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How to: Optimize Wi-Fi Coverage in Multi-Floor Facilities

Sooner or later, you’ll need to install a wireless LAN in a multi-floor facility. Learn tips on how to assess the environment and possibly use inter-floor signal propagation in your favor.

We’re all accustomed to thinking about 802.11 radio signals propagating from an antenna in directions horizontal to the floor of a building. When deploying a wireless network in a multi-floor facility, however, you must consider that radio waves propagate up and down vertical to the floor, as well. The range is not as far in the vertical directions, but it’s often enough to penetrate the floors and ceilings in most buildings. This inter-floor signal propagation makes a site survey more complex, but you might be able to take advantage of the situation.

Analyzing inter-floor signal propagation

The first thing you need to do is gain a good understanding of how 802.11 signals propagate through the floors of the facility where you’re deploying the wireless network. Every building is different, so it’s a good idea to do some testing and determine how much the floors actually attenuate radio signals. Keep in mind that 2.4GHz and 5GHz signals propagate differently through the same materials, so perform testing with the frequencies you plan to deploy. If you’re deploying 802.11n, then consider testing both 2.4GHz and 5GHz frequencies. Perform this testing at the beginning of an RF site survey so that you have a better idea of how to position test access points based on the inter-floor signal propagation. 

The following are steps you should complete when analyzing inter-floor signal propagation:

1. Assess the construction of the facility. Walk through the building to observe and understand the construction between the floors. You might need to consult with the building manager and possibly peek above the ceiling tiles. Make notes about the general construction, such as wood, concrete, steel, etc. Identify areas where the construction varies because you should perform testing in each area where the inter-floor construction is significantly different.

2. Activate an access point. Do this toward the center of the building, away from exterior walls, so that you have room to see the impacts of inter-floor propagation. Ideally, position the access point where there are floors above and below the access point location (e.g., the 2nd floor). All you need to do is supply power to the access point and ensure that it’s configured with the same radio (2.4GHz or 5GHz) and transmit power that you plan to install. Similar to site survey testing, you’ll be measuring the access point beacons as the basis for signal strength. Be sure to mark the position of the test access point on the building diagrams.

3. Measure inter-floor signal propagation. The main idea here is to see how well the signal propagates through the floors in the areas above and below the access point. For example, with the use of the building diagrams, go to the floor above the test access point, and measure the signal strength just above and around the test access point. Also take measurements on the floor below the access point, as well. In most large buildings, acceptable signal propagation generally doesn’t occur beyond the floor immediately above and below the access point, but you should check higher and lower floors, as well. Move the access point to other areas of the facility where the inter-floor construction is very different, and repeat the testing.

4. Characterize the inter-floor propagation. Make notes regarding the diameter of the inter-floor signal propagation (on the floor above and below the access point) for each of the areas tested. Be sure to take into account the minimum signal level that constitutes acceptable signal coverage. You’ll need to keep this information and use it for spacing access points when performing an RF site survey.

Note: Some tools, such as AirMagnet Survey, have special features that allow you to record inter-floor signal coverage and generate applicable coverage maps. These tools are very beneficial, especially when performing a site survey of a large facility with varying inter-floor construction.

Determining access point placement

If you find that signals propagate well through the floors of the facility, you can take advantage of the inter-floor propagation in a way that reduces the number of access points necessary to cover the facility. For example, AP-1 and AP-4 in the illustration below can provide coverage on the 2nd floor (where they are installed), as well some coverage on the 1st and 3rd floors. AP-2 (installed on the 3rd floor) and AP-3 (installed on the 1st floor) both provide some coverage on the 2nd floor. This allows the spacing between the access points, such as AP-1 and AP-4, to be farther apart than if the inter-floor propagation is not taken into account. Of course this reduces the cost of the deployment.


Assigning RF channels

As with any wireless network deployment, you should alternate the use of RF channels in a manner that minimizes inter-access-point interference. The limitation of three non-overlapping channels in the 2.4GHz band makes it very difficult and sometimes impossible to keep inter-access-point interference down to acceptable levels in multi-floor buildings, especially when supporting high-capacity voice systems. As a result, it’s advisable to migrate to 5GHz 802.11n, as soon as possible. The 5GHz spectrum includes a relatively large number of non-overlapping channels, which significantly reduce channel reuse problems and improves the performance of the network.

Jim Geier provides independent consulting services and training to companies developing and deploying wireless networks for enterprises and municipalities. He is the author of a dozen books on wireless topics.