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Tuesday, July 30, 2013

Hybrid location technologies: GPS, cellular ID, and triangulation - trilateration

The handset and the base-stations interact to implement
observed TDOA positioning.  The RTT measurement
is used for eCID mobile location.
As continued from our previous reportCell ID referencing is a network-based positioning method where the user's equipment (UE) or mobile device position is estimated depending on the cell it is connected with. In the enhanced cell ID (eCID) method, the handset sends cell measurements to the network, which can be used to better estimate the position. Measurements can include (depending on the radio access technology or RAT) round-trip time (RTT) and timing advance. These methods can be used in any cellular network, but the specific eCID position estimation techniques may be proprietary. They are accurate from 250 to 1000 m.

Location measurements involving distances is referred to as 'trilateration' and measurements involving angles is called 'triangulation'.  A third method is called multilateration which uses difference in distances or absolute measurements of time-of-flight from three or more sites at known locations.  GPS position location uses a type of trilateration.  eCID can use one or more of these techniques as a hybrid mobile tracking solution.

For some eCID location techniques, fingerprinting methods are similar to the approach used for Wi-Fi positioning. A central database is created with geotagged cell information, such as received signal strength indication (RSSI), cell ID, real-time difference (RTD), and network measurement record (NMR) in GSM. The mobile tracking device reports currently observed network information from the serving cell and all other nearby cells. This information is used as an input to a fingerprinting algorithm that estimates the mobile device position. Accuracy is 100 to 300 meters.

In time difference of arrival (TDOA) methods (downlink only), the mobile device detects the TDOA of signals between the serving base-station and one or more neighbors. The time differences can be used to estimate the mobile tracking device position. Typically, TDOA methods are more effective in synchronized networks. Accuracy ranges from 25 to 300 m. Common applications include:
  • CDMA/advanced forward link trilateration (AFLT): Time differences in pilot signals from the serving cell and the reference cell are detected and used for positioning.
  • LTE/observed TDOA (OTDOA): Special cell-specific reference signals, called positioning reference signals (PRS), are used for the time difference measurements.
The key handset factors that enable good performance vary depending on the technology. They all require the mobile device to be able to continuously measure the signals necessary for enabling the technology, while at the same time handling all the other call and data functionality required.

Positioning must be conducted in connected mode at the same time as various network procedures such as handovers, circuit-switched fall back (CSFB), and cell reselections. Cellular positioning typically does not provide as high-grade accuracy as A-GNSS, but it can assist in deriving a quick, coarse location, as well as provide an alternative for GPS or GNSS when indoors.