05-13-2013 06:41 PM
We are implementing RTLS over several access points (AP) to track Wi-Fi enabled mobile phones in a shopping mall. We have implemented the Aeroscout version, where we set the update frequency to be one second. We have succeeded in receiving updates from the AP's.
The problem is the updates are not fine-grained enough to localize a mobile device accurately. When the mobile device just turns on the Wi-Fi, all AP's report this mobile device, based on the probe messages sent. But as soon as the mobile associates to one particular AP, only the associated AP send updates based on management packets (null packets/ RTS), but other AP's stop sending any update about this associated mobile device. So the localization accuracy of this mobile device drops to the radius of the associated AP, which can be a large number of shops in a shopping mall scenario.
We have experimented by turning Wi-Fi on and off on the mobile device, keeping it on long enough to scan but not long enough to associate. In this case, all AP's continue to send updates about this mobile device, because of the continued probe requests, and the localization accuracies based on frequent updates from multiple AP's is pretty good. But turning on-off Wi-Fi on client mobiles to localize them is a hack, and not a desirable solution.
I have attached the RTLS document which justifies the current behavior, saying unassociated AP's report probe messages and associated AP's report management messages other than probes. But then what about the low localization accuracy of associated devices? Has Aruba thought on this and is there any workaround to get more frequent updates from all AP's?