Multipath Distortion
Occurs when an RF signal has more than one path between a receiver and a transmitter
RF take more than one path
Multiple signals cause distortion of the signal
Can cause high signal strength yet low signal quality
© 2003, Cisco Systems, Inc. All rights reserved. AWLF v3.1—2-34
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 of objects, so does RF. This means there can be more than one path that RF takes when going from a TX to an 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.
You can relate this to a common occurrence in your car. As you pull up to a stop, you may notice static on the radio. But as you move forward a few inches or feet, the station starts to come in more clearly. By rolling forward, you move the antenna slightly, out of the point where the multiple signals converge.
- ___, Received Signals
Combined Results
Combined Results
Cisco.com
Cisco.com
In a multipath environment, signal null points are located throughout the area
Moving the antenna slightly will allow you to
• Receive the signal correctly
Ceiling
Ceiling
RX1 Obstacle
RX1 Obstacle
Dual diversity antennas typically mean if one antenna is in a null, the other one will not be, therefore providing better performance in multipath environments
When a radio wave bounces back on itself 180 degrees out of phase, a dead spot or "null" is created. Null points are a fact of life with RF and will be located throughout the coverage area.
Two things can change the multipath null point:
■ Change the location of the antenna.
■ Change the type of antenna, moving the main lobe of energy and affecting the reflected energy.
A diversity antenna system can be compared to a switch that selects one antenna or another, never both at the same time. The radio in receive mode will continually switch between antennas listening for a valid radio packet. After the beginning sync of a valid packet is heard, the radio will evaluate the sync signal of the packet, on one antenna, then switch to the other antenna and evaluate. Then the radio will select the best antenna, and use only that antenna for the remaining portion of that packet.
On transmit, the radio will select the same antenna it used the last time it communicated to that given radio. If a packet fails, it will switch to the other antenna and retry the packet.
Changing the type of antenna and/or the location of the antenna can eliminate multipath interference. The shortest distance between two points is a straight line. As other signals take longer paths, they will arrive later and weaker. All of the signals will combine in the RX to form a single received signal, and the result is distorted.
Caution: Diversity is not designed for using two antennas, covering two different coverage cells. The problem in using it this way, is that if antenna #1 is communicating to device #1, while device #2 (which is in the antenna #2 cell) tries to communicate, antenna #2 is not connected (due to the position of the switch), and the communication fails. Diversity antennas should cover the same area, from only a slightly different location.
Continue reading here: Cisco Aironet Access Point Quality of Service Feature
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