Solutions

What can we do about it?

When planning WiFi networks it is essential to place the access points and choose their channels carefully. Although it seems strange, interference and bandwidth sharing can be reduced by lowering the transmit power at the access points (view example). Alternatively using WiFi blocking walls in buildings can reduce interference. When unwanted signals are stopped, the interference goes down and a few users inside a screened room can share the entire WiFi bandwidth to themselves (view example). Similarly, people on the other side of the screen can do the same. Using highly channel screening metal sheets in the walls would provide very good signal blocking between rooms. Building regulations and good practice require that metal foils are included in external and some internal walls for thermal insulation and to prevent moisture penetration. These can reduce both interference as well as wanted signals for mobile devices etc.

Obviously this can lead to poor radio, TV, mobile cellular and wireless reception in buildings and could be serious in the event of a fire or emergency when the emergency services radios would not work. Therefore to improve WiFi systems, a solution is required that allows walls to retain metal sheeting for insulation (and to contain WLAN channels) while allowing through broadcast, cellular and emergency service radio signals. A technology that can do this is known as Frequency Selective Surfaces (FSS). These can be designed to block WiFi signals (like mirrors) and pass other radio channels (like windows). The following illustration shows how an FSS reflects or transmits a radio signal.

In the figure below, the wall has been modified with FSS to pass some frequencies while reflecting others. The image shows WiFi channel 1 is blocked by the wall and therefore gives good coverage around both Access Points. Hover over the image (or click if using touch screen devices). This image shows that the emergency services channel passes through the wall offering reception throughout the building. It can be seen that the WLAN channels are contained within the FSS lined walls while a single police/fire service transmitter can cover the entire floor area. It is not necessary to cover all the walls with the Frequency Selective Screens. As the walls are already covered with foil, we can create ‘radio windows’ which let in some signal power through a small opening in the foil. See right-hand image. This should reduce the cost significantly of including FSS in walls.

Hover on the image to change. Both Access Points are on channel 1 but the wall blocks out the interference and so good signal can be achieved. However, because of FSS in the wall, the emergency services signal is not blocked and so can achieve good signal throughout the building.

The figure shows an FSS inserted into a gap in a foil screen. The rest of the wall is foil covered on the other side.

These radio windows are invisible to people, but they can act as mirrors to WLAN while allowing through other radio systems. As well as being useful to maximise the performance of WiFi networks this technology could be used in prisons to reduce illicit mobile phone use, or in theatres and cinemas to prevent performances being interrupted. Other examples could include secure government buildings and hospitals. In all these cases it is important that emergency service radios should be able to work if necessary.