Wireless-N / Diversity and Multipath

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Jun 28th, 2010
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I have a question/concern with the installation of Cisco Wireless-N access points, specifically with the use of higher gain antenna's.  There was an older doc that gave specific instructions to use multiples of 4.92" (NOT TO EXCEED 4 INTERVALS) as a theoretical distance between 2 antenna's on the 2.4 Ghz side.  On the 5 Ghz side it states to use 2.36" with the same caveat.


My question is -- what are the ground rules for the newer access points that require 3 antenna's ?


Does Cisco have any guidance with regards to the installation of the higher gain antenna's ?


My gut feeling tells me that the rules are the same since the signal on 2.4 and 5 Ghz has not changed.  I also wanted to know how far apart should the 5 Ghz be from the 2.4 Ghz antenna's to avoid any issues.


I attached the older doc as a point of reference. 

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Rob Huffman Tue, 06/29/2010 - 06:47
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Hi there,


Excellent question


Great answer from Fred Niehaus TME Cisco WNBU.


https://supportforums.cisco.com/message/1314938#1314938



Cheers!
Rob




Please support CSC Helps Haiti


https://supportforums.cisco.com/docs/DOC-8895


https://supportforums.cisco.com/docs/DOC-8727

ca4199 Tue, 06/29/2010 - 18:57
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I read that posting and what I gather is that from 802.11b/g to 802.11n antenna placement is not as critical.  In the past we have been keeping to 19.75" apart.... to keep diversity and multipath in check.


Would it be safe to assume that I can keep each 2.4 Ghz antenna 9.84" apart and each 5.0 Ghz antenna 4.72" apart.  It actually works out nice cause the 2 furthest antennas would be 19.75" apart on the 2.4 side and 9.44" on the 5 Ghz side.  They would not exceed the older requirements.

abersven Sat, 07/03/2010 - 12:04
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/* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}

Remember that the optimal distances between the antennas are calculated and used in order to get correct phasing on the RF signals. The cable lengths must be a part of this equation in order to achieve optimal distance. So don’t cut the original cables – keep the same cable lengths on all antennas – or do some more advanced calculations.


A short rule is that the distance between the antennas should be a multiple of half the wavelength.


For 11n that uses MIMO you should use three equal antennas for each band covering the same area. You should not try to extend the coverage area by separating the antennas too much or i.e. use three sector antennas in different or slightly different directions.


/* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-qformat:yes; mso-style-parent:""; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;} Your gut feeling about distance between 2,4GHz antennas and 5GHz antennas is good. You should have some physical distance but since there is 2,6GHz between these bands the inbuilt filters should give enough separation.

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