wireshark + geolocation if poss?
Basically you'd need to monitor the traffic going through your hotspot (easy enough) then comes the hard bit. You could possibly do it through geolocation somehow, no idea how to implement that through a router though.
Additionally you could create a highly accurate method of defining a users distance from router by using the ping as a reference.
Wireshark will let you see a multitude of information about all the traffic running through your router and all the devices connected to it. Regardless of which method you end up using/creating I'd definitely start with wireshark. It's a great tool for anything to do with networks.