Showing papers on "Interference (wave propagation) published in 1995"

A framework for uplink power control in cellular radio systems

Abstract: In cellular wireless communication systems, transmitted power is regulated to provide each user an acceptable connection by limiting the interference caused by other users. Several models have been considered including: (1) fixed base station assignment where the assignment of users to base stations is fixed, (2) minimum power assignment where a user is iteratively assigned to the base station at which its signal to interference ratio is highest, and (3) diversity reception where a user's signal is combined from several or perhaps all base stations. For the above models, the uplink power control problem can be reduced to finding a vector p of users' transmitter powers satisfying p/spl ges/I(p) where the jth constraint p/sub j//spl ges/I/sub j/(p) describes the interference that user j must overcome to achieve an acceptable connection. This work unifies results found for these systems by identifying common properties of the interference constraints. It is also shown that systems in which transmitter powers are subject to maximum power limitations share these common properties. These properties permit a general proof of the synchronous and totally asynchronous convergence of the iteration p(t+1)=I(p(t)) to a unique fixed point at which total transmitted power is minimized. >

Optical multi-mode interference devices based on self-imaging: principles and applications

Abstract: This paper presents an overview of integrated optics routing and coupling devices based on multimode interference. The underlying self-imaging principle in multimode waveguides is described using a guided mode propagation analysis. Special issues concerning the design and operation of multimode interference devices are discussed, followed by a survey of reported applications. It is shown that multimode interference couplers offer superior performance, excellent tolerance to polarization and wavelength variations, and relaxed fabrication requirements when compared to alternatives such as directional couplers, adiabatic X- or Y-junctions, and diffractive star couplers. >

Interchannel interference analysis of OFDM in a mobile environment

Abstract: Orthogonal frequency division multiplexing with a suitably chosen guard interval is an effective means of eliminating intersymbol interference for high-rate transmission over fading dispersive channels. Time variations of the channel, however, lead to a loss of subchannel orthogonality, resulting in interchannel interference (ICI). The article examines the effects of ICI through analysis and simulation, in the context of a system design for HDTV digital video broadcasting to mobile receivers. It is shown that ICI can be modeled as an additive Gaussian random process that leads to an error floor which can be determined analytically as a function of the Doppler frequency. Antenna diversity and trellis coding are then examined as methods for reducing this error floor.

An algorithm for combined cell-site selection and power control to maximize cellular spread spectrum capacity

Abstract: There is much current interest in spread spectrum wireless mobile communications and in particular the issue of spread spectrum wireless capacity. We characterize spread spectrum cellular capacity and provide a combined power control, cell-site selection algorithm that enables this capacity to be achieved. The algorithm adapts users' transmitter power levels and switches them between cell-sites, and it is shown that the algorithm converges to an allocation of users to cells that is optimal in the sense that interference is minimized. The algorithm is decentralized, and can be considered as a mechanism for cell-site diversity and handover. We provide numerical examples to show how effectively the algorithm relieves local network congestion, by switching users in a heavily congested cell to adjacent, less congested cells. >

Experimental demonstration of laser oscillation without population inversion via quantum interference in Rb

Abstract: Laser oscillation without population inversion is demonstrated experimentally in a V-type atomic configuration within the ${D}_{1}\mathrm{and}{D}_{2}$ lines of Rb vapor. It is shown that the effect is due to the atomic interference. The experimental results, as first predicted by careful theoretical analysis, are in a good agreement with detailed calculations.

CDMA overlay situations for microcellular mobile communications

Abstract: Direct sequence code division multiple access communications is a promising approach to cellular mobile communications, which operates in an environment characterized by multipath Rician fading. In this paper, the CDMA network is assumed to share common spectrum with a narrowband microwave user. Because of the presence of the narrowband waveform, an interference suppression filter at each CDMA receiver is employed to reject the narrowband interference. The problem of interference from adjacent cells is also considered. Average power control is assumed to combat the near/far problem, and multipath diversity, in conjunction with simple interleaved channel coding, is considered for improving the performance of the CDMA system. >

Multitone spread spectrum multiple access communications system in a multipath Rician fading channel

Abstract: The paper proposes a new multiple access communications system based on the combination of multitone transmission and direct sequence spectrum spreading (DS/SS). Multitone transmission is proposed because the associated larger symbol duration is favorable to combat the effects of multipath propagation. The multiple access capability of the system is achieved by means of DS/SS which also helps to combat the presence of several paths. On the other hand, DS/SS is also attractive to decrease the multiple access interference. The performance of this system is investigated for a multipath, slowly Rician fading and frequency selective channel, and a suboptimal receiver based on matched filters. The multiple access interference is also taken into account, A model for the computation of the bit error probability is derived for QPSK modulation of the orthogonal tones by the spread symbols. The system performance is investigated for selection diversity and maximal ratio combining (RAKE reception). Then the influence of the number of tones is investigated for given bandwidth, bit rate and transmission power. The positive effect of a multitone spread spectrum transmission is demonstrated. >

Atom interferometer based on Bragg scattering from standing light waves.

Abstract: We have constructed an atom interferometer by Bragg deflecting a collimated beam of metastable neon atoms from three parallel standing waves. Interference fringes have been observed using atoms Bragg scattered at up to the third order, giving a maximum of $6\ensuremath{\Elzxh}k$ transverse momentum difference between the two arms of the interferometer. In the first order case we have achieved a fringe contrast of 62% and a peak to peak signal of 1700 atoms/s. We believe this to be the highest fringe contrast that has been achieved in atom interferometers.

2-Phase approach to global tool interference avoidance in 5-axis machining

Abstract: In this paper, application of convex hull is proposed to solve the problem of global tool interference avoidance in 5-axis sculptured surface machining. Two different methods for interference checking have been developed. Quick Feasibility Checking is used to find a conservative feasible tool orientation by using the control mesh of a sculptured surface. Detailed Feasibility Checking uses exact surface to detect tool interference. A correction method for removing tool interference has also been developed. In this paper both the algorithms and the implementation are presented.

Using interference in the frequency domain for precise determination of thickness and refractive indices of normal dispersive materials

Abstract: We present an experimental demonstration of the spectral interference phenomenon, using a spectrally broad white-light source in a Michelson interferometer configuration. We also present a theoretical analysis of the spectral interference law for the general case of amplitude-division interferometers. When a dispersive medium is introduced into one of the arms of the interferometer, the spectral interference pattern changes drastically, with a change in the frequency modulation corresponding to the dispersive nature of the medium. A zero-order fringe appears at a wavelength where the net path difference between the two arms of the interferometer is zero. We relate the thickness of the dispersive medium to the width of the zero-order fringe. From the experimental data over the entire visible region of the spectrum we obtain the refractive index n(λ) and the thickness t of the dispersive medium, calculated to an accuracy of the order of 10−5.

Bandwidth efficient QPSK in cochannel interference and fading

Abstract: The performances of QPSK in the presence of cochannel interference in both nonfading and fading environments are analyzed. Three approaches for representing the cochannel interference are investigated. These are a precise error probability method, a sum of sinusoids (sinusoidal) model, and a Gaussian interference model. In addition to determining precise results for the performance of QPSK in cochannel interference, we examine the validity of these two interference models in both additive white Gaussian noise (AWGN) environments and in different flat fading environments; Rayleigh, Ricean, and Nakagami. Nyquist pulse shaping is considered and the effects of cross channel ISI produced by the cochannel interference are accounted for in the precise interference model. Also accounted for are the random symbol and carrier timing offsets of the interfering signals. Two performance criteria are considered. These are the average bit error rate and the interference penalty. The latter is defined as the increase in signal-to-noise power ratio (SNR) required by a system with cochannel interference in order to maintain the same BER as a system without interference. Attention is given, in particular, to the outdoor microcellular fading environment. In this environment, the fading experienced by the interfering signals may be represented by a Rayleigh-fading model while the fading experienced by the desired signal may be represented by a Ricean or a Nakagami-fading model. >

Subarray configurations for digital beamforming with low sidelobes and adaptive interference suppression

Abstract: The paper investigates how subarrays in a large phased array should be arranged. The aim is to form sum and difference beams with low sidelobes with and without adaptation against external interference. At each array element phase shifting and amplitude tapering for one beam with low sidelobes is assumed. The other beams (e.g. difference beams) have to be formed digitally at the subarray outputs. Problems with grating lobes are considered.

Experimental results of localization of moving underwater signal by adaptive beamforming

Abstract: The problem of weak moving signal localization and tracking in the presence of single motionless strong interference is investigated using real data of an underwater experiment in the Baltic sea (Sept. 1990) with a horizontal receiving array of 64 hydrophones and with two independent powerful narrowband sources imitating the signal and interference. Three simple adaptive beamforming methods were employed for the experimental data processing. The first one is based on the well-known projection approach to adaptive beamforming, the second method uses the adaptive canceler approach (also termed the dipole pattern method), and the third method combines these approaches. The signal-to-interference power ratio (SIR) threshold of the signal localization and tracking is evaluated by a special technique, which allows examination of the considered algorithms with change of the SIR in consecutive order. The results of the data processing show the high possibilities of signal localization in the presence of strong interference. The combined method performs better than the methods considered and enables localization of the signal source up to an SIR/spl sime/-25 dB. >

Spread spectrum modulation communication apparatus for narrow band interference elimination

Abstract: A wireless communication apparatus employing spread spectrum modulation which detects and eliminates interference waves and temporarily stops and restarts communication for maintaining fine communication status. If the interference wave frequency band is known, a reception signal of the frequency band is eliminated. Communication is temporarily halted and restarted corresponding to interference occurrence period and/or radiowave propagation condition, and communication status is notified to a user. Further, if the interference wave occurrence period is unknown, the frequency of the interference wave is detected and a signal of the detected frequency is eliminated. In a case where communication is performed in packet form, the number of packets which can be transmitted sequentially without receiving reception confirmation is controlled based on the radiowave propagation condition. Moreover, a reception signal can be divided into a plurality of frequency band signals and interference wave detection can be performed in band signal units.

Demonstration of a power-recycled Michelson interferometer with Fabry-Perot arms by frontal modulation.

Abstract: Large-scale gravitational-wave detectors currently under construction such as the LIGO detectors use multiple-mirror resonant optical systems containing several surfaces at which the relative phase of interfering light beams must be controlled. We describe a tabletop experiment that demonstrates a scheme for extracting signals in such an interferometer corresponding to deviations from perfect interference.

Young's double-slit interferometry within an atom.

Abstract: An experiment is described which is an analog of Young's double-slit interferometer using an atomic electron instead of light. Two phase-coherent laser pulses are used to excite a single electron into a state of the form of a pair of Rydberg wave packets that are initially on opposite sides of the orbit. The two wave packets propagate and spread until they completely overlap, then a third phase-coherent laser pulse probes the resulting fringe pattern. The relative phase of the two wave packets is varied so that the interference produces a single localized electron wave packet on one side of the orbit or the other.

Characterisation and modelling of artificial light interference in optical wireless communication systems

Abstract: Wireless indoor infrared transmission systems are affected by noise and interference induced by natural and artificial ambient light. While the shot noise induced on the receiver photodiode by steady ambient light has been extensively described and included in system models, the interference produced by artificial light has only been mentioned as a source of degradation and quite simple descriptions have been presented. This paper presents a characterisation (through extensive measurements) of the interference produced by artificial light and proposes a simple model to describe it. These measurements show that artificial light can introduce significant in-band components for systems operating at bit rates up to several Mbit/s. Therefore it is essential to include it as part of the optical wireless indoor channel. The measurements show that fluorescent lamps driven by solid state ballasts produce the wider band interfering signals, and are then expected to be the more important source of degradation in optical wireless systems.

A frequency diversity spread spectrum system for communication in the presence of in-band interference

Abstract: Protection against in-band interference is required in antijam systems and when low level spread spectrum signals are overlaid on top of existing narrow-band users. We propose a secure spread-spectrum system whose optimum receiver is easy to build. We use frequency diversity which allows the receiver to distinguish unjammed signal replicas from their jammed versions. The system can also resist bandlimited partial-time jamming. The only choice left to a wise jammer to maximize our error probability is to spread his signal like ours. We derive the optimum receiver which jointly performs symbol detection and interference rejection. Side information, needed by this receiver, can easily be estimated. However, if the interference bandwidth is narrow enough compared to the signal bandwidth, side information is not needed by a simpler and near-optimum receiver. The bit error probability is evaluated for QPSK modulation and compared with that of direct sequence spread-spectrum.

Picture-word interference increases with target-set size

Abstract: In the picture-word interference task the naming of a picture is hampered by the presence of a distractor word that is to be ignored. Two main components of this interference effect can be distinguished: an interference effect induced by an unrelated distractor word in comparison with a nonword control, and an additional interference effect that is due to a semantic similarity between target and distractor (calledsemantic interference). We examine whether the size of these two interference effects is affected by the number of different target pictures in an experiment. The results show that both interference effects increase with the size of the target set. This finding has two implications. First, at an empirical level, the use of a relatively small number of target pictures may account for remarkably small, or even nonsignificant, picture-word interference effects in a number of previous studies. Second, at a theoretical level, the present finding is in accordance with a name-retrieval account of picture-word interference.

Determination of horizontal and vertical structure of an unusual pattern of short period gravity waves imaged during ALOHA-93

Abstract: An all-sky CCD imager has been used to measure the properties of short period gravity waves present over the Hawaiian Islands during the ALOHA-93 campaign. Observations of emissions from four different altitudes provided a capability to describe the vertical as well as the horizontal structure of the wave field. On several occasions during this campaign an unusual morphology wave pattern was detected that consisted of a group of small-scale waves oriented in the same direction. These were most noticeable in the OI (557.7 nm) emission, altitude ∼96 km, and were usually observed in association with a larger scale gravity wave. This paper presents a preliminary analysis of data recorded on the night of 22 October during which both types of waves were prominent. The small-scale waves exhibited highly coherent phase structures at each emission altitude, consistent with a ducted wave motion. The spatial intensity and phase modulation of this display is indicative of interference between two waves with similar characteristics and slightly different propagation directions. The larger scale wave motion was observed to propagate perpendicular to the small-scale waves, and showed evidence of phase progression with altitude, implying upward energy propagation. These data have been interpreted in the context of simultaneous wind measurements from an MF radar.

Optical parametric loop mirror

Abstract: A novel configuration for four-wave mixing (FWM) is proposed that offers the remarkable feature of inherently separating the FWM wave from the input pump and signal waves and suppressing their background amplified stimulated emission without optical filtering. In the proposed configuration, an optical parametric loop mirror, two counterpropagating FWM waves generated in a Sagnac interferometer interfere with a relative phase difference that is introduced deliberately. FWM frequency-conversion experiments in a polarization-maintaining fiber achieved more than 35 dB of input-wave suppression against the FWM wave.

Performance of a coded multi-carrier DS-CDMA system in multi-path fading channels

Abstract: Multi-carrier DS-CDMA has been considered as an effective scheme for reducing multiple access interference in quasi-synchronous transmission. The scheme allows the reduction of multiple access interference by transferring the orthogonality property of the signals into the frequency domain where the orthogonality property is robust to relative chip offsets between the spreading codes of the various users. However in multi-path channels, the multi-carrier technique results in frequency non-selective fading in the sub-channels, due to the narrower bandwidth, hence a reduction of the capability of the spread spectrum signal to mitigate the effect of multi-path propagation. In this paper, we consider the use of a Reed-Muller code with soft decision decoding to regain the corresponding loss in performance, and compare the resulting system with a single carrier DS-CDMA system. The effect of system parameters such as the number of sub-channels is investigated through numerical calculation and simulation, from which a number of system design criteria are arrived at.

Interchannel interference in multiwavelength operation of integrated acousto-optical filters and switches

Abstract: Multiple wavelength filtering/switching can be obtained using integrated acousto-optical wavelength filters and switches in LiNbO/sub 3/ by driving them with several RF signal simultaneously. As a consequence, interchannel interference occurs due to the interaction of several acoustic waves within the same mode converter resulting in an intensity modulation of the transmitted optical waves. System performance can be degraded as this interchannel interference leads to a power penalty equivalent to an additional cross talk. We present the coupled-mode analysis of the interchannel interference in the multiwavelength operation of acousto-optical filters and switches. Theoretical models are developed and confirmed by experimental results. Furthermore, the impact on WDM system performance is studied by measuring bit error rate characteristics and comparing the results with the theory. >

Fringe visibility, irradiance, and accuracy in common path interferometers for visualization of phase disturbances.

Abstract: Common-path interferometers have been used to perform phase visualization for over 40 years. A number of techniques have been proposed, including dark central ground, phase contrast (π/2 and π), and field-absorption interferometers. The merits of the interferometers have been judged ad hoc by either tests with a small number of phase objects or by computer simulation. Three standardized criteria, which consolidate the work of others, are proposed to evaluate common-path interferometers: fringe visibility, fringe irradiance, and fringe accuracy. The interferometers can be described as one generic class of Fourier-plane filters and can be analyzed for all input conditions. Closed-form expressions are obtained for visibility and irradiance under the forced condition that little inaccuracy is tolerated. This analysis finds that the π-phase-contrast interferometer is a good choice if the optical phase disturbance is at least 2π; for smaller disturbances, the Π/2 filter selected by Zernike is near optimum. It is shown mathematically that the resulting fringe visibility is highly object dependent, and good results are not ensured. By allowing the optical beam to be 50% larger than the phase object, the interferometer performs well under all conditions. With this approach and a combination π-phase/field-absorption filter, interference fringe visibility is greater than 0.8 for all phase objects.

Adjacent cell interference analysis of reverse-link in CDMA cellular radio systems

Abstract: The effect of adjacent cell interference on the system capacity is investigated. Hexagonal and circular cell approaches are used. The amount of interference from the six adjacent cells of the first tier is obtained by a numerical method in the hexagonal cell approach. We use a circle to approximate the hexagonal cell and a closed-form solution is obtained. By the circular cell approach, the adjacent cell interference function is derived. The total adjacent cell interference is found by the adjacent cell interference function obtained. Generalization of the analysis to an irregular cell structure is also presented.

Performance of FH SS radio networks with interference modeled as a mixture of Gaussian and alpha-stable noise

Abstract: This paper considers the performance of FH SS radio networks in a Poisson field of interfering terminals using the same modulation and power. Assuming 1/r/sup m/ attenuation of signal strength over distance, the interference in the network is modeled as a mixture of Gaussian and circularly symmetric /spl alpha/-stable noise. The problem is relevant to mobile communication systems where the mobility of users requires random modeling of transmitter positions in the network. We derive formulas for the probability distributions of the interference in FH SS radio networks. We generalize the results of Sousa (see IEEE Trans. I.T., vol.38, no.6, p.1743-1754, 1992) where attention was limited to Cauchy random variables, a special subclass of stable distributions, and where the effect of a background (Gaussian) noise was neglected. Based on the formulas derived, we calculate the probability of symbol error for radio links in environments varying from urban settings to office buildings. The results obtained allow the prediction of the performance of wireless systems under a wide range of conditions.

Modulated interference effects: use of photoelastic modulators with lasers

Abstract: When photoelastic modulators (PEMs) are used with lasers as light sources, modulated interference effects may appear as spurious signals at the fundamental and harmonic frequencies of the PEM. They are correlated with the modulator reference signal and are at precisely the same frequencies as the polarization modulation effects being studied. This modulated interference does not appear to be a problem with any light source other than lasers. The modulated interference effects arise because of interference between light reflected at the surfaces of the modulator optical element and the primary beam and relative motion of these two surfaces synchronized with the modulator oscillations. Interference in modulator optical elements, which is similar to multiple beam interference in thin films, is examined. Criteria for estimating the strength of the modulated interference are presented and a simple test for the presence of modulated interference in an optical system that includes a PEM is described. Several strategies are presented for reducing or eliminating these troublesome effects including (1) careful positioning of the PEM, (2) use of antireflective (AR) coatings, and (3) techniques that physically separate the primary beam from multiple reflected beams. Data are given for suppression of the modulated interference by using AR coatings and for one method of beam separation.