Interference (wave propagation)

About: Interference (wave propagation) is a research topic. Over the lifetime, 26086 publications have been published within this topic receiving 321110 citations.

Blind adaptive interference suppression for the near-far resistant acquisition and demodulation of direct-sequence CDMA signals

Abstract: Two key operations required of a receiver in a direct-sequence (DS) code division multiple access (CDMA) system are the timing acquisition of transmissions that are starting up or have lost synchronization, and the demodulation of transmissions that have been acquired. The reliability of both these operations is limited by multiple-access interference, especially for conventional matched filter-based methods, whose performance displays an interference floor and is vulnerable to the near-far problem. Recent work has shown that, provided timing information is available for a given transmission, it can be demodulated reliably using blind or training-sequence-based adaptive interference suppression techniques. These techniques are near-far resistant, unlike the matched filter demodulator, and do not require explicit knowledge of the interference parameters, unlike nonadaptive multiuser detectors. In this paper, we present a blind adaptive interference suppression technique for joint acquisition and demodulation, which has the unique feature that the output of the acquisition process is not simply the timing of the desired transmission, but a near-far resistant demodulator that implicitly accounts for knowledge of the timing and amplitudes of all transmissions to suppress the multiple-access interference. The only knowledge required by the scheme is that of the desired transmission's signature sequence, so that it is amenable to a decentralized implementation. On the other hand, it can be efficiently implemented as a centralized scheme in which the bulk of the computations for the adaptation are common to all transmissions that need to be acquired or demodulated.

A novel interference suppression scheme for global navigation satellite systems using antenna array

Abstract: This paper considers interference suppression and multipath mitigation in Global Navigation Satellite Systems (GNSSs). In particular, a self-coherence anti-jamming scheme is introduced which relies on the unique structure of the coarse/acquisition (C/A) code of the satellite signals. Because of the repetition of the C/A-code within each navigation symbol, the satellite signals exhibit strong self-coherence between chip-rate samples separated by integer multiples of the spreading gain. The proposed scheme utilizes this inherent self-coherence property to excise interferers that have different temporal structures from that of the satellite signals. Using a multiantenna navigation receiver, the proposed approach obtains the optimal set of beamforming coefficients by maximizing the cross correlation between the output signal and a reference signal, which is generated from the received data. It is demonstrated that the proposed scheme can provide high gains toward all satellites in the field of view, while suppressing strong interferers. By imposing constraints on the beamformer, the proposed method is also capable of mitigating multipath that enters the receiver from or near the horizon. No knowledge of either the transmitted navigation symbols or the satellite positions is required.

Interferometric measuring device

Abstract: The invention relates to an apparatus for an interferometric measuring device comprising a first interferometer and a second interferometer, wherein short-coherent radiation is fed to the first interferometer by means of a radiation source, which radiation is split into two partial beams by means of a first beam splitter, and wherein the optical path length is longer in one partial beam than in the other partial beam to the effect that the optical path difference is greater than the coherence length of the radiation, wherein the two partial beams are recombined prior to exit from the first interferometer and are fed to the second interferometer, which splits the radiation into two further partial beams, wherein the optical path lengths of the two partial beams are different to the effect that the optical path difference written in the first interferometer is compensated again, wherein the optical path length for a respective partial beam in the first and second interferometer can be set by means of at least one moveable optical component and the moveable optical components are mechanically coupled to one another. The invention furthermore relates to a method for compensating for an optical path difference in such an interferometric measuring device, wherein the optical path differences between the partial beams in the two interferometers are altered simultaneously and by the same magnitude by means of mechanically coupled moveable optical components. It is thereby possible to vary path differences in the partial beams of the interferometers in one work step, the conditions for forming interference still being complied with.

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.

Suppression of nonlinear frequency sweep in an optical frequency-domain reflectometer by use of Hilbert transformation

Abstract: A compensation technique for reducing the effect of nonlinear optical frequency swept in an optical frequency-domain reflectometer (OFDR) is proposed. The instantaneous sweep optical frequency of an OFDR laser source is directly obtained by analysis of the interference signal from an auxiliary interferometer with a Hilbert transformation. Beating OFDR data from a main interferometer are regenerated with respect to the measured instantaneous optical frequency. We show that this technique dramatically improves the spatial resolution of a conventional OFDR and can be applied to an optical frequency-domain medical imaging system to eliminate the problem of a nonlinear frequency sweep effect.