Zeroing in on wireless location

Different vendors take different approaches to measuring the wireless environment to locate a client device. As mentioned last time, triangulation can be used in conjunction with “nearest sensor” information to pinpoint location to usually about 50 meters. Triangulation is used both in wireless LANs and cellular networks.

On the cellular side, GPS systems combine triangulation with a measurement called time difference of arrival (TDOA) over a network of satellites. TDOA measures the relative time delay of signals arriving at different sensors and can be used with triangulation in 802.11 networks, too. Because time is proportional to the distance traveled, the distance to each sensor within range can be estimated and, consequently, the location of the client. In addition to TDOA measurements, received signal strength indication (RSSI) can be used to measure the RF power loss between transmitter and receiver to calculate distance.

To date, GPS isn’t used much in 802.11 WLANs because a GPS chipset cost is expensive, compared to using information radiating from a Wi-Fi client, and satellite reception within buildings can be iffy.

A more sophisticated category of location tracking used in 802.11-based WLANs is called RF fingerprinting. This technique uses intelligent algorithms to improve location-tracking precision by accounting for the environmental effects – such as an object, human, mirrors, windows, attenuation and multi-path – on the wireless signal. A “fingerprint” of the wireless environment is calculated by a physical walk-around using a handheld spectrum analysis device. These measurements are later compared to deviations in the real-time environment to locate the client device.

Several Wi-Fi system vendors support a form of RF fingerprinting in their management systems to enable security and management of their own WLANs. So do a number of third-party location specialists. We’ll take a look at those companies next time.