wave phenomena and thermal convection in the planetary boundary layer. The system is in use for a variety of field measurements. In some, a research aircraft is radio directed into the region of meteorological interest. Other experiments involve the development of user-oriented color display techniques for ready interpretation of air pollution related parameters. [Research supported by EPA Grant R800397.]

Physically Realizable Filtering for Data Transmission Systems

In this paper, the design of physically realizable rational fuction transmitting or receiving filters for use in pulse transmission systems operating in the presence of Gaussian noise and intersymbol interference is explored. For the design, the three iteria considered are 1) mean-square error (MSE), 2) error probability, and 3) a weighted sum of the squares of the signal-to-noise ratios corresponding to all possible received signal patterns (MSSN). Expressions are obtained for the various error criteria in terms of the transnmission system poles and residues (coefficients of a partial fraction expansion), assuming that the transmitting and receiving filters and the transmission medium are given by physically realizable rational function forms. It is shown that optimization of the MSE criterion under a received signal amplitude constraint with respect to the receiving filter residues, for a fixed set of poles, leads to a set of linear equations readily solvable for the optimal residues. A suboptimal technique is used to specify "reasonable" pole values, thereby the poles are constrained to belong to some "standard" set of all-pole transmission functions, as for example, maximally flat delay or maximally flat magnitude. The bandwidth of the given pole configuration is determined to optimize the given error criterion. Numemical examples are presented to illustrate the filter design techniques developed. The results indicate that, in many cases, filter design under the MSE or MSSN error criteria leads to optimal or near optimal design under an error-probability criterion. A brief discussion is also given of the filter sensitivity to parameter variations.

The effects of polarization rotation and phase delay with frequency on ionospherically propagated signals

When a CW skywave signal is received on a linearly polarized antenna, polarization (Faraday) rotation produces a variation of received signal strength with radio frequency. The resulting dependence of received signal amplitude on radio frequency may impose a bandwidth limitation on pulsed signals where waveform preservation is important. A measure of this limitation, termed polarization bandwidth, is defined to correspond to the bandwidth in which the plane of polarization rotates <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">90\deg</tex> . Computer ray-tracing calculations were performed using a single Chapman-layer ionospheric model to determine the 1-hop polarization bandwidth as a function of geomagnetic azimuth and radio frequency. The polarization bandwidth was found to decrease with increasing radio frequency and with increasingly close alignment of the propagation path with the longitudinal component of the earth's magnetic field. Assuming a critical frequency of 9 MHz and a path length of 2000 km, the polarization bandwidth increased from a minimum of 140 kHz at 10.5 MHz and from a minimum of 70 kHz at 17.5 MHz, as the propagation direction varied from geomagnetic north to east. A model for the 1-hop ionospheric signal channel is proposed whose parameters are the rate of change of polarization rotation with frequency and the phase versus frequency characteristic of the path. These two parameters are shown to be readily determined from FM-CW or equivalent oblique-path sounding records. Using this model, predictions are made of the effects of polarization rotation with frequency, and also of ionospheric dispersion or phase distortion, on the envelope shape of short-pulse signals (of from 1.5 to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50 \mu</tex> s duration). A pronounced waveshape distortion due to the effects of polarization rotation on the pulse envelope was observed when the signal bandwidth appreciably exceeded the "polarization bandwidth" for the path.

A Model For Mobile Radio Fading Due to Building Reflections: Theoretical and Experimental Fading Waveform Power Spectra

Fluctuations in received signal amplitude occur during mobile communications because of the motion of the mobile station through the spatial standing-wave pattern resulting from the interaction of direct and reflected signals. A model is presented which permits a theoretical calculation of the power spectrum of these fluctuations and satisfactorily predicts the features of spectra computed from experimental fading data except for an observed rise at low frequencies. The model is based on the geometry of the reflections from nearby randomly placed vertical plane reflectors. Vertical polarization is assumed. Both the standing-wave pattern and the Doppler shift view of fading are used to obtain nearly identical results. The detailed shape and in particular the sharp cutoff frequency of the spectrum are shown to depend crucially on the angle a between the direction of vehicle motion and the direction to the fixed station. Detailed comparisons are made of theoretical spectra with experimental spectra representing a range of the angle α. The collection, digitization, calibration, plotting, and digital processing to obtain power spectra of actual recorded fading waveforms are described.