This paper presents an overview of ultrawideband (UWB) propagation channels. It first demonstrates how the frequency selectivity of propagation processes causes fundamental differences between UWB channels and "conventional" (narrowband) channels. The concept of pathloss has to be modified, and the well-known WSSUS model is not applicable anymore. The paper also describes deterministic and stochastic models for UWB channels, identifies the key parameters for the description of delay dispersion, attenuation, and directional characterization, and surveys the typical parameter values that have been measured. Measurement techniques and methods for extracting model parameters are also different in UWB channels; for example, the concepts of narrowband channel parameter estimation (e.g., maximum-likelihood estimation) have to be modified. Finally, channel models also have an important impact on the performance evaluation of various UWB systems.

Ultrawideband propagation channels-theory, measurement, and modeling

Understanding ultra-wide-band (UWB) propagation channels is a prerequisite for UWB system design as well as communication-theoretic and information-theoretic investigations. This paper surveys the fundamental properties of UWB channels, pointing out the differences to conventional channels. If the relative bandwidth is large, the propagation processes, and therefore path loss and shadowing, become frequency-dependent, and the well-known wide-sense stationary uncorrelated scattering model is not applicable anymore. If the absolute bandwidth is large, the shape of the impulse responses as well as the fading statistics change. This paper also describes methods for measuring UWB channels and extracing channel parameters. Throughout this paper, the relationship between channel properties and other areas of UWB research are pointed out.

Ultra-Wide-Band Propagation Channels

Time-domain ultra-wideband (UWB) channel characterization with deconvolution has become increasingly popular. However, owing to the nature of the frequency-selective non-line-of-sight channel on a general wideband channel-sounding pulse, the a priori assumption of the CLEAN deconvolution algorithm does not necessarily hold. Moreover, the assumption of a fixed a priori can easily be violated since the radio-wave propagation mechanisms experienced by the sounding pulse can be frequency-selective. This paper first summarizes the use of the CLEAN algorithm in UWB channel characterization, including a discussion about the a priori assumption, enhancements made to the basic algorithm, and the effect of the a priori assumption on channel characteristics. A study of the a priori dependence is then presented along with real-world measurements to demonstrate the shortcomings of the single-template CLEAN algorithm. Finally, a high-resolution, multi-template deconvolution algorithm is proposed to enhance the channel estimation performance. This algorithm incorporates realistic frequency-dependent pulse distortions and is shown to supersede its predecessors by being computationally efficient, simple, accurate, and deployable for point-to-point measurements.

A high-resolution, multi-template deconvolution algorithm for time-domain UWB channel characterization

The variation of channel fading and dispersion statistics between the ultrawideband (UWB) channel subbands is characterized experimentally. Within the FCC-allocated UWB frequency range (3.1-10.6 GHz), the multipath characteristics in various subbands are found to be highly dissimilar. It is demonstrated that the higher subbands suffer greater attenuation, and the rms delay spread varies significantly with the center frequency. The impact of this variation on multiband-OFDM UWB system performance is evaluated, and a large performance variation across the band is reported

/pdf/frequency-dependence-of-fading-statistics-for-ultrawideband-m9vwaltzem.pdf

Frequency Dependence of Fading Statistics for Ultrawideband Systems

High-resolution time-domain ultra-wideband (UWB) channel characterization with CLEAN deconvolution has become increasingly popular. However, due to the extremely wide bandwidth of the pulse, the assumption of negligible pulse distortion no longer holds. In this paper, we first summarize the current works in channel characterization with the CLEAN algorithm. We then show the a priori dependence of CLEAN, and propose a modified multi-template approach to overcome these issues while retaining its high-resolution capability.

A High-Resolution, Multi-Template Deconvolution Algorithm for Time-Domain UWB Channel Characterization

The problem of a pulsed short-duration ultra-wideband (UWB) electromagnetic field incident on a multiple-layer lossy dielectric wall is considered. The electromagnetic fields are computed utilizing both frequency-domain (FFT) and finite-difference time-domain (FDTD) formalisms. Computed results, pertaining to internal and external EM fields, based on FFT and FDTD formulations are compared. The effects of layer thickness and material composition on the reflected and transmitted EM pulse are analyzed. The application of UWB technology to nondestructive testing (NDT) and evaluation of multilayer structures is discussed.

Ultra-wideband (UWB) incidence on multiple dielectric interfaces

The radiation characteristics of ultra-wideband (UWB) monopulse arrays are discussed in this paper. The UWB arrays considered here consist of ultra-wideband sources radiating coded sequences of ultra-narrow Gaussian mono-pulses with very low duty cycle. A pseudorandom (PN) time-hopping spread spectrum technique is utilized for further spreading the signal spectrum and providing multiple access interference rejection, and secure communication. Pulse position modulation (PPM) at the rate of many pulses per data bit is used for data modulation. At the receiver correlation is applied and the PN code is used to discriminate between the desired and interfering signals. A model for the general UWB monopulse volume array has been developed. Mathematical formulas for parameters of interest have been derived and computed results have been presented.

http://ieeexplore.ieee.org/iel5/7598/20720/00959764.pdf

Ultra-wideband antenna arrays

This paper presents a discrete scattering model for the ultra-wideband (UWB) propagation channel in dense multipath environments. A method is introduced whereby the complex and highly cluttered UWB propagation channel is replaced with a finite impulse response (FIR) filter. Subtractive deconvolution method is applied to obtain the filter parameters from channel transfer function measurement data. Channel transfer characteristics obtained from the discrete scattering model are then compared to the experimental data based on the UWB channel measurements

A physical scattering model for the ultra-wideband (UWB) propagation channel

The problem of a pulsed electric current source comprising an infinitely long filament has been solved using both time-domain and frequency-domain techniques Also formulated is the problem of an infinitely long electrically conducting (PEC) thin cylinder, illuminated by a uniform normally impinging, pulsed plane wave excitation Computed results for both problems illustrating the radiated field, induced current, and scattered field are presented, and physical interpretations of the mathematical formulae and numerical data are provided Numerical results based on time-domain and frequency domain formulations, for each problem, are compared

Ultra-wideband radiation and scattering from a thin filament

The two-dimensional scattering problem of an electrically conducting (PEC) plane perforated by an infinite array of uniform apertures has been formulated. The problem is transformed to an equivalent one, in which the apertures are covered with electric conductors, and impressed magnetic surface current sources of equal magnitude and opposite polarity are placed on two opposing sides of each conductor. An integral equation for the magnetic current density function is obtained by enforcing the continuity of tangential magnetic field across the apertures, which is subsequently solved using moment method. Computationally efficient formulas for impedance elements have been proposed. Due to periodicity of the structure all elements of the impedance matrix are obtained form a single row. The formulation of this problem can be applied to the electromagnetic characterization of metal studded walls in wireless communication channels.

K. Heidary

Papers

Ultra-wideband (UWB) incidence on multiple dielectric interfaces

Ultra-wideband antenna arrays

A physical scattering model for the ultra-wideband (UWB) propagation channel

Ultra-wideband radiation and scattering from a thin filament

Transmission and reflection characteristics of a conducting wall with periodic apertures (TE case)