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Accurate RF Survey Measurements

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Written by Jim Geier

Monday, 02 April 2007

In most cases, the downstream signals from access point (measured at the wireless client) will be higher than the upstream signals (measured at the access point) because the radio cards and antennas in wireless clients generally offer lower radiated transmit power. The client radios usually have lower gain antennas. As a result, measuring only the downstream signal strength may offer coverage that's better than what applies to typical wireless clients. In order to correct for this, you can perform RF surveys by only measuring the uplink signal strength. You can log into the access point statistics page, though, and read the signal strength of your wireless client. Most survey tools only support downstream measurements. Alternately, you can measure the downstream signal strength and take into account the lower radiated power of the applicable radio card making the measurements.

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WLAN consultant, CWNA

Rene Hinsch (IP:83.93.153.3) 2007-07-02 18:56:12

I prefer setting the AP transmit power to what i expect (or require) a station to have. Then I can use my survey tool to measure what the uplink signal coverage is.

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reddle (Registered) 2007-07-11 20:07:33

I think Jim’s article above applies when using a hotspot model but should never apply when using an ISP model to deliver a WiFi service. In an ISP model built for reliable service you would perform the standard RF site survey which should graphically show your service boarders for signal strength, data rate, and SNR. Armed with this define exactly where your fringe customers are going to be against the tech spec of the CPE you plan to deploy (you are going to use an outdoor CPE aren’t you) then design the antenna system and power levels to match your most difficult to reach customers.

Having worked this out go into the field and test, then test, then test again, if you accurately define your fringe network coverage the rest will (almost) take care of itself the key is to make sure your customers and core network are a balanced system with both sides presenting equivalent signal levels, quality, SNR etc.

In a hotspot model you have no control over your clients and Jim’s method is a good guide, all I would add is that you should get the worst client device (laptop, pda etc) you can find and do live tests in the worst locations you know you cover and then you can again define your fringe service area and set your customers expectations accordingly. It will also save you time later working out why client X keeps dropping of the network, you will already know he is out of your reliable service area, or sat in the middle of that black hole that eats RF for no reason