Showing papers on "Multipath propagation published in 2003"

Distributed space-time-coded protocols for exploiting cooperative diversity in wireless networks

Abstract: We develop and analyze space-time coded cooperative diversity protocols for combating multipath fading across multiple protocol layers in a wireless network. The protocols exploit spatial diversity available among a collection of distributed terminals that relay messages for one another in such a manner that the destination terminal can average the fading, even though it is unknown a priori which terminals will be involved. In particular, a source initiates transmission to its destination, and many relays potentially receive the transmission. Those terminals that can fully decode the transmission utilize a space-time code to cooperatively relay to the destination. We demonstrate that these protocols achieve full spatial diversity in the number of cooperating terminals, not just the number of decoding relays, and can be used effectively for higher spectral efficiencies than repetition-based schemes. We discuss issues related to space-time code design for these protocols, emphasizing codes that readily allow for appealing distributed versions.

A survey of various propagation models for mobile communication

Abstract: In order to estimate the signal parameters accurately for mobile systems, it is necessary to estimate a system's propagation characteristics through a medium. Propagation analysis provides a good initial estimate of the signal characteristics. The ability to accurately predict radio-propagation behavior for wireless personal communication systems, such as cellular mobile radio, is becoming crucial to system design. Since site measurements are costly, propagation models have been developed as a suitable, low-cost, and convenient alternative. Channel modeling is required to predict path, loss and to characterize the impulse response of the propagating channel. The path loss is associated with the design of base stations, as this tells us how much a transmitter needs to radiate to service a given region. Channel characterization, on the other hand, deals with the fidelity of the received signals, and has to do with the nature of the waveform received at a receiver. The objective here is to design a suitable receiver that will receive the transmitted signal, distorted due to the multipath and dispersion effects of the channel, and that will decode the transmitted signal. An understanding of the various propagation models can actually address both problems. This paper begins with a review of the information available on the various propagation models for both indoor and outdoor environments. The existing models can be classified into two major classes: statistical models and site-specific models. The main characteristics of the radio channel - such as path loss, fading, and time-delay spread - are discussed. Currently, a third alternative, which includes many new numerical methods, is being introduced to propagation prediction. The advantages and disadvantages of some of these methods are summarized. In addition, an impulse-response characterization for the propagation path is also presented, including models for small-scale fading, Finally, it is shown that when two-way communication ports can be defined for a mobile system, it is possible to use reciprocity to focus the energy along the direction of an intended user without any explicit knowledge of the electromagnetic environment in which the system is operating, or knowledge of the spatial locations of the transmitter and the receiver.

Channel models for ultrawideband personal area networks

Abstract: This article describes the modeling of ultrawideband wireless propagation channels, especially for the simulation of personal area networks. The IEEE 802.15.3a standards task group has established a standard channel model to be used for the evaluation of PAN physical layer proposals. We discuss the standard model, the measurements that form its basis, and the possibilities for future improvements. This article points out the important differences between UWB channels and narrowband wireless channels, especially with respect to fading statistics and time of arrival of multipath components. The impacts on the different propagation conditions on system design, like RAKE receiver performance, are elaborated.

A robust timing and frequency synchronization for OFDM systems

Abstract: A robust symbol-timing and carrier-frequency synchronization scheme applicable to orthogonal frequency-division-multiplexing systems is presented. The proposed method is based on a training symbol specifically designed to have a steep rolloff timing metric. The proposed timing metric also provides a robust sync detection capability. Both time domain training and frequency domain (FD) training are investigated. For FD training, maintaining a low peak-to-average power ratio of the training symbol was taken into consideration. The channel estimation scheme based on the designed training symbol was also incorporated in the system in order to give both fine-timing and frequency-offset estimates. For fine frequency estimation, two approaches are presented. The first one is based on the suppression of the interference introduced in the frequency estimation process by the training symbol pattern in the context of multipath dispersive channels. The second one is based on the maximum likelihood principle and does not suffer from any interference. A new performance measure is introduced for timing estimation, which is based on the plot of signal to timing-error-induced average interference power ratio against the timing estimate shift. A simple approach for finding the optimal setting of the timing estimator is presented. Finally, the sync detection, timing estimation, frequency estimation, and bit-error-rate performance of the proposed method are presented in a multipath Rayleigh fading channel.

Particle filtering algorithms for tracking an acoustic source in a reverberant environment

Abstract: Traditional acoustic source localization algorithms attempt to find the current location of the acoustic source using data collected at an array of sensors at the current time only. In the presence of strong multipath, these traditional algorithms often erroneously locate a multipath reflection rather than the true source location. A recently proposed approach that appears promising in overcoming this drawback of traditional algorithms, is a state-space approach using particle filtering. In this paper we formulate a general framework for tracking an acoustic source using particle filters. We discuss four specific algorithms that fit within this framework, and demonstrate their performance using both simulated reverberant data and data recorded in a moderately reverberant office room (with a measured reverberation time of 0.39 s). The results indicate that the proposed family of algorithms are able to accurately track a moving source in a moderately reverberant room.

Experimental characterization of the MIMO wireless channel: data acquisition and analysis

Abstract: Detailed performance assessment of space-time coding algorithms in realistic channels is critically dependent upon accurate knowledge of the wireless channel spatial characteristics. This paper presents an experimental measurement platform capable of providing the narrowband channel transfer matrix for wireless communications scenarios. The system is used to directly measure key multiple-input-multiple-output parameters in an indoor environment at 2.45 GHz. Linear antenna arrays of different sizes and construction with up to ten elements at transmit and receive are utilized in the measurement campaign. This data is analyzed to reveal channel properties such as transfer matrix element statistical distributions and temporal and spatial correlation. Additionally, the impact of parameters such as antenna element polarization, directivity, and array size on channel capacity are highlighted. The paper concludes with a discussion of the relationship between multipath richness and path loss, as well as their joint role in determining channel capacity.

Deficiencies of 'Kronecker' MIMO radio channel model

Abstract: Evidence that the ‘Kronecker’ model underestimates the channel capacity of an 8×8 multiple input, multiple output (MIMO) system in indoor non-line-of-sight (NLOS) scenarios is presented. Moreover, the model does not render the multipath structure correctly, which is the cause for the systematic capacity mismatch.

Optimal OFDM design for time-frequency dispersive channels

Abstract: Transmission over wireless channels is subject to time dispersion due to multipath propagation and to frequency dispersion due to the Doppler effect. Standard orthogonal frequency-division multiplexing (OFDM) systems, using a guard-time interval or cyclic prefix, combat intersymbol interference (ISI), but provide no protection against interchannel interference (ICI). This drawback has led to the introduction of pulse-shaping OFDM systems. We first present a general framework for pulse shape design. Our analysis shows that certain pulse shapes proposed in the literature are, in fact, optimal in a well-defined sense. Furthermore, our approach provides a simple way to adapt the pulse shape to varying channel conditions. We then show that (pulse-shaping) OFDM systems based on rectangular time-frequency lattices are not optimal for time- and frequency-dispersive wireless channels. This motivates the introduction of lattice-OFDM (LOFDM) systems which are based on general time-frequency lattices. Using results from sphere packing theory, we show how to design LOFDM systems (lattice and pulse shape) optimally for timeand frequency-dispersive channels in order to minimize the joint ISI/ICI. Our theoretical analysis is confirmed by numerical simulations, showing that LOFDM systems outperform traditional pulse-shaping OFDM systems with respect to robustness against ISI/ICI.

Optimal and suboptimal receivers for ultra-wideband transmitted reference systems

Abstract: The optimal receiver for an ultra-wideband transmitted reference (UWB TR) system in a single user multipath environment is derived, based on knowledge of channel properties. The performances of this optimal receiver and other crosscorrelation receivers are analyzed and compared. The usual crosscorrelation receiver which is often used in UWB TR systems is shown to be suboptimal. In addition, an UWB differential transmitted reference (UWB DTR) system is also proposed and its performance is evaluated.

Maximum-diversity transmissions over doubly selective wireless channels

Abstract: High data rates and multipath propagation give rise to frequency-selectivity of wireless channels, while carrier frequency offsets and mobility-induced Doppler shifts introduce time-selectivity in wireless links. The resulting time- and frequency-selective (or doubly selective) channels offer joint multipath-Doppler diversity gains. Relying on a basis expansion model of the doubly selective channel, we prove that the maximum achievable multipath-Doppler diversity order is determined by the rank of the correlation matrix of the channel's expansion coefficients, and is multiplicative in the effective degrees of freedom that the channel exhibits in the time and frequency dimensions. Interestingly, it turns out that time-frequency reception alone does not guarantee maximum diversity, unless the transmission is also designed judiciously. We design such block precoded transmissions. The corresponding designs for frequency-selective or time-selective channels follow as special cases, and thorough simulations are provided to corroborate our theoretical findings.

Full Spectrum Inversion of radio occultation signals

Abstract: [1] Temperature, pressure, and humidity profiles of the Earth's atmosphere can be derived through the radio occultation technique. This technique is based on the Doppler shift imposed, by the atmosphere, on a signal emitted from a GNSS satellite and received by a low orbiting satellite. The method is very accurate with a temperature accuracy of 1°K, when both frequencies in the GPS system are used. However, difficulties arise when the signal consists of multiple frequencies generated by multipath phenomena in the atmosphere. We demonstrate that, in general, it is possible to determine the arrival times of the different frequency components in a radio occultation signal simply as the derivatives of the phases of the conjugated Fourier spectrum of the entire occultation signal. Based on this property, a novel Full Spectrum Inversion technique for radio occultation sounding capable of disentangling multiple rays in multipath regions is presented. As the entire signal is used in the Fourier transform, a high spatial resolution in the Doppler frequency, and hence in the retrieved temperature, pressure, and humidity profiles, can be achieved. The method is conceptual and computational simple and thus easy to implement. The performance of the Full Spectrum Inversion is demonstrated by applying the technique to simulated signals generated by solving Helmholtz equation with use of the multiple phase-screen technique. Excellent agreement is found between computed bending angle profiles and corresponding solutions to the Abel integral.

Method and apparatus for adapting antenna array using received predetermined signal

Abstract: An antenna apparatus that can increase capacity in a wireless communication system is disclosed. The antenna operates in conjunction with a station and comprises a plurality of antenna elements (101-n), each coupled to a respective weight control component (111-n) to provide a weight to the signal transmitted from (or received by) each element. The weight for each antenna element is adjusted to achieve 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 station, 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 and intercell interference are greatly reduced. The weights are adjusted in a coarse and a fine mode. In the coarse mode all the weight control components are jointly adjusted or changed so that the antenna beam scans through a predetermined sector of a circle until a signal quality metric of the received signal is optimized. The coarse adjustment mode is followed by a fine adjustment mode during which the weights of are independently adjusted to further optimize the signal quality metric.

A new statistical wideband spatio-temporal channel model for 5-GHz band WLAN systems

Abstract: In this paper, a new statistical wideband indoor channel model which incorporates both the clustering of multipath components (MPCs) and the correlation between the spatial and temporal domains is proposed. The model is derived based on measurement data collected at a carrier frequency of 5.2 GHz in three different indoor scenarios and is suitable for performance analysis of HIPERLAN/2 and IEEE 802.11a systems that employ smart antenna architectures. MPC parameters are estimated using the super-resolution frequency domain space-alternating generalized expectation maximization (FD-SAGE) algorithm and clusters are identified in the spatio-temporal domain by a nonparametric density estimation procedure. The description of the clustering observed within the channel relies on two classes of parameters, namely, intercluster and intracluster parameters which characterize the cluster and MPC, respectively. All parameters are described by a set of empirical probability density functions (pdfs) derived from the measured data. The correlation properties are incorporated in two joint pdfs for cluster and MPC positions, respectively. The clustering effect also gives rise to two classes of channel power density spectra (PDS)-intercluster and intracluster PDS-which are shown to exhibit exponential and Laplacian functions in the delay and angular domains, respectively. Finally, the model validity is confirmed by comparison with two existing models reported in the literature.

An accurate ultra wideband (UWB) ranging for precision asset location

Abstract: This paper investigates a ranging method employing ultra wideband (UWB) pulses under the existence of the line of sight (LOS) path in a multipath environment Our method is based on the estimation of time of arrival of the first multipath It averages the received pulses over multiple time frames, performs a correlation operation on the averaged signal, and detects the peak of the correlated signal Our method reduces the ranging accuracy over conventional methods, and its accuracy is close to the Cramer-Rao lower bound (CRLB) on even for a low SNR

Acoustic (Underwater) Communications

Abstract: The need for underwater wireless communications exists in off-shore oil industry, environmental monitoring, speech transmission between divers, control of autonomous underwater vehicles, and mapping of the ocean floor for detection of objects and discovery of new resources. Wireless underwater communications can be established by transmission of acoustic waves. However, the communication channels have limited bandwidth, and often cause severe multipath dispersion and time-variability. Despite these limitations, research efforts of the 90's have culminated in the development of underwater acoustic modems that are capable of transmitting information at rates on the order of several kilobits per second systems over varying distances. The emerging scenario is that of an underwater communication network consisting of both stationary and mobile nodes. Current research is focusing on the design of efficient modulation and coding schemes, adaptive signal processing algorithms for equalization and diversity combining, multiple-access communication methods and network protocols suited for low bandwidth, long propagation delays and strict power requirements encountered in the underwater environment. Keywords: underwater communications; acoustic communications; bandwidt-efficient modulation; adaptive equalization; diversity combining; reduced-complexity receivers; phase-coherent detection; channel estimation; multipath propagation; multiuser detection; underwater networks

Two EM-type channel estimation algorithms for OFDM with transmitter diversity

Abstract: We study channel estimation for orthogonal frequency-division multiplexing (OFDM) systems utilizing transmitter diversity and operating over multipath fading channels. Two expectation-maximization (EM)-type algorithms are introduced and compared with each other in terms of convergence rate. At each iteration and for every OFDM link, the EM-type algorithms partition the problem of estimating a multi-input channel into independent channel estimations for each transmit-receive antenna pair, therefore avoiding the matrix inversion encountered in the joint least-square estimation. The EM-type algorithms can also be used to efficiently implement a recently proposed algorithm, termed the significant-tap-catching estimator, so that the system performance is more robust to different multipath channel delay profiles.

Single-carrier space-time block-coded transmissions over frequency-selective fading channels

Abstract: We study space-time block coding for single-carrier block transmissions over frequency-selective multipath fading channels. We propose novel transmission schemes that achieve a maximum diversity of order N/sub t/N/sub r/(L+1) in rich scattering environments, where N/sub t/ (N/sub r/) is the number of transmit (receive) antennas, and L is the order of the finite impulse response (FIR) channels. We show that linear receiver processing collects full antenna diversity, while the overall complexity remains comparable to that of single-antenna transmissions over frequency-selective channels. We develop transmissions enabling maximum-likelihood optimal decoding based on Viterbi's ( 1998) algorithm, as well as turbo decoding. With single receive and two transmit antennas, the proposed transmission format is capacity achieving. Simulation results demonstrate that joint exploitation of space-multipath diversity leads to significantly improved performance in the presence of frequency-selective fading channels.

Joint channel estimation and data detection in space-time communications

Abstract: The paper considers joint channel estimation and data sequence detection for multipath radio channels with multiple antennas at the transmitter and/or receiver. An iterative space-time receiver based on the expectation-maximization algorithm is proposed. We examine the performance of this receiver for transmit diversity and space-time coding methods over Rayleigh fading channels. Simulation results show that the receiver can often achieve near-coherent performance with modest complexity and using very few pilot symbols.

A low-complexity ML channel estimator for OFDM

Abstract: Orthogonal frequency-division multiplexing with cyclic prefix enables low-cost frequency-domain mitigation of multipath distortion. However, to determine the equalizer coefficients, knowledge of the channel frequency response is required. While a straightforward approach is to measure the response to a known pilot symbol sequence, existing literature reports a significant performance gain when exploiting the frequency correlation properties of the channel. Expressing this correlation by the finite delay spread, we build a deterministic model parametrized by the channel impulse response and, based on this model, derive the maximum-likelihood channel estimator. In addition to being optimal (up to the modeling error), this estimator receives an elegant time-frequency interpretation. As a result, it has a significantly lower complexity than previously published methods.

A simple approach to near-optimal multiuser detection: interleave-division multiple-access

Abstract: This paper presents an asynchronous interleave-division multiple-access (IDMA) scheme, in which users are distinguished by different chip-level interleaving methods instead of by different signatures as in a conventional code-division multiple -access (CDMA) scheme. A very low-cost iterative detection algorithm is derived for the IDMA scheme based on a chip-by-chip detection principle. The proposed scheme can achieve nearly optimal performance for systems with a large number of users. Furthermore, receiver simplicity as well as high performance can be maintained in multipath environments.

A first-order statistical method for channel estimation

Abstract: Multipath is a major impairment in a wireless communications environment, and channel estimation algorithms are of interest. We propose a superimposed periodic pilot scheme for finite-impulse response (FIR) channel estimation. A simple first-order statistic is used, and any FIR channel can be estimated. There is no loss of information rate but a controllable increase in transmission power. We derive the variance expression of our linear channel estimate and compare with the Cramer-Rao bound. Numerical examples illustrate the effectiveness of the proposed method.

Receiver processing systems

Abstract: Spread spectrum receiver architectures and methods are described for reducing interference, particularly the interference observed at a user-end terminal in a W-CDMA 3G mobile communications system. Interpath interference which arises due to non-zero cross and auto correlation of more than one spreading code is suppressed by estimating a transmitted signal stream, or a plurality of such signal streams in the case of a plurality of multipath components, respreading this estimated signal and subtracting non-orthogonal interference contributions from a received signal. The techniques provide an improved bit error rate or equivalently, enhanced capacity for a digital mobile communications network.

Comparison of Multipath Mitigation Techniques with Consideration of Future Signal Structures

Abstract: Code and carrier multipath is still a major issue for high-precision GNSS applications. While many system-inherent errors or errors resulting from the signal propagation characteristics can be modeled, calibrated or eliminated by use of special observation methods (e.g. by differencing code and/or carrier observations), multipath errors are not correlated between two receivers (even if the receivers are separated by only a few meters) and will thus not cancel out by differencing observations. Multipath is thus the dominating error source for many GNSS applications. In order to minimize the influence of multipath signals, several approaches can be taken into account. Among them are the use of special multipath limiting antennas (e.g. choke-ring or multi-beam antennas), post processing techniques to reduce carrier multipath (e.g. multi-antenna spatial processing [7]) or carrier smoothing to reduce code multipath. However, the most important multipath mitigation approaches are receiver-internal correlation techniques. This paper provides a survey of current receiver-internal correlation techniques that are able to minimize code and/or carrier multipath. The mitigation techniques discussed in this paper are the Narrow CorrelatorTM, the Double-Delta correlation techniques, the Early/Late Slope Technique (ELS) and the Early1/Early2 Tracker. While all of these techniques are able to reduce code multipath, only a few of them can also be used to mitigate carrier multipath. These correlation techniques are first introduced with respect to their basic functionality and implementation. In a second step, the performance of these multipath mitigation concepts is illustrated in the form of multipath error envelopes. Assuming a dedicated multipath scenario with one multipath component and a constant signal attenuation factor, this approach allows an intuitive comparison of the aforementioned mitigation techniques. In addition, the performance analysis for these mitigation techniques will also be carried out for different signal types. For this purpose, not only the current GPS signals are considered, but also signals of the modernized GPS and the future Galileo system (namely BOC signals). The main purpose of this paper is to summarize the most important efforts of (receiver-internal) multipath mitigation, to compare the performance of these approaches with respect to each other and to compare their performance when using different types of signals.

An ultra-wideband space-variant multipath indoor radio channel model

Abstract: We present a generic ultra-wideband (UWB) multipath radio channel model that has been designed for link-level simulations and is suited for indoor environments, e.g. for WPAN applications. It covers the FCC frequency band [FCC, Feb. 2002] and is capable of producing space-variant impulse responses (IR) and transfer functions. The model is of a hybrid type and combines a statistical approach to model dense multipath and a simple quasi-deterministic method to model strong individual echoes. These echoes are important to obtain a realistic spatial (Doppler) behavior. The architecture of the model facilitates the application to UWB MIMO analyses. For a few scenarios (e.g., intra-office and inter-office transmission), site-specific parameter profiles are given that have been derived from measured data.

An efficient space-frequency coded OFDM system for broadband wireless communications

Abstract: We propose an efficient space-frequency coded orthogonal frequency-division multiplexing (OFDM) system for high-speed transmission over wireless links. The analytical expression for the pairwise probability of the proposed space-frequency coded OFDM system is derived in slow, space- and frequency-selective fading channels. The design criteria of trellis codes used in the proposed system are then developed and discussed. It is shown that the proposed space-frequency coded OFDM can efficiently achieve the full diversity provided by the fading channel with low trellis complexity, while for traditional space-frequency coded OFDM systems, we need to design space-time trellis codes with high trellis complexity to exploit the maximum achievable diversity order. The capacity properties of space-frequency coded OFDM over multipath fading channels are also studied. Numerical results are provided to demonstrate the significant performance improvement obtained by the proposed space-frequency coded OFDM scheme, as well as the excellent outage capacity properties.

Application of successive interference cancellation to the GPS pseudolite near-far problem

Abstract: Ground-based transmitters called pseudolites have been proposed to augment the basic Global Positioning System (GPS) in environments where satellite visibility is limited. One difficulty in their use is the so-called near-far problem, where in close proximity to the ground transmitter, the pseudolite signal can be orders of magnitude stronger than the satellite signals. This large range of signal levels prevents a conventional receiver from simultaneously detecting both types of signals. This paper describes the application of a signal processing technique, known as successive interference cancellation (SIC), to the acquisition and tracking of weak satellite signals in the presence of a nearby pseudolite and possible multipath reflections of this pseudolite signal. The SIC architecture is implemented on simulated and experimental near-far data sets. The results are compared with a conventional detector and improvements in acquisition and tracking performance are illustrated.

Error performance of a pulse amplitude and position modulated ultra-wideband system over lognormal fading channels

Abstract: In this paper, the error performance of an ultra-wideband (UWB) system with a hybrid pulse amplitude and position modulation (PAPM) scheme over indoor lognormal fading channels is analyzed. In the PAPM UWB system, input data is modulated onto both the pulse amplitudes and pulse positions. The receiver employs a RAKE to combine energy contained in the resolvable multipath components. Derivation of closed-form error rate expressions of the system in lognormal fading channels is based on approximating a sum of independent lognormal random variables (RV) as another lognormal RV using the Wilkinson method. Given the same delay spread of the channel, the proposed PAPM scheme can provide a higher throughput than the binary pulse amplitude or pulse position modulation scheme.

Generalized transmitted-reference UWB systems

Abstract: Transmitted-reference (TR) ultra-wideband (UWB) wireless communication systems [R. Hoctor et al., Jan. 2002] can relax the difficult UWB timing requirements and can provide a simple receiver that gathers the energy from the many resolvable multipath components. However, TR-UWB's relatively poor bit error rate (BER) performance and low data rate have limited its application. In this paper, the TR-UWB idea is generalized to address both of these issues. In particular, the aspects of the system that provide the desirable multipath gathering ability and timing attributes are retained, while the remainder of the system is optimized, resulting in a significantly different signaling scheme and receiver back-end. Numerical results for two examples indicate a significant BER improvement over standard TR-UWB under the same timing requirement.

Simple iterative chip-by-chip multiuser detection for CDMA systems

Abstract: This paper examines several low-cost iterative chip-by-chip multiuser detection algorithms for chip-interleaved code-division multiple-access (CDMA) systems in multipath and multiple antenna environments. The complexities (per user) of these algorithms are very low and increase either linearly or quadratically with path number. It is shown that a system employing a rate-1/2 16-state convolutional code and a length-8 spreading sequence can support more than 100 users with two receive antennas.