Antenna Diversity - Diff Antennas for Transmit and Recieve

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Nov 11th, 2009
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Hi,


There is a wierd question wandering i my mind as I read abt Antenna Diversity.

- It eleminates multipath distortions


1. Now if we turn off diverity. Can I use one antenna for Transmit and the Other Antenna for Recive?


2. When We say Antenna Diversity, is it Transmit Diversity or Receive Diversity or both?


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jeff.kish Wed, 11/11/2009 - 05:48
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1. No, access points cannot use one for transmit and one for receive. The reason is that both antennas connect to the same radio, and the radio itself can only transmit or listen, never both at once.


2. Diversity is receive diversity. Transmitting the same signal twice would just make multipath worse :)


A few other points that should hopefully make things a bit clearer:


- Only one antenna is active at a time. The radio uses one to transmit a signal, and it flips between the two antennas very quickly while receiving.

- Diversity works by comparing two similar signals and seeing which one is stronger. The real advantage gained from diversity is the physical separation of the antennas. Since they're located in two different spots, the signal will vary between these two spots, and the AP can pick which one of these spots has the better signal at any given time.

- Diversity antennas should never be deployed to create two separate coverage areas. The two antennas should be identical, be deployed identically, and be intended to create a single coverage area. They should be as close together as possible, and they should be some multiplier of the frequency's wavelength. Using rubber duck antennas on a Cisco AP takes care of all of these points, but you need to keep them all in mind if you ever use detached antennas.


I hope that helps!


Jeff

Rob Huffman Wed, 11/11/2009 - 13:55
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Hi Queenie,


Just to add a note to the excellent tips from Jeff (+5 points here my friend :)



Have a look @ this good doc;


Multipath and Diversity


http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008019f646.shtml



Here is the best description of why to use Diversity Antennas (in laymans terms) from a great NetPro ScottMac. When I read his reference to being at a stadium watching a sporting event, this all finally made sense.Have a look at his great explanation;


http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless - Mobility&topic=WLAN Radio Standards&CommCmd=MB?cmd=pass_through&location=outline@^1@@.1dd7905b/6#selected_message



Hope this helps!

Rob


Robert.N.Barrett_2 Thu, 11/12/2009 - 04:36
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Rob,


Great article in the first link. I think something is off in the second link.


The "Case Study" from the first link has interesting wording that conflicts with what I've heard from Cisco in the past (and what Jeff mentions in his reply). According to the article, diversity will use the appropriate antenna for transmit, too.


"Thus, when the access point cannot receive the client signal, it switches. The access point evaluates and uses the best antenna to receive client data. The access point then uses that same antenna when it transmits data back to the client. If the client does not respond on that antenna, the access point tries to send the data out the other antenna."

Rob Huffman Thu, 11/12/2009 - 05:33
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Hey Robert,


Good stuff my friend! I've always understood the process this way, and it makes sense to me (but I could be crazy ;-)


In the link I tried to send (we'll try once more, if it doesn't work just copy/paste the link into your browser)


http://forum.cisco.com/eforum/servlet/NetProf?page=netprof&forum=Wireless%20-%20Mobility&topic=WLAN%20Radio%20Standards&topicID=.ee6e8c2&fromOutline=&CommCmd=MB%3Fcmd%3Ddisplay_location%26location%3D.1dd7905b



Think about Scott's excellent "Ballpark" analogy. If the Antenna cannot Receive from the client on one side, it wouldn't make any sense to try and Transmit back out that same side.



Cheers!

Rob

Philip Mon, 03/28/2011 - 11:27
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Apologies for resurrecting, but I was just searching this same question, found an answer contrary to the thread's mainline, and thought I'd weigh in just in case someone else is curious.  When working with the wireless interface the 'antenna' command appears to provide some nice functionality, relevant to this thread, and also allows control of Gain.


conf t

interface dot11Radio 0 (for 80211bg or 1 for 80211a radio)

antenna receive/transmit left/right


or, to turn diversity back on (or perhaps on/off for either receive/transmit? haven't tested)


antenna receive/transmit diversity


Cheers,

-Phil

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