4.1 Introduction

The channel model is the bread and butter for the telecommunications engineer.

This is how I used to emphasize in my lectures the importance of the channel model for designing radio interfaces for wireless communications systems. And this was not just a phrase to keep the students' attention. The channel models really are the foundation on which mobile communications systems are built.

As the main topic of this book is the design of OFDM air interfaces, this chapter describes and discusses an appropriate channel model for such systems [1 44]. This model must address two general requirements:

1. Analytical treatment of OFDM-related problems;

2. Efficient computer simulation schemes.

According to the system's key specifications, it should fit into physical radio channels in the millimeter-wave frequency band for indoor (in-room) and short-range outdoor environments.

Radio propagation in a mobile radio channel is determined mainly by its multi-path nature. Multiple reflections and sometimes an LOS component of the transmitted signal arrive at the receiver via different propagation paths and, therefore, with different amplitudes and delay times. As an effect of this, the narrowband received power fluctuates dramatically when observed as a function of location (or time) and frequency. In the early days of mobile systems, the communications engineer was mainly interested in the time-variability of narrowband channels, which were thus studied extensively (see, e.g., [1]). By that time, transmission bandwidths were small; thus, flat-fading was a reasonable assumption. As the systems have evolved, demand for higher transmission rates has been increasing, making the...