Help With Antenna Diversity

Dave Edwards Dave.Edwards at
Mon Dec 9 12:13:48 EST 2002

Hi Michael,

In my experience Antenna Diversity only really works inside a building, 
the net effect is around +3dB improvement on the receive sensitivity for 
equal antennas.

There is also an issue with the Prism II diversity state machine. I 
don't know if it's been fixed with the later versions of the chipset but 
here is what we observed with the Prism I and Prism II devices:

Firstly the receive process starts with either the last good antenna (ie 
the last port to sucessfully receive a packet) or the default TX/RX port 
(see the prism manual for Antenna port selection) being selected.

Antenna diversity operates when the receiver detects enough energy on 
that port and a decision is then made about which port to use for the 
rest of the packet. The chipset will switch ports for a number of cycles 
until it makes the decision

The problem comes when you are in a situation where the port being 
selected does not receive enough energy to trigger the process, when 
this occurs the radio will effectively become deaf until there is enough 
energy at the port.

We've seen this problem when using a sectored antenna approach, you will 
need some overlap to try and trigger the RX, and in a situation where 
one antenna was completely nulled out by multipath for about 1/2 an hour 
every 8 or 9 hours.

The problem can be solved by forcing the chipset to change antenna ports 
but I'm not sure how you would do this automatically under hostap.

Hope this helps


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