Multipath Situations Research Articles

BimodalPS: Causes and Corrections for Bimodal Multi-Path in Phase-Shifting Structured Light Scanners.

Structured light illumination is an active 3D scanning technique based on projecting and capturing a set of striped patterns and measuring the warping of the patterns as they reflect off a target object's surface. As designed, each pixel in the camera sees exactly one pixel from the projector; however, there are multi-path situations where a camera pixel sees light from multiple projector positions. In the case of bimodal multi-path, the camera pixel receives light from exactly two positions, which occurs along a step edge where the edge slices through a pixel which, therefore, sees both a foreground and background surface. In this paper, we present a general mathematical model to address this bimodal multi-path issue in a phase-shifting or so-called phase-measuring-profilometry scanner to measure the constructive and destructive interference between the two light paths, and by taking advantage of this interference, separate the paths and make two decoupled depth measurements. We validate our algorithm with both simulations and a number of challenging real-world scenarios, significantly outperforming the state-of-the-art methods.

Performance Comparison of TR and FSRUWB System Using Particle Filter: Effects of Frequency, Data Rate, Multi-Path and Multi-Channel Communication

In this study, we introduced a novel scheme based on Transmitted References (TR) and Frequency Shifted Reference (FSR) for ultra-wideband (UWB) system. By taking into account tracking loop-based particle filtering together with a data collecting approach for single and multi-path channel situations, the suggested method is an enhanced model. Each particle's location is determined using this filtering technique, which is then utilised to calculate the timing inaccuracy and regulate the UWB system's timing pulse. Also, it can tackle the multimodal distribution of errors then effectively approximate the optimal solution. The data distribution is discretised via a number of particles that are weighted samples evolving concerning time duration. The simulation results show that, in terms of error rate, number of particles, and delay response, the recommended model of FSR-UWB with particle filter performs better than the TR-UWB with and without considering particle filter.

Synchronization in DSSS system

AbstractDirect sequence spread spectrum (DSSS) communication systems offer huge performance focal points in perspective on their low probability of block, improved performance in multipath fading situations and their capacity to stay away from interference by spreading the signal over a wide bandwidth subsequently conveying the power. For the transmitted sequence to be effectively received and demodulated, the spreading sequence utilized at the receiver ought to be like that utilized in the transmitter. This paper uses MATLAB Simulink to show a technique for synchronizing the code clock at the receiver with the code clock at the transmitter. This fine arrangement procedure is known as code tracking.

Decoherence and visibility enhancement in multipath interference

We theoretically analyze the observations reported in a four-path quantum interference experiment via multiple beam Ramsey interference [Phys. Rev. Lett. 86, 559 (2001)]. In this experiment, a selective scattering of photons from just one interfering path causes decoherence. However, contrary to expectations, there is an increase in the contrast of the interference pattern, demonstrating that path selective decoherence can not only lead to a decrease, but under certain conditions, to an increase of the fringe contrast. Here we explain this seemingly counter-intuitive effect based on a model for a multi-path interference, with four to six slits, in the presence of decoherence. The effect of the environment is modeled via a coupling to a bath of harmonic oscillators. When decoherence is introduced in one of the multiple paths, an enhancement in fringe contrast is seen under certain conditions. A similar effect is shown to appear if instead of path-selective decoherence, a selective path detector is introduced. Our analysis points to the fact that while traditional fringe visibility captures the wave nature in the two-path case, it can fail in multi-path situations. We explain the enhancement of fringe visibility and also show that quantum coherence based on the l1 norm of coherence, in contrast to traditional visibility, remains a good quantifier of wave nature, even in such situations. The enhancement of fringe contrast in the presence of environmental decoherence underscores the limitations of traditional visibility as a good measure for wave nature in quantifying complementarity and also makes it an unlikely candidate for quantifying decoherence. Our analysis could lead to better insight in ways to quantify decoherence in multi-path interference, and in studies that seek to exploit quantum superpositions and quantum coherence for quantum information applications.

Optimization of Recursive Least Square-Based Adaptive Linear Equalizer for ZigBee Transceiver

An efficient technique to compensate for the channel detrimental effects in ZigBee systems is introduced in this paper. The proposed methodology relies on adding a recursive least square (RLS) based adaptive linear equalizer (ALE) to the physical layer of the receiver side. The performance of the RLS based ALE is investigated inside the ZigBee system under different multipath fading situations: Rician and Rayleigh. Moreover, the paper proposes a methodology for deciding the RLS based ALE’s design parameters. The design procedure depends on solving multiple objectives optimizing function based on genetic algorithms (GAs). The ALE’s parameters are chosen, such that the system experiences minimum bit error rate (BER) with fast convergence response. For design verification purposes, the ZigBee transceiver is modeled in MATLAB Simulink and tested under different fading and signal to noise ratios. In addition, the performance of the RLS adaptation algorithm is compared with the least mean square (LMS) one. The results show that the RLS based ALE provides better ZigBee performance with less BER and fast adaptation response.

An Adaptive Low-Cost INS/GNSS Tightly-Coupled Integration Architecture Based on Redundant Measurement Noise Covariance Estimation.

The main objective of the introduced study is to design an adaptive Inertial Navigation System/Global Navigation Satellite System (INS/GNSS) tightly-coupled integration system that can provide more reliable navigation solutions by making full use of an adaptive Kalman filter (AKF) and satellite selection algorithm. To achieve this goal, we develop a novel redundant measurement noise covariance estimation (RMNCE) theorem, which adaptively estimates measurement noise properties by analyzing the difference sequences of system measurements. The proposed RMNCE approach is then applied to design both a modified weighted satellite selection algorithm and a type of adaptive unscented Kalman filter (UKF) to improve the performance of the tightly-coupled integration system. In addition, an adaptive measurement noise covariance expanding algorithm is developed to mitigate outliers when facing heavy multipath and other harsh situations. Both semi-physical simulation and field experiments were conducted to evaluate the performance of the proposed architecture and were compared with state-of-the-art algorithms. The results validate that the RMNCE provides a significant improvement in the measurement noise covariance estimation and the proposed architecture can improve the accuracy and reliability of the INS/GNSS tightly-coupled systems. The proposed architecture can effectively limit positioning errors under conditions of poor GNSS measurement quality and outperforms all the compared schemes.

Intelligent identification and mitigation of GNSS multipath errors using adaptive BOCPD

Reliable knowledge of the complete navigation states is a key requirement for the mobile mapping and land localization applications. And it is the system integrating strapdown inertial navigation system (SINS) and global navigation satellite system (GNSS) that can seamlessly provide the position, velocity and orientation. In the SINS/GNSS integration, GNSS serves as the accurate reference, however, it is susceptible to the multipath propagations in the dense urban environment where GNSS signals may be reflected by buildings or bridges. When multipath situations occur, the positioning quality of GNSS is dramatically degraded, which also has a negative effect on the stable performance of the integration. Therefore, in order to degrade the negative effect caused by GNSS multipath errors, this paper proposes an intelligent identification and mitigation of GNSS multipath errors method, which employs adaptive bayesian online change point detection (BOCPD) based on the high-precision characteristics of the SINS in the short sampling period. The real tests are conducted, which demonstrate that the proposed approach can effectively identify and mitigate GNSS multipath errors, and then improve the accuracy and reliability of the integration.

Estimation of PN Code of DS-CDMA Signals over Frequency selective Fading Channel in presence of Impulsive noise

Spread spectrum signals are widely used for secure communications, as well as for multiple access interference (MAI). They have many interesting properties, including low probability of interception. Indeed, direct sequence spread spectrum systems (DSSSS) use a periodical pseudo-random sequence to modulate the baseband signal before transmission. Interception of DSSS signals is a difficult task due to the hiding of the PN sequence, especially if the accompanied noise is non-Gaussian and the channel is frequency selective. In this paper, a receiver is proposed to intercept asynchronous direct sequence code division multiple access (DS-CDMA) signals received from different users and estimate the PN sequences of each one of them. The signals are assumed to be transmitted over frequency selective fading channel and contaminated with Class-A impulsive noise. The proposed interception receiver is based on Independent Component Analysis (ICA), the pseudo random sequences is estimated from the eigenvector subspace in case of single path. In multipath situations, the estimation is the overlapped form of the pseudo random sequences, a triple correlation function (TCF) is introduced to eliminate the influence of multipath. The performance of proposed receiver is evaluated in terms of the number of correct estimated codes for m-sequence over typical urban channel in presence of impulsive noise. The results show that the interception receiver performs is 100% code estimation at -8dB signal to noise ratio.

Modeling outlier formation in scanning reflective surfaces using a laser stripe scanner

Scanning reflective surfaces using 3D laser scanners is a challenging task since reflective surfaces of complex geometry promote the formation of unwanted data outliers. These outliers are characterized with large measurement errors, which significantly deteriorate the quality of the scanned point cloud data. This paper experimentally investigates the formation of outliers in relation to scanning reflective surfaces using a commercial laser stripe scanner. Two outlier formation models are developed: mixed reflection and multi-path reflection. The undesirable specular reflections in both mixed reflection and multi-path reflection scanning situations cause multiple peaks in the image sensing arrays. The false image peaks are recorded and will eventually be observed as outliers because they are not part of the scanned object surface geometry. A series of scanning experiments have been conducted and the results confirm the validity of the developed outlier formation models. Potential applications of the developed models such as scan path evaluation and outlier filter design are also discussed.

Joint Blind Frame Synchronization and Encoder Identification for Low-Density Parity-Check Codes

In this paper, the problem involving joint blind frame synchronization and encoder identification for binary low-density parity-check (LDPC) codes of binary phase-shift keyed (BPSK) signals is tackled. The unknown encoder and time-delay can be blindly estimated simultaneously using the average log-likelihood ratios (LLRs) of syndrome a posteriori probability (APP). A new two-stage search method with a search step of size q by exploiting the quasi-cyclic nature of the parity-check matrix is devised to reduce the complexity of the blind frame synchronization algorithm. Numerous Monte Carlo simulations are evaluated to demonstrate the effectiveness of the new joint blind frame-synchronization and encoder-identification scheme for multi-path situations.

Doppler Characterization of a Mobile GNSS Receiver in Multipath Fading Channels

Global Navigation Satellite Systems (GNSS) Doppler measurements are commonly used for velocity-based relative positioning and aiding Inertial Navigation Systems (INS) in signal degraded environments. The aim of this paper is to characterise the Doppler measurements in GNSS harsh multipath environments. In multipath fading situations such as indoor and urban canyon environments, multipath components arrive to the receiver antenna from different paths and directions. These give rise to various Doppler shifts that cause errors in the velocity solution. In this work the Doppler measurements discrepancy characterised by Doppler spread in multipath environments is investigated. By assuming a ‘sphere of scatterers’ model and considering the antenna gain pattern, the theoretical Power Spectral Density (PSD) observed by a receiver is formulated. The theoretical findings are examined using two sets of measurements in dense multipath environments. Global Positioning System (GPS) live signals using non-isotropic antennas with different orientations are used for this purpose. Different motion directions are also examined using different data sets. An Assisted GPS (A-GPS) approach is utilised where the code phase and the navigation data bits are provided by a nearby outdoor antenna. By applying a ‘Block Processing’ technique, an epoch-by-epoch Doppler and velocity estimation is implemented. Herein, the Doppler and velocity measurements accuracy in addition to the Doppler spread characterization are studied. As shown both theoretically and experimentally, in harsh multipath environments the PSD of the observed signals is a function of the scatterers' geometry and the antenna gain pattern. The Doppler estimation accuracies in multipath and multipath-free cases are compared for different ranges of Carrier-to-Noise ratio (C/N0). Theoretical and experimental results revealed inaccurate Doppler estimation and poor Doppler-derived velocity solutions in dense multipath environments.

Blind Separation of DS-CDMA Signals with ICA Method

The estimation of pseudo noise sequence and information sequence is of great importance in the security of DS-CDMA system, which remains a hot research problem in reconnaissance and supervision of wireless communication. In DS-CDMA system, the pseudo noise sequences of different users are uncorrelated and the information sequences of different users are statistical independent, thus independent component analysis (ICA) could be introduced to separate the DS-CDMA signals with little prior knowledge. In multipath situations, the recovered pseudo noise sequence with ICA is overlapped. For DS-CDMA signals which utilize m-sequence as pseudo noise sequence, the triple correlation function (TCF) is introduced to eliminate the influence of overlap in this paper , which increases the estimation correct ratio greatly. Under the non - cooperative conditions, it is very difficult for the interceptor to aim at the very beginning of the transmitted signal. Certain offset s between the interceptor and the transmitter deteriorate the blind separation method obviously. The relationship between none convergence and the offset is found out and explained from eigen value decomposition. The validity of the proposed method and theory analysis is proved well by the simulation results at last.

Geometric Source Separation Method Using Nonnegative Matrix Factorization and Interference Suppression

In this paper, a geometric source separation system using nonnegative matrix factorization (NMF) is proposed. The adaptive beamformer is the best method for geometric source separation, but it suffers from a “target signal cancellation” problem in multi-path situations. We modified the HALS-NMF algorithm for decomposition into bases, and developed an interference suppression module in order to cancel the interference bases. A performance comparison between the proposed and subband GSC-RLS algorithm using a MATLAB® simulation was executed; the results show that the proposed system is robust in multi-path situations.

A robust to multi-path ranging technique over IEEE 802.11 networks

In this paper, we present a two-way TOA technique for distance estimation (i.e., ranging) over IEEE 802.11 networks. It is based on repetitively measuring the Round Trip Time (RTT) of IEEE 802.11 MAC frames and carrying out statistical processing in order to identify the multi-path condition of the channel and to improve accuracy. The study demonstrates that a multi-path condition has a negative impact on the ranging accuracy in Non-Dominant Direct Path (NDDP) and Undetectable Direct Path (UDP) situations. In this paper, an approach is presented to detect and overcome this impact. The proposed approach identifies the channel profile (i.e., the multi-path condition) from the statistical features of a set of RTT measurements and uses a ranging algorithm that is sensitive to multi-path. Results demonstrate the feasibility of channel profile identification in real time and a ranging accuracy of around one meter in all possible multi-path situations.

정상 환경 상태에서 다중 빔 탐색 레이다의 표적 고도 추출

다중 빔 탐색 레이다는 디지털 빔 형성 기술을 이용하여 수신시 적층 빔을 형성하는 최신의 3D(3 Dimensional) 레이다 기술이다. 본 논문에서는 다중 빔 3D 레이다의 빔 형성기 설계 개념 및 다중 경로 현상이 없는 정상 환경 상태에서 표적 고도 추출 방법을 제안하고 고찰해 보고자 한다. FFT(Fast Fourier Transform)수신 빔형성기에서 안테나 정현 공간(sine space) 좌표계 기반으로 표적 고도 추출 알고리듬에 대해 기술하였다. 제안된 알고리듬은 1개의 look-up 테이블을 이용하여 다양한 고도 및 레이다 주파수 대역에 대해서 일치하는 결과가 나오는 것을 시뮬레이션으로 확인하였다. The multibeam surveillance radar is a state-of art of 3D radar technology. It applies the stacked beam-on-received realized by a digital beamformer. In this paper, a design concept of beamformer and a method of target altitude extraction for multibeam surveillance radar in the normal environmental condition considering no multipath situations are proposed and investigated. The extraction algorithm based on antenna sine space coordinated system in a FFT digital beamformer is described. The proposed algorithm is simulated by 1 look-up table data and confirmed to have consistent results in accordance with a variety of target altitudes and a full radar frequency range.

Comparative analysis of radio occultation processing approaches based on Fourier integral operators

We analyze and compare two approaches to processing radio occultation data: (1) canonical transform method and (2) full spectrum inversion method. We show that these methods are closely related and can be explained from two view points: (1) both methods apply a Fourier transform like operator to the entire radio occultation signal, and the derivative of the phase of the transformed signal is used for the computation of bending angles, and (2) they can be explained from a signal processing view point as the location of multiple tones constituting the complete signal. The full spectrum inversion method is a composition of phase correction and Fourier transform, which makes the numerical algorithm computationally more efficient as compared to the canonical transform method. We investigate the relative performance of the two methods in simulations using a wave optics propagator. We use simple analytical models of the atmospheric refractivity as well as radiosonde data in order to reproduce complex multipath situations. The numerical simulations as well as the analytical estimations indicate that a resolution of 60 m (or even higher) can be achieved.

Diffractive vector and scalar integrals for bistatic radio holographic remote sensing

We introduce new vector diffractive integrals, which can be used for the radio holographic remote sensing of the atmosphere and terrestrial surfaces. These integrals are exact relationships connecting the electromagnetic fields known at some interface or curve in space with radio fields on the terrestrial surface or inside the atmosphere. They allow one to restore the radio image of the atmosphere or Earth surface in the investigated regions using a radio hologram registered in space by a small instrument installed on the low Earth orbit satellite. The high‐precision radio signals of the Global Positioning System (GPS) navigational satellites can be used as a source of the radio emission for radio holograms. We indicated a connection between the vector diffractive integrals and scalar diffractive integral, which is now applied for the GPS occultation investigation of Earth's atmosphere under an assumption of the spherical symmetry. For the atmosphere itself the accuracy of the scalar theory corresponds to the accuracy of the GPS occultation measurements. The most significant factor that affects the polarization is the reflection from the surface. The use of vector theory can thus be useful for the investigation of Earth's atmosphere by detecting the reflected rays. We show that the reference signal needed for restoration of the radio field from the registered radio hologram is coinciding with the Green function of the scalar wave equation corresponding to a three‐dimensional inhomogeneous medium. This substantiates the radio holographic–focused synthetic aperture principle (RFSA) in its application to the atmosphere and surface research. We validated the high vertical resolution of the RFSA method by obtaining radio image of the atmosphere and Earth's surface. Zverev's diffractive integral is used to compare the canonical transform (CT), back propagation (BP), and RFSA methods. For comparison, a general inverse operator (GIO) is introduced. The CT and BP transforms can be obtained by application of the GIO transform to Zverev's diffractive integral. The CT method can resolve physical rays in multipath situations under an assumption of the global spherical symmetry of the atmosphere and ionosphere. The RFSA method can account for the multipath in the case when the global spherical symmetry is absent by using the appropriate model of the refractivity and has a promise to be effective for operational data analysis.

Adaptive acquisition of PN code in multipath fading mobile channels

The adaptive acquisition processor (AAP), which controls false alarms and reduces miss detection of multipath signals by estimating the threshold level after censoring some highest samples in the sliding window, was introduced by the present author. In addition, an AAP was analysed in a single path situation. Here, the detection performance of an AAP in multipath situations, which may exist in the sliding window, is investigated. Results show that the AAP has good performance if the number of excision samples, Ne, is greater than or equal to the number of multipath signals in the window.

MC‐CDMA with quadrature spreading for wireless communication systems

AbstractMulti‐Carrier CDMA (MC‐CDMA) systems using the Orthogonal Frequency Division Division Multiple Access (OFDM) technique resolve the frequency selectivity in multipath fading channels and have good spectral properties. The addition of a spread spectrum component to the OFDM introduces a frequency diversity gain that can combat deep multipath fading situations. In this paper a MC‐CDMA system operating over frequency selective, slowly fading channels is considered and its performance is studied analytically and by computer simulations. We consider the downlink of a cellular radio system where for each user a BPSK modulation and a quadrature (complex) spreading code is used. The spreading codes are arranged in a way that reduces the effect of the multipath fading channel and restores some of the orthogonality losses between users. The obtained results show considerable performance improvement compared to conventional OFDM and to MC‐CDMA that uses single spreading codes. The effect of frequency offsets on the system performance is also addressed in this paper. Its superior performance and low complexity receiver make MC‐CDMA with quadrature spreading codes suitable for future wireless communication systems to achieve the requirements of high quality services, high speed, and low cost‐receiver.

Pattern translation and rotation in uncorrelated source distributions for multiple beam antenna design

In the uncorrelated scenario model, a continuous source distribution illuminates a receiver with waves bearing signals that are uncorrelated with respect to their angle of arrival. This model is used for multipath situations such as scintillating atmospheres, but also can be applied to optical beams for indoor communications, acoustic beamforming, and, of particular interest here, mobile communications. For diversity antennas operating in such a scenario, patterns that produce uncorrelated signals are required. An angular separation of directive beams in multipath scenarios acts to decorrelate the received signals. It is of interest to quantify the minimum angular spacing required of beams in order to provide a framework for the design and configuration of the antennas. The approach is to consider only the main lobe of the antenna pattern and to take it as a real function. This is reasonable as long as most of the energy is conveyed via the main beam. In practice, the sidelobe structure and nonuniformity in the phase of an actual pattern act to improve the situation in the sense that the decorrelation angles become smaller. The conditions for angular diversity result in a simple rule-of-thumb for the minimum beam separation requirement, which is essentially independent of the directivity. Finally, both scalar and vector elliptic beams rotated about their boresight axes are analyzed for the decorrelation rotation angle as a function of the ratio of the ellipticity. The resulting design curves offer a guideline to beam configuration for multipath scenarios.