There’s a recent paper Review of Indoor Positioning: Radio Wave Technology that provides a great overview of indoor positioning technologies.
From a hardware perspective it covers, RFID, UWB, Bluetooth, ZigBee, IR, WiFi, ultrasonic and hybrid systems. There’s a useful comparison table of the various technologies:
The paper describes methods of using radio signals to determine position such as RSSI ranging, trilateration, angle of arrival (AOA), round trip time of flight (RTOF), phase of arrival (POA) and time of arrival (TOA).
It also describes methods such as fingerprint localization.