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Building a 100Mbps wireless bridge - page 2

Doing the Sums

It's now time to get down and starting looking at the numbers so we can work out what size antenna we are going to need to build this link. The main figure we need to calculate is the signal loss between the two ends of the bridge, i.e. the attenuation in free space. We are not going to bother with a theoretical figure but are going to look at real examples to calculate it.

From the existing 11Mbps link we should be able to work out how much the signal is lost over the distance. When the link was initially put in, it would not hold 11Mbps and would frequently drop to 5.5Mbps, so by adding just a 3db antenna to one access point only, we managed to stabilise the connetion, and it's been reliable ever since. Knowing that the receive sensitivity is -83db, I can assume that with the 3db antenna we must be getting around -82db or -81db signal strength. Given we are using access points that are mounted inside, we need to take into account also the signal loss in going through the office windows at either end, so the math is ...

Transmit power + antenna gain - loss through window - freespace loss - loss through window > receive sensitivity.

Plugging in our figures we get

15db + 3db -3db -freespaceloss -3db = -82db

Which gives us a freespace loss of

freespace loss = 15 +3 -3 -3 +82 = 94db

This shows that over the 120m of our link, which is far from ideal, with trees and poles almost blocking line of sight, we are losing 94db. If we now look at the estimated loss over 50m for the 'average' 54Mbps kit.

15db + 2db -freespace loss = -67db

Giving a freespace loss in ideal conditions over 50m of

freespace loss = 15 + 2 + 67 = 84db

These figures seem to tie up, as an 84db attenuation over an ideal 50m gives us a 90db attenuation over an ideal 100m which is close to our loss in less than ideal conditions over 120m. These figures mean we can be fairly confident we can calculate the antenna size we need to get a stable connection. So if we use a 94db freespace loss, and standard 802.11g kit at 54Mbps, we can calculate how much antenna gain we need.

transmit power + antenna gain - loss through glass - freespace loss - loss through glass + antenna gain = -67db

15 + ( 2 x antenna gain ) - ( 2 x 3db ) - 94 = -67

Giving an antenna gain of

antenna gain = ( -67 + 94 + ( 2 x 3 ) - 15 ) / 2 = 9db

So we need 9db antenna at each end using standard 802.11g kit to get a signal through at 54Mbps. Hopefully this is a worse case scenario, as by using newer more sensitive kit (with a receive sensitivity of -72 or -71 db ) we could reduce the transmit power by 3 db and still have a couple of db more signal strength than we need for a 54Mbps connection.

Given there are offices at both end of the link, I really don't just want to use massive antenna and large transmit power to 'cook' to folk who work in each office. Ideally I want to keep the transmit power down to around 20db including the antenna gain.

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