Impacts of Multipath Propagation
Delays caused by multipath propagation result in the information symbols represented in the radio wave smearing (see Figure 4-12). This is often referred to as intersymbol interference (ISI). Because the shape of the signal conveys the information being transmitted, the receiver will make mistakes when demodulating the signal’s information. If the delays are great enough, bit errors in the packet will occur, especially when data rates are high. The receiver will not be able to distinguish the symbols and interpret the corresponding bits correctly. When multipath strikes in this way, the receiving station must have an error-checking process to ensure that the data it receives is okay. WLANs employ a protocol that causes the source transceiver to retransmit the data if the destination transceiver detects errors in the data at the destination.
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Figure 4-12 Multipath Propagation Decreases the Quality of the Signal at the Receiver
Because of retransmissions, users encounter lower performance when multipath is significant. For example, 802.11 signals in homes and offices might encounter 50 nanoseconds of multipath delay, while for users in a manufacturing plant it could be as high as 300 nanoseconds. Metal machinery and racks in a plant provide a lot of reflective surfaces that cause radio waves to bounce around and take erratic paths. As a result, be wary of multipath problems in warehouses, processing plants, and other areas full of irregular metal obstacles.
Wireless LAN manufacturers compensate for the effects of multipath propagation by using special processing techniques. For example, equalization and antenna diversity are methods for reducing the number of problems arising from multipath propagation. See Chapter 12 for more details on using diversity to combat multipath propagation.