Section 3.1. Introduction

3.1. Introduction

The received signal in any communications system is an attenuated, delayed, and possibly distorted version of the signal that was transmitted plus noise and (possibly) interference. The relationship between the received signal and the transmitted signal is typically called the "channel." In order to evaluate and design wireless systems, we must create models of the channel. In the following sections, we will discuss how the channel is modeled for UWB systems. We should make a careful distinction about the type of modeling being examined. In general, there are two prevalent types of modeling of electromagnetic wave propagation. The first is what might be termed "site-specific modeling" or "deterministic modeling," and attempts to model the exact interaction of the EM wave in the specific environment of interest. This type of modeling is discussed in Chapter 2, "Channel Measurement and Simulation," for UWB signals and is often used to predict coverage patterns in wireless systems when detailed information concerning the environment is available. A second type of modeling attempts to model the relevant statistics of the received signal and may be called "statistical modeling." Statistical modeling, discussed in this chapter, is particularly useful in communication system development where the system must work in a wide variety of environments.

In our discussion of the UWB channel, we can conveniently divide the effects into three categories: (a) large-scale effects, (b) small-scale effects, and (c) undesired signals. The third category includes the impact of noise sources and interfering signals. We will defer the discussion of these until Chapter 6, "Receiver Design Principles." The phrase "large-scale" typically refers to the impact that the channel has on the transmit signal over large distances and generally includes only average attenuation effects due to distance and large objects that are in the propagation path. Conversely, "small-scale" typically refers to attenuation changes over small distances and the distortion to the waveform introduced by the channel. We will examine how the two effects are modeled separately.