Cisco Cisco Aironet 350 Wireless Bridge Hoja De Datos

 

 

© 2014 Cisco and/or its affiliates. All rights reserved. This document is Cisco Public Information. 

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Figure 4.    Multipath Distortion 

 

You can relate multipath distortion 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. 

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, and 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. 

One caution with diversity antenna systems is that they are not designed for using two antennas covering two 

different coverage cells. The problem in using it this way is that if antenna number 1 is communicating to device 

number 1 while device number 2 (which is in the antenna number 2 cell) tries to communicate, antenna number 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. 

With the introduction of the latest direct-spread physical layer chips, and the use of diversity antenna systems, 

direct-spread systems have equaled or surpassed frequency-hopping systems in handling multipath interference. 

While the introduction of WBFH does increase the bandwidth of frequency-hopping systems, it drastically affects 

the ability to handle multipath issues, further reducing its range compared to present direct-spread systems in sites 

that are highly RF reflective. 

Wireless LAN Design 

Before the physical environment is examined, it is critical to identify the mobility of the application, the means for 

coverage, and system redundancy. An application such as point-to-point, which connects two or more stationary 

users, may be best served by a directional antenna, while mobile users will generally require a number of 

omnidirectional micro cells. These individual micro cells can be linked together through the wired LAN 

infrastructure or by using the wireless repeater functionality built into every Cisco Aironet access point.