An Introduction to Ultra Wideband Communication Systems

2.1. Introduction

The development of channel models for UWB communication systems requires extensive data on UWB signal propagation. Both experimental and simulation techniques can be used to examine the propagation of UWB signals in indoor and indoor-outdoor environments. The advantage of experimental methods is that all system and channel parameters affecting the propagation of UWB signals are accounted for without preassumptions. However, these methods are usually expensive, time consuming, and limited by the characteristics of available equipment, such as sensitivity, bandwidth, dispersion, and the attenuation of the connecting cables. On the other hand, simulation techniques are free from the limitations of experimental approaches, are cost effective, and are less time consuming. Simulators can serve as a useful tool for predicting the behavior of UWB communication systems. The accuracy of simulation results depends on the amount of details included in the simulation model. However, more details lead to more complex computer programs, and require more computational time. Thus, a compromise between the required accuracy and the available computational resources should be made in designing simulators for UWB communication systems.

In narrowband wireless communication systems, the information signal modulates a very high frequency sinusoidal carrier; thus, along each propagation path the signal suffers very little distortion because the system elements such as antennas, reflecting walls, diffracting objects in the channel, and so on, have essentially constant electromagnetic properties over the narrow bandwidth of the radiated signal. The only signal degradation is caused by multipath components. On the other hand, in UWB systems, the information signal may suffer significant distortion due to the transmitting/receiving antennas not meeting the necessary bandwidth requirements, and also due to the dispersive behavior of building materials in the propagation channel. Of course, multipath components are also present in UWB channels. But, unlike narrowband signals, UWB signals do not suffer fading due to the destructive interference of multipath components.

In this chapter, both measurement and simulation of UWB signal propagation are addressed. Section 2.2 surveys time domain and frequency domain measurement techniques. Important issues such as triggering, calibration, interference rejection, and noise are discussed. Typical indoor measurement results in both time domain and frequency domain are presented in Section 2.3. The role of antennas, as well as the impacts of building architecture and dispersive properties of building materials, are also discussed in this section. The electromagnetic simulation of UWB signal propagation in indoor environments is detailed in Section 2.4. Channel models, based on data collected from extensive UWB propagation measurements in two buildings on the Virginia Tech campus, are discussed in Chapter 3, "Channel Modeling."