Showing papers on "Interference (wave propagation) published in 2002"

Double-slit quantum eraser

Abstract: We report a quantum eraser experiment which actually uses a Young double slit to create interference. The experiment can be considered an optical analogy of an experiment proposed by Scully, Englert, and Walther [Nature (London) 351, 111 (1991)]. One photon of an entangled pair is incident on a Young double slit of appropriate dimensions to create an interference pattern in a distant detection region. Quarter-wave plates, oriented so that their fast axes are orthogonal, are placed in front of each slit to serve as which-path markers. The quarter-wave plates mark the polarization of the interfering photon and thus destroy the interference pattern. To recover interference, we measure the polarization of the other entangled photon. In addition, we perform the experiment under ``delayed erasure'' circumstances.

On the UWB system coexistence with GSM900, UMTS/WCDMA, and GPS

Abstract: This paper evaluates the level of interference caused by different ultra-wideband (UWB) signals to other various radio systems, as well as the performance degradation of UWB systems in the presence of narrowband interference and pulsed jamming. The in-band interference caused by a selection of UWB signals is calculated at GSM900, UMTS/wideband code-division multiple-access (WCDMA), and Global Position System (GPS) frequency bands as a function of the UWB pulsewidth. Several short-pulse waveforms, based on the Gaussian pulse, can be used to generate UWB transmission. The two UWB system concepts studied here are time hopping and direct sequence spread spectrum. Baseband binary pulse amplitude modulation is used as the data modulation scheme. Proper selection of pulse waveform and pulsewidth allows one to avoid some rejected frequency bands up to a certain limit. However, the pulse shape is also intertwined with the data rate demands. If short-pulses are used in UWB communication the high-pass filtered waveforms are preferred according to the results. The use of long pulses, however, favors the generic Gaussian waveform instead. An UWB system suffers most from narrowband systems if the narrowband interference and the nominal center frequency of the UWB signal overlap. This is proved by bit-error rate simulations in an additive white Gaussian noise (AWGN) channel with interference at global system for mobile communication (GSM) and UMTS/WCDMA frequencies.

A nonlinear adaptive method of elimination of power line interference in ECG signals

Abstract: A new method of elimination of power line noise in electrocardiogram signals is presented. The proposed method employs, as its main building block, a recently developed signal processing algorithm capable of extracting a specified component of a signal and tracking its variations over time. Design considerations and performance of the proposed method are presented with the aid of computer simulations. Superior performance is observed in terms of effective elimination of noise under conditions of varying powerline interference frequency. The proposed method presents a simple and robust structure which complies with practical constraints involved in the problem such as low computational resource availability and low sampling frequency.

Wireless systems and interference avoidance

Abstract: Motivated by the emergence of programmable radios, we seek to understand a new class of communication system where pairs of transmitters and receivers can adapt their modulation/demodulation method in the presence of interference to achieve better performance. Using signal to interference ratio as a metric and a general signal space approach, we present a class of iterative distributed algorithms for synchronous systems which results in an ensemble of optimal waveforms for multiple users connected to a common receiver (or colocated independent receivers). That is, the waveform ensemble meets the Welch (1974) bound with equality and, therefore, achieves minimum average interference over the ensemble of signature waveforms. We derive fixed points for a number of scenarios, provide examples, look at ensemble stability under user addition and deletion as well as provide a simplistic comparison to synchronous code-division multiple-access. We close with suggestions for future work.

Performance of ultra-wideband communications in the presence of interference

Abstract: We analyze the performance of ultra-wideband (UWB) communications in the presence of interference. Closed-form expressions are provided for the jam resistance of UWB with binary pulse position modulation utilizing rectangular pulses. A simple approximation is obtained for the special case of tone interference. The jam resistance analysis is extended to more practical UWB waveforms such as Gaussian and Rayleigh monocycles. A comparison between the interference suppression capabilities of UWB and direct-sequence spread-spectrum (DS-SS) is carried out under conditions similar to both systems. It is shown that in most cases, the jam suppression of UWB is superior to that of DS-SS.

Multiuser detection for DS-CDMA UWB in the home environment

Abstract: We demonstrate the effectiveness of multiuser detection for an ultra-wideband (UWB) pulse based direct sequence spread spectrum system using code division multiple access. Extensive simulations were run using channel soundings of the 2-8 GHz band collected in a residential setting and characterized by a high level of multipath fragmentation. We show that the adaptive minimum mean square error (MMSE) multiuser detection (MUD) receivers are able to gather multipath energy and reject intersymbol and interchip interference for these channels to a much greater extent than RAKE receivers with 4 and 8 arms. We also demonstrate the adaptive MMSE is able to reject a narrowband IEEE 802.11a OFDM interferer, even for signal-to-interference ratio as severe as -30 dB. We show the adaptive MMSE exhibits only a 6 dB penalty relative to the single user case for the heavy multi-access interference (number of asynchronous users equal to spreading code length). The practical RAKE receivers were incapable of effectively rejecting either the strong narrowband interference or the heavily loaded wideband interference. Even more moderate levels of interference caused significant degradation in the performance of the practical RAKE receivers.

Beamforming In Acoustic Testing

Abstract: Non-intrusive measurements of wind tunnel model noise can be made in non-acoustic, hardwall wind tunnels, as well as free jets and other challenging settings, by using beamforming techniques with sparse, wide-band phased arrays of microphones The process, which was developed for this application at Boeing in the last few years, begins by measuring a full array cross-spectral matrix for each frequency of interest A grid of potential source locations in the test section is defined A complex array steering vector is computed for each grid point with non-uniform flow, microphone imperfections, and installation effects taken into account Beamforming combines the cross-spectral matrices and the steering vectors to produce maps of the wind tunnel model’s acoustic source distribution Several beamforming algorithms are described with intuitive motivation and expected performance Practical details of steering vector construction are presented Techniques for removing various types of interference are detailed

All-digital impulse radio with multiuser detection for wireless cellular systems

Abstract: Impulse radio is an ultrawideband system with attractive features for baseband asynchronous multiple-access, multimedia services, and tactical wireless communications. Implemented with analog components, the continuous-time impulse radio multiple-access model utilizes pulse-position modulation and random time-hopping codes to alleviate multipath effects and suppress multiuser interference. We introduce a novel continuous-time impulse radio transmitter model and deduce from it an approximate one with lower complexity. We also develop a time-division duplex access protocol along with orthogonal user codes to enable impulse radio as a radio link for wireless cellular systems. Relying on this protocol, we then derive a multiple-input/multiple-output equivalent model for full continuous-time model and a single-input/single-output model, for the approximate one. Based on these models, we finally develop design composite linear/nonlinear receivers for the downlink. The linear step eliminates multiuser interference deterministically and accounts for frequency-selective multipath while a maximum-likelihood receiver performs symbol detection. Simulations are provided to compare performance of the different receivers.

Electronic security systems and active implantable medical devices.

Abstract: IRNICH, W.: Electronic Security Systems and Active Implantable Medical Devices. How do active implantable medical devices react in the presence of strong magnetic fields in the frequency range between extremely low frequency (ELF) to radiofrequency (RF) as they are emitted by electronic security systems (ESS)? There are three different sorts of ESSs: electronic article surveillance (EAS) devices, metal detector (MDS) devices, and radiofrequency identification (RFID) systems. Common to all is the production of magnetic fields. There is an abundance of literature concerning interference by ESS gates with respect to if there is an influence possible and if such an influence can bear a risk for the AIMD wearers. However, there has been no attempt to study the physical mechanism nor to develop a model of how and under which conditions magnetic fields can influence pacemakers and defibrillators and how they could be disarmed by technological means. It is too often assumed that interference of AIMD with ESS is inevitable. Exogenous signals of similar intensity and rhythm to heart signals can be misinterpreted and, thus, confuse the implant. Important for the interference coupling mechanism is the differentiation between a “unipolar” and a “bipolar” system. With respect to magnetic fields, the left side implanted pacemaker is the most unfavorable case as the lead forms approximately a semicircular area of maximum 225 cm2 into which a voltage can be induced. This assumption yields an interference coupling model that can be expressed by simple mathematics. The worst-case conditions for induced interference voltages are a coupling area of 225 cm2 that is representative for a large human, a homogeneous magnetic field perpendicular to the area formed by the lead, and a unipolar ventricular pacemaker system that is implanted on the left side of the thorax and has the highest interference sensitivity. In bipolar systems the fields must be 17 times larger when compared to a unipolar system to have the same effect. The magnetic field for interfering with ICDs must be 1.7 stronger than that of the most sensitive unipolar pacemaker. The lowest interference thresholds measured over the last 10 years in the low frequency range (16 2/3 Hz–24 kHz) together with thresholds > 24 kHz that were supplied by the CETECOM study are listed. Both sets of data together with the coupling model, allow for judging which fields of ESSs could influence AIMDs. From measurements at gate antennas, it is possible to derive a “maximum allowed field” curve over the whole frequency range, below which no interference will occur. Comparison of data from literature with these maximum allowed fields confirm the correctness of the calculations. Thus, it is possible to predict interference situations in gates if the magnetic field is known. If all future pacemakers were to have the immunity against interference of the better 50% of today's pacemakers, the magnetic field ceiling values could be at least four times higher. The same is true if the ventricular sensitivity is routinely set at 7 mV. Pacemaker manufacturers should consider filter improvement with modern technology, but gate manufacturers should not claim the privilege of being out of bounds.

Adaptive detector arrays for optical communications receivers

Abstract: An optimal adaptive array receiver for use in groundbased optical communications is investigated. Kolmogorov phase screen simulations are used to generate realistic focal-plane distributions of the received optical fields in the presence of turbulence. The array detection concept reduces interference from background radiation by effectively assigning higher confidence levels at each instant of time to those detector elements that contain significant signal energy and suppressing those that do not. A simpler suboptimum structure that replaces the continuous weighting of the optimal receiver by a hard decision over each detector element is also described. It is shown that, for photon counting receivers observing Poisson distributed signals, performance improvements of up to 5 dB can be obtained over conventional single-detector photon counting receivers when observing turbulent optical fields in high background environments.

Multirate optical fast frequency hopping CDMA system using power control

Abstract: This paper addresses the problem of real-time multimedia transmission in fiber-optic networks using code division multiple access (CDMA). We present a multirate optical fast frequency hopping CDMA (OFFH-CDMA) system architecture using fiber Bragg gratings (FBGs). In addition, we argue that, in multimedia applications, different services have different quality of service (QoS) requirements; hence, the user only needs to use the minimum required power to transmit the signal, such that the required signal-to-interference ratio (SIR) is met. We show that a variable bit rate optical communication system with variable QoS can be implemented by way of power control with great efficiency. Present-day multirate optical CDMA systems concentrate on finding the code structure that supports a variable rate system, neglecting the importance of the transmission power of active users on the multiple access interference (MAI) and, therefore, on the system capacity. We assign different power levels to each rate through a power control algorithm using variable optical attenuators, which minimizes the interference and, at the same time, provides variable QoS constraints for different traffic types. Although we are using a code family that preserves good correlation properties between codes of different lengths, simulations show a great improvement in the system capacity when power control is used.

Code-constrained blind detection of CDMA signals in multipath channels

Abstract: In this letter, a constant-modulus-algorithm-based multiuser detection scheme is proposed for a communication system under multipath propagation. To mitigate channel distortion and multiuser interference, we integrate multiple constraints into the optimization criterion. According to our analysis, the ability of the detector to remove all interference is ensured in the absence of noise when the constraints are properly preselected. However, in the presence of noise, the constraints highly affect the performance of the receiver. In order to optimally combine signals from different paths to achieve performance gains, those constraints can also be treated as variables and jointly optimized with the receiver, as verified by numerical examples.

Iterative interference cancellation for high spectral efficiency satellite communications

Abstract: The problem of efficient utilization of the frequency spectrum for satellite systems is investigated; one which results as a consequence of highly crowding adjacent channels. An analytical characterization of the resulting interference channel is introduced and then exploited for interference cancellation. Two classes of cancelers are investigated. The first approach does not benefit from the forward error control (FEC) coding information which limits the performance gain. This motivates the second approach where a joint implementation of interference cancellation and decoding is developed using soft-input-soft-output (SISO) modules along with the iterative structure. It is shown that iterative interference cancellation techniques can achieve significant gains compared with the single-user matched filter receiver.

Spatial filtering of RF interference in radio astronomy

Abstract: We investigate spatial filtering techniques for interference removal in multichannel radio astronomical observations. The techniques are based on the estimation of the spatial signature vector of the interferer from short-term spatial covariance matrices followed by a subspace projection to remove that dimension from the covariance matrix, and by further averaging. The projections will also modify the astronomical data, and hence a correction has to be applied to the long-term average to compensate for this. As shown by experimental results, the proposed technique leads to significantly improved estimates of the interference-free covariance matrix.

Power control by interference prediction for broadband wireless packet networks

Abstract: A Kalman-filter method for power control is proposed for broadband, packet-switched time division multiple access wireless networks. By exploiting the temporal correlation of co-channel interference, a Kalman filter is used to predict future interference power. Based on the predicted interference and estimated path gain between the transmitter and receiver, the transmission power is determined to achieve a desired signal-to-interference-plus-noise ratio (SINR). A condition to ensure power stability in the packet-switched environment is established and proven for a special case of the Kalman-filter method. The condition generalizes the existing one for a fixed path-gain matrix, as for circuit-switched networks. Performance results reveal that the Kalman-filter method for power control provides a significant performance improvement. Specifically, when messages consist of ten packets on average, the 90th and 95th percentile of the SINR by the new method are 3.79 dB and 5.46 dB above those when no power control is in use, and lie just 0.96 dB and 1.14 dB below the upper-bound performance of the optimal power control, respectively, in a system with four-sector cells and an interleaved frequency assignment of a reuse factor of 2/8. In addition, the new method performs noticeably better than the delta-modulation method and a simple scheme that uses the last measurement as predicted interference power. In an example of 8-PSK modulation and average message length of 20 packets, the SINR performance gain by the new method improves the network throughput by about 150% and 70%, relative to no power control and the simple scheme, respectively.

Interferometer with angled beam entry

Abstract: An interferometer has an angled beam entry to a multimode waveguide that can be formed of parallel plane mirrors. The beam is input at a specified primary angle α so as to generate interference between populated modes. The primary angle preferably satisfies the condition α>arcsin(λ/d), where d is a width of the waveguide and λ is a wavelength of the primary beam. The interferometer can provide a small distance between transmission fringes and is useful for precise position measurements. The fringe positions are determined by wavelength, which permits use in spectrum analysis and the like. The waveguide can include an active optical material and be controlled, for example for switching between output channels.

Successive interference cancellation algorithms for downlink W-CDMA communications

Abstract: In this paper, successive intracell interference cancellation (IIC) of the wideband-code division multiple access (W-CDMA) signal at the mobile unit is considered. Three new interference cancellation techniques suitable for the downlink of any CDMA system with orthogonal spreading are proposed. No prior knowledge of users' spreading codes or even their spreading factors are required for interference cancellation. A new term, effective spreading code, has been introduced, which is defined as the interfering user physical code as seen by the desired user within the desired user symbol duration. The mobile receiver estimates the effective spreading codes of the interfering users regardless of their spreading factors using fast Walsh transform (FWT) correlators (instead of the regular correlators) and uses this information to suppress the intracell multiuser interference. Three different interference-suppressing techniques are studied: subtraction; combined interfering signal projection; and separate interfering signal subspace projection. The complexity of the proposed techniques is low compared to conventional interference cancellation techniques. For a W-CDMA system and the IMT-2000 vehicular channel model, a capacity increase of up to 150% of the original (without IIC) system capacity is shown.

Updating of form factor tabulations for coherent scattering of photons in tissues.

Abstract: An updating of photon transport modelling in tissues is carried out by including the effect of molecular interference in the coherent (Rayleigh) scattering. To this end, the present tabulations—which permit us to obtain the linear differential scattering coefficient of compounds from a simple weighted sum of the elemental components—are integrated by adding files for a limited set of molecular interference functions. This set originates from a four-component model which is found to be capable of reproducing human tissues in situations involving bony and soft tissues. The proposed procedure overcomes, in the computation, the hindrance that the dependence on molecular interference effects leads every tissue to have its own diffraction pattern, which is not easily obtained by means of measurements or calculations.

An interference suppressing OFDM system for wireless communications

Abstract: The orthogonal frequency division multiplexed (OFDM) system introduced here, termed interference suppressing OFDM (IS-OFDM), has the capabilities of suppressing narrowband interference in wideband wireless applications. The IS-OFDM encodes each transmitted symbol in all frequency bins. Each frequency bin then "contains" all transmitted symbols which are distinguished and separated from each other by orthogonal Hadamard sequences. The IS-OFDM can provide a point-to-point wireless link without spreading the incoming data rate. In addition, the IS-OFDM has all the advantages of the ordinary OFDM, which is shown to be a special case of the IS-OFDM. We present the IS-OFDM system design and evaluate its performance in the presence of narrowband interference and AWGN channel.

Interference subspace rejection: a framework for multiuser detection in wideband CDMA

Abstract: We present a unifying framework for a new class of receivers that employ linearly-constrained interference cancellation (IC). The associated multiuser detectors operate in various modes and options ranging in performance from that of IC detectors to that of linear receivers, yet provide more attractive performance/complexity tradeoffs. They exploit both space and time diversities as well as the array-processing capabilities of multiple antennas and carry out simultaneous channel and timing estimation, signal combining and interference rejection. Additionally, they can operate on both links and in multiple mixed-rate traffic scenarios. The improved performance can be translated to increased utilization of wideband code division multiple access networks, particularly at high data rates.

A novel iterative multiuser detector for complex modulation schemes

Abstract: A novel multiuser detector for direct sequence code division multiple access is proposed. The receiver performs iterated soft decision interference cancellation (ISDIC) based on multiuser interference suppression filters designed for minimization of the mean-square error. Assuming a complex modulation format, we show that the multiuser interference becomes rotationally variant in the course of the iterations. Regarding this rotational variance in the design of the multiuser interference suppression filter, the presented iterative multiuser detector achieves significant performance gains compared with conventional ISDIC employing a standard minimum mean-squared error filter which is optimum only for rotationally invariant multiuser interference.

Multiple-beam interference lithography with electron beam written gratings

Abstract: Interference lithography with multiple beams provides two-dimensional periodic patterns in a single exposure step. It is possible to obtain various symmetries and shapes such as sparse hole arrays or dots on a hexagonal grid. However, when the number of interfering beams exceeds three, the intensity pattern depends strongly on the relative phases of the beams. In this article we show that this problem can be overcome with a scheme where transmission diffraction gratings written on a single substrate create the interfering beams possessing the required phase relations. The relative phases of the diffracted beams are controlled by the relative positions of the gratings in the substrate plane. We used electron beam lithography with an interferometrically controlled stage to obtain the required precise placement of the gratings. This method enabled us to produce hexagonal and square arrays of holes where we used three and four beam configurations, respectively. Moreover, we created sparse hole arrays that have hexagonal and square symmetries with six and eight beam interference. The experiments were carried out in the extreme ultraviolet region with synchrotron radiation and with laser light in the visible spectrum. The technique requires spatially coherent light but it is achromatic and it is not sensitive to small errors in the alignment of the incoming beam.

Quantum interference and supercurrent in multiple-barrier proximity structures

Abstract: We analyze an interplay between the proximity effect and quantum interference of electrons in the hybrid structures superconductor--normal-metal--superconductor which contain several insulating barriers. We demonstrate that the dc Josephson current in these structures may change qualitatively due to quantum interference of electrons scattered at different interfaces. In junctions with few conducting channels mesoscopic fluctuations of the supercurrent are significant and its amplitude can be strongly enhanced due to resonant effects. In the many-channel limit averaging over the scattering phase effectively suppresses interference effects for systems with two insulating barriers. In that case a standard quasiclassical approach describing scattering at interfaces by means of Zaitsev boundary conditions allows us to reproduce the correct results. However, in systems with three or more barriers the latter approach fails even in the many-channel limit. In such systems interference effects remain important in this limit as well. For short junctions these effects result in additional suppression of the Josephson critical current, indicating the tendency of the system towards localization. For relatively long junctions interference effects may---on the contrary---enhance the supercurrent with respect to the case of independent barriers.

Adaptive Interference Suppression for the Downlink of a Direct Sequence CDMA System with Long Spreading Sequences

Abstract: A simple approach for adaptive interference suppression for the downlink (base-to-mobile link) of a direct sequence (DS) based cellular communication system is presented. The base station transmits the sum of the signals destined for the different mobiles, typically attempting to avoid intra-cell interference by employing orthogonal spreading sequences for different mobiles. However, the signal reaching any given mobile passes through a dispersive channel, thus destroying the orthogonality. In this paper, we propose an adaptive linear equalizer at the mobile that reduces interference by approximately restoring orthogonality. The adaptive equalizer uses the pilot's spreading sequence (which observes the same channel as the spreading sequence for the desired mobile) as training. Simulation results for the linear Minimum Mean Squared Error (MMSE) equalizer are presented, demonstrating substantial performance gains over the RAKE receiver. Long spreading sequences (which vary from symbol to symbol) are employed, so that the equalizer adapts not to the time-varying spreading sequences, but to the slowly varying downlink channel. Since the inter-cell interference from any other base station also has the structure of many superposed signals passing through a single channel, the adaptive equalizer can also suppress inter-cell interference, with the tradeoff between suppression of intra- and inter-cell interference and noise enhancement depending on their impact on the Mean Squared Error (MSE).

Phase-shifting interferometry method and system

Abstract: An interferometry method including: i) forming an optical interference image by combining different portions of an optical wave front (140) reflected from multiple surfaces (109); ii) recording (170) an interference signal at different locations of the optical interference image in response to varying a property of the optical wave front that causes pairs of the multiple surfaces that have different optical path separations to contribute differently to the interference signal; iii) transforming (180) the interference signal for at least one of the locations to produce a spectrum having a peak at a spectral coordinate corresponding to each pair of the multiple surfaces; and iv) identifying (190) the spectral coordinate of the peak corresponding to a selected pair of the multiple surfaces.

Spectral efficiency of FDMA/TDMA wireless systems with transmit and receive antenna arrays

Abstract: The ever growing need for higher capacity in wireless systems has fueled the interest in exploiting the spatial dimension-through the use of antennas arrays-to improve the utilization of the available radio spectrum. As a result, a large number of space-time techniques have been proposed wherein arrays are used to mitigate interference and enhance signal levels. More recently, information theory has shown that, with spatial data multiplexing, very large spectral efficiencies can be attained in multipath channels using transmit and receive antenna arrays. In this paper, the system benefit of using transmit and receive arrays in multicell scenarios is evaluated as a function of both the propagation environment and the number of antennas. Our results confirm the potential for very large system spectral efficiencies associated with the use of transmit and receive arrays, in particular in interference-limited rich-multipath conditions wherein the ability to perform interference mitigation-leading to tighter spectral reuse-and spatial data multiplexing grows with the number of antennas. In environments free of multipath, the potential is smaller but still very significant, associated with interference mitigation and signal enhancement only, since spatial data multiplexing is no longer possible.

Structured adaptive frequency hopping

Abstract: Embodiments of the invention reduces the computational complexity of adaptive frequency hopping mechanisms while mitigating the effects of interference. Embodiments of the invention can provide structured adapted hopping sequences that reduce transitions between good and bad channels and/or assign good channels to slots where data is to be transmitted and bad channels to idle slots. Embodiments of the can be designed to comply with current FCC regulations, yet remain adaptable to changing rules.

Adaptive interference suppression for DS-CDMA

Abstract: We propose an adaptive scheme to reject multiuser interference using as little as 20-30 taps in direct-sequence code-division multiple-access systems employing arbitrary processing gains (31-400 considered here). We demonstrate that our receiver using this scheme does not require training signals, either initially when the desired user powers on in the presence of several strong interferers, or during transmission, as other users (possibly near-far interferers) gain access to the system. Moreover, knowledge of interfering users spreading codes, timing, or phase information is not required with our adaptive receiver.