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Gathering Wall Attenuation Values for a Predictive Model

For a predictive model to be valuable, attenuation values for objects in the environment such as walls must be obtained.

The tools needed are a signal generator, a signal receiver, a battery pack, a floor plan of the area being evaluated and a clipboard and pencil.

Signal generator

To generate a wireless signal I am using a TP-Link AC750 Wireless Portable Nano Travel Router (TL-WR902AC). The settings on the travel router don’t matter as long as they stay consistent throughout the process of measuring attenuation.

Battery pack

Next you will need a battery pack to power the AP. The smaller the better as long as the pack has enough power to last you the time it will take you to measure. I like the Portable Charger Anker PowerCore 20100mAh.

Signal receiver(s)

I use an app on my Android phone to capture the RSSI of the travel router. It is called WiFi Analyzer (open-source) by VREM. In settings set the refresh rate to 3 seconds. The app output looks like this.

If you're an iPhone user, you can use the AirPort Utility to obtain measurements. You'll have to enable the scanner mode feature first, if you haven't already done so. To enable the scanner:

  1. Open the Settings app.

  2. Scroll down to the bottom where you will find a listing for the AirPort Utility and tap it.

  3. Next to Wi-Fi Scanner at the bottom, move the slider over to enable it (the slider should be highlighted in green).

You'll now be able to see the "Wi-Fi Scan" link at the top right of the screen when you open the AirPort Utility. Set how long you want to scan for (Continuous is probably best, but be mindful of battery) and then click "Scan" to begin scanning. You can use the search field to focus on a specific SSID and eliminate nearby networks from the list.

Now power on the router with the battery pack, collect your floor plan, pencil and clipboard and let’s measure.

For each obstruction you will take two measurements, one on the near side of the obstruction and one on the far side. An obstruction is anything that attenuates a wireless signal such as walls, a row of lockers or a bunch of kitchen appliances. Using the 5GHz network for measuring attenuation is preferable because the smaller aperture size of the antenna requires a higher SNR than the 2.4 GHz but, for our purposes it doesn’t really matter. How we measure matters a lot. Specifically, where we measure matters a lot. Stand approximately 12-15 feet away from the travel router when measuring the RSSI. If you measure closer to the travel router the reading may vary wildly with just the smallest movement due to the logarithmic nature of free space path loss (FSPL). If you can't get that far away from the router move as far away as you can.

To take the measurement of an obstruction perform the following steps. In this example, we are measuring the attenuation of a wall.

  1. Walk inside the room. Close the door. Place the travel router next to the wall. Walk about 15 feet away from the travel router. Use your phone to measure the RSSI.

  2. Leave the room. Close the door. Measure the RSSI right next to the wall from the same area that you measured the RSSI inside of the room. Measure the RSSI. Record the difference in RSSI measurements on the floor plan.

  3. There’s no need to measure every wall, door etc. Simply measure the ones that are different than the ones for which you already have measurements.

  4. However,I caution you that some materials can be deceiving. For example, regular glass will typically attenuate about 2dB while glass with an energy conservation coating can be 3 times as much. If in doubt measure!

Note: PDF Expert and an iPad mini is an excellent alternative to the paper map and pencil for recording attenuation values.

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