We're working on an application that needs ultra-short range BLE, on the order of 50cm radius or less. I've found little discussion anywhere on that topic, likely because most applications are looking for the longest possible BLE range. The issue we have is that there will be dozens or possible a few hundred beacons in the same general area, with spacing as little as 1m between beacons, while my receiver (a typical smartphone) needs to pick up only on the beacon that the user is standing right next to. Using RSSI hasn't been good, due to the orientation of the user's body causing attenuation and making the nearest beacon appear to be the "next nearest", for example.
If anyone has any thoughts on this topic or links that you are aware of that explore this, possibly with custom directional antenna, etc., I'd be grateful.