Using only the power of a received radio signal, the accuracy of 0.1–0.5 m can not be obtained. Different ways of holding the phone in your hand, and then a user between devices is sufficient to substantially affect the received signal.
Graphs can be viewed here: "Accurate Extraction of Face-to-Face Proximity Using
Smartphones and Bluetooth
" [pdf]. With Wi-Fi will be about the same.
Alternatively, you can use sound signals. Microsoft Research Asia claim to have achieved centimeter accuracy ("BeepBeep: A High Accuracy Acoustic Ranging System using COTS Mobile Devices