Showing papers on "Multipath propagation published in 2001"

Highly-resilient, energy-efficient multipath routing in wireless sensor networks

Abstract: Previously proposed sensor network data dissemination schemes require periodic low-rate flooding of data in order to allow recovery from failure. We consider constructing two kinds of multipaths to enable energy efficient recovery from failure of the shortest path between source and sink. Disjoint multipath has been studied in the literature. We propose a novel braided multipath scheme, which results in several partially disjoint multipath schemes. We find that braided multipaths are a viable alternative for energy-efficient recovery from isolated and patterned failures.

Channel capacity of MIMO architecture using the exponential correlation matrix

Abstract: Multiple-input multiple output (MIMO) communication architecture has recently emerged as a new paradigm for wireless communications in rich multipath environment, which has spectral efficiencies far beyond those offered by conventional techniques. The channel capacity of the MIMO architecture in independent Rayleigh channels scales linearly as the number of antennas. However, the correlation of a real-world wireless channel may result in a substantial degradation of the MIMO architecture performance. In this letter, we investigate the MIMO channel capacity in correlated channels using the exponential correlation matrix model. We prove that, for this model, an increase in correlation is equivalent to a decrease in signal-to-noise ratio (SNR). For example, r=0.7 is the same as 3-dB decrease in SNR.

Intercarrier interference self-cancellation scheme for OFDM mobile communication systems

Abstract: For orthogonal frequency-division multiplexing (OFDM) communication systems, the frequency offsets in mobile radio channels distort the orthogonality between subcarriers resulting in intercarrier interference (ICI). This paper studies an efficient ICI cancellation method termed ICI self-cancellation scheme. The scheme works in two very simple steps. At the transmitter side, one data symbol is modulated onto a group of adjacent subcarriers with a group of weighting coefficients. The weighting coefficients are designed so that the ICI caused by the channel frequency errors can be minimized. At the receiver side, by linearly combining the received signals on these subcarriers with proposed coefficients, the residual ICI contained in the received signals can then be further reduced. The carrier-to-interference power ratio (CIR) can be increased by 15 and 30 dB when the group size is two or three, respectively, for a channel with a constant frequency offset. Although the redundant modulation causes a reduction in bandwidth efficiency, it can be compensated, for example, by using larger signal alphabet sizes. Simulations show that OFDM systems using the proposed ICI self-cancellation scheme perform much better than standard systems while having the same bandwidth efficiency in multipath mobile radio channels with large Doppler frequencies.

An efficient protocol for realizing cooperative diversity in wireless networks

Abstract: We develop two variants of an energy-efficient cooperative diversity protocol that combats fading induced by multipath propagation in wireless networks, The underlying techniques build upon the classical relay channel and related work and exploit space diversity available at distributed antennas through coordinated transmission and processing by cooperating radios. While applicable to any wireless setting, these protocols are particularly attractive in ad-hoc or peer-to-peer wireless networks, in which radios are typically constrained to employ a single antenna. Substantial energy-savings resulting from these protocols can lead to reduced battery drain, longer network lifetime, and improved network performance in terms of, e.g., capacity.

The double-directional radio channel

Abstract: We introduce the concept of the double-directional mobile radio channel. It is called this because it includes angular information at both link ends, e.g., at the base station and at the mobile station. We show that this angular information can be obtained with synchronized antenna arrays at both link ends. In wideband high-resolution measurements, we use a switched linear array at the receiver and a virtual-cross array at the transmitter. We evaluate the raw measurement data with a technique that alternately used estimation and beamforming, and that relied on ESPRIT (estimation of signal parameters via rotational invariance techniques) to obtain superresolution in both angular domains and in the delay domain. In sample microcellular scenarios (open and closed courtyard, line-of-sight and obstructed line-of-sight), up to 50 individual propagation paths are determined. The major multipath components are matched precisely to the physical environment by geometrical considerations. Up to three reflection/scattering points per propagation path are identified and localized, lending insight into the multipath spreading properties in a microcell. The extracted multipath parameters allow unambiguous scatterer identification and channel characterization, independently of a specific antenna, its configuration (single/array), and its pattern. The measurement results demonstrate a considerable amount of power being carried via multiply reflected components, thus suggesting revisiting the popular single-bounce propagation models. It turns out that the wideband double-directional evaluation is a most complete method for separating multipath components. Due to its excellent spatial resolution, the double-directional concept provides accurate estimates of the channel's multipath-richness, which is the important parameter for the capacity of multiple-input multiple-output (MIMO) channels.

Channel estimation for OFDM transmission in multipath fading channels based on parametric channel modeling

Abstract: We present an improved channel estimation algorithm for orthogonal frequency-division multiplexing mobile communication systems using pilot subcarriers. This algorithm is based on a parametric channel model where the channel frequency response is estimated using an L-path channel model. In the algorithm, we employ the ESPRIT (estimation of signal parameters by rotational invariance techniques) method to do the initial multipath time delays acquisition and propose an interpath interference cancellation delay locked loop to track the channel multipath time delays. With the multipath time delays information, a minimum mean square error estimator is derived to estimate the channel frequency response. It is demonstrated that the use of the parametric channel model can effectively reduce the signal subspace dimension of the channel correlation matrix for the sparse multipath fading channels and, consequently, improve the channel estimation performance.

A transmitter diversity scheme for wideband CDMA systems based on space-time spreading

Abstract: We present a transmit diversity technique for the downlink of (wideband) direct-sequence (DS) code division multiple access (CDMA) systems. The technique, called space-time spreading (STS), improves the downlink performance by using a small number of antenna elements at the base and one or more antennas at the handset, in conjunction with a novel spreading scheme that is inspired by space-time codes. It spreads each signal in a balanced way over the transmitter antenna elements to provide maximal path diversity at the receiver. In doing so, no extra spreading codes, transmit power or channel information are required at the transmitter and only minimal extra hardware complexity at both sides of the link. Both our analysis and simulation results show significant performance gains over conventional single-antenna systems and other open-loop transmit diversity techniques. Our approach is a practical way to increase the bit rate and/or improve the quality and range in the downlink of either mobile or fixed CDMA systems. A STS-based proposal for the case of two transmitter and single-receiver antennas has been accepted and will be included as an optional diversity mode in release A of the IS-2000 wideband CDMA standard.

Link-optimal space-time processing with multiple transmit and receive antennas

Abstract: Previous information theory results have demonstrated the enormous capacity potential of wireless communication systems with multiple transmit and receive antennas. To exploit this potential, a number of space-time architectures have been proposed which transmit parallel data streams, simultaneously and on the same frequency, in a multiple-input multiple-output fashion. With sufficient multipath propagation, these different streams can be separated at the receiver. Mostly, these space-time schemes have been studied only in the presence of spatially white noise. We present an architecture that is optimal, in the sense of maximum link spectral efficiency, in the presence of spatially colored interference. We evaluate this new architecture and compare it, under various propagation conditions, to other adaptive-antenna techniques with equal number of antennas.

Limitations of sum-of-sinusoids fading channel simulators

Abstract: Rayleigh signal fading due to multipath propagation in wireless channels is widely modeled using sum-of-sinusoids simulators. In particular, Jakes' (1994) simulator and derivatives of Jakes' simulator have gained widespread acceptance. Despite this, few in-depth studies of the simulators' statistical behavior have been reported in the literature. Here, the extent to which Jakes' simulator adequately models the multipath Rayleigh fading propagation environment is examined. The results show that Jakes' simulator does not reproduce some important properties of the physical fading channel. Some possible improvements to Jakes' simulator are examined. The significances of the number and the symmetries of the Doppler frequency shifts on the validity of the simulator's reproduction of the physical fading channel are elucidated.

A multicarrier CDMA architecture based on orthogonal complementary codes for new generations of wideband wireless communications

Abstract: This article is a review of our ongoing research effort to construct a new multicarrier CDMA architecture based on orthogonal complete complementary codes, characterized by its innovative spreading modulation scheme, uplink and downlink signaling design, and digital receiver implementation for multipath signal detection. There are several advantages of the proposed CDMA architecture compared to conventional CDMA systems pertinent to current 2G and 3G standards. First of all, it can achieve a spreading efficiency (SE) very close to one (the SE is defined as the amount of information bit(s) conveyed by each chip); whereas SEs of conventional CDMA systems equal 1/N, where N denotes the length of spreading codes. Second, it offers MAI-free operation in both upand downlink transmissions in an MAI-AWGN channel, which can significantly reduce the co-channel interference responsible for capacity decline of a CDMA system. Third, the proposed CDMA architecture is able to offer a high bandwidth efficiency due to the use of its unique spreading modulation scheme and orthogonal carriers. Lastly, the proposed CDMA architecture is particularly suited to multirate signal transmission due to the use of an offset stacked spreading modulation scheme, which simplifies the rate-matching algorithm relevant to multimedia services and facilitates asymmetric traffic in up- and downlink transmissions for IP-based applications. Based on the above characteristics and the obtained results, it is concluded that the proposed CDMA architecture has a great potential for applications in future wideband mobile communications beyond 3G, which is expected to offer a very high data rate in hostile mobile channels.

Unified design of iterative receivers using factor graphs

Abstract: Iterative algorithms are an attractive approach to approximating optimal, but high-complexity, joint channel estimation and decoding receivers for communication systems. We present a unified approach based on factor graphs for deriving iterative message-passing receiver algorithms for channel estimation and decoding. For many common channels, it is easy to find simple graphical models that lead directly to implementable algorithms. Canonical distributions provide a new, general framework for handling continuous variables. Example receiver designs for Rayleigh fading channels with block or Markov memory, and multipath fading channels with fixed unknown coefficients illustrate the effectiveness of our approach.

On the performance of ultra-wide-band signals in Gaussian noise and dense multipath

Abstract: An ultra-wide-band (UWB) signal is characterized by a radiated spectrum with a very wide bandwidth around a relatively low center frequency. In this paper, we study the reduced fading margin property of UWB signals. To evaluate the fading margin, we compare the performance of UWB signals in an environment with only additive white Gaussian noise (AWGN) versus the performance of UWB signals in a dense multipath environment with AWGN. The assumption here is that the presence of multipath causes a small increase in the signal-to-noise ratio required to achieve reasonable levels of bit error rate. A numerical example confirms this assumption, more specifically, the example shows that to achieve a bit error rate equal to 10/sup -5/, we require about 13.5 dB in the AWGN case and about 15 dB in the multipath case, resulting in a fading margin of just 1.5 dB. This small fading margin can be understood by the ability of the UWB signal to resolve the dense multipath.

Asymptotic error probability analysis of quadratic receivers in Rayleigh-fading channels with applications to a unified analysis of coherent and noncoherent space-time receivers

Abstract: A general, asymptotic (high signal-to-noise (SNR)) error analysis is introduced for quadratic receivers in frequency-flat and multipath Rayleigh-fading channels with multiple transmit and receive antennas. Asymptotically tight expressions for the pairwise error probabilities are obtained for coherent, noncoherent, and differentially coherent space-time receivers. Not only is our unified analysis applicable to more general modulation schemes and/or channel models than previously considered, but it also reveals a hitherto unrecognized eigenvalue structure that is common to all of these problems. In addition to providing an easy recipe for computing the asymptotic pairwise error rates, we make some conclusions regarding criteria for the design of signal constellations and codes such as (a) the same design criteria apply for both correlated and independent and identically distributed (i.i.d.) fading processes and (b) for noncoherent communications, unitary signals are optimal in the sense that they minimize the asymptotic union bound.

The effects of multipath interference on the performance of UWB systems in an indoor wireless channel

Abstract: Multipath propagation for the indoor wireless channel is very complicated and difficult to predict. Several papers have been written attempting to apply an analytical model for this channel to help systems engineers evaluate various architectural components of a wireless system, including modulation, multiple access, and waveform design techniques, without having to resort to empirical data collection. This paper attempts to apply one of these analytical models, known as the /spl Delta/-K model, to help study the performance of an ultra wide band (UWB) system and look at performance trade-offs for different multilevel modulations as well as the benefits of RAKE reception.

Blind adaptive algorithms for minimum variance CDMA receivers

Abstract: Constrained optimization methods have received considerable attention as a means to derive blind multiuser receivers with low complexity. The receiver's output variance is minimized subject to appropriate constraints which depend on the multipath structure of the signal of interest. When multipath is present, the constraint equations can be written in parametric form, and the constraint parameters jointly optimized with the linear receiver's parameters. We develop adaptive solutions for this joint, constrained optimization problem. Both stochastic gradient and recursive least-square-type algorithms are developed. The performance of the proposed methods is compared with other blind and trained methods and turns out to be close to the trained minimum mean-square-error receiver.

Transmit antennae space-time block coding for generalized OFDM in the presence of unknown multipath

Abstract: Transmit antenna diversity has been exploited to develop high-performance space-time coders and simple maximum-likelihood decoders for transmissions over flat fading channels. Relying on block precoding, this paper develops generalized space-time coded multicarrier transceivers appropriate for wireless propagation over frequency-selective multipath channels. Multicarrier precoding maps the frequency-selective channel into a set of flat fading subchannels, whereas space-time encoding/decoding facilitates equalization and achieves performance gains by exploiting the diversity available with multiple transmit antennas. When channel state information is unknown at the receiver, it is acquired blindly based on a deterministic variant of the constant-modulus algorithm that exploits the structure of space-time block codes. To benchmark performance, the Cramer-Rao bound of the channel estimates is also derived. System performance is evaluated both analytically and with simulations.

Analysis of channel effects on direct-sequence and frequency-hopped spread-spectrum acoustic communication

Abstract: Multiuser underwater acoustic communication is one of the enabling technologies for the autonomous ocean-sampling network (AOSN). Multiuser communication allows vehicles, moorings, and bottom instruments to interact without human intervention to perform adaptive sampling tasks. In addition, multiuser communication may be used to send data from many autonomous users to one buoy with RF communications capability, which will then forward the information to shore. The two major signaling techniques for multiuser acoustic communication are phase-shift keying (PSK) direct-sequence spread-spectrum (DSSS) and frequency-shift keying (FSK) frequency-hopped spread-spectrum (FHSS). Selecting between these two techniques requires not only a study of their performance under multiuser conditions, but also an analysis of the impact of the underwater acoustic channel. In the case of DSSS, limitations in temporal coherence of the channel affect the maximum spreading factor, leading to situations that may be better suited to FHSS signals. Conversely, the multipath resolving properties of DSSS minimize the effects of frequency-selective fading that degrade the performance of FSK modulation. Two direct-sequence receivers potentially suitable for the underwater channel are presented. The first utilizes standard despreading followed by decision-directed gain and phase tracking. The second uses chip-rate adaptive filtering and phase tracking prior to despreading. Results from shallow water testing in two different scenarios are presented to illustrate the techniques and their performance.

A statistical model for the UWB indoor channel

Abstract: We evaluate a measurement campaign for ultrawideband indoor channels in a typical modern office building. We show that the power delay profile can be well modeled by a single exponential decay with a statistically distributed decay-time constant. We also analyze path loss, amplitude distribution function, and temporal correlation between adjacent multipath components, and give statistical distributions for all those parameters. These results form the basis of a stochastic tapped-delay-line model of the ultra-wideband indoor channel.

Method and apparatus for performing directional re-scan of an adaptive antenna

Abstract: An antenna apparatus that can increase capacity in a cellular communication system. The antenna operates in conjunction with a mobile subscriber unit and provides a plurality of antenna elements, each coupled to a respective signal control component such as a phase shifter. The phase shift for each antenna element is programmed for optimum reception during, for example, an idle mode when a pilot signal is received. The antenna array creates a beam former for signals to be transmitted from the mobile subscriber unit, and a directional receiving array to more optimally detect and receive signals transmitted from the base station. By directionally receiving and transmitting signals, multipath fading is greatly reduced as well as intercell interference. The phase shifters are adjusted in a coarse and a fine mode. In the coarse mode all phase shifters are jointly incremented through several phase shift values until a signal quality metric is optimized. The coarse adjustment mode is followed by a fine adjustment mode during which the phase shifters are independently adjusted to further optimize the signal quality metric.

Method and apparatus for interference suppression in orthogonal frequency division multiplexed (OFDM) wireless communication systems

Abstract: A method and apparatus for interference suppression in wireless communication systems, especially Orthogonal Frequency Division Multiplexed (OFDM) systems. The array apparatus includes a two-tier adaptive array system which provides for both spatial diversity and beamforming at the uplink. The adaptive array is comprised of sub-arrays spaced at a distance sufficient to provide spatial diversity, ideally 5 to 15 wavelengths at the frequency of operation. Each sub-array is composed of at least two antenna elements spaced in proximity sufficient to provide effective beamforming or scanning, ideally less then one-half of one wavelength at the frequency of operation. The Direction of Arrival (DOA) of signals impinging upon the array can be calculated by comparing signals from sub-array elements. Each sub-array can then be filtered or beamformed so as to provide high gain to desired signals received from the DOA (which may be a multipath signal) while at the same time damping out undesired signals such as co-channel interference (CCI) in the frequency band of operation. The DOA is also used in a method of allocating frequency bins for data signals, such as in an OFDMA system, to provide weighted guidelines for bin allocation to maximize received signal power.

Method and apparatus for canceling pilot interference in a wireless communication system

Abstract: Techniques for canceling pilot interference in a wireless (e.g., CDMA) communication system. A received signal typically includes a number of signal instances (i.e., multipaths). For each multipath desired to be processed, the other multipaths act as interference on the desired multipath. If the pilot is generated based on a known data pattern (e.g., all zeros) and covered with a known channelization code (e.g., a Walsh code of zero), then the pilot in an interfering multipath may be estimated as the spreading sequence at a time offset corresponding to the arrival time of that multipath. The pilot interference from each interference multipath may be estimated based on the spreading sequence for the interfering multipath and the despreading sequence for the desired multipath. The total pilot interference from a number of interfering multipaths may be subtracted from the data component in the desired multipath to provide pilot-canceled data having improved performance.

Efficient ray-tracing methods for propagation prediction for indoor wireless communications

Abstract: The application of several ray-tracing techniques, in combination with the uniform theory of diffraction, is presented for efficient prediction of propagation in the UHF (communication) band in an indoor environment First, we improve the computational efficiency of the two-dimensional (2D) ray-tracing method by reorganizing the objects in an indoor environment into irregular cells Second, by making use of the two-dimensional ray-tracing results, a new three-dimensional (3D) propagation-prediction model is developed, which can save 99% of the computation time of a traditional three-dimensional model This new hybrid model is more accurate than two-dimensional models, and more efficient in computing the path loss to any point in the building than traditional three-dimensional models In this model, reflection and refraction by layered materials, and diffraction for the corners of the walls, are considered A patched-wall model is used to improve the accuracy of prediction in the method Finally, a comparison between simulation and measurements shows good agreement

Performance comparison of multipath mitigating receiver architectures

Abstract: As the Global Positioning System (GPS) has matured over the last decade, expectations regarding system performance have grown steadily. This has placed pressure on receiver manufacturers and system providers to be ever more creative in their efforts to mitigate error sources. Historically, multipath has been the dominant error source in differential GPS (DGPS). However, with the recent decision to deactivate Selective Availability, multipath has become a significant error source for all GPS users. In 1991, the narrow correlator was introduced to the market and was shown to reduce multipath errors by as much as 90% over conventional receivers. Over the past several years, a number of multipath-mitigation techniques have been developed and promoted. This paper explores the theory behind each technique and provides a performance comparison. The inherent assumptions and limitations of each technique are discussed as well.

Underwater acoustic communication using time reversal

Abstract: Time reversal, or phase-conjugation, refocuses energy back to a probe source location despite the complexity of the propagation channel. A probe source pulse is transmitted and a complicated multipath signal is measured by an array of source/receiver elements. The signal is time reversed and retransmitted into the ocean. The time-reversal process recombines this temporal multipath at the original probe source range and depth. The ability of time reversal to reduce dispersion and its simplicity of implementation makes it ideal for underwater acoustic communications, which must mitigate the inter-symbol interference caused by the time-varying multipath dispersion. Furthermore, time reversal focuses energy at the desired depth, thus mitigating the effects of channel fading. An experiment was conducted in June 2000 demonstrating that the time-reversal process recombined the temporal multipath resulting in reduced bit errors for communication. Communication sequences were transmitted over a distance of 10 km both in range independent and range dependent environments north of Elba Island, Italy. The range independent transmissions were made in 110-m deep water and the range dependent transmissions were made upslope from 110-m deep water into 40-m deep water. Single source transmissions were also measured in the same channels. Quantitative bit error results are shown for BPSK (binary phase shift keying) and QPSK (quadrature phase shift keying).

Identifiability results for blind beamforming in incoherent multipath with small delay spread

Abstract: Several explicit identifiability results are derived for deterministic blind beamforming in incoherent multipath with small delay spread. For example, it is shown that if the sum of spatial and fractional sampling diversities exceeds two times the total number of paths, then identifiability can be guaranteed even for one symbol snapshot. The tools come from the theory of low-rank three-way array decomposition (commonly referred to as parallel factor analysis (PARAFAC) and data smoothing in one and two dimensions. New results regarding the Kruskal-rank of certain structured matrices are also included, and they are of interest in their own right.

Broadband packet wireless access appropriate for high-speed and high-capacity throughput

Abstract: This paper evaluates the optimum broadband packet wireless access scheme using a 50-100 MHz bandwidth to achieve highspeed packet transmission taking into consideration the realization of high-speed and high-capacity packet transmission, the accommodation of variable rate services with different quality requirements, and the extendibility for and commonality to the IMT-2000. In the forward link, the presented simulation results associated with the evaluation of key features show that multi-carrier CDMA (MC-CDMA) is the most promising candidate, since it achieves the highest-capacity packet transmission in a severe multipath fading channel due to a lower symbol-rate with numerous sub-carriers and a channel coding gain accompanied by the frequency diversity effect. Meanwhile, single-carrier/DS-CDMA and multi-carrier/DS-CDMA (SC/DS-CDMA and MC/DS-CDMA) are severely degraded due to significantly increasing multipath interference, and orthogonal frequency division multiplexing (OFDM) lacks efficient cell deployment due to the necessity of frequency reuse in a multi-cell environment without complicated dynamic channel assignment (DCA). Furthermore, in the reverse link, we conclude that MC(SC)/DS-CDMA is a promising candidate because it has advantages such as an effective random access method, soft handover, and an inherent property for accurate location detection.

Space-time coding with maximum diversity gains over frequency-selective fading channels

Abstract: We study space-time (ST) block coding for single carrier zero-padded (ZP) block transmissions through finite impulse response (FIR) multipath fading channels of order L. We prove that the maximum diversity of order 2(L+1) is achieved with two transmit and one receive antenna. Simulations demonstrate that joint exploitation of multi-antenna diversity and multipath diversity leads to significantly enhanced performance.

Multipath processing for GPS receivers

Abstract: A GPS receiver system determines the presence of trackable signals at code delays less than the prompt delay being tracked for a particular signal and changes the prompt delay to correspond to the smallest code delay having a trackable signal. Trackable signals at large code delays are multipath signals and may be separately tracked to aid in dead reckoning. The trackable signals at code delays not adjacent to the currently tracked prompt delay may be tracked in the same channel as the prompt delay so that all satellite channels are continuously evaluated for multipath signals being tracked or a non-satellite specific channel may be used to sequentially step through the satellite signals to evaluate multipath on a satellite by satellite basis.

GPS code and carrier multipath mitigation using a multiantenna system

Abstract: Multipath is a major source of error in high precision Global Positioning System (GPS) static and kinematic differential positioning. Multipath accounts for most of the total error budget in carrier phase measurements in a spacecraft attitude determination system. It is a major concern in reference stations, such as in Local Area Augmentation Systems (LAAS), whereby corrections generated by a reference station, which are based on multipath corrupted measurements, can significantly influence the position accuracy of differential users. Code range, carrier phase, and signal-to-noise (SNR) measurements are all affected by multipath, and the effect is spatially correlated within a small area. In order to estimate and remove code and carrier phase multipath, a system comprising a cluster of five GPS receivers and antennas is used at a reference station location. The spatial correlation of the receiver data, and the known geometry among the antennas, are exploited to estimate multipath for each satellite in each antenna in the system. Generic receiver code and carrier tracking loop discriminator functions are analyzed, and relationships between receiver data, such as code range, carrier phase, and SNR measurements, are formulated and related to various multipath parameters. A Kalman filter is described which uses a combination of the available information from the antennas (receivers) in the multiantenna cluster to estimate various multipath parameters. From the multipath parameters, the code range and carrier phase multipath is estimated and compensated. The technique is first tested on simulated data in a controlled multipath environment. Results are then presented using field data and show a significant reduction in multipath error.