Detection of PPM-UWB random signals

This paper focuses on the symbol detection problem of random pulse-position modulation (PPM) ultrawideband (UWB) signals in the absence of interframe interference. Particular attention is devoted to severely time-varying channels where optimal detectors are proposed for both uncorrelated and correlated scattering scenarios. This is done by assuming the received waveforms to be unknown parameters. In UWB communication systems, the assumption of unknown random waveforms is consistent with the fact that the received waveform has very little resemblance with the original transmitted pulse. In order to circumvent this limitation, a conditional approach is presented herein by compressing the likelihood ratio test with the information regarding the second-order moments of the end-to-end channel response. Both full-rank and rank-one detectors are derived. For the reduced complexity rank-one detector, an iterative procedure is presented that maximizes the J-divergence between the hypotheses to be tested. Finally, simulation results are provided to compare the performance of the proposed detectors in different propagation environments.Peer Reviewe

Distributed Power Control Techniques Based on Game Theory for Wideband Wireless Networks

This thesis describes a theoretical framework for the design and the analysis of distributed (decentralized) power control algorithms for high-throughput wireless networks using ultrawideband (UWB) technologies. The tools of game theory are shown to be expedient for deriving scalable, energy-efficient, distributed power control schemes to be applied to a population of battery-operated user terminals in a rich multipath environment. In particular, the power control issue is modeled as a noncooperative game in which each user chooses its transmit power so as to maximize its own utility, which is defined as the ratio of throughput to transmit power. Although distributed (noncooperative) control is known to be suboptimal with respect to the optimal centralized (cooperative) solution, it is shown via large-system analysis that the game-theoretic distributed algorithm based on Nash equilibrium exhibits negligible performance degradation with respect to the centralized socially optimal configuration. The framework described here is general enough to also encompass the analysis of code division multiple access (CDMA) systems and to show that UWB slightly outperforms CDMA in terms of achieved utility at the Nash equilibrium

Ultra-Wideband Wireless Channels - Estimation, Modeling and Material Characterization

This licentiate thesis is focused on the characterization of ultra-wideband wireless channels. The thesis presents results on ultra-wideband communications as well as on the ultra-wideband characterization of materials. The communications related work consisted in the measurement and modeling of outdoor scenarios envisioned for infostation systems. By infostation, we mean a communication system covering a small area, i.e., ranging up to 20 m, where mobile users can pass by or stop while receiving large amounts of data in a short period of time. Considering the expected (but perhaps overly optimistic) 480 Mbps for UWB systems, it should be possible to download a complete DVD in roughly two minutes, which is something not realizable with any of the current wireless technologies. Channel models, commonly based on measurements, can be used to evaluate the performance of such systems. We therefore, we started by performing measurements at one of the scenarios where infostation systems can exist in the future, namely, petrol stations. The idealized model, was one that could correctly describe the continuous evolution of the channel impulse response for a moving user within the system’s range, and therefore it was deemed necessary to track the multipath components defining the impulse responses along a path of several meters. To solve this problem we designed a novel high-resolution scatterer detection method, which is described in Paper I, capable of tracking individual multipath components for a moving user by identifying the originating point scatterers in a two dimensional geometrical space. The same paper also gives insight on some properties of clusters of scatterers, such as their direction-selective radiated power. The scatterer detection method described in Paper I provided us with the required tools to create the channel model described in Paper II. The proposed channel model has a geometrical basis, i.e., each realization of the channel is based on a virtual map containing point scatterers that contribute to the impulse response by multipath components. Some of the particular characteristics of the model include non-stationary effects, such as shadowing and cluster’s visibility regions. At the end of Paper II, in a simple validation step, the output of the channel model showed a good match with the measured impulse responses. The second part of our work, documented in Paper III, consisted on the dielectric characterization of soil samples using microwave measurements. This project was made in cooperation with the Department of Physical Geography and Ecosystem Analysis at Lund University, which had been developing research work on methane emissions from the wetlands in Zackenberg, Greenland. In recent years, a lot of attention has been put into the understanding of the methane emissions from soils, since methane is a greenhouse gas 20 times stronger than carbon dioxide. However, whereas the methane emissions from natural soils are well documented, the reason behind this effect is an open issue. The usage of microwave measurements to monitor soil samples, aims to address this problem by capturing the sub-surface changes in the soil during gas emissions. An experiment consisting on the monitoring of a soil sample was performed, and a good correlation was found between the variations of the microwave signals and the methane emissions. In addition, the soil dielectric constant was calculated, and from that, the volumetric fractions of the soil constituents which provided useful data for the elaboration of models to describe the gas emission triggering mechanisms. Based on this laboratory experiment, a complete soil monitoring system was created and is at the time of writing running at Zackenberg, Greenland

Information Theory of underspread WSSUS channels

The chapter focuses on the ultimate limit on the rate of reliable communication through Rayleigh-fading channels that satisfy the wide-sense stationary (WSS) and uncorrelated scattering (US) assumptions and are underspread. Therefore, the natural setting is an information-theoretic one, and the performance metric is channel capacity. The family of Rayleigh-fading underspread WSSUS channels constitutes a good model for real-world wireless channels: their stochastic properties, like amplitude and phase distributions match channel measurement results. The Rayleigh-fading and the WSSUS assumptions imply that the stochastic properties of the channel are fully described by a two-dimensional power spectral density (PSD) function, often referred to as scattering function. The underspread assumption implies that the scattering function is highly concentrated in the delay-Doppler plane. Two important aspects need to be accounted for by a model that aims at being realistic: neither the transmitter nor the receiver knows the realization of the channel; and the peak power of the transmit signal is limited. Based on these two aspects the chapter provides an information-theoretic analysis of Rayleigh-fading underspread WSSUS channels in the noncoherent setting, under the additional assumption that the transmit signal is peak-constrained

Channel-based antenna synthesis for improved in-vehicle UWB MB-OFDM communications

Ultra-wide band (UWB) is an attractive technology for innovative in-vehicle wireless communications requiring high data rates and multiband orthogonal frequency division multiplexing (MB-OFDM) a suitable scheme for the accomplishment due to its high performance, low-power and low-cost characteristics. To contribute toward improved UWB MB-OFDM communications inside vehicles, a channel-based antenna synthesis technique to customise in-vehicle UWB antennas that reduce ‘blind spots’ in the communication channel is proposed and presented. For the realisation, a comprehensive analysis was utilised and comprised an in-car channel evaluation including bit-error-rate (BER) estimations and radiation pattern-and-source syntheses. The channel was measured using a standard antenna to set up the base of the experiments and the distribution of the impulse responses and signal-to-noise ratios in the vehicle's passenger plane shown. The currently available IEEE 802.15.3a channel models were perceived unrealistic for the in-vehicle application and the reason for measuring the channel practically. Using these specific channel measurements, the synthesised pattern is unveiled and consequently the channel-based antenna synthesis technique used to predict the antenna source. The antenna with optimised pattern-and-source showed an improved BER performance compared with the standard antenna in this application; that is, a figure of merit of 37.73% minimised ‘blind spots’

Using Akaike Information Criterion for Selecting the Field Distribution in a Reverberation Chamber

Previous studies on modeling the random field (amplitude) in a reverberation chamber (RC) were conducted either by fitting a given distribution to measured data or by comparing different distributions using goodness-of-fit (GOF) tests. However, the GOF tests are inappropriate for comparing different distribution candidates in that they are meant to check if a given distribution provides an adequate fit for a set of data or not and they cannot provide correct relative fitness between different candidate distributions in general. A fair comparison of different distributions in modeling the RC field is missing in the literature. In this paper, Akaike's information criterion (AIC), which allows fair comparisons of different distributions, is introduced. With Rayleigh, Rician, Nakagami, Bessel K, and Weibull distributions as the candidate set, the AIC approach is applied to measured data in an RC. Results show that the Weibull distribution provides the best fit to the field in an undermoded RC and that the Rayleigh distribution provides the best approximation of the field in an overmoded RC. In addition, it is found that both the Rician and Weibull distributions provide improved approximations of the field in an RC loaded with lossy objects. This study provides correct complementary results to the previous RC studies

Artifact paths removal algorithm for ultra-wideband channels

Ultra-wideband (UWB) is a promising technology for achieving high data rate communications. When UWB channel measurements are conducted, channel impulse responses (CIRs) are extracted from measured UWB waveforms using CLEAN deconvolution algorithm. However, artifact paths that represent unreal received multipath components (MPCs) are generated during this process. These artifact paths are registered as part of the measured CIRs representing a reflected signal from a scatterer. In reality, these paths do not represent a real scattering environment and this affects accurate channel modeling. Therefore, removal of the artifact paths is important to conserve better and have a more real scattering environment. In this work, an algorithm was developed to remove artifact paths from measured CIRs. The algorithm development was achieved based on the concept of geometric elliptical modeling applied to wideband channels, where the effective path in each ellipse is utilized to represent the channel response of the ellipse. Several UWB channel measurements were conducted to obtain the measured UWB waveforms. In addition, the characteristics of the UWB channels were analyzed in terms of CIRs properties and their stationarity regions. The algorithm performance was evaluated by comparing the single-template CLEAN CIRs with the CIRs result from the application of the developed algorithm on single-template CLEAN CIRs. Results showed that the developed algorithm can successfully remove the artifact paths. Besides that, an enhancement in the received power was achieved. For a specific measured channel, the received power enhancement obtained was more than 5%. The algorithm is beneficial for enhancing accuracy of CIRs extracted from a single-template CLEAN algorithm. Consequently, more accurate channel characteristics are gained leading to improved channel modelling and different parameter extractions