Here we review how the typical AP layout for location grade WLAN is laid out - but also take a first look at the Cisco Hyperlocation module and how that might help you achieve the highest levels of location accuracy.
Wi-Fi has an increasing array of use cases, with the days of a single use Wi-Fi network behind us. Many customers are looking to use their network for multiple purposes and one of these is to location track tags and devices.
What is a Wi-Fi Location Tag? Tags are small Wi-Fi transmitters, which can be attached to objects to allow them to be tracked.
If you're interested in a bit of a background read on how to make Wi-Fi work, take a look at the Top 8 Secrets to Great Wi-Fi.
Traditional wireless location tracking is based solely on RSSI (Received Signal Strength Indication). This is how loud the AP can hear the clients, and is used to work out the client position.
APs use the RSSI to estimate distance from itself to a client. Since each AP only knows the distance to the client but not the direction, one AP is not enough to identify the location of the client; a minimum of 3 APs with good signal strength is recommended in order to tri-laterate the location.
For a good Location Grade design, sub 5m accuracy is pretty achievable, although this can drift in wide open areas. Typically quoted figures are 5 - 10m of accuracy with a good Location Grade design.
Location Grade Predictive Wi-Fi Survey
Predictive Surveys are the starting point for any RF design and that includes Location Grade Wi-Fi.
The recommended AP design/layout is to place the APs in a staggered manner (rather than a grid or just down the centre of corridors) and also around the perimeter of the building.
In reality, the RF environment is affected by many factors, especially wall position and wall type and this affects the AP positioning as well.
For the purposes of this article though, we will keep it simple and assume the RF attenuation to be constant across the whole building to illustrate the rationale behind the staggered design.
Recommended AP Locations
In this recommended design example, AP 1 and AP 4 will see approximately similar RSSI value from the client – so the client is somewhere between 1 and 4.
The circles around APs 1 and 4 illustrate there are two possible locations where the circles intersect each other. AP2 would also see a (fainter) signal from the client, which adds further information to narrow down the location of the client.
Non-Recommended AP Locations
While it is possible to cover the whole building with reduced number of APs as shown in the design to the right, the location tracking readiness is greatly reduced. AP 1 can see the client, and determined the distance of the client.
This gives a radius, but not where on that radius . AP 2 will only resolve the clients location to two possibilities. AP 3 is more than likely too far away to provide any meaningful RSSI data, so the location cannot be resolved with this design.
As with all RF designs, a pre-install survey should be conducted to confirm the AP placement in the predictive survey is correct.