Coherent Multipath Research Articles

Passive source localization based on multipath arrival angles with a vertical line array using sparse Bayesian learning.

In deep water, multipath time delays or frequency-domain interference periods of the acoustic intensity combined with multipath arrival angles are typically used for source localization. However, depth estimate is hard to achieve for a narrowband source at a remote part of the direct arrival zone as the required bandwidth increases with the source range. In this paper, a passive source localization method with a vertical line array, suitable for both broadband and narrowband sources, is proposed. Based on the variation trends of multipath angles with source range and depth, source localization is achieved by only matching the measured angles of the direct path and surface-reflected path with model-based values of a predefined grid of potential source locations. Considering the angle resolution limited by the array aperture and the presence of coherent multipath, sparse Bayesian learning is used and compared with the conventional beamforming and the minimum-variance distortionless-response beamforming to resolve and estimate the multipath angles. Simulations and experimental data of explosive sources collected by a vertical line array in the South China Sea are carried out to illustrate the method and demonstrate the performance.

Applying the skew-normal distribution to model coherent MPI and to evaluate its impact on PAM signals

Abstract We statistically model the coherent multipath interference (MPI) using an extended skew-normal distribution, with the results showing a good agreement with published experimental data. This distribution is very flexible and permits to model data with large skewness values, which is a typical feature in coherent MPI histograms. Furthermore, the developed model was applied to estimate the impact of coherent MPI on PAM-4 systems, with the results showing a 1-dB Q-penalty for an MPI level of −24 dB and a bit error ratio of 10−3.

DOA Estimation for Highly Correlated and Coherent Multipath Signals with Ultralow SNRs

In a typical multipath propagation environment, there exists a strong direct path signal accompanying with several weak multipath signals. Due to the strong direct path interference and other masking effects, the Direction-of-Arrival (DOA) of a weak multipath signal is hard to be estimated. In this paper, a novel method is proposed to estimate the DOA of multipath signals with ultralow signal-to-noise ratio (SNR). The main idea is to increase the SNR and signal-to-interference ratio (SIR) of the desired multipath signal in time-delay domain before DOA estimation processing. Firstly, the cross-correlation functions of the direct path signal and the received array signal are calculated. Then, they are combined and constructed to an enhanced array signal. Under certain conditions, the SNR and SIR of the desired signal can be significantly increased. Finally, the DOAs of multipath signals can be estimated by conventional technologies, and the associated time delays can be measured on the DOA-time-shift map. The SNR and SIR gains of the desired signal are analyzed theoretically, and theoretical analysis also indicates that the Cramer–Rao bound can be reduced. Simulation examples are presented to verify the advantages of the proposed method.

M-ary Cyclic Shift Keying Spread Spectrum Underwater Acoustic Communications Based on Virtual Time-Reversal Mirror.

Underwater acoustic communications are challenging because channels are complex, and acoustic waves when propagating in the ocean are subjected to a variety of interferences, such as noise, reflections, scattering and so on. Spread spectrum technique thus has been widely used in underwater acoustic communications for its strong anti-interference ability and good confidentiality. Underwater acoustic channels are typical coherent multipath channels, in which the inter-symbol interference seriously affects the performance of underwater acoustic communications. Time-reversal mirror technique utilizes this physical characteristic of underwater acoustic channels to restrain the inter-symbol interference by reconstructing multipath signals and reduce the influence of channel fading by spatial focusing. This paper presents an M-ary cyclic shift keying spread spectrum underwater acoustic communication scheme based on the virtual time-reversal mirror. Compared to the traditional spread spectrum techniques, this method is more robust, for it uses the M-ary cyclic shift keying spread spectrum to improve the communication rate and uses the virtual time-reversal mirror to ensure a low bit error rate. The performance of this method is verified by simulations and pool experiments.

Research on equalisation scheduling algorithm for network channel under the impact of big data

In order to improve the equalisation scheduling ability of network channel, an equalisation scheduling algorithm for big data network communication based on the baud-spaced equilibration and decision feedback modulation technology is proposed in this paper. With this algorithm, a model for network communication channel under the impact of big data is constructed to analyze the multipath characteristics of network channel; coherent multipath channel modulation method is used to conduct wave filtering to intersymbol interference and adaptive baud-spaced equilibration technology is also used to design channel equalisation; the model for tap delay line of channel is used for multipath suppression of network channel and the decision feedback modulation technology is also used for network channel equalisation scheduling to overcome the impact of phase shift caused by big data impact on channel and improve channel equalisation. Simulation results show that when the proposed algorithm is used for network channel equalisation scheduling, the fidelity of symbols output through network communication is good and the bit error rate is low and the performance of network channel equalisation scheduling under the impact of big data and multipath is good, which improves the robustness of network channel.

Multiplicative and min processing of experimental passive sonar data from thinned arrays.

Sparse arrays reduce the number of sensors required to achieve a specific angular resolution by using sensor spacing greater than the half-wavelength. These undersampled sparse arrays require processing algorithms to eliminate aliasing ambiguities. Thinned arrays are sparse arrays whose sensor positions lie on an underlying equally spaced grid. Using data from a shallow water passive sonar experiment, this paper investigates two thinned array geometries (coprime and nested) along with two processing algorithms (multiplicative and min). Coprime and nested arrays consist of two interleaved Uniform Line Arrays (ULAs) where one or both of the ULAs are undersampled. Multiplicative and min processors combine the outputs of the conventionally-beamformed subarrays to estimate the spatial spectrum. While these nonlinear processors can suppress aliasing, they are often plagued by high sidelobes and cross term interference. This paper presents sparse array designs for a shallow waveguide that require 33% fewer sensors than a fully-sampled ULA and provide significant sidelobe attenuation. Experimental data analysis reveals that cross term interference dominates the spectral estimates for the coprime and nested multiplicative processors and the coprime min processor. The nested min processor outperforms its sparse counterparts due to its ability to contend with coherent multipath in the environment.

Coherent Multipath Direction-of-Arrival Resolution Using Compressed Sensing

For a sound field observed on a sensor array, performance of conventional high-resolution adaptive beamformers is affected dramatically in the presence of coherent multipath signals, but the directions-of-arrival (DOAs) and power levels of these arrivals can be resolved with compressed sensing (CS). When the number of multipath signals is sufficiently small, a CS approach can be used by formulating the problem as a sparse signal recovery problem. CS overcomes the difficulty of resolving coherent arrivals at an array by directly processing the sensor outputs without first estimating a sensor covariance matrix. CS is compared to the adaptive minimum-variance-distortionless-response (MVDR) spatial processor with spatial smoothing. Though spatial smoothing produces improved results by preprocessing the sensor array covariance matrix to decorrelate the coherent multipath components, it reduces the effective aperture of the array and hence reduces the resolution. An empirical study with a uniform linear array (ULA) demonstrates that CS outperforms MVDR beamformer with spatial smoothing in terms of spatial resolution and bias and variance of DOA and power estimates. Analysis of the shallow-water HF97 ocean acoustic experimental data shows that CS is able to recover the DOAs and power levels of the multipath signals with superior resolution compared to MVDR with spatial smoothing.

Bearing estimation from vector sensor intensity processing for autonomous underwater gliders

Data have been collected on acoustic vector sensors mounted on autonomous underwater gliders in the Monterey Bay during 2012–2013. In this work, we show results of intensity processing to estimate bearing to impulsive sources of interest. These sources included small explosive shots deployed by local fisherman, and humpback whale vocalizations. While the highly impulsive shot data produced unambiguous bearing estimations, the longer duration whale vocalizations showed a fairly wide spread in bearing. The causes of the ambiguity in bearing estimation are investigated in the context of the highly variable bathymetry of the Monterey Bay Canyon, as well as the coherent multipath interference in the longer duration calls.

BER Penalty Induced by Coherent MPI Noise in FTTH Optical Links

We measure the coherent multipath interference noise and the associated bit error rate (BER) penalty affecting an optical link in which a fraction of the signal is scattered from the fundamental into the first-order mode of a bend insensitive fiber, to mimic a situation that can be encountered in fiber to the home deployments. Our set up allows changing the relative weights and the phase difference between the two interfering modes, and measuring the BER in stationary and dynamical settings, namely with the interference conditions kept fixed or continuously varied during the measurement cycle, respectively. We show that the BER penalty detected in the dynamical measurements can be entirely ascribed to the stochastic variations of the received intensity, and we demonstrate a good agreement between the experimental data and the results of our numerical simulations.

Throughput-Optimal Training for a Time-Varying Multi-Antenna Channel

A lower bound on the capacity of trained space-time modulation is presented for the case of a time-varying channel. We consider the case where the channel consists of two components of variable strength: a specular component from line-of-sight or strong coherent multipath, and a time-varying diffuse component due to a mobile receiver or scatterers. The training period and the data period are assumed quasi-static, with the time-autocorrelation function of the channel used to model the variation between each sub-block. The training signal, training signal length, power allocation and training frequency that optimize the capacity bound are derived. We compare the effective SNR of this bound to that of differential modulation at high SNR, and find it to be lower. We find the best number of antennas to use at high SNR with differential modulation, and compare it with the optimal value for trained modulation. For our time-varying channel model, trained modulation often has a higher achievable rate than differential modulation. Our results are supported by several numerical examples.

A Space/Fast-Time Adaptive Monopulse Technique

Mainbeam jamming poses a particularly difficult challenge for conventional monopulse radars. In such cases spatially adaptive processing provides some interference suppression when the target and jammer are not exactly coaligned. However, as the target angle approaches that of the jammer, mitigation performance is increasingly hampered and distortions are introduced into the resulting beam pattern. Both of these factors limit the reliability of a spatially adaptive monopulse processor. The presence of coherent multipath in the form of terrain-scattered interference (TSI), although normally considered a nuisance, can be exploited to suppress mainbeam jamming with space/fast-time processing. A method is presented offering space/fast-time monopulse processing with distortionless spatial array patterns that can achieve improved angle estimation over spatially adaptive monopulse. Performance results for the monopulse processor are obtained for mountaintop data containing a jammer and TSI, which demonstrate a dramatic improvement in performance over conventional monopulse and spatially adaptive monopulse.

Evolution and systems impact of coherent distributed multipath interference

We demonstrate an emulator that captures the coherent distributed nature of multipath interference in few-mode fiber devices. Coherent interference in these devices manifests in slow power fluctuations as well as relative intensity noise. Slow bit-error-rate drifts and attendant systems power penalty drifts, in timescales similar to the slow power fluctuations (/spl sim/10 s), are the dominant manifestations of this impairment.

Frequency-hopping versus interference cancellation for TD-CDMA

This work considers asynchronous time division code division multiple access (TD-CDMA) systems with RAKE receivers and one stage of parallel interference cancellation (PIC) or serial interference cancellation (SIC). A general method based on the concept of spherically symmetric signals is presented for the evaluation of the average probability of error of uncoded TD-CDMA systems. Slow frequency-hopping (SFH) with frequency overlap (FO) between adjacent carriers and interference cancellation are also included in the analysis, which considers the multipath Rayleigh fading channel (which models indoor and outdoor vehicular radio propagation). We analyze quadrature phase shift keying (QPSK) modulation with coherent demodulation and multipath (frequency) diversity with maximal ratio combining (MRC). Power control, adaptive SFH, and interference cancellation are employed for improving the bit-error rate (BER) performance. It is found that the scheme with SFH, in spite of the FO, always improves the performance substantially, and interference cancellation, in general, provides the highest BER improvement.

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.

A sliding window decorrelating receiver for multiuser DS-CDMA mobile radio networks

We detail the development of a near-far (NF) resistant sliding window decorrelating algorithm (SLWA) that overcomes the near-far problem (NFP) pertaining to the conventional linear correlation receiver (CLCR) and alleviates some of the limitations of the standard decorrelator. The SLWA architecture is extended to incorporate differentially coherent multipath (RAKE) diversity combining techniques and hence achieves simultaneous rejection of multiuser and multipath interference. This paper also presents a novel algorithm for updating the decorrelator coefficients using a fully parallel architecture. We present a mathematical model for the performance of a sliding window scheme, the main contribution of which is the analysis of finite sequence length multipath decorrelation under the practical limitation of incomplete RAKE combining. In addition to numerical results, we present results pertaining to capacity improvements achieved over the CLCR, derived from a simulation of a multiuser direct-sequence code-division multiple-access (DS-CDMA) mobile radio system.

High resolution coherent source location using transmit/receive arrays

A general approach to super resolution imaging of point sources using active arrays of transmit/receive elements is presented. The usual techniques of high resolution imaging using single transmitters and passive receive arrays fail in the presence of sets of coherent point sources, which often arise due to coherent multipath. However, data obtained from transmit/receive arrays may be arranged into matrices to which eigenspace direction of arrival estimation may be successfully applied, even int he presence of coherent sources. Each such matrix may be thought of as corresponding to a different transmit/receive array; this may be either the actual transmit/receive array or a virtual transmit/receive array whose effect is synthesized. This approach provides great flexibility, since a large number of different synthetic or virtual arrays may be available for a given transmit/receive array, and each can provide a different tradeoff between the total number of resolvable targets and the largest number of mutually coherent targets which can be resolved.

Fast signal acquisition by an adaptive antenna moving in a multipath field

An adaptive antenna algorithm is described which acquires a signal of unknown direction in the presence of uncorrelated interference and coherent multipath by using a sequential power search procedure on a small number of stored vector samples of the field. A set of candidate weighting vectors is generated, one of which, when used to filter the antenna circuit, will usually produce the desired signal at a sufficient signal-tonoise ratio for communications. The system is tested by simulation on a mobile receiver at 1 GHz in heavy multipath conditions with several interference sources and the effects of motion and bandwidth are assessed.