For one of my major projects I’m tasked with figuring out how we can locate mobile wireless devices within a limited location, and it seems a lot of other people I’ve talked to are in the process of figuring out the same thing, so here’s an overview of what I’ve learned so far.
Wireless Technologies / Protocols
- WiFi (802.11)
- XBee/ZigBee (802.15.4)
- BlueTooth (802.15.1)
- RFID
- Near Field Communication (Two-Way RFID)
These are four of the principal wireless technologies in mainstream use today. The goal is to determine within some degree of accuracy the position in 2 or 3 dimensions of a wireless node. Most, if not all, of the available approaches utilize the RSSI (Received Signal Strength Indicator) of the target node’s connections to nearby nodes.
If you have fixed routers with known locations, you can use the RSSI values of their connections to the target node to calculate the distances from the fixed nodes to the target node. However, any RSSI value will be heavily influenced by a number of factors that may be out of your control: interfering structures, interfering people (we are walking bags of water, which readily absorbs 2.4ghz radiation) and other interfering radio signals or noise.
Due to these issues, some smart folks developed a system called RADAR that uses ‘location fingerprinting’ to take a bunch of readings of signal strengths under varying conditions and associate them with known locations. There are a variety of algorithms to tabulate this information, one of which uses neural networks.
Here is a collection of extremely useful papers and links that describe these various approaches, techniques and technologies:
Resources
- Survey of Wireless Indoor Positioning Systems [pdf]
- Indoor Location using 802.15.4
- Bluetooth Triangulation [pdf]
- CS/RADAR: Indoor Location Discovery and Tracking [pdf]
- Indoor location tracking using RSSI readings from a single Wi-Fi access point [pdf]
- Location Estimation in ZigBee Network Based on Fingerprinting [pdf]
- Experimental Analysis of RSSI-based Location Estimation in Wireless Sensor Networks [pdf]
- Position Location Monitoring Using 802.15.4/ZigBee technology [pdf]
