Coastal multipath fading

While flight testing airborne satellite communication systems, two-ray multipath fading has been regularly encountered at low satellite elevation angles during over water flights. In addition, an unexpectedly fast multipath fading was experienced as the aircraft transitioned from over land to over water. This unusual multipath fading occurred at medium and high elevation angles where the antenna discrimination should have reduced the reflected signal to an insignificant value. The coastal multipath has been recorded on over twenty occasions by different aircraft using a variety of antennas and flying over various north and central American coastlines. The rate of the multipath fading and fading amplitude suggests that the ocean has a very high reflection coefficient near shore and that some other mechanism such as dielectric wedge diffraction may play a part in the unexplained multipath fading.     

A multipath self-routing switch

The switching technology, the associated distributed control architecture, and the performance of the multipath self-routing (MPSR) group switching network are discussed. The MPSR group switching network is a folded three-stage multiplane structure smoothly expandable by stage or plane additions as required for capacity or throughput increase, respectively. The transport equipment, including its embedded distributed control processors, is implemented in a number of identical racks, each rack terminating 256 asynchronous transfer mode (ATM) links at 155 MB/s. The equipment configuration can grow up to 64 racks, for a capacity of 16 K ATM links. It is shown that the multipath, self-routing technique associated with a multislot cell transfer mode reduces the switching element complexity and allows for the realization of all of its functions in a single-chip integrated switching element (ISE) of a relatively large size. Results are presented on the ISE buffer size engineering for a 10^-10 overall through-switch cell loss ratio based on analytical methods verified by extensive simulations, on the derived total cell transfer delay, and on the engineering of the output buffer capacity     

Toward internet-wide multipath routing

The Internet would be more efficient and robust if routers could flexibly divide traffic over multiple paths. Often, having one or two extra paths is sufficient for customizing paths for different applications, improving security, reacting to failures, and balancing load. However, support for Internet-wide multipath routing faces two significant barriers. First, multipath routing could impose significant computational and storage overhead in a network the size of the Internet. Second, the independent networks that comprise the Internet will not relinquish control over the flow of traffic without appropriate incentives. In this article, we survey flexible multipath routing techniques that are both scalable and incentive compatible. Techniques covered include: multihoming, tagging, tunneling, and extensions to existing Internet routing protocols.     

Real-time adaptive multipath compensation

Multipath propagation interference in urban mobile communications can be minimised by using spread spectrum modulation and a RAKE filter. The Letter describes the simulation and practical operation of an adaptive or programmable RAKE filter, employing maximal ratio combining, which can handle time varying multipath channels in either the base station or mobile receiver.<>     

相对时延的码片内的多径分离

本文首先通过分析多径传播对单频信号和调制信号的影响,指出在扩频方式下,存在进一步分离码片内多径的可能性。然后基于导出的相关解扩输出的解析表达式,在忽略噪声的理想条件下,求解码片内多径的问题可以近似转化为对一线性方程组的求解。最后对接收滤波器输出为矩形脉冲序列和三解脉冲序列两种情况进行了仿真,仿真结果表明,码片内多径的幅值、相应能够被精确地分离出来,多径时延的估计误差为信号的采样间隔。     

Angles of arrival of indoor multipath

Some preliminary results of the angles of arrival of multipath components in indoor radio channel measurements are reported. The results show that strong multipath components arrive at the receiver in various directions.<>     

Analytical investigation of LOS multipath propagation

An improved and comprehensive version of the multipath model suggested by Ruthroff is presented. The model can be used to determine the different ray path parameters and to investigate both the frequency and space selectivity aspects of this phenomenon.     

Performance of multipilot symbol-aided multipath fadingcompensation

The frequency selective fading compensation technique described in the Letter compensates for a maximum delay time of up to one symbol duration. Computer simulation results show that distortion caused by frequency selective fading can effectively be compensated for by using the channel impulse response measured by three pilot symbols     

Adaptive beam-space nulling of multipath signals

A beam-space adaptive nulling technique that has potential for applications in communication circuits degraded by multipath signals is described. The technique is based on a well-known vector relationship in beam space. A linear transformation is used to prevent the degradation of the desired signal reception. Results are presented from tests of the technique that were conducted using real multipath data. The data used in these tests were recorded using a 32-element sampled aperture antenna (SAMPAR). These measurements were carried out on an over-water path. The direct signal, i.e. the signal that propagates via the shortest path from the transmitter to the receiver antenna, is selected as the desired signal; its reception is enhanced by suppressing the unwanted signals, i.e. the multipath signals that propagate via a reflection from the surface of the water. Examples are given in which the wanted and unwanted signals are separated by less than a beamwidth     

Adaptive equaliser for multipath channels

An adaptive equalising system is described which makes possible efficient communication of serial digital data over a multipath channel, within certain limitations of the severity of the multipath. This technique is particularly suitable for two paths of comparable strength.     

Eigenvalues and troposcatter multipath analysis

Performance predictions for troposcatter channels with multipath are developed using the eigenvalues in the Karhunen-Loeve representation of the tropo signal. Flat Rayleigh fading predictions, in the presence of jamming, are easily extended to multipath channels with this technique     

Strange attractors in multipath propagation

The estimation of the correlation dimension of multipath propagation signals in an indoor environment, indicates the presence of a strange attractor of dimension between four and five, suggesting deterministic chaos. The authors stress that although the profiles they examined show saturation in all three experimental configurations, indicating that they would be able to describe their data with a set of at most five nonlinear differential equations, investigation in other propagation situations is needed. An example of the detection of the presence of strange attractors in radar clutter is presented     

Capon estimation of covariance sequences

Estimating the covariance sequence of a wide-sense stationary process is of fundamental importance in digital signal processing (DSP). A new method, which makes use of Fourier inversion of the Capon spectral estimates and is referred to as theCapon method, is presented in this paper. It is shown that the Capon power spectral density (PSD) estimator yields an equivalent autoregressive (AR) or autoregressive moving-average (ARMA) process; hence, theexact covariance sequence corresponsing to the Capon spectrum can be computed in a rather convenient way. Also, without much accuracy loss, the computation can be significantly reduced via an approximate Capon method that utilizes the fast Fourier transform (FFT). Using a variety of ARMA signals, we show that Capon covariance estimates are generally better than standard sample covariance estimates and can be used to improve performances in DSP applications that are critically dependent on the accuracy of the covariance sequence estimates.This work was supported in part by National Science Foundation Grant MIP-9308302, Advanced Research Project Agency Grant MDA-972-93-1-0015, the Senior Individual Grant Program of the Swedish Foundation for Strategic Research and the Swedish Research Council for Engineering Sciences (TFR).     

Modeling of perturbed covariance sequences

An algorithm for obtaining a state-space (Markovian) model of a random process from a finite number of estimated covariance lags is presented and compared to other SVD-based methods. The new algorithm performs well for small data sets in which the estimated covariance sequence is perturbed by estimation errors, and may not even be positive definite. Simulation results are presented for a second-order Markov process.     

Estimation of structured covariance matrices

Covariance matrices from stationary time series are Toeplitz. Multichannel and multidimensional processes have covariance matrices of block Toeplitz form. In these cases and many other situations, one knows that the actual covariance matrix belongs to a particular subclass of covariance matrices. This paper discusses a method for estimating a covariance matrix of specified structure from vector samples of the random process. The theoretical foundation of the method is to assume that the random process is zero-mean multivariate Gaussian, and to find the maximum-likelihood covariance matrix that has the specified structure. An existence proof is given and the solution is interpreted in terms of a minimum-entropy principle. The necessary gradient conditions that must be satisfied by the maximum-likelihood solution are derived and unique and nonunique analytic solutions for some simple problems are presented. A major contribution of this paper is an iterative algorithm that solves the necessary gradient equations for moderate-sized problems with reasonable computational ease. Theoretical convergence properties of the basic algorithm are investigated and robust modifications discussed. In doing maximum-entropy spectral analysis of a sine wave in white noise from a single vector sample, this new estimation procedure causes no splitting of the spectral line in contrast to the Burg technique.     

Multidimensional maximum-entropy covariance extension

A universal characterization of maximum-entropy covariances for multidimensional signals is presented. It is shown that the maximum-entropy extension of an arbitrary partial covariance of a nonstationary multidimensional signal always has a banded inverse, i.e the inverse is sparse and has the same support as the given partial covariance. A dual formulation of the problem that makes it possible to approximate maximum-entropy extensions with models selected from suitably constrained model sets is introduced. It is proved that the best approximation in terms of multidimensional recursible autoregressive models can be determined by solving a set of linear equations. A simple graph-theoretic criterion is introduced to characterize those partial covariances whose maximum-entropy extension coincides with its autoregressive approximation, as in the conventional (one-dimensional stationary) maximum-entropy problem     

Reliability analysis of a multipath switching network

Multipath switching networks are encountered in areas such as communications, energy transmission, and transportation. The reliability of such networks usually cannot be calculated using conventional probability arguments for series and parallel systems. In this paper we describe a new Markov chain method for the reliability calculation. The method is easily implemented and has already been applied successfully. A case application in communications is used to illustrate the method. The paper generalizes and extends work reported in whitmore, Pullum and Chown [4].     

Measurement of time delays in multipath systems

In a recent letter, Gouriet proposed a method of measuring time delay, which uses the fact that the time delay of a linear system is the group delay at zero frequency. Based on the same method, a system is proposed in this letter to measure the time delays in a linear system which provides more than one transmission path between a single output and a single input. The system assumes that a rough knowledge of the magnitudes of time delays is available and that the individual multipath responses do not overlap in the time domain.     

Estimation of multipath parameters in wireless communications

In a parametric multipath propagation model, a source is received by an antenna array via a number of rays, each described by an arrival angle, a delay, and a fading parameter. Unlike the fading, the angles and delays are stationary over long time intervals. This fact is exploited in a new subspace-based high-resolution method for simultaneous estimation of the angle/delay parameters from multiple estimates of the channel impulse response. A computationally expensive optimization search can be avoided by using an ESPRIT-like algorithm. Finally, we investigate certain resolution issues that take the fact that the source is bandlimited into account     

Bandwidth scaling for fading multipath channels

We show that very large bandwidths on fading multipath channels cannot be effectively utilized by spread-spectrum systems that (in a particular sense) spread the available power uniformly over both time and frequency. The approach is to express the input process as an expansion in an orthonormal set of functions each localized in time and frequency. The fourth moment of each coefficient in this expansion is then uniformly constrained. We show that such a constraint forces the mutual information to 0 inversely with increasing bandwidth. Simply constraining the second moment of these coefficients does not achieve this effect. The results suggest strongly that conventional direct-sequence code-division multiple-access (CDMA) systems do not scale well to extremely large bandwidths. To illustrate how the interplay between channel estimation and symbol detection affects capacity, we present results for a specific channel and CDMA signaling scheme