Showing papers on "Interference (wave propagation) published in 1996"

Performance of multicarrier DS CDMA systems

Abstract: In this paper, we apply a multicarrier signaling technique to a direct-sequence CDMA system, where a data sequence multiplied by a spreading sequence modulates multiple carriers, rather than a single carrier. The receiver provides a correlator for each carrier, and the outputs of the correlators are combined with a maximal-ratio combiner. This type of signaling has the desirable properties of exhibiting a narrowband interference suppression effect, along with robustness to fading, without requiring the use of either an explicit RAKE structure or an interference suppression filter. We use bandlimited spreading waveforms to prevent self-interference, and we evaluate system performance over a frequency selective Rayleigh channel in the presence of partial band interference; we also compare system performance with that of a single-carrier RAKE system.

Spectral Line Elimination and Spontaneous Emission Cancellation via Quantum Interference

Abstract: Quantum interference in spontaneous emission from a four-level atom is investigated. The atom has two upper levels coupled by the same vacuum modes to a common lower level and is driven by a coherent field to an auxiliary level. Interference can lead to the elimination of a spectral line in the spontaneous emission spectrum and spontaneous emission cancellation in steady state.

Can Two-photon Interference be Considered the Interference of Two Photons?

Abstract: We report on a ``postponed compensation'' experiment in which the observed two-photon entangled state interference cannot be pictured in terms of the overlap of the two individual photon wave packets of a parametric down-conversion pair on a beam splitter. In the sense of a quantum eraser, the distinguishability of the different two-photon Feynman amplitudes leading to a coincidence detection is removed by delaying the compensation until after the output of an unbalanced two-photon interferometer.

Digital recording and numerical reconstruction of holograms: reduction of the spatial frequency spectrum

Abstract: Direct recording of Fresnel holograms on a charge-coupled device and their numerical reconstruction is possible if the maximum spatial frequency of the holographic microstructure is adapted to the spatial resolution of the detector array. The maximum spatial frequency is determined by the angle between the interfering waves. For standard CCDs with spatial resolutions of ;100 lines/mm, the angle between ref- erence and object wave is limited to a few degrees. This limits the size of the objects to be recorded or requires a great distance between object and CCD target. A method is described in which the primary object angle is optically reduced so that objects with larger dimensions can be re- corded. The principle is demonstrated for the example of deformation analysis. Two Fresnel holograms, which represent the undeformed and the deformed states of the object, are generated on a CCD target, stored electronically, and the wave fields are reconstructed numerically. The interference phase can be calculated directly from the digital holograms, without generating an interference pattern. As an application of this method, we present the transient deformation field of a plate that is loaded by an impact. © 1996 Society of Photo-Optical Instrumentation Engineers. Subject terms: digital holography; hologram interferometry; numerical hologram reconstruction; charge-coupled devices.

Continuous observation of interference fringes from Bose condensates

Abstract: We use continuous measurement theory to describe the evolution of two Bose condensates in an interference experiment. It is shown how in a single run the system evolves into a state with a fixed relative phase, without violating particle number conservation.

Analysis of adaptive multistage interference cancellation for CDMA using an improved Gaussian approximation

Abstract: We consider a simple model for adaptive multistage interference cancellation within a CDMA system, and seek to develop an accurate analytical expression for the performance of this system. Previous work on interference cancellation has relied heavily on simulation techniques or a simple Gaussian approximation (GA). The standard GA ran lead to bit-error rate (BER) results which are optimistic for the conventional receiver, and this also occurs when the approximation is applied to the interference cancellation problem. Additionally, this approximation does not allow the second order effects of the multiple access interference (MAI) to be included in the performance estimates. Several improvements on the standard GA have been suggested which result in accurate performance results for a standard CDMA receiver. This paper presents an analytical expression for the probability of bit error for an adaptive multistage interference canceller, using an improved Gaussian approximation (IGA) for MAI. The BER at any stage of interference cancellation can be recursively computed from the signal-to-noise ratio (SNR), the statistics of the random powers of users, and the processing gain of the CDMA system. The performance of the resulting EER expression is compared with simulation results. Since the second order effects of MAI can be included, the analytical framework presented here can also be used to evaluate the performance of multistage interference cancellation in arbitrary fading environments, and we present results for the performance of interference cancellation in lognormal fading environments.

Heterodyne interferometer with two‐wave mixing in photorefractive crystals for ultrasound detection on rough surfaces

Abstract: Heterodyne interferometers using two‐wave mixing in photorefractive cubic crystals for ultrasound detection on rough surfaces are demonstrated. The speckled scattered beam from a rough surface sample interferes with a planar coherent pump beam inside a photorefractive crystal. A third frequency‐shifted beam is used to read the grating. The diffracted readout beam and the transmitted signal beam are wavefront matched, resulting in an optimal heterodyne interference signal. The signal to noise ratio for the two commonly used crystallographic configurations with cubic crystals, G∥〈110〉∥ and G∥〈001〉, where G is the grating wave vector, are investigated. Very good sensitivity is demonstrated for the detection of small amplitude ultrasonic surface displacements.

Subspace based estimation of the signal to interference ratio for CDMA cellular systems

Abstract: The signal-to-interference ratio (SIR) has been highlighted in the literature to be a most efficient criterion for several methods aiming at reducing the effects of cochannel interference, e.g. diversity reception, dynamic channel allocation and power control. In this paper we address the problem on how to obtain fast and accurate measurements of this parameter in a practical context. We develop a general SIR estimation technique for narrow-band cellular systems, that is based on a signal subspace approach using the sample covariance matrix of the received signal. Simulation results for a GSM-like system show that the SIR can be estimated to within an error of 0.3 dB after only 200 ms, or within an error of 0.1 dB after only 0.6 seconds.

Two different philosophies in CDMA-a comparison

Abstract: There has been considerable interest recently in the subject of multiuser interference cancellation techniques for CDMA wireless systems. It is widely believed that a system design using interference cancellation techniques has considerably better capacity than the design methodology adopted in the IS-95 CDMA cellular standard, which does not use such techniques. We examine this issue, and in particular focus on linear interference cancellation techniques. Linear techniques are simpler and require fewer assumptions, such as accurate knowledge of amplitude and phase information of the users to be canceled, than nonlinear interference cancellers. However, linear interference cancellation techniques require a fundamentally different CDMA system design philosophy than IS-95. In IS-95, the coding gain from error control codes bears the burden of handling interference, while a linear canceller uses the dimensional separation of users. This paper compares and contrasts the two design philosophies. Our comparison is based on overall user capacity (defined as the number of users per Hz per unit area) as well as robustness and fairness. The focus of the paper is on the mobile-user to base station reverse link which is modeled by the classical many-to-one multiple-access problem.

Increasing the capacity of a cellular radio network

Abstract: A cellular radio network and a method for increasing traffic carrying capacity in a cellular network in which the operating frequency spectrum of the cellular network has been divided so that typically both regular frequencies and super-reuse frequencies are employed in each cell. The regular frequencies use a conventional frequency reuse pattern to provide seamless overall coverage (overlay). A very tight frequency reuse pattern is used for the super-reuse frequencies to provide additional capacity (underlay). The cellular network controls the division of traffic into regular and super-reuse frequencies by radio resource allocation at the call set-up phase and later on during the call by handover procedure. The cellular network continuously monitors the downlink co-channel interference of each super-reuse frequency in the cell separately for each ongoing call. The call is handed over from a regular frequency to a super-reuse frequency when the co-channel interference level on the super-reuse frequency is sufficiently low. When the co-channel interference level on the super-reuse frequency deteriorates, the call is handed over from the super-reuse frequency back to the regular frequency.

Performance of infrared transmission systems under ambient light interference

Abstract: Optical transmission systems are mainly impaired by the shot noise induced by ambient light, the transmitted optical power limitations (high path losses), the channel bandwidth limitations owing to multipath dispersion and the interference produced by artificial light sources. Several modulation and encoding schemes have been proposed for this channel and their performance has been studied and presented by several authors while neglecting the effects of the artificial light interference. The work reported extends the previous analysis by taking into account the optical power penalty induced by artificial light interference. An analytical approach is used to estimate this. In practical systems, the effect of the interference is usually mitigated using electrical highpass filters. In the paper the combined effect of interference and highpass filter is evaluated. The presented results show that the interference produced by fluorescent lamps driven by electronic ballasts induce high power penalties in OOK and L-PPM systems, even when electrical highpass filtering is used, for data rates up to 10 Mbit/s. For the interference produced by incandescent lamps and fluorescent lamps driven by conventional ballasts, the power penalty induced in OOK systems can be effectively reduced using highpass filtering, while PPM is very tolerant to that interference even without any highpass filtering. The major conclusion is that artificial light interference have to he considered both in system design and performance evaluation.

Diversity receiver and its control method

Abstract: Enabling reception quality to be improved and the capacity in terms of the number of simultaneous users in a cell to be increased, and speeding up the establishment of spreading code synchronization and the convergence of weighted coefficients, by feedback controlling weighted coefficients such that desired signal power to interference power ratio (SIR) becomes maximum. A coherent adaptive diversity configuration is employed to which absolute coherent detection is applicable. Adaptively feeding weighted coefficients back to diversity branches makes it possible to reduce the influence of the interference power from other users, and thus to increase the capacity in terms of the number of simultaneous users in the cell. Furthermore, the direction that provides the maximum received SIR is determined while rotating antennas in a state of having directivity with a predetermined angle range. Setting initial values of the weighted coefficients at values corresponding to that direction enables the weighted coefficients to converge quickly.

Interference rejection combining for GSM

Abstract: Digital cellular communication systems in urban areas are today beginning to be limited in performance by interference. To further increase the capacity in these areas, more advanced detection schemes could be used. We study how the performance of the MLSE equalizer can be improved by utilizing interference suppression. Simulation results for a GSM system under realistic conditions are presented.

Linear multipath-decorrelating receivers for CDMA frequency-selective fading channels

Abstract: Analytical expressions for the error probability of linear multipath-decorrelating receivers with coherent and differentially coherent reception are derived. Both multi-user receivers have superior performance compared to the RAKE receiver and eliminate error probability floor caused by multiple-access interference on a code-division multiple access (CDMA) reverse link. Results also emphasize benefits of using coherent multi-user reception with maximal-ratio combining in frequency-selective fading channels.

All-fiber three-path Mach-Zehnder interferometer

Abstract: We report the realization of a three-path Mach-Zehnder interferometer using single-mode fibers and two integrated 3 x 3 fiber couplers. We observed enhanced phase sensitivity, as compared with two-path interferometers, with a visibility of the interference pattern of more than 97%. This interferometer has an analog in two-photon interferometry, and we believe it to be the first nontrivial example of N x N multiport interferometers.

Method and apparatus for eliminating interference using transmission delay and code multiplexing in digital radio system

Abstract: At a transmitter side, signals are providcd with a delay difference with a plurality of branches, coded multiplexed by spectrum spreaders and a combination unit, and transmitted by single antenna. At a receiver side, the signals are received by a single antenna, and diversity branches are extracted and separated by first and second spectrum de-spreaders. They are subject to linear combination so that the mean square of the decision error signals is minimized. The output passes through an adaptive matched filter and an adaptive equalizer so as to provide an output from which interference waves are eliminated. Thus, interference is eliminated and diversity gain for a signal is ensured, while reducing the scale and cost of a system without using a plurality of antennae.

Measurement of subpicosecond electron pulses.

Abstract: A bunch-length measuring method has been developed to measure the subpicosecond electron pulses generated at the Stanford University Short Intense Electron Source (SUNSHINE) facility. This method utilizes a far-infrared Michelson interferometer to measure coherent transition radiation emitted at wavelengths longer than or equal to the bunch length via optical autocorrelation. To analyze the measurement, a simple and systematic way has also been developed, which considers interference effects on the interferogram caused by the beam splitter; hence the electron bunch length can be easily obtained from the measurement. This simple, low-cost, frequency-resolved autocorrelation method demonstrates subpicosecond resolving power that cannot be achieved by existing time-resolved methods.

Absolute interferometer measuring process and apparatus having a measuring interferometer, control interferometer and tunable laser

Abstract: An absolute measuring interferometer having a measuring interferometer, a tunable laser emitting a laser beam and a control interferometer for adjusting the air wavelength of the laser beam. The control interferometer adjusts the air wavelength of the laser beam to a specific wavelength value at the ends of each measuring cycle. The wavelength of the tunable laser is continually tuned within the specific wavelength interval where the phase change of the interference signal is continually detected during the wavelength modulation process. An absolute measurement is determined by a simple mathematical relationship between the measured wavelength and phase changes.

Iterative interference cancellation for FFH/MFSK MA systems

Abstract: Iterative interference cancellation is introduced to improve system spectral efficiency in fast frequency hopping multiple access systems. The proposed algorithm iteratively cancels interference from users whose symbols have already been estimated. Both an interference-only channel and a frequency selective fading channel are considered. Noncoherently detected M-ary frequency shift keying is employed and all users are time-aligned at the receiver. Analytical results and results obtained from computer simulations are presented. It is shown that iterative interference cancellation may either substantially reduce the bit error probability or allow for a large increase in the number of simultaneously transmitting users.

Phase control of absorption in large polyatomic molecules

Abstract: The phenomenon of interference of the amplitude for absorption of one photon of frequency 3ω and the amplitude for absorption of three photons of frequency ω was theoretically predicted by Shapiro, Hepburn, and Brumer. The interference was demonstrated experimentally by varying the relative phase between the tripled frequency photon and three photons with the fundamental frequency by the groups of Elliott and Gordon in atoms and small molecules. In order to see how general this phenomenon is, five compounds were studied, ammonia, trimethylamine, triethylamine, cyclooctatetraene, and 1,1‐dimethylhydrazine. CH3I was used as the tripling gas for light in the range 604–600 nm. Interference was observed in all cases. The last four compounds have low ionization potentials and interference was observed between a 3+1 and a 1+1 ionization process with a maximum modulation of 22%. NH3 with a higher ionization potential requires absorption of 3+2 or 1+2 photons and exhibits a maximum modulation of 33%. We conclude that molecular size is no obstacle and that as long as a molecule has sufficiently strong absorption at the tripled frequency, and sufficient vapor pressure, and the laser fundamental beam is very strong, phase control of interference is observable.

Pulse coding techniques for ST/MST radar systems: a general approach based on a matrix formulation

Abstract: An approach based on matrix formulation used for the representation and analysis of the signals and processes in ST/MST radar systems using a series of polyphase code sequences is presented. Expressions of the appropriate signals at various stages of the transmitting/receiving and processing channels of the system are given, using mostly matrix formulation, which has the advantage, among other things, of compactness. The expressions of the powers of the various components of the received signal, including the geophysical signal and interference signals, are obtained in order to derive expressions for performance parameters such as those related to the degradation of signal-to-noise ratio in the truncated ranges and interference suppression. Analysis tools for obtaining optimum solutions to the full decoding of truncated ranges, interference suppression, and reduction of sidelobes resulting from atmospheric characteristics are developed.

Interference issues in multi-code CDMA networks

Abstract: Multi-code direct-sequence code division multiple access (MC-CDMA) has been proposed as a flexible multiple access scheme for wireless packet networks that support a large variety of mobiles with different and even time-varying rates. However, the interference model typically used with conventional DS-CDMA cellular systems - a large number of low-power mobiles uniformly distributed in cells - may not hold. In MC-CDMA cellular systems, it is possible for a high-rate mobile, perhaps near the boundary of a cell, to have all the codes for high-rate transmission - therefore transmitting at high power and causing significant interference to the neighboring cell. In this study, we analyze the effects of MC-CDMA intercell interference on the reverse link (i.e., mobile to cell site) and investigate a maximum capacity power allocation (MCPA) criterion that exploits the sub-code concatenation property of MC-CDMA transmission. When the transmission rate is high, power control with MCPA criterion can significantly reduce reverse-link MC-CDMA intercell interference. However, with only four codes per mobile (e.g., as in IS-95), the reduction in intercell interference is less than 1 dB.

Appliance for measuring polarization mode dispersion and corresponding measuring process

Abstract: An appliance for measuring polarization mode dispersion (PMD) of a waveguide (2) comprises a source (1, 6) of wide-band polarized light, an interferometer (5) capable of receiving a light beam (21) sent by the source (1, 6), a detector (3) capable of detecting the light beam (25) from the interferometer (5), and an electronic processing unit (4) connected to the detector (3), capable of extracting a value τ representative of the polarization mode dispersion of the waveguide (2). The interferometer (5) divides a measuring light beam (22) sent by the source (1) into two interference light beams (23, 24). The measuring appliance comprises at least one birefringent element (17) positioned on the interferometer (5), capable of producing an algebraic difference of relative phase shifts in each of the two interference light beams (23, 24) equal to π.

Localization of light in three-dimensional random dielectric media

Abstract: A systematic approach to the localization of light waves in three-dimensional dielectric media is developed. A general definition of localization of electromagnetic waves is proposed and its consequences are elaborated. A significant amount of localization of the energy density of the electromagnetic field is predicted in finite systems of randomly distributed dielectric particles modeled by dipoles linearly coupled to the electric field of the incident wave. Although in this case it is not possible to achieve perfect localization, the predicted phenomenon is experimentally indistinguishable from a complete localization. Our approach is based directly on the Maxwell equations; the vector character of the electromagnetic waves is fully taken into account. The concepts presented in our previous paper [M. Rusek and A. Or\l{}owski, Phys. Rev. E 51, R2763 (1995)] are now generalized to the three-dimensional case. Instead of using the Kirchhoff integral formula for scalar waves, we now analyze light scattering by pointlike dielectric particles as the special case of general considerations dealing with elastic scattering of electromagnetic waves by arbitrary localized charges and currents.

Crosstalk and interference penalty in all-optical networks using static wavelength routers

Abstract: The crosstalk and interference penalty in an all-optical network using static wavelength routers is analyzed in this paper. A worst case methodology is used to derive the upper bound of the penalty. We show that the penalty strongly depends on the linewidth of the laser source. Up to -20 dB in crosstalk can be tolerated in a moderate-size network (/spl ap/10/sup 5/ nodes), with the ratio of the laser linewidth to the electrical bandwidth less than or equal to unity. Larger linewidth has the advantage of reducing the power penalty incurred by phase-to-amplitude noise conversion. However, the number of wavelength channels will he reduced as well. The maximum tolerable component crosstalk for a network with arbitrary size is reduced to -30 dB.

Coherent Frequency Shift of Atomic Matter Waves.

Abstract: We demonstrate how Bragg diffraction of atomic matter waves at a time-modulated thick standing light wave can be used to coherently shift the de Broglie frequency of the diffracted atoms. The coherent frequency shift is experimentally confirmed by interferometric superposition of modulated and unmodulated atoms resulting in time-dependent interference fringes. Our frequency shifter for atomic matter waves is a generalization of an acousto-optic frequency shifter for photons. [S0031-9007(96)01871-6]

Apparatus for rapid interference cancellation and despreading of a CDMA waveform

Abstract: A receiver performs rapid adaptive interference canceling for use in despreading multiple CDMA channels sharing the same RF front end. The receiver includes a buffer (22) for providing for overlapping time samples and rate adaptation, a windowing function (24) for improving interference rejection capability and a FFT (26) for calculating the input power spectrum. The receiver further includes a circuit (28, 30) to whiten the input power spectrum. The whitened power spectrum is multiplied (32) against the frequency domain version of different spreading sequences and the inverse FFT (40) of the product is performed. The output of this inverse FFT is buffered (44) to provide multiple despread output channels, with a plurality of code phases for further processing.

Joint power control and beamforming for capacity improvement in wireless networks with antenna arrays

Abstract: The interference reduction capability of antenna arrays and the power control algorithms have been considered separately as means to increase the capacity in wireless communication networks. The minimum variance beamformer maximizes the carrier to interference ratio (CIR) when it is employed in the receiver of a wireless link. In a system with omnidirectional antennas, power control algorithms are used to maximize the CIR as well. We consider a system with power control capabilities which uses antenna arrays at the receivers. An iterative algorithm is proposed to jointly update the transmission powers and the beamformer weights so that it converge to the jointly optimal beamforming and transmission power vector. The algorithm is distributed and uses only local interference measurements. In an uplink transmission scenario it is shown how base assignment can be incorporated in addition to beamforming and power control such that a globally optimum solution is obtained. The network capacity increase and the saving in mobile power achieved by beamforming are evaluated through a numerical study.

CHAPTER 4 – Phase Imaging

Abstract: This chapter focuses on phase imaging techniques developed for the confocal scanning optical microscope (CSOM) and interference optical microscope. The phase information can be extracted from the fringe structure of the interferogram. Conventional optical microscopes measure only the amplitude component of an optical wave by detecting the reflected or transmitted intensity. However, other important information can be obtained by measuring its phase. The extremely high frequency of an optical wave inhibits the direct measurement of its phase with a single light beam. Instead, phase images are formed by converting the phase information into amplitude variations or by heterodyning the signal so that the phase appears on a carrier at a lower frequency. In phase-contrast imaging, both the amplitude and phase changes of the sample appear as intensity (or contrast) changes in the image. This technique makes it possible to observe small changes in the thickness and refractive index of biological samples. The chapter concludes with a brief review on heterodyne phase imaging techniques.

Detection of space-time fluctuations by a model matter interferometer

Abstract: In papers on primary state diffusion (Percival 1994, 1995), numerical estimates suggested that fluctuations in the space-time metric on the scale of the Planck time (10^-44s) could be detected using atom interferometers. In this paper we first specify a stochastic metric obtained from fluctuations that propagate with the velocity of light, and then develop the non-Markovian quantum state diffusion theory required to estimate the resulting decoherence effects on a model matter interferometer. Both commuting and non-commuting fluctuations are considered. The effects of the latter are so large that if they applied to some real atom interferometry experiments they would have suppressed the observed interference. The model is too crude to conclude that such fluctuations do not exist, but it does demonstrate that the small numerical value of the Planck time does not alone prevent experimental access to Planck-scale phenomena in the laboratory.