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.
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.
Laying out Access Points
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 as shown on the first image below. 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 Location
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 Location
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.
The Cisco Hyperlocation Advantage
As shown in the figure on the right, the array antenna used by the Cisco Hyperlocation module allows an AP to identify a client’s direction. Note the antennas all around the perimeter of the module - using these, the module can determine the relative distance, but also the angle the client is transmitting from.
This, combined with the traditional RSSI based location estimation, provides a much higher accuracy in location tracking, as a single AP is now more accurate than two APs using the traditional method.
If you happen to have a site that utilises an AP layout that is not location grade (and if your APs happen to be compatible with the Cisco Hyperlocation module), adding the module will greatly improve the location readiness. Using the above recommended design as an example, the additional relative direction data would mean only APs 1 and 4 would be needed for location resolution.
We'd recommend you to utilise the recommended design and then adding the Cisco Hyperlocation module will enhance the results you can achieve.
We understand that different sites may have different budgets and needs - contact us if you would like to discuss your requirements and leave the design with us so we can provide you with options.