Showing papers on "Interference (wave propagation) published in 1984"

Optimum Combining in Digital Mobile Radio with Cochannel Interference

Abstract: This paper studies optimum signal combining for space diversity reception in cellular mobile radio systems. With optimum combining, the signals received by the antennas are weighted and combined to maximize the output signal-to-interference-plus-noise ratio. Thus, with cochannel interference, space diversity is used not only to combat Rayleigh fading of the desired signal (as with maximal ratio combining) but also to reduce the power of interfering signals at the receiver. We use analytical and computer simulation techniques to determine the performance of optimum combining when the received desired and interfering signals are subject to Rayleigh fading. Results show that optimum combining is significantly better than maximal ratio combining even when the number of interferers is greater than the number of antennas. Results for typical cellular mobile radio systems show that optimum combining increases the output signalto-interference ratio at the receiver by several decibels. Thus, systems can require fewer base station antennas and/or achieve increased channel capacity through greater frequency reuse. We also describe techniques for implementing optimum combining with least mean square (LMS) adaptive arrays.

Adaptive Cancellation of Nonlinear Intersymbol Interference for Voiceband Data Transmission

Abstract: Two related cancellation techniques based on "tentative" decisions are proposed for removing intersymbol interference, due to channel nonlinearities, for voiceband data transmission. Cancellation of intersymbol interference avoids the need for equalizing or "inverting" analog nonlinear operations as in the decision-feedback method previously proposed by Falconer. The nonlinear cancellation approach removes nonlinear interference terms without excessive noise enhancement and allows effective implementation with ROM's to generate the needed nonlinear signal variables that are to be weighted and summed. The first technique can be incorporated as an "add-on" unit to a present-day modem; the second technique requires a different receiver architecture but offers somewhat enhanced improvement. Simulation results for 9.6 kbit/s data transmission over nonlinear channel models show that both techniques can offer substantial performance improvements for 16-point QAM systems.

Performance Analysis of Narrow-Band Interference Rejection Techniques in DS Spread-Spectrum Systems

Abstract: Linear least squares estimation (LLSE) techniques can provide an effective means of suppressing narrow-band interference in direct sequence (DS) spread-spectrum systems. In the results presented here, analytical expressions for bit error rate are derived for two DS spread-spectrum systems under the conditions of either tone or narrowband Gaussian interference. It is shown that the most common LLSE filter design can lead to performance inferior to that of various other filter designs. However, results are also presented demonstrating that an LLSE filter design motivated by the structure of the maximum-likelihood receiver leads to consistently superior performance. The performance of a system using this new design criterion is compared with that of an approximation to the maximum-likelihood (ML) receiver for the tone interference model and with that of the exact ML receiver for the Gaussian interference. Finally, it is shown that the bit error rate estimate obtained from application of a Gaussian approximation for the test statistic is overly pessimistic for the systems studied here.

Multichannel phase-shifted interferometer

Abstract: A new real-time interferometer based on diffraction phenomena is discussed. It consists of a point-diffraction interferometer fabricated on a transmission grating. The real-time data-analysis capability is achieved by simultaneously introducing a phase shift (piston) on the three separate channels of diffracted interferograms. Mathematical analysis and preliminary observational results are included.

Closed-Form Analytical Results for the Rejection of Narrow-Band Interference in PN Spread-Spectrum Systems--Part II: Linear Interpolation Filters

Abstract: The performance of PN spread-spectrum communication systems in the presence of narrow-band interference is studied when linear interpolation filters are employed for the estimation and subsequent suppression of the interference. Closed-form analytical expressions for the system's performance are established for a broad class of interference processes. The advantages of linear interpolation filters over predictive filters with identical number of taps are examined analytically and some unexpected results are obtained. The analytical results are illustrated by examples.

Frequency modulated lasar radar

Abstract: An apparatus and a method for measuring the distance to an arbitrary target includes a radiation source producing a beam of coherent radiation the frequency of which is continuously varied. The beam is divided into a ranging beam and a reference beam. The ranging beam is coupled to a ranging interferometer, which directs a portion of the ranging beam at the target. The ranging interferometer produces a first signal indicative of the phase difference between a portion of the ranging beam directed at and scattered by the target and another portion of the ranging beam which has traveled over a path of fixed length. The reference beam is coupled to a reference interferometer. A portion of the reference beam is directed by the reference interferometer along a reference path of a predetermined length, and the phase difference between the portion of the reference beam directed along the reference path and another portion of the reference beam which has travelled over a path of a fixed length is measured. The number of fringes resulting from the wave interference produced in the ranging interferometer and the number of fringes in the wave interference pattern produced by the reference interferometer are counted and used, together with the known length of the reference path, to determine the distance of the target from the ranging interferometer.

Acoustic Microscopy of Elastic Discontinuities

Abstract: In the reflection-scanning acoustic microscope, interference fringes may be obtained from cracks and grain boundaries running at an angle to the surface. The periodicity of these fringes suggests that while sometimes they are caused by reflection of longitudinal waves within the specimen, when the feature is approximately normal to the surface, Rayleigh waves are responsible, and this confirms the dominant role played by Rayleigh waves in the contrast in acoustic microscopy. The variation of contrast with defocus may be expressed as a Fourier transform of the reflectance function of a specimen; oscillations in V(z) then arise as the transform of the change in phase around the Rayleigh angle. When Rayleigh waves strike a surface-breaking discontinuity, they may be strongly reflected even though the discontinuity is much less than a wavelength thick. This enables fine cracks and other features which would not be resolved according to conventional criteria to be revealed very clearly in acoustic micrographs.

Progressing and oscillatory waves for hybrid synthesis of source excited propagation and diffraction

Abstract: Progressing and oscillatory waves provide alternative building blocks for constructing source-excited time-harmonic or transient fields in various propagation and scattering environments. Progressing waves describe the field in terms of direct and multiple reflected-diffracted wavefront or ray arrivals, while oscillatory waves describe the field in terms of body resonances and (or) guided modes. Each description is convenient and physically incisive when it requires few constituent elements but inconvenient and physically more obscure when it requires many elements. Collective summation of many inconvenient elements into fewer convenient ones, when possible, provides a means of switching from a poorly to a more rapidly convergent field representation. Rays (wavefronts) and modes (resonances) fall into the category of such bilaterally convertible wave fields. They not only have complementary convergence properties but furnish, respectively, local and global environmental discriminants. When combined in self-consistent hybrid form, where the number retained of the one uniquely determines the grouping required of the other, there emerges a rigorous theory of propagation and diffraction with high versatility. Groupings can be chosen so as to eliminate "difficult" elements (for example, caustic forming or transitional ray fields) and replace these by "well behaved" ones (modes) of the complementary species, or to model multiple interference in the one as simple interference in the other. The theory, based on Poisson summation or on alternative treatment of wave spectral representations, is presented and applied to numerous coordinate separable, but also weakly nonseparable, environments in electromagnetics, underwater acoustics and elastic motion, with numerical comparisons to highlight the salient features of the hybrid appraoch.

Aspherical Surface Testing With Shearing Interferometer Using Fringe Scanning Detection Method

Abstract: A technique for accurately measuring the wavefront aberration of aspherical optical surfaces with a lateral-shearing interferometer is described. A computer-controlled interference phase measuring technique is employed, which provides greater accuracy and real-time data analysis. Key elements of the present system are a lateral-shearing interferometer with a parallel plate, a piezoelectric-driven mirror, an areal image detector, and a microcomputer system with a graphic display. The shearing interferometer gives a fringe pattern corresponding to the derivative of the wavefront, which is analyzed by the fringe scanning method. By integrating the derivative of the analyzed data, we have the wavefront aberration of the test optics over an aperture containing a 32 X32 element array. A rms accuracy of measurement of 1/32 wavelength is achieved on the evaluation of an f/4 aspherical mirror.

Threshold Detection in Non-Gaussian Interference Environments: Exposition and Interpretation of New Results for EMC Applications

Abstract: The critical problem of developing optimum weak-signal detection algorithms for general classes of man-made and natural interference is concisely examined. This paper includes a summary of new results for both canonical and specific types of desired signals and electromagnetic interference (EMI) environments. A suitable bias term, missing in most earlier analyses, is essential to preserve the asymptotically optimum (AO) character of these algorithms, as sample size (or observation period ~n) necessarily becomes large, to enable the detection of weak signals with acceptably small probabilities of error. Canonical optimum threshold algorithms are obtained for three principal modes of detection: 1) coherent, 2) incoherent, and 3) composite. The latter 3) is AO, when it consists of the sum of (optimum) algorithms for 1) and 2); it can give noticeable improvement over either 1) or 2) alone. Various (usually) suboptimum algorithms, e.g., simple correlators and clipper correlators, are also considered. Performance measures (n > > 1), e.g., error probabilities, etc., and asymptotic relative efficiencies (ARE's), are noted, along with the useful concepts of processing gain and minimum detectable signal. Interpretations of the algorithmic structure and the implications of these results, generally, for EMC, are concisely discussed.

Error Probability Bounds and Approximations for DS Spread-Spectrum Communication Systems with Mutiple Tone or Multiple Access Interference

Abstract: Upper and lower bounds on the average probability of error of direct sequence (DS) spread-spectrum communication systems operating in the presence of either multiple tone or multiple access interference are presented. The bounds are quite tight and can be used without the restriction that the peak level of the interference be less than the desired signal level. A simple method for evaluating approximations to the bounds for multiple access systems, in which the peak interference does indeed exceed the desired signal level, is presented as well. The tightness of both the bounds and the approximations are illustrated with numerical results.

Broad-source fringes in grating and conventional interferometers

Abstract: A higher-order analysis of broad-source fringe formation in the Mach–Zehnder interferometer and its diffraction-grating counterpart is carried out. The results are considerably different for the two cases, with the grating interferometer being much more favorable. Broad-spectrum effects are also considered.

Model studies of barrier performance in the presence of ground surfaces. Part II—Different shapes

Abstract: Modeling experiments are reported which have investigated the frequency dependence of barrier insertion loss for various noise barrier designs. The effect of ground surfaces has been studied, treating both grass‐covered ground and asphalt. Results show that interference effects are an important feature of observed behavior. Use is made of this knowledge in the design and testing of new barrier designs, which deliberately introduce a beneficial destructive interference phenomenon to increase insertion losses over well‐defined ranges of frequency.

Multiple Spacecraft Michelson Stellar Interferometer

Abstract: An alternative to a monolithic space Michelson interferometer is a device composed of three separate spacecraft. Such a device consists of two telescopes which collect light from a source and transmit it to a third spacecraft positioned so that interference fringes are detectable in an onboard interferometer. The contrast of the fringes is then measured as a function of separation of the telescopes. A multiple spacecraft design allows extremely large interferometer baselines. The resulting high angular resolution permits fundamental astrophysical measurements different from those possible with foreseeable monolithic devices. We describe an orbital analysis and assessment of performance for a particular device design, SAMSI, Spacecraft Array for Michelson Spatial Interferometry. The device we consider includes two one-meter telescopes in orbits which are identical except for slightly differing inclinations. The telescopes achieve separations as large as 10 kilometers and relay starlight to a central station which has a one-meter optical delay line in one interferometer arm. Our four key findings are as follows: 1) a 1000 kilometer altitude, zero mean inclination orbit affords natural scanning of the 10 km baseline with departures from optical pathlength equality which are well within the corrective capacity of the optical delay line; 2) electric propulsion is completely adequate to provide the required spacecraft motions (principally those needed for repointing); 3) all necessary technology is already in a high state of development; and 4) resolution and magnitude limits of 10-5 arcsecond and my = 15 to 20 are achievable.© (1984) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

An improved eigenvector beamformer

Abstract: Most techniques for computing beamformer tap weights do so by maximizing the overall signal-to-noise ratio at the system output, where noise includes both thermal noise and directional interference. This is the optimal solution if the two noise sources count equally. However, there do exist scenarios where directional interference is more harmful to system performance. For this reason, the authors introduced a new beam-former based on eigenvector techniques[1] which is able to exactly null interference even when thermal noise is present. Unfortunately, however, the new method can also cause signal cancellation, depending on the geometry of the interference. This cancellation can be reduced by introducing additional eigenvectors in the tap weight solution. We develop a criterion for choosing these additional vectors which allows the user to set the maximum tolerable signal cancellation. Given this signal cancellation constraint, the directional interference power is minimized. Simulation results are presented which demonstrate the modified eigenvector beamformer. We also comment on the extension of the method to wide-band signals.

Optical spatial frequency filtering using interferences

Abstract: Several recent publications describe optical image-processing experiments in which the spatial-filtering operation is performed by an interference phenomenon. In this paper we derive the general conditions allowing interferences to play the role of a spatial filter. It is shown that, although a spatially coherent illumination is necessary, the interferometer must be able to produce localized fringes, and that the localization locus must coincide with the Fourier spectra formed through the two (or more) arms of the interferometer. The filtering operation may be understood either as an interference between these Fourier spectra or as an interference between the images of the object formed by each of the arms of the interferometer. In the case of most usual interferometers, these images are simply geometrical images of the object that are shifted with respect to each other, and the shift is responsible for the filtering effect. Two classes of interferometric setups can be distinguished, depending on whether the shift is perpendicular or parallel to the image plane. Examples are given, and experimental results are presented.

Two-wavelength holographic interferometer.

Abstract: It has been demonstrated that a Bi12SiO20 crystal is a good holographic recording medium. Using this crystal as a real-time recording device, a two-wavelength holographic interferometer has been constructed. The 488- and 514.5-nm lines of an Ar-ion laser were used as the source to yield an equivalent wavelength of 9.47 μm.

Optical image processor

Abstract: @ An optical processing system based on noncoherent light processing employs both geometrical and diffraction optical systems. One portion of the optical system (25, 26) preforms a subtraction operation of the image from itself where one of the subtracted images is relatively defocused and one is delayed in time from the other. The subtracted image is edge- enhanced (32) and then applied to a spatial filter employing an interferometer (56) in which spatial filters (61, 62) are contained in the interferometer iris plates (59, 60). Additional spatial filtering (72) is performed on the interference image and the processed image is displayed in an appropriate display (78, 81) which can selectively view the processed image, the unprocessed image, or combinations of the two. Light storage elements are employed for storing the processed images in appropriate buffers (23, 24, 64) over a given length of time. The buffers are read out by a flashlamp. Suitable shutters (15, 16, 80, 82) are closed during the readout process.

The Coherence of Light

Abstract: Interference and diffraction of light are phenomena known from various experiments. Whereas a dark room used to be necessary to observe interference and diffraction with the use of a monochromatic spectral light source, it is now possible to observe them in an ordinary room in daylight by using a laser light source. The phenomena of interference and diffraction of light are due to the wave nature of light, which will be discussed in this chapter.

Focus tracking system

Abstract: A method and apparatus are disclosed for tracking the position of surfaces during certain processing such as optical projection printing. Light from a line or point source is made incident on the surface at a small angle to produce interference fringes between the reflected light and a reference beam. Any movement of the surface causes a shifting of the fringe pattern which can be detected by appropriate means such as a slit or grating and photodiode combination.

Dynamic two-frequency interferometry for small displacement measurements

Abstract: Two-frequency interferometry for small displacement measurements is examined from a practical point of view. A laser beam that is modulated sinusoidally in intensity by an acousto-optic Bragg cell provides signal and reference optical waves to produce dynamic interference. The output signal of a photodetector receiving the interference is composed of optical homodyne and heterodyne components. The heterodyne component carries a vibrational displacement to be detected on its amplitude. A sinusoidal vibration amplitude is measured down to 0.5 nm.

A resolution of the classical wave-particle problem

Abstract: The classical wave-particle problem is resolved in accord with Newton's concept of the particle nature of light by associating particle density and flux with the classical wave energy density and flux. Point particles flowing along discrete trajectories yield interference and diffraction patterns, as illustrated by Young's double pinhole interference. Bound particle motion is prescribed by standing waves. Particle motion as a function of time is presented for the case of a “particle in a box.” Initial conditions uniquely determine the subsequent motion. Some discussion regarding quantum theory is preseted.

Laser Unequal Path Interferometer Configurations By Grating Splitting At The Fourier Plane

Abstract: Simple interferometric configurations are considered in which the beam splitter is made of a diffraction grating placed at the Fourier plane of the source system. Under coherent illumination, interference patterns are produced that account for the phase difference between the source wavefront and the back-reflected probe wavefront. A practical laser unequal path interferome-ter (LUPI) setup is presented, along with calibration results from comparison with well-established interferometric configurations.

Decision Feedback Techniques for Interference Cancellation in PN Spread Spectrum Communication Systems

Abstract: A new type of nonlinear interference canceller with applications in PN spread spectrum communication systems is described, and performance results are given. This interference cancelling technique can be implemented in a one stage or two stage form. Both forms are discussed, and their performance compared and related to that of linear interference cancellation methods. These results demonstrate that decision feedback techniques yield significantly improved performance relative to previously reported linear interference cancellation methods.

Method and apparatus for optically measuring three-dimensional coordinates

Abstract: The disclosed apparatus provides a point optical radiation source (i.e. point light source), at one or more distant points, the three-dimensional coordinates of which are to be measured relative to a known frame of reference. Each point light source emits light waves of a known wavelength which are received by the front plate of a multiple-beam interferometer means having a known angle of light acceptance. With respect to the light waves transmitted by each point light source and received by the interferometer means, a set of non-localized multiple-beam interference fringes are created within the interferometer means. A photo-electric detection means is provided a known distance from the back plate of the multiple-beam interferometer means and is positioned so that the interferometer means and photo-electric detection means have a common optical center axis. The intersection of such optical center axis and the plane defined by the back plate of the interferometer means establishes the origin of the known frame of reference. The photo-electric detection means both detects the two-dimensional position of at least the innermost fringe of each set of interference fringes, and transmits the detected positional information to a processor means by electric signals. The processor means employs the electric signals to determine the coordinate values for the center point of each set of interference fringes relative to the common optical center axis, and determines the radius of the innermost fringe of each set of interference fringes. Such determinations allow the processor means to calculate the three-dimensional coordinates of the distant points relative to the known frame of reference.

Automation of length measurements which involve analysis of interference patterns

Abstract: Calibration of gauges having parallel plane ends against the wavelength of light involves the use of interference patterns which have to be observed visually by an operator. In this paper an automatic method of measurement is described and compared to the visual method.