Multipath Distortion

This topic describes multipath distortion.

Multipath Distortion

Multipath distortion (a form of radio degradation) occurs when radio signals bounce off metal objects in a room, such as metal cabinets or ceiling lights.

OFDM overcomes multipath distortion through parallel frequency use.

Multiple signals at receiver cause distortion of the signal.

As radio waves bounce, they arrive at the receiver slightly delayed, combining with the original signal, causing distortion.

Diversity systems use two antennas in different positions to reduce the degradation.

OFDM overcomes multipath distortion through parallel frequency use.

Diversity systems use two antennas in different positions to reduce the degradation.

Multipath interference occurs when an RF signal has more than one path between a receiver and a transmitter. Just as light and sound bounce off objects, so do RF waves. RF waves can take more than one path when going from a transmitting (Tx) antenna to a receiving (Rx) antenna. These multiple signals combine in the Rx antenna and receiver to cause distortion of the signal.

Multipath interference can cause high signal strength yet low signal quality, whereby the data would be unreadable. One indication that you are getting multipath interference is the drastically fluctuating signal strength and signal quality when you move the client only very minor amounts (inches).

When an antenna transmits, it radiates RF energy in more then one definite direction. This transmission causes RF to move between the transmitting and receiving antenna in the most direct (desired) path and to take other routes that include reflecting or bouncing off metallic and other RF reflective surfaces. The process of reflecting the RF waves causes several things to occur:

■ The reflected waves travel farther than the desired direct RF wave, which causes them to get to the receiving antenna later.

■ Because of the longer transmission route, the signal loses more RF energy while traveling than the direct route signal.

■ The signal loses some energy as a result of the reflection or bounce.

When these reflected signals are combined at the receiver, although RF energy (signal strength) may be high, the data would be unrecoverable. In the end, the desired wave is combined with many reflective waves in the receiver. As these different waveforms combine, they cause distortion to the desired waveform, which can affect the decoding capability of the receiver, resulting in poor performance. It is also possible that the radio signals can cancel each other out, causing what is known as a radio null, or dead spot.

6-132 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

Changing the location of the antenna can change these reflections and diminish the chance of multipath interference and nulls. Diversity systems use two antennas, and the access point samples each of the antennas, choosing the antenna with the best performance.

The pattern in which signals reflect is greatly affected by the physical wavelength of the signal. Because the wavelength is inversely proportional to the frequency, each frequency has differing multipath effects (fading). In a location where one frequency has a large multipath interference issue, another, even close frequency, will typically not have multipath interference.

Because orthogonal frequency-division multiplexing (OFDM) is based on many different frequencies, all operating in parallel, the odds are good that some of the information in at least some of the frequencies will be communicated successfully. This approach provides much greater performance in multipath environments.

© 2006 Cisco Systems, Inc. Wireless LANs 6-133

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