Enhanced Observed Time Difference (E-OTD) is a standard for the location of mobile telephones. The location method works by

multilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multilateration, for e ...

. The standardisation was first carried out for

GSM The Global System for Mobile Communications (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) to describe the protocols for second-generation ( 2G) digital cellular networks used by mobile devices such ...

by the GSM standard committees (T1P1.5 and ETIS) in LCS Release 98 and Release 99. The standardisation was continued for 3G and

WCDMA The Universal Mobile Telecommunications System (UMTS) is a 3G, third generation mobile cellular system for networks based on the GSM standard. Developed and maintained by the 3GPP (3rd Generation Partnership Project), UMTS is a component of the I ...

mobile telephones by 3GPP. Conceptually, the method is similar to U-TDOA, however, it involves time difference measurements being made in the handset rather than the network, and a mechanism to pseudo-synchronise the network. The handset makes an observation of the time difference of arrival of signals from two different base stations. These observations are known as Observed Time Difference (OTD). The handset measures the OTD between a number of different base stations. If the base stations were synchronised, then a single OTD defines a hyperbolic locus. A second, independent OTD, for which one of the observed base stations is spatially distinct from those in the first OTD, would provide a second hyperbolic locus, and the intersection of the two loci gives an estimate of the location of the mobile. If more than two independent OTDs are available, then the measurements can be combined to yield a more accurate measurement. However, GSM and 3G networks are not necessarily synchronised, so further information is needed. The E-OTD standard provides a method for pseudo-synchronisation. A Location Measurement Unit (LMU) can be used to estimate the transmission time offset between two base stations. This measurement is known as the Real Time Difference (RTD). The RTD for two base stations can then be subtracted from the OTD for the same two base stations to produce the Geometric Time Difference (GTD). The GTD is the time difference that would have been measured by the mobile if the network was perfectly synchronised. Accordingly, the application of the RTD provides a pseudo-synchronisation. An LMU is a receiver that is placed in a position in the network that is able to report the RTDs of a number of different base stations. If the base station clocks are not synchronised to a common source, then it is necessary to continuously update the RTDs, as the time offsets will be changing due to the clock drift in each base station. The deployment of LMUs can be expensive, and so is a drawback of E-OTD. However, a 2003 paper describes a method of operating E-OTD without LMUs, and presents results of an operational trial. In essence, if there are sufficient independent OTD measurements such that the equation system is over-determined, then the additional information can be used to estimate the RTDs. E-OTD was considered for the

Enhanced 911 Enhanced 911, E-911 or E911 is a system used in North America to automatically provide the caller's location to 911 dispatchers. 911 is the universal emergency telephone number in the region. In the European Union, a similar system exists known as ...

mandate, but ultimately was not a successful contender for this application. An active proponent and developer of E-OTD was Cambridge Positioning Systems (CPS).“Last Chance for E911 Technology”
/ref> In 2007, CPS was acquired by CSR. In 2009, CSR merged with

SIRF SiRF Technology, Inc. was a pioneer in the commercial use of GPS technology for consumer applications. The company was founded in 1995 and was headquartered in San Jose, California. Notable and founding members included Sanjai Kohli, Dado Banat ...

. Because E-OTD requires a software modification to be included in the mobile phone, E-OTD positioning system has been less commonly used than the U-TDOA positioning system.

References

External links

Encyclopedic article from Phone Scoop

Advertisement of the E-OTD capability

Manual for the E-ODT compliance check

E-OTD compliance check in lab environment

References

External links

See also