Radio Wave Propagation
Multipath

Imagine a sandwich of several layers of transparent materials. Imagine the center layer, the core, has a higher index of refraction than the two outer layers. Light rays traveling at certain angles through the core medium will be reflected off of the interfaces, according to the law of total internal reflection. Now imagine a light source emitting at several angles, which would all be reflected. This is multipath distortion or interference .

In many common WLAN installations, the radio waves emitted from a transmitter are traveling at different angles. They can reflect off of different surfaces and end up arriving at the receiver at slightly different times. All waves are traveling at approximately the speed of light. However, only a small amount of time difference is needed, to result in a distorted microwave signal. Multipath interference can cause high RF signal strength, but poor signal quality levels. This is an important issue to consider when installing WLANs.

Utilize the interactive activity to learn more about multipath distortion. The path of two reflected rays, from a laptop to an AP, will be shown. Notice that the rays take different paths. The longer the path, the longer time required to reach the destination. At the destination, the two rays of light can interfere with each other, through constructive and destructive interference. If this interference is destructive enough, the messages will not get through. This is what can occur with multimode optical fibers.


Interactive Media Activity

Interactive Activity: Multipath

This activity demonstrates how a laptop transmits signals and how those signals can be received by the Access Point.