ULTRA-WIDE-BAND PROPAGATION CHANNELS

INTRODUCTION

There is a general trend in wireless communications to increase the bandwidth occupied by the employed signals. This trend is caused on one hand by the constantly increasing demand for data rates V while 10 kbit/s is sufficient for speech communications, new applications like video-on-demand require 10 Mbit/s and more. On the other hand, multiple-access schemes like code-division multiple access (CDMA) require signals with larger bandwidth in order to achieve advantages like robustness to fading, improved multiple-access capabilities, and immunity to interference. Ultra-wide-band (UWB) radio pushes this trend to the limit by occupying bandwidths of 500 MHz or more (UWB with large absolute bandwidth) and/or using a bandwidth that is 20% or larger than the carrier frequency (UWB with large relative bandwidth).

UWB systems can thus exploit the advantages of large bandwidths to the hilt. UWB communications has gathered great interest from the academic research community and the industry, especially in the last 15 years. This interest is due to a confluence of factors:

i)                    theoretical breakthroughs, especially the introduction of time-hopping impulse radio by Win and Scholtz in the early 1990s;

ii)                  ii) new frequency regulations, in particular the 2002 decision of the Federal Communications Commission in the United States, that allow the unlicensed operation of UWB radios in the microwave range;

iii)                iii) advances in digital and analogue circuitry that made generating and processing of UWB signals feasible at a reasonable price;

iv)                iv) the development of new applications that required the unique features that UWB signals offer, e.g., extremely high data rates (500 MBit/s for wireless USB), precise ranging and geolocation (for many sensor networks), and covert high-data-rate communications (e.g., for military video transmission).

These developments in turn resulted in more than 5000 research papers on the topic, as well as the development of several communications standards based on UWB technology.