Showing papers on "Interference (wave propagation) published in 1992"

Performance of a spread spectrum packet radio network link in a Poisson field of interferers

Abstract: Results on the modeling of interference in a radio communication network and performance measures for the link as a function of distance are presented. It is assumed that a transmitter-receiver pair in a radio network is affected by a set of interferers, using the same modulation and power, whose positions are modeled as a Poisson field in the plane. Assuming a 1/r/sup gamma / propagation power loss law, the probability distributions for the noise at the receiver are found to be the stable distributions. Results are given for the probability of symbol error and link capacity as a function of the distance between the transmitter and receiver for direct sequence and frequency hopping spread spectrum schemes. It is found that the frequency hopping schemes are inherently superior and their performance is not dependent on the synchronization of the hopping times for the different users. >

Signal-selective time-difference-of-arrival estimation for passive location of man-made signal sources in highly corruptive environments. I. Theory and method

Abstract: For the problem of estimating time difference of arrival (TDOA) of radio waves impinging on a pair of antennas for the purpose of passively locating the source of a communications or telemetry signal in the presence of interfering signals and noise, a new class of signal-selective algorithms that are highly tolerant to interference and noise is introduced. Unlike conventional methods, the new methods cross correlate frequency-shifted as well as time-shifted versions of the received data in order to exploit the unique cyclostationarity property of the signal of interest. >

Assessing the contributions of surface waves and complex rays to far-field Mie scattering by use of the Debye series

Abstract: The contributions of complex rays and the secondary radiation shed by surface waves to scattering by a dielectric sphere are calculated in the context of the Debye series expansion of the Mie scattering amplitudes. Also, the contributions of geometrical rays are reviewed and compared with the Debye series. Interference effects between surface waves, complex waves, and geometrical waves are calculated, and the possibility of observing these interference effects is discussed. Experimental data supporting the observation of a surface wave-geometrical pattern is presented.

Dispersion cancellation and high-resolution time measurements in a fourth-order optical interferometer

Abstract: Group-velocity dispersion (GVD) places a fundamental limitation on the resolution possible when measuring the propagation time of short optical pulses through dielectric media. We show that due to a nonlocal, quantum-mechanical effect, a surprising cancellation can occur in fourth-order interference, allowing the time interval between photons emitted in spontaneous parametric down-conversion to be measured without significant degradation of the resolution due to the spreading of the individual photon wave packets. This may also prove useful for measuring the higher-order contributions to GVD

Frequency-domain interferometer for femtosecond time-resolved phase spectroscopy

Abstract: A new femtosecond time-resolved interferometer was developed that utilizes interference fringes in the frequency domain to obtain simultaneously difference phase spectra (DPS) and difference transmission spectra with a multichannel spectrometer. For the first time to our knowledge, transient oscillations were observed in DPS and the spectral shift of a probe pulse was time resolved together with the rise in DPS, which is clear evidence for induced phase modulation in absorptive materials.

A storeless model of aliasing and its abstractions using finite representations of right-regular equivalence relations

Abstract: The problem of interference and aliasing in programming languages with structured, dynamically allocated data is studied The author starts from a novel semantic model of data aliasing, then elaborates new results in formal language theory in order to represent its invariants, and finally uses these results to derive an efficient and online algorithm for statically determining dynamic aliasing properties of structured data >

Signal-selective time-difference of arrival estimation for passive location of man-made signal sources in highly corruptive environments. II. Algorithms and performance

Abstract: For ptI see ibid, vol40, no5 p1168 (1992) For the problem of estimating time difference of arrival (TDOA) of radio waves impinging on a pair of antennas for the purpose of passively locating the source of a communications or telemetry signal in the presence of interfering signals and noise, a new class of signal-selective algorithms that are highly tolerant to interference and noise is introduced By virtue of the fact that the multiple-signal resolution problem is essentially eliminated by the signal selectivity of the algorithms, performance advantages are gained The new algorithms exhibit their signal selectivity regardless of the extent of temporal, spectral, or spatial overlap among received signals, yet their computational complexity is no more than that of conventional algorithms Algorithmic implementations of some of the new methods are given, and theoretical arguments given in ptI of this work are corroborated by quantitative evaluation of their performance >

Interference of diffusive light waves.

Abstract: We examine interference effects resulting from the superposition of photon-density waves produced by coherently modulated light incident upon a turbid medium Photon-diffusion theory is used to derive expressions for the ac magnitude and phase of the aggregate diffusive wave produced in full- and half-space volumes by two sources Using a frequency-domain spectrometer operating at 410 MHz, we verify interference patterns predicted by the model in scattering samples having optical properties similar to those of skin tissue Potential imaging applications of interfering diffusive waves are discussed in the context of the theoretical and experimental results

A gas sensor based on an integrated optical Mach-Zehnder interferometer

Abstract: In this paper we demonstrate a sensor system for measuring gaseous compounds using an integrated optical Mach-Zehnder interferometer (IO-MZ chip). One of the surface waveguide arms of the IO device is covered with a polysiloxane layer, sensitive to organic solvents. The refractive index of the polymer layer changes continuously when exposed to the vapours of such solvents. The influence on the evanescent field of the guided mode causes a change in the optical pathlength in the covered arm. Incoupled light of a semiconductor laser diode results in an interference pattern at the output of the IO-MZ chip. The phase shift of the signal is dependent on the gas concentration and the solvent type. We have determined the values for some hydrocarbons, chlorohydrocarbons and aromatic compounds. The extremely short response time of the system allows it to be used like a gas chromatographic detector. The applicationn of Maxwell's equations for the propagation of light in waveguides offers a simulation of the interference signals. By this means, the experimental signal obtained can be correlated to changes in refractive index of the polymer layer. The dependence of the interference patterns on wavelength is explained. White light interference is obtained by use of a xenon lamp.

Photoplethysmographics using phase-division multiplexing

Abstract: First and second carrier signals, distinguishable by phase, are respectively applied to infrared and red energy emitters. A detector receives the sum of the energy after modulation at the infrared and red wavelengths. The signal received by the detector is then demultiplexed into its original first and second components, thereby allowing determining of both the infrared and red modulation components. The first and second carrier signals may comprise time-varying periodic signals with identical frequency and frequency spectra, such as a pair of sine waves which are indistinguishable except by phase and amplitude. A 90° phase difference is preferred, but any phase other than 0 or an integer multiple of 180° is workable. A carrier frequency which avoids excessive interference from ambient light is preferred.

New theoretical and experimental results in fresnel optics with applications to matter-wave and X-ray interferometry

Abstract: We present new methods and formulae for calculating the image amplitude and image spatial power spectral density produced by monochromatic point-source illumination of a finite (and/or infinite) periodic complex transmission grating. At specific finite-width resonances the image amplitude is seen to display periodic complex amplitude self-imaging of the grating, with interlaced alias images. The finite width grating resonances (as a function of spatial frequency) are broadened (from zero width) and displaced in frequency relative to those produced by an infinite grating, although the finite resonance width relative to illumination wavelength variation persists with infinite gratings. In the Fresnel domain the self images are generalizations of the Talbot and von Lau effects, while in the Fraunhofer to Fresnel transition domain, our formulae demonstrate the formation of these structures from Fraunhofer diffraction order side-lobes. Using these results, design criteria are provided for constructing lens-free three-grating interferometers with spatially diffuse illumination and detection. Such interferometers have a wide variety of applications for both X-rays and matter-waves, including a phase sensitive imaging device and/or narrow-band interference filter. For wavelengths in the Angstrom to sub-Angstrom range they feature high throughput and ease of fabrication. Experimental results using light with such an interferometer are presented. Our results conclusively demonstrate interference and image aliasing in such a device with spatially diffuse illumination and detection. The experiment is readily reproducible in any undergraduate physics laboratory.

Fourier transform evaluation of interference patterns: demodulation and sign ambiguity

Abstract: The Fourier-transform method determines the interference phase distribution from an interference pattern. If evaluating only a single pattern, the sign remains unknown. Methods for interactive sign correction and for determination of the sign by two phase shifted interferograms are presented. For demodulation of the wrapped phase, a path independent algorithm is described.

Error probabilities of fast frequency-hopped FSK with self-normalization combining in a fading channel with partial-band interference

Abstract: Error probability analysis is performed for a binary orthogonal frequency-shift-keying (FSK) receiver using fast frequency-hopped (FFH) spread-spectrum waveforms transmitted over a frequency-nonselective slowly fading channel with partial-band interference. Diversity is performed using multiple hops per data, bit. A nonlinear combination procedure referred to as self-normalization combining is used by the receiver to minimize partial-band interference effects. Diversity is found to completely negate degradation of the self-normalized receiver caused by partial-band interference and offers definite receiver performance improvement when the direct signal component is weak. The self-normalized receiver is sensitive to fading channels. For severe channel fading, the performance of a conventional noncoherent binary FSK receiver is generally either equivalent or superior to that of the self-normalized receiver. >

Error probability and near-far resistance of minimum mean squared error interference suppression schemes for CDMA

Abstract: Interference suppression schemes for direct-sequence spread-spectrum (DS/SS) code division multiple access (CDMA) systems using the minimum mean squared error (MMSE) criterion are considered. When compared to other interference suppression schemes, such as maximum-likelihood decoding, this criterion has the advantage of being amenable to adaptive implementations that do not require knowledge of the interference parameters, such as their relative strengths and spreading sequences. These schemes are shown to be near-far resistant to varying degrees, depending on their complexity. That is, the error probability remains relatively low no matter how strong the interference. Numerical results showing error probability vs. interference power demonstrate that the proposed schemes offer substantial performance gains relative to the matched filter receiver. >

Direct-sequence spread spectrum in a shadowed Rician fading and land-mobile satellite channel

Abstract: The performance of a direct-sequence spread-spectrum land-mobile satellite transmission system, using binary phase-shift keying (BPSK) modulation, is analysed. The satellite channel is modeled as having Rician fading characteristics. The bit-error probability is evaluated, considering both the envelope and the phase variation. Assuming a Gaussian approximation for the interference, numerical results are obtained for both spread-spectrum and narrowband land-mobile satellite communication systems with BPSK modulation. A comparison of the two systems is made for light, average, and heavy shadowing. >

High-accuracy length-measuring interferometer using the two-colour method of compensating for the refractive index of air

Abstract: A high-accuracy length-measuring two-colour interferometer for the compensation of the refractive index of air is described. It counts two types of interference fringes which are in phase quadrature by means of a lambda /8 plate. The light source is a diode laser pumped YAG laser of about 40 mW output power; its second harmonic wave is generated by a KTiOPO4 (KTP) crystal. The resolution in determining the refractive index of air is improved by averaging the numbers of the interference fringes counted. The interferometer is evaluated to be better than 2*10-7 in accuracy.

Performance of fast frequency-hopped MFSK receivers with linear and self-normalization combining in a Rician fading channel with partial-band interference

Abstract: An error probability analysis is performed for both self-normalized and conventional M-ary orthogonal frequency-shift-keying (MFSK) noncoherent receivers using fast frequency-hopped (FFH) spread-spectrum waveforms transmitted over a Rician fading channel with partial-band interference. The self-normalization receiver uses a nonlinear combination procedure to minimize performance degradation due to partial-band interference. The performance of the conventional receiver is significantly degraded by worst-case partial-band interference regardless of the modulation order or number of hops per data symbol used, while the self-normalization receiver can provide a significant immunity to worst-case partial-band interference for many channel conditions when diversity is used, provided the signal-to-thermal-noise ratio is large enough to minimize degradation due to nonlinear combining losses. The improvement afforded by higher modulation orders is dependent on channel conditions. >

Computer-assisted holographic image formation technique which determines interference pattern data used to form the holographic

Abstract: A computer-generated hologram recording apparatus includes a diffraction image generator which receives an input image signal representing an object and computes a corresponding diffraction pattern with a first sampling density. An interpolation processor is connected to the diffraction image generator via an intermediate page memory. The interpolation processor subjects the diffraction pattern to the interpolation process to create an interpolated diffraction pattern with an increased second sampling density. An interference pattern generator is connected to the interpolation processor to compute an interference caused pattern between the interpolated diffraction pattern and a reference wave by converting amplitude and phase distributions of the input image signal into the intensity distribution. A multi-beam scan printing apparatus records the interference pattern on a previously selected recording medium for later reproduction by use of white light.

Beamforming microphone arrays for speech enhancement

Abstract: It is shown that beamforming with microphone arrays can be used to significantly reduce the effects of noise and reverberation. The Frost algorithm and a new vectorized algorithm called the row-action projection (RAP) method are shown to yield approximately 11 dB of interference suppression. Computer simulations demonstrate the performance of the beamforming techniques for various locations of interference sources. The scenarios used for the computer simulations consist of a speech signal arriving normal to the array contaminated with speech interference from an unknown direction off the normal. The direction of the desired speaker is taken to the normal to the array for the simulation; however, it could be at any specified 'look direction'. >

Extrinsic Fabry-Perot Optical Fiber Sensor

Abstract: Phase-modulated fiber optic sensors have been shown to possess high sensitivities for the measurement of strain, temperature, vibration, pressure and other parameters.1 Fabry-Perot (FP) sensors that are based on multiple beam interference eliminate the need for a reference arm and do not require sophisticated stabilization techniques as in the case of Mach-Zehnder and Michelson interferometers.2 Several techniques to create intrinsic optical fiber Fabry-Perot interferometers have been described in the past.3-5 In a recent paper, we described an optical fiber extrinsic FP interferometer and used it as a sensor of microdisplacements and thermally- induced strain.6 The fiber interferometer was classified as extrinsic because the FP cavity was an air-gap between two fiber ends and the sensor output was immune to perturbations in the input/output fiber.

Parallel N-junction superconducting interferometer with enhanced flux-to-voltage transfer function

Abstract: A physical configuration for a parallel multi-junction superconducting quantum interference device that can be used for a variety of applications involving the detection of magnetic flux, including applications where it is desired to measure the absolute magnitude of the flux. The device of this invention features a novel geometry for a multi-junction interference device which significantly enhances the flux-to-voltage transfer function, thereby yielding a significant improvement in the device sensitivity in its use in a magnetometer, gradiometer, or other applications.

Frequency hopping CDMA for future cellular radio

Abstract: A code-division multiple access (CDMA) system based on slow frequency hopping (FH) is proposed. The hopping sequences are selected such that all users of each cell are using mutually orthogonal codes, and intercell correlation of hopping sequences is minimal. FEC and interleaving are used to mitigate the remaining interference. It is shown that with proper selection of system parameters, FH-CDMA enjoys the advantages so far claimed by DS-CDMA, i.e. FH-CDMA capacity benefits from interference averaging and can take advantage of the duty cycle associated with voice activity. It also yields a sectorization capability and one-cell frequency reuse pattern. Moreover, due to the orthogonal operation, the interference from cell co-users is eliminated. Since this is the major source of interference in nonorthogonal systems like DS-CDMA, FH-CDMA yields higher capacity capabilities (35 times AMPS). Other advantages of FH-CDMA with its implementation as a cellular system are discussed. >

Manipulation of optical fields by atomic interferometry : quantum variations on a theme by young

Abstract: We analyze the principle of a very general and conceptually simple method for manipulating optical fields by coupling them into a matter waves Young double slit apparatus. The field, non resonant with the atoms, acts as a “phase-retarding” medium in one of the arms of the interferometer and shifts the atomic fringe pattern. The method constitutes a simple quantum nondemolition measuring scheme of the photon number. Non classical states such as Schrodinger cats and Fock states of the field are generated in the measurement process. The analysis of the atomic interferometer with optical “retarding” fields provides a very simple and striking illustration of basic concepts of the quantum measurement theory and of the principle of complementarity. This scheme, which would be very difficult to implement in the optical domain, is equivalent to a more feasible and recently proposed Ramsey interference method to measure small microwave fields with beams of Rydberg atoms.

Simultaneous cancellation of cross interference and ISI in CDMA mobile radio communications

Abstract: In this paper, data estimation in the uplink of a synchronous mobile radio system applying CDMA is considered. In mobile radio systems applying CDMA, multipath propagation leads on the one hand to ISI and on the other hand, together with time-variance, to cross interference between the signals of different users, regardless whether the user codes are chosen orthogonal or not. A linear unbiased data estimation algorithm is presented which cancels both ISI and cross interference perfectly by jointly detecting the different user signals, thus leading to unbiased estimates of the transmitted data symbols. This algorithm is less complex than MLSE performed e.g. by the Viterbi algorithm and furthermore, the complexity does not depend on the kind of the applied linear modulation scheme, which opens a simple way to offer services with different data rates in one system. By theoretical analysis and simulation, the performance of the unbiased data estimation algorithm is examined under the assumption that the radio channel impulse responses are known at the receiver. The price to be paid for the interference cancellation is SNR-degradations, which are calculated for typical mobile radio situations in urban areas. The resulting average uncoded bit error probabilities lead to the conclusion that systems applying the unbiased data estimation algorithm are well-suited for mobile radio applications. >

Data receiving system

Abstract: According to the present invention, a large compensation effect can be obtained in a small operation volume regardless of which one of a distortion due to a frequency selective fading and a distortion due to an interference of adjacent waves is ruling. In the present invention, a principal wave signal and signals of channels adjacent to both sides of said principal wave are taken out, and power of these signals is compared to select either one of the fractional interval equalizer and the linear decision feedback equalizer.