03.713GPPFunctional descriptionLocation Services (LCS)Release 1999Stage 2TS
The Enhanced Observed Time Difference (E-OTD) positioning method is developed from the Observed Time Difference (OTD) feature. For synchronised networks, the MS measures relative time of arrival of the signals from several BTSs. For unsynchronised networks, the signals are also received by a fixed measuring point known as the Location Measurement Unit (LMU) whose location is known. The position of the MS is determined by deducing the geometrical components of the time delays to an MS from the BTSs.
Measurements are performed by the MS without any additional hardware. For OTD measurements synchronization, normal and dummy bursts can be used. When the transmission frames of BTSs are not synchronized, the network needs to measure the Real Time Differences between them. To obtain accurate triangulation, OTD measurements and, for non-synchronized BTSs, RTD measurements are needed for at least three geographically distinct BTSs. Based on the measured OTD values, the location of the MS can be calculated either in the network or, if all the needed information is available in MS, in the MS itself. The terms ‘MS-assisted’ applies to the former method and ‘MS-based’ to the latter.
NOTE: In this document, the term OTD is used to refer to a quantity of time, whereas the E-OTD is used to refer to a positioning method.