Showing papers on "Interference (wave propagation) published in 1999"

Principles of Optics

Abstract: The book is comprised of 15 chapters that discuss various topics about optics, such as geometrical theories, image forming instruments, and optics of metals and crystals. The text covers the elements of the theories of interference, interferometers, and diffraction. The book tackles several behaviors of light, including its diffraction when exposed to ultrasonic waves.

Iterative (turbo) soft interference cancellation and decoding for coded CDMA

Abstract: The presence of both multiple-access interference (MAI) and intersymbol interference (ISI) constitutes a major impediment to reliable communications in multipath code-division multiple-access (CDMA) channels. In this paper, an iterative receiver structure is proposed for decoding multiuser information data in a convolutionally coded asynchronous multipath DS-CDMA system. The receiver performs two successive soft-output decisions, achieved by a soft-input soft-output (SISO) multiuser detector and a bank of single-user SISO channel decoders, through an iterative process. At each iteration, extrinsic information is extracted from detection and decoding stages and is then used as a priori information in the next iteration, just as in turbo decoding. Given the multipath CDMA channel model, a direct implementation of a sliding-window SISO multiuser detector has a prohibitive computational complexity. A low-complexity SISO multiuser detector is developed based on a novel nonlinear interference suppression technique, which makes use of both soft interference cancellation and instantaneous linear minimum mean-square error filtering. The properties of such a nonlinear interference suppressor are examined, and an efficient recursive implementation is derived. Simulation results demonstrate that the proposed low complexity iterative receiver structure for interference suppression and decoding offers significant performance gain over the traditional noniterative receiver structure. Moreover, at high signal-to-noise ratio, the detrimental effects of MAI and ISI in the channel can almost be completely overcome by iterative processing, and single-user performance can be approached.

Linear multiuser receivers: effective interference, effective bandwidth and user capacity

Abstract: Multiuser receivers improve the performance of spread-spectrum and antenna-array systems by exploiting the structure of the multiaccess interference when demodulating the signal of a user. Much of the previous work on the performance analysis of multiuser receivers has focused on their ability to reject worst case interference. Their performance in a power-controlled network and the resulting user capacity are less well-understood. We show that in a large system with each user using random spreading sequences, the limiting interference effects under several linear multiuser receivers can be decoupled, such that each interferer can be ascribed a level of effective interference that it provides to the user to be demodulated. Applying these results to the uplink of a single power-controlled cell, we derive an effective bandwidth characterization of the user capacity: the signal-to-interference requirements of all the users can be met if and only if the sum of the effective bandwidths of the users is less than the total number of degrees of freedom in the system. The effective bandwidth of a user depends only on its own SIR requirement, and simple expressions are derived for three linear receivers: the conventional matched filter, the decorrelator, and the MMSE receiver. The effective bandwidths under the three receivers serve as a basis for performance comparison.

Area spectral efficiency of cellular mobile radio systems

Abstract: A general analytical framework quantifying the spectral efficiency of cellular systems with variable-rate transmission is introduced. This efficiency, the area spectral efficiency, defines the sum of the maximum average data rates per unit bandwidth per unit area supported by a cell's base station. Expressions for this efficiency as a function of the reuse distance for the worst and best case interference configurations are derived. Moreover, Monte Carlo simulations are developed to estimate the value of this efficiency for average interference conditions. Both fully loaded and partially loaded cellular systems are investigated. The effect of random user location is taken into account, and the impact of lognormal shadowing and Nakagami (1960) multipath fading is also studied.

Interference cancellation in W-CDMA cellular structures using statistical processing

Abstract: A statistical processing method is considered for interference suppression in W-CDMA systems. The major first-order statistic of interference, namely, the pdf function is estimated by use of higher-order statistics (HOS) of the received samples. This estimate, in turn, leads to deriving the best non-linear processor of the samples for enhanced detection performance of the desired user. A mathematical model for the process is presented and the relative asymptotic efficiency of the detector with respect to the conventional linear detector is computed. Some numerical results are also provided to show the achievable enhancement by the proposed nonlinear processing as applied to UMTS-based receivers.

Adaptive antenna arrays for OFDM systems with cochannel interference

Abstract: Orthogonal frequency-division multiplexing (OFDM) is one of the promising techniques for future mobile wireless data systems. For OFDM systems with cochannel interference, adaptive antenna arrays can be used for interference suppression. This paper focuses on a key issue for adaptive antenna arrays, that is, parameter estimation for the minimum mean square error (MMSE) diversity combiner (DC). Using the instantaneous correlation estimation approach developed in the paper, an original parameter estimator for the MMSE-DC is derived. Based on the original estimator, we propose an enhanced parameter estimator. Extensive computer simulation demonstrates that the MMSE-DC using the proposed parameter estimators can effectively suppress both synchronous and asynchronous interference in OFDM systems for packet and continuous data transmission.

Spin-interference device

Abstract: We propose a spin-interference device which works even without any ferromagnetic electrodes and any external magnetic field. The interference can be expected in the Aharonov–Bohm (AB) ring with a uniform spin-orbit interaction, which causes the phase difference between the spin wave functions traveling in the clockwise and anticlockwise direction. The gate electrode, which covers the whole area of the AB ring, can control the spin-orbit interaction, and therefore, the interference. A large conductance modulation effect can be expected due to the spin interference.

Interference and Interaction in Multiwall Carbon Nanotubes

Abstract: We report equilibrium electric resistance R and tunneling spectroscopy dI/dV measurements obtained on single multiwall nanotubes contacted by four metallic Au fingers from above. At low temperature quantum interference phenomena dominate the magnetoresistance. The phase-coherence and elastic-scattering lengths are deduced. Because the latter is of order of the circumference of the nanotubes, transport is quasi-ballistic. This result is supported by a dI/dV spectrum which is in good agreement with the density-of-states (DOS) due to the one-dimensional subbands expected for a perfect single-wall tube. As a function of temperature T the resistance increases on decreasing T and saturates at approx. 1-10 K for all measured nanotubes. R(T) cannot be related to the energy-dependent DOS of graphene but is mainly caused by interaction and interference effects. On a relatively small voltage scale of order 10 meV, a pseudogap is observed in dI/dV which agrees with Luttinger-Liquid theories for nanotubes. Because we have used quantum diffusion based on Fermi-Liquid as well as Luttinger-Liquid theory in trying to understand our results, a large fraction of this paper is devoted to a careful discussion of all our results.

Phase-coherent amplification of atomic matter waves

Abstract: Atomic matter waves, like electromagnetic waves, can be focused, reflected, guided and split by currently available passive atom-optical elements. However, the key for many applications of electromagnetic waves lies in the availability of amplifiers. These active devices allow small signals to be detected, and led to the development of masers and lasers. Although coherent atomic beams have been produced1,2,3,4, matter wave amplification has not been directly observed. Here we report the observation of phase-coherent amplification of atomic matter waves. The active medium is a Bose–Einstein condensate, pumped by light that is far off resonance. An atomic wave packet is split off the condensate by diffraction from an optical standing wave, and then amplified. We verified the phase coherence of the amplifier by observing interference of the output wave with a reference wave packet. This development provides a new tool for atom optics and atom interferometry, and opens the way to the construction of active matter-wave devices.

Method and system for profiling objects having multiple reflective surfaces using wavelength-tuning phase-shifting interferometry

Abstract: The invention features methods and systems for interferometrically profiling a measurement object having multiple reflective surfaces, e.g., to profile a selected one of the multiple reflective surfaces. The methods and systems involve: positioning the measurement object within an unequal path length interferometer (e.g., a Fizeau interferometer) employing a tunable coherent light source; recording an optical interference image for each of multiple wavelengths of the light source, each image including a superposition of multiple interference patterns produced by pairs of wavefronts reflected from the multiple surfaces of the measurement object and a reference surface; and extracting phases of a selected one of the interference patterns from the recorded images by using a phase-shifting algorithm that is more sensitive (e.g., at least ten times more sensitive) to a wavelength-dependent variation in the recorded images caused by the selected interference pattern than to wavelength-dependent variations in the recorded images caused by the other interference patterns.

A 10 Gb/s optical code division multiplexing using 8-chip optical bipolar code and coherent detection

Abstract: bOptical code division multiplexing (OCDM) using optical bipolar code and coherent detection is a new multiplexing method for future fiber-optic communication networks. Some optical bipolar codes, which are described by the combination of phase of optical chip pulses, with a repetition frequency 10 GHz are generated experimentally. Temporal matched filtering for the 10 Gb/s OCDM is demonstrated using optical encoder and decoder, and their fundamental properties are investigated. Effect of interference code is shown experimentally and discussed. The novel coherent detection system with autocorrelation sidelobe suppression using balanced detector and local light source is proposed. The pseudocoherent detection is demonstrated experimentally by using a clock pulse. Advantageous properties of the coherent detection are shown. The fundamental functions to realize the gigabit rate OCDM system is revealed.

Adaptive multistage parallel interference cancellation for CDMA

Abstract: Although the multistage interference cancellation detector is simple in structure, its performance degrades when the number of active users becomes large. In some cases, the performance is even worse than that without cancellation, due to the lack of the exact knowledge of the interfering signal in cancellation. Partial interference cancellation suggested by Divsalar and Simon (see IEEE Trans. Commun., vol.46, p.258-68, 1998) tries to remedy this weakness by reducing the cost of a wrong interference estimation through a weight in each stage. This paper presents an adaptive multistage structure based on the partial interference cancellation approach. In this structure, the weights are obtained by minimizing the mean-square error between the received signal and its estimate through a least mean square (LMS) algorithm. The resulting weights contain reliability information for the hard decisions made in the previous stage. Neither a training sequence nor a pilot signal is needed in the proposed scheme, and its complexity is much lower than that of linear multiuser detectors. Simulation results show that the proposed scheme can outperform some of the existing interference cancellation methods in both the additive white Gaussian noise (AWGN) and the multipath fading channels.

Frequency locking a laser to an optical cavity by use of spatial mode interference.

Abstract: We present a novel technique to frequency lock a laser to an optical cavity. This technique, tilt locking, utilizes a misalignment of the laser with respect to the cavity to produce a nonresonant spatial mode. By observing the interference between the carrier and the spatial mode one can obtain a quantum-noise-limited frequency discriminator. Tilt locking offers a number of potential benefits over existing locking schemes, including low cost, high sensitivity, and simple implementation.

Optical properties of planar colloidal crystals: Dynamical diffraction and the scalar wave approximation

Abstract: We present a quantitative comparison between two analytic theories for the propagation of electromagnetic waves in periodic dielectric structures. These theories have both been used extensively in the modeling of optical spectra of colloidal crystals exhibiting photonic band gap behavior. We demonstrate that dynamical diffraction theory is equivalent to the scalar wave approximation, in the limit of small dielectric contrast. This equivalence allows us to place quantitative limits on the validity of dynamical diffraction, relative to the predictions of the more accurate scalar wave theory. We also note that dynamical diffraction is often applied with boundary conditions which neglect the strong interference between the incident and diffracted waves within the periodic medium. These boundary conditions lead to expressions for the transmission spectrum which cannot be generalized to the case of normal-incidence propagation. We provide a corrected form for these expressions, and use them in comparisons with experimental spectra. Excellent agreement between theory and experiment is obtained for the widths of optical stop bands, for both positive and negative values of the dielectric contrast. These are among the first quantitative comparisons between theoretical and experimental optical spectra of colloidal photonic crystals.

Single-Photon Interference in Sidebands of Phase-Modulated Light for Quantum Cryptography

Abstract: We report single-photon interference in the sidebands of modulated light. The relative phase of interacting quantum states is reliably controlled by the phase of a low-frequency modulating signal. We show how this type of interference can be used to build a robust system for quantum cryptography. An experiment was implemented at 1540 nm, over a 20-km-long standard single-mode fiber, using a germanium singlephoton avalanche photodiode.

Review of laboratory experiments on Alfvén waves and their relationship to space observations

Abstract: Hannes Alfven predicted the existence of a hydrodynamic wave in a perfectly conducting fluid in 1942. It took 6 years before this discovery was accepted and 10 years before Alfven waves were first observed in the laboratory. Now it is widely recognized that these waves are ubiquitous in space plasmas and are the means by which information about changing currents and magnetic fields are communicated. Alfven waves have been observed in the solar wind, are thought to be prevalent in the solar corona, may be responsible for parallel electric fields in the aurora, and can cause particle acceleration over large distances in interstellar space. They have also been considered as a candidate for heating thermonuclear plasmas and are potentially dangerous to confinement. Alfven waves have been difficult to observe in basic laboratory experiments because of their low frequencies and long wavelengths. In this paper we present a review of plasma Alfven wave experiments performed in recent years. The quality of the laboratory data have paralleled advances in plasma sources and diagnostics. In the past few years the quantum jump in data collection on the Freja and FAST missions have lead to the reevaluation of the importance of these waves in the highly structured plasma that was probed. Recent laboratory experiments have examined, in great detail, shear waves generated by filamentary currents in both spatially uniform and striated plasmas. Tone bursts, short pulses, and interference effects have been studied with emphasis on structures of the order of the skin depth, c/ωpe. These are features of significant interest to the space community. In fact, it appears that the phenomena observed in laboratory experiments show striking similarities to what has been observed in space. A comparison of these results will be given.

Optimum interference excision in spread spectrum communications using open-loop adaptive filters

Abstract: A generalized approach for interference suppression in PN spread spectrum communications using open-loop adaptive excision filtering is introduced. The excision filter coefficients under this technique depend on the jammer power and its instantaneous frequency (IF) information, and both values can be gained in the time-frequency domain. The dependency of the excision filter characteristics on the interference power, which was absent in past contributions in this area, is of significant importance as it allows optimum tradeoff between interference removal and the amount of self-noise generated from the induced correlation across the PN chip sequence, due to filtering. This tradeoff is bounded by the two extreme cases of no self-noise, which implies preprocessing disabled, and full interference excision, which the case previously considered. In this paper, we derive the FIR excision filters that maximize the receiver signal-to-noise ratio for narrowband interference and discuss the generalization to nonstationary jamming environment.

Optical frequency-domain imaging microprofilometry with a frequency-tunable liquid-crystal Fabry-Perot etalon device.

Abstract: An optical frequency-domain interference microscope with a liquid-crystal Fabry–Perot interferometer as an optical frequency-scan device was developed for microscopic three-dimensional shape measurements. The proposed system can perform absolute measurement of the discontinuous surface profile of a microscopic object without use of mechanically moving components such as a piezoelectric transducer or a grating spectrometer. Experimental results are presented that demonstrate the validity of the principle.

Observation of Four-Photon Interference with a Beam Splitter by Pulsed Parametric Down-Conversion

Abstract: When four photons arrive at a beam splitter, two from each side, a four-photon, six-path interference effect occurs to yield a sixfold enhancement of the probability for all four photons to exit together from the beam splitter. We produce the four-photon state by using the stimulated emission process in a pulsed parametric down-conversion and measure the probability for all four photons to exit from one side of the beam splitter. The observed enhancement factor is in good agreement with a multimode treatment of pulsed down-conversion.

Adaptive interference suppression in multiuser wireless OFDM systems using antenna arrays

Abstract: This paper considers the problem of mitigating fading and interference in wireless orthogonal frequency division multiplexing (OFDM) multiple access communication systems. Applications include cellular mobile radio, wireless local loop, and wireless local area networks. The effect of interchannel interference (ICI) arising from time-selective fading and frequency offsets and co-channel interference (CCI) is analyzed. A loop-timing method that enables a synchronous uplink between multiple mobile transceivers and a base-station is described. Adaptive antenna arrays are utilized at the base for uplink reception, and optimum array combining based on the maximum SINR criterion is used for each subchannel over slowly time-varying channels. For operation over fast time-varying channels, a novel two-stage adaptive array architecture that incorporates combined spatial diversity and constraint-based beamforming is presented. While ICI alone is most effectively overcome by spatial diversity, combined beamforming and diversity are most effective to combat CCI in the presence of fading. The overall method is suitable for real-time implementation and can be used in conjunction with traditional coding schemes to increase the link-margin.

Method and system for power control in wireless networks using interference prediction with an error margin

Abstract: An enhanced power control method for use in a wireless packet-switched network having an interference prediction algorithm which includes an error margin. In particular, the method can measure an interference power and a path gain between an intended receiver and transmitter. Based upon the past performance of the network, a future interference value may be predicted by using a prediction algorithm. Furthermore, based upon the prior accuracy of the interference prediction, the method can also estimate an error margin for the interference prediction. Finally, a transmission power for the transmitter can be calculated using the predicted interference power, the estimated error margin for the predicted interference power, the path gain, and the target SINR.

Optimum space-time processors with dispersive interference: unified analysis and required filter span

Abstract: We consider optimum space-time equalizers with unknown dispersive interference, consisting of a linear equalizer that both spatially and temporally whitens the interference and noise, followed by a decision-feedback equalizer or maximum-likelihood sequence estimator. We first present a unified analysis of the optimum space-time equalizer, and then show that, for typical fading channels with a given signal-to-noise ratio (SNR), near-optimum performance can be achieved with a finite-length equalizer. Expressions are given for the required filter span as a function of the dispersion length, number of cochannel interferers, number of antennas, and SNR, which are useful in the design of practical near-optimum space-time equalizers.

Reflectance measurements of layered media with diffuse photon-density waves: a potential tool for evaluating deep burns and subcutaneous lesions.

Abstract: The basic principles of a non-contact, near-infrared technique for the mapping of layered tissues are discussed theoretically and verified experimentally The propagation properties of diffuse photon-density waves in tissues depend on the optical properties of the tissue When a layered medium is irradiated by amplitude modulated light, the difference in optical properties between the layers is evident in the phase and amplitude of the diffuse reflection coefficient, which is a result of the interference of the partial waves propagating in the different layers Thus, diffuse photon-density waves are applicable to the analysis of the structure of layered tissue The probing depth is determined by the modulation frequency of the incident light For modulation frequencies between several hundred megahertz and a few gigahertz, this allows us to analyse the properties of muscle tissue of up to 4-8 mm below the surface Experimental results based on chicken breast muscle are given As an example, the technique might be of use for evaluating the depth of necrosis and the blood volume fraction in deep burns

Quantum interference by a nonlocal double slit

Abstract: We report an interference experiment in which photon pairs generated by spontaneous parametric down-conversion produce a Young-type fourth-order interference pattern after being scattered by two different and spatially separated apertures, whose superposition defines a double slit. The experiment is compared with previous ones based on parametric down-conversion, and its nonlocal nature is discussed. A theoretical explanation is also provided.

Effects of frequency offset on MC/CDMA system performance

Abstract: The effects of frequency offset on a multicarrier code-division multiple-access system are theoretically analyzed and verified by computer simulations for the downlink channel. Both equal gain combining and maximal ratio combining are considered in combining subcarrier signals in the analysis.

Unequal error protection for digital broadcasting using channel classification

Abstract: The invention provides methods and apparatus for processing information, e.g., audio, video or image information, for transmission in a communication system. In an illustrative embodiment, interference characteristics are determined for a set of n channels to be used to transmit audio information bits, where n is greater than or equal to two. The audio information bits are separated into n classes based on error sensitivity, for example, the impact of errors in particular audio data bits on perceived quality of an audio signal reconstructed from the transmission. The classes of bits are then assigned to the n channels such that the classes of bits having the greatest error sensitivity are transmitted over the channels which are the least susceptible to interference. The interference characteristics associated with the n channels can be determined by, for example, measuring interference levels for one or more of the channels, or obtaining information regarding known interference levels for one or more of the channels. The channels may correspond to different frequency bands, time slots, code division slots or any other type of channels. The invention can provide UEP for different classes of audio information bits even in cases in which the same convolutional code, or the same complementary punctured pair convolutional (CPPC) code pair, is used to encode the classes. The assignment of the classes of bits to the channels, as well as the characteristics of the classes and the channels, may be fixed or dynamic.

Performance of optimum and suboptimum combining at the antenna array of a W-CDMA system

Abstract: The performance of an array of antennas combined with a RAKE receiver at the base station of a wideband code division multiple access system with quaternary phase shift keying modulation is investigated for single-user reception. This receiver structure is also known as a multidimensional RAKE receiver. Expressions for the interference correlation matrix are derived, and subsequently, an analysis of the error probability is provided. With regard to antenna weights, the optimum combining linear receiver, which takes into account the spatial distribution of all the users in the system, is compared with the suboptimum combining receiver (SOC), which exploits knowledge of the desired user array vector only. It is shown how the two receivers, ideal in the sense that perfect channel estimation is assumed, achieve very similar performances under realistic conditions. Consequently, the investigated pilot assisted combining receiver, which estimates the channel based on limited overhead, calculates weights according to the SOC strategy by a very simple algorithm.

Twin waveguide based design for photonic integrated circuits

Abstract: An asymmetric twin waveguide (ATG) structure is disclosed that significantly reduces the negative effects of inter-modal interference in symmetric twin-waveguide structures and which can be effectively used to implement a variety of optical devices. The ATG structure of the invention can be monolithically fabricated on a single epitaxial structure without the necessity of epitaxial re-growth. To achieve the ATG structure of the invention, the effective index of the passive waveguide in the ATG is varied from that of a symmetric twin waveguide such that one mode of the even and odd modes of propagation is primarily confined to the passive waveguide and the other to the active waveguide. The different effective indices of the two coupled waveguides result in the even and odd modes becoming highly asymmetric. As a result, the mode with the larger confinement factor in the active waveguide experiences higher gain and becomes dominant. In a further embodiment, the active waveguide is tapered to reduce coupling losses of the optical energy between the passive waveguide and the active waveguide. In a further embodiment, a grating region is incorporated atop the passive waveguide to select certain frequencies for transmission of light through the passive waveguide.

Reverse link capacity and interference statistics of a fixed-step power-controlled DS/CDMA system under slow multipath fading

Abstract: Power control (PC) on the reverse link of a direct-sequence code-division multiple-access system is essential to increase system capacity. Perfect PC eliminates fluctuations in the received signal level and hence reduces the required signal-to-interference ratio. However, a perfect PC algorithm tracks multipath fading accurately, which results in increasing the intercell interference level. A fixed step PC algorithm becomes almost perfect when the power command rate is too fast compared to the Doppler rate, which is the case for low-mobility users. We investigate the statistics of the intercell interference assuming users are moving slowly. These statistics are then used to find the system capacity. Three parameters that can affect the capacity are considered: the number of the fading process resolvable paths, the maximum transmitted power, and soft handoff.

Iterative multiuser detection using antenna arrays and FEC on multipath channels

Abstract: This paper investigates a multiple-access communication receiver system that receives coded data modulated using either direct-sequence code division multiple access or narrowband binary phase shift keying, with an antenna array in a multipath propagation environment. We describe an iterative receiver that improves the initial estimates from the antenna array, and therefore reduces the multiple access interference. Simulation results show that the bit error rate performance approaches that obtained when only one user's signal is incident on the array. This occurs even with a large number of users in comparison to the product of the spreading gain and array size.