**2. Broadband wireless channel models**

One of the more intriguing aspects of wireless channels is fading. Unlike path loss or shad‐ owing, which are large-scale attenuation effects owing to distance or obstacles, fading is caused by the reception of multiple versions of the same signal. The multiple received versions are caused by reflections that are referred to as *multipath*. The reflections may arrive nearly simultaneously— for example, if there is local scattering around the receiver—or at relatively longer intervals— for example, owing to multiple paths between the transmitter and the receiver (Figure 1).

When some of the reflections arrive at nearly the same time, their combined effect is as in Figure 2. Depending on the phase difference between the arriving signals, the interference can be either constructive or destructive, which causes a very large observed difference in the amplitude of the received signal even over very short distances. In other words, moving the transmitter or the receiver even a very short distance can have a dramatic effect on the received amplitude, even though the path loss and shadowing effects may not have changed at all.

One of the key parameters in the design of a transmission system is the maximum delay spread value that it has to tolerate.

In order to design and benchmark wireless communication systems, it is important to develop channel models that incorporate their variations in time, frequency, and space. Models are

**Figure 1.** A channel with a few major paths of different lengths, with the receiver seeing a number of locally scattered versions of those paths.

classified as either *statistical* or *empirical*. Statistical models are simpler and are useful for analysis and simulations. Empirical models are more complicated but usually represent a specific type of channel more accurately. There are several channels models which are explained in the following,
