Section 3.2. What s Different About UWB?

3.2. What's Different About UWB?

Channel modeling for wireless systems is a well-investigated topic and is the subject of several texts [2, 3, 26]. Thus, an important first step in this chapter is to address the reasons why we need to revisit channel modeling for UWB. The main reason to revisit channel modeling is the extremely large bandwidth associated with UWB signals. Traditional channel models for path loss assume that diffraction coefficients, attenuation due to materials, and other propagation effects are constant over the band of interest. When the fractional bandwidth (see Chapter 1, "Introduction," for a definition of fractional bandwidth) is 0.01 or less, this is a safe assumption. Additionally, narrowband models often incorporate antenna effects, such as the effective aperture, into the path loss. Again, this is acceptable when the change in these antenna effects is negligible over the band. Neither of these assumptions is correct for a UWB system.

There are also small-scale assumptions that may no longer hold for UWB systems. Most importantly, narrowband, and even wideband, channel models assume that the received signal is the sum of delayed, phase shifted, and attenuated copies of the received signal. The interaction of these multiple signals results in fading and possible frequency distortion. However, it is assumed that the individual copies are not distorted. When UWB signals are being used, this may no longer be true. Individual components of the received signal may be distorted, thus introducing pulse-level frequency distortion in addition to the distortion seen in the total received signal. This means that we must revisit the traditional model.

In the following sections, we will address each of these assumptions and show how UWB affects traditional assumptions and how we need to either adjust our modeling approach or adjust our understanding of the modeling approach. Specifically, we will address the two main areas of channel modeling, large-scale modeling and small-scale modeling. Section 3.3 addresses large scale modeling, while Section 3.4 addresses small-scale modeling. The spatial behavior (important for multiple antenna systems) of UWB signals is addressed in Section 3.5. The impact of the discrete time channel model is examined in Section 3.6. Conclusions are made in Section 3.7. UWB channel modeling efforts are fairly recent and remain an active area of research [1]. Throughout this chapter, we will attempt to highlight recent work, although the goal is to provide a basic understanding rather than a comprehensive survey.