Presence Of Multipath Research Articles

Capture effects of wireless CSMA/CA protocols in Rayleigh and shadow fading channels

We investigate the capture effects for a wireless local area network (LAN) system in the presence of multipath, shadowing, and near-far effects. The performance of wireless media access control (MAC) protocols with Rayleigh fading, shadowing, and capture effect are analyzed. We consider carrier-sense multiple-access/collision-avoidance (CSMA/CA) protocols as the wireless MAC protocols, since CSMA/CA protocols are based on the standard for wireless LAN's IEEE 802.11. We analyze and compare the channel throughput and packet delay for three types of CSMA/CA protocols: basic CSMA/CA, stop-and-wait automatic repeat request (SW ARQ) CSMA/CA, and four-way handshake (4-WH) CSMA/CA. We calculate the capture probability of an access point (AP) in a channel with Rayleigh fading, shadowing, and near-far effects, and we derive the throughput and packet delay for the various protocols. We have found that the performance of CSMA/CA in a radio channel model is 50% less than in an error-free channel model in low-traffic load, while the throughput and packet delay of CSMA/CA in a radio channel model show better performance than in an error-free channel model in high-traffic load. We also found that the 4-WH CSMA/CA protocol is superior to the other CSMA/CA protocols in high-traffic load.

Direct-sequence spread-spectrum chip tracking in the presence of unresolvable multipath components

Delay lock loop (DLL) chip time tracking in the presence of unresolvable multipath components is known to be suboptimum and prone to tracking errors. In the present contribution, the behavior of noncoherent and coherent digital DLL (DDLL) for a bandlimited direct-sequence spread-spectrum (DS-SS) system in the presence of multipath is analyzed under slow and fast fading assumptions. Due to the fading channel random nature, the tracker performance is evaluated according to a statistical metric. Numerical results show that the noncoherent DLL outperforms the coherent DLL in terms of demodulator losses caused by unresolvable multipath.

Synchronous DS-CDMA downlink with frequency selective fading

Synchronous code-division multiple-access (CDMA) techniques possess intrinsic protection against co-channel interference due to the orthogonal spreading codes used. However, in the presence of multipath, the signals lose their orthogonality property, leading to increased cross correlation. In these cases, channel estimation may be needed in order to improve the detection of the desired symbols. We propose a subspace-based algorithm for blind channel estimation of a synchronous CDMA downlink. We compare the channel estimation accuracy with the Cramer-Rao bound, and we show that the estimation accuracy can be increased considerably using a decision feedback approach.

Bias compensation and tracking with monopulse radars in the presence of multi path

The problem of tracking targets in the presence of reflections from sea or ground is addressed. Both types of reflections (specular and diffuse) are considered. Specular reflection causes large peak errors followed by an approximately constant bias in the monopulse ratio, while diffuse reflection has random variations which on the average generate a bias in the monopulse ratio. Expressions for the average error (bias) in the monopulse ratio due to specular and diffuse reflections and the corresponding variance in the presence of noise in the radar channels are derived. A maximum maneuver-based filter and a multiple model estimator are used for tracking. Simulation results for five scenarios, typical of sea skimmers, with Swerling III fluctuating radar cross sections (RCSs) indicate the significance and efficiency of the technique developed in this paper-a 65% reduction of the rms error in the target height estimate.

Efficient maximum likelihood DOA estimation for signals with known waveforms in the presence of multipath

We present a large-sample maximum likelihood (ML) algorithm for estimating the directions of arrival (DOA's) and signal amplitudes of known, possibly coherent signals impinging on an array of sensors. The algorithm is an extension of the DEML method of Li et al., that handles coherent multipath that may be present in the signals. The algorithm is computationally efficient because the nonlinear minimization step decouples into a set of minimizations of smaller dimension.

GNSS CODE AND CARRIER TRACKING IN THE PRESENCE OF MULTIPATH

It is well known that multipath represents a major error source in differential GNSS positioning. Errors produced are unique to each antenna position and so are uncorrelated between the reference station and the mobile receiver. Reduction techniques, such as narrow correlator spacing1 and the multipath estimating delay lock loop (MEDLL)2 have improved the situation. However, reflected signals emanating close to the antenna are not reduced by either technique. Carrier multipath is a particular problem because short delay multipath signals cause maximum carrier errors. This study describes the analysis that has been carried out into code and carrier tracking in the presence of multipath. Code tracking in this situation produces a non-zero mean error,3 and the source of this bias has been investigated. Code and carrier multipath errors of the global orbiting navigation satellite system (GLONASS) have been compared with those of the global positioning system (GPS). The effects of pre-correlation filtering on code and carrier multipath errors have been determined using data from a measured correlation function. The multipath fading bandwidth is also discussed, and fast and slow fading effects are presented. A comparison has been made of the performance of typical coherent and non-coherent code discriminators in the presence of multipath. The performance of the GPS and GLONASS P code signals have been determined with respect to code and carrier multipath errors. © 1997 by John Wiley & Sons, Ltd.

Blind deconvolution of polynomial-phase signals using the high-order ambiguity function

We consider the problem of estimating the parameters of a complex constant-modulus polynomial-phase signal that has undergone convolution with a linear time-invariant FIR channel. Such a signal is a sum of polynomial-phase signals, with special relationships among the parameters of the various components. Those relationships are used to develop a simple non-iterative algorithm for estimating the signal parameters. The algorithm is based on the recently developed high-order ambiguity function. The estimated parameters can be optionally supplied as initial conditions to a maximum likelihood estimation algorithm, thereby reducing the biases of the estimates and improving their statistical accuracy. As a by-product, estimates of the channel parameters are also obtained. The Cramér-Rao bound for this problem is also derived, and performance is illustrated by some numerical examples. Possible applications of the algorithms developed in the paper include the estimation of sonar, radar and communications signals in the presence of multipath.

Performance analysis of subspace based method for source localization in shallow water

The present paper aims at studying the performance characteristics of a subspace based algorithm for source localization in shallow water such as coastal water. Specifically, we study the performance of Multi Image Subspace Algorithm (MISA). Through first-order perturbation analysis and computer simulation it is shown that MISA is unbiased and statistically efficient. Further, we bring out the role of multipaths (or images) in reducing the error in the localization. It is shown that the presence of multipaths is found to improve the range and depth estimates. This may be attributed to the increased curvature of the wavefront caused by interference from many coherent multipaths.

A subspace method for signature waveform estimation in synchronous CDMA systems

Synchronous code-division multiple-access (CDMA) techniques possess intrinsic protection against co-channel interference due to orthogonal codes employed and thus, offers higher capacity than existing frequency-division multiple-access (FDMA) or time-division multiple-access (TDMA) systems. In the presence of multipath, however, each signal is subject to frequency-selective fading and the orthogonality condition does not necessarily hold leading to increased cross correlation. In these scenarios, multiuser detection need to be performed to suppress interference and recover the message symbols. To implement such a technique, explicit knowledge of the (nonorthogonal) signature waveforms of all users is required. We propose a blind estimation scheme that provides closed-form estimates of the signature waveforms by exploiting the structure information of the data output. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of the signature waveforms. Based on this observation, a multiple signal classification (MUSIC)-like algorithm is derived. Performance analysis of the new approach is also presented.

Shallow water bottom scattering strength at low frequencies

Determinations of acoustic scattering strength for sand bottoms have been made at several shallow water sites under downward refracting sound propagation conditions in the frequency band 50 Hz to 1 kHz. The measurements have been made using explosive sources detonated at mid-water depth and received on two collocated, bottom-mounted hydrophone line arrays, with one array in vertical and the other in horizontal orientation. The major focus of this work has been the variation of scattering strength with frequency and with bottom grazing angle. For the scattering strength measurements, the presence of multipaths in shallow water prevents a direct path scattering geometry from being used at low frequencies and, consequently, scattering strength must be extracted from the full reverberation field. Two techniques for obtaining scattering strength integrated over grazing angle and for using the vertical array to examine the angular dependence of the scattering strength will be described. The results will be compared with other determinations of scattering strength for shallow water sand bottoms and will be discussed in terms of the role of scattering from structure within the bottom sediment.

Robust spreading code detection in multipath

It is demonstrated that the detection of direct-sequence spread-spectrum signals by triple correlation is robust in the presence of multipath. The detection performance is illustrated by means of a typical receiver operating curve.

Recursive super-resolution algorithm for low-elevation target angle tracking in multipath

Conventional methods of tracking a target using monopulse radar are severely limited in the presence of multipath, particularly at low-elevation angles. A technique using recursive eigendecomposition together with frequency-agile waveforms for tracking low-elevation targets in multipath has been developed. Frequency agility decorrelates the coherence of the direct and specular components on a pulse-to-pulse basis. This frequency agility also provides some robustness to the severe signal cancellation that arises when the direct and specular components are 180 degrees out of phase. The proposed high-resolution, eigenstructure-based algorithm for solving the low-angle tracking problem can be implemented in three steps: (i) updating the covariance matrix, (ii) updating the eigenvalue decomposition of the covariance matrix, and (iii) updating the angle estimates by searching for the peaks of the updated MUSIC spectrum, or solving the zeros derived by the minimum-norm polynomial coefficient. In addition, a phased-array implementation using three orthogonal simultaneous beams placed at half null-beamwidth apart has been developed. The angles can be determined by explicit computation or by a calibration curve in a lookup table such as in monopulse processing for angle estimation using two simultaneous beams.

Target localisation in presence of multipaths

A delayed version of a bandlimited signal can be represented by convolving a sine function and the signal itself. By using this property, a novel constrained LMS algorithm that estimates the difference in the arrival times of signals at two spatially separated receivers with multipath propagations is developed. The time delay obtained can be used to localise targets in sonar and radar systems.

Angle-of-arrival estimation in presence of multipath

The Thomson method of multiple-windows is applied to the problem of angle-of-arrival (AOA) estimation at low grazing angles, and in the presence of combined specular and diffuse multipath. Experimental data are used, collected by means of a 32 element sampled aperture antenna system operating at X band. Additional improvement is observed when multiple frequencies and a priori information is incorporated in the AOA estimator.

Parabolic equation modelling of the effects of multipath and ducting on radar systems

The parabolic equation approach to the modelling of clear-air propagation, and in particular to the assessment of radar performance in the presence of multipath and ducting, is described. The derivation and solution of the parabolic equation by means of the split-step algorithm are described. A discussion of the radar equation is given to provide a connection with conventional assessment methods. The model is applied to the problem of anomalous propagation and the influence of the ground. One of the main advantages of the parabolic equation approach over alternative ray and mode methods is its ability to make quantitative predictions for two-dimensional refractiveindex structures without the need for a priori identification of the structures. New applications, such as wideband effects and the influence of refractive-index fluctuations and measurement errors on prediction accuracy, are discussed. Current work on clutter modelling and other extensions to the model are briefly mentioned.

Joint estimation of PN code delay and multipath using the extended Kalman filter

The problem of delay estimation in the presence of multipath is considered. It is shown that the extended Kalman filter (EKF) can be used to obtain joint estimates of time-of-arrival and multipath coefficients for deterministic signals when the channel can be modeled as a tapped-delay line. Simulation results are presented for the EKF joint estimator used for synchronization in a direct-sequence spread-spectrum system operating over a frequency-selective fading channel. A simplified model of the EKF joint estimator is considered for analysis purposes. The evolution in time of the tracking error probability density function and the nonlinear tracking error variance are examined through numerical solution of the Chapman-Kolmogorov equation. The nonlinear tracking error variance is compared to both the linear error variance estimate directly provided by the EKF and the Cramer-Rao lower bound.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Source level estimation of a monopole source at rest in the presence of multipath

The problem of estimating the source level of a broadband cw monopole source in coastal waters using a vertical array of hydrophones was investigated. The statistics of the sound field in a particular coastal region were modeled. The medium was idealized as a wedge-shaped volume bounded by a pressure release surface and a lossy inclined plane. The transfer function between source and receiver positions was treated as a random process. The signals from the hydrophone array were processed to give an estimate of the source level of the sound source. The efficiencies of a number of estimators were analyzed including beamforming, focusing, and linear and log power averaging. The bias and standard deviation of each estimator were measured as a function of the number of elements in the array. A numerical model implemented on a computer and a sea test provided the data samples on which the statistics were based.

Large area characterisation of urban UHF multipath propagation and its relevance to the performance bounds of mobile radio systems

A large number of small-area statistical channel descriptors in the form of average-power/time-delay profiles have been obtained in urban and suburban areas. The average excess delay and the delay spread are meaningful parameters which adequately describe the dispersiveness of the channel, and the variability of these parameters over a large area has been used as part of a two-stage model for characterisation of a UHF mobile radio channel. Suburban and urban areas exhibit quite different characteristics, reflecting the different nature of the environment. The variability of the statistical parameters is also a useful estimator of the performance bounds of radio systems. Using the measured data, together with relationships available in the literature for the performance of a given system in the presence of multipath, consideration is given to several system, such as DPSK data, SSB with pilot, FM voice and a phase ranging automatic vehicle location system. It is concluded that system techniques such as diversity reception can often lead to useful improvements in performance.

Adaptive canceller for elevation angle estimation in the presence of multipath

The paper describes a new adaptive canceller for estimating the elevation angle in a low-angle tracking radar environment in the presence of specular and diffuse multipath. The adaptation is achieved using the complex least-mean-squares (LMS) algorithm. Computer-simulation results are included to demonstrate the usefulness of this scheme for estimating the elevation angle when it is a small fraction of a standard beamwidth.

Improved Three Subaperture Method for Elevation Angle Estimation

The three subaperture method is a technique for estimating the arrival angles of two targets located in the main beam of a radar. The problem of estimating the arrival angle of a target in the presence of multipath is considered, and it is shown how the accuracy of the three subaperture method can be improved by making use of the information that the amplitude of the direct ray is larger than that of the reflected ray.