50m-150m 50-150m Route Finder Emergency Service Car Assistance Localized Menu Local Content Social Networking Point of Interest Fleet Management Figure 9.25 LBS positioning methods or representing the area in some manner. In a dedicated state the Serving MLC sends a TA_REQUEST to request the TA value from the serving BSC. The BSC responds with a TA_RESPONSE carrying the TA value. TA is used to assist all positioning mechanisms and as a fall-back procedure [38]. Time of Arrival: The uplink TOA positioning method is based on measuring the Time of Arrival (TOA) of a known signal sent from the mobile and received at three or more measurement units (LMU). The known signal is the access burst generated by having the mobile perform an asynchronous handover. The method requires additional measurement unit (LMU) hardware in the network at the geographical vicinity of the mobile to be positioned to accurately measure the TOA of the bursts. Since the geographical coordinates of the measurement units are known, the mobile position can be calculated via hyperbolic triangulation. This method will work with existing mobiles without any modification [38]. TOA can offer accuracy of location of around 125-200 m. The cost benefit analysis is not very much in favor of the usage of this technology, because of the large number of LMUs required [35]. E-OTD: The Observed Time Difference (OTD) is the time interval that is observed by a handset between the receptions of bursts from two BTSs in the cellular network. In the Enhanced OTD (E- OTD) the handset measures the differences of arrival time of signals transmitted from a minimum of three synchronized base stations. E-OTD is a modification of the TOA method and for E-OTD measurement synchronization; normal and dummy bursts are used. When the transmission frames of BTSs are not synchronized, the network needs to measure the Relative or Absolute Time Differences (RTDs or ATDs) between them. To obtain accurate triangulation, E-OTD measurements and, for non-synchronized BTSs, RTD or ATD me