The positioning method known as uplink time of arrival (TOA) is based on the time of arrival of a known signal sent from the mobile device and received by three or more base stations (see Figure 5.9). The signal is the access burst created by having a mobile device perform an asynchronous handover.
The TDOA values are calculated by pairwise subtracting the TOA values at the SMLC. The position of the mobile device is then calculated by hyperbolic trilateration (see the earlier section "How GPS Works"), provided that the geographic coordinates of the measurement units are known and the timing offset between the measurement units used in the measurement are known.
Additional technical details are presented by the 3rd Generation Partnership Project (http://www.3gpp.org) in GSM 03.72: "Digital Cellular Telecommunications System (Phase 2+); Location Services (LCS); (Functional Description) ? Stage 2":
Access bursts are used for detecting the TOA at the listening measurement units. At a positioning request, the units which should measure the TOA of the Mobile Station (MS) signal are selected and configured to listen at the correct frequency. The MS is then forced to perform an asynchronous handover. Under such circumstances, the MS is transmitting up to 70 access bursts (320 ms) with specified power on a traffic channel (which may be frequency hopping).
The TOA measurements are performed at each measurement unit by integrating the received bursts to enhance the sensitivity, and therefore increasing the detection probability and measurement accuracy, and by applying a multipath rejection technique to accurately measure the arrival time of the Line of Sight component of the signal. The presence of diversity, e.g., antenna diversity and frequency hopping will improve the multipath rejection capability and therefore the measurement accuracy.
When an application requires the position of a mobile, it has to send a request to SMLC for the identification of the mobile and the accuracy level parameter. Depending on this accuracy level, SMLC decides how many measurement units to include in the positioning request. The measured TOA values together with the accuracy parameter of the TOA value are collected and transmitted to the SMLC. The SMLC utilizes the TOA measurements in combination with information about the coordinates of the measurement units and the RTD (Relative Time Difference) values (a and b above) to produce a position estimate. The SMLC delivers the position estimate together with an uncertainty estimate to the application.
The uplink TOA method requires additional hardware (LMUs) to accurately measure the arrival time of the bursts. Different implementation options exist for this positioning method. For instance, it is possible to either integrate the measuring units in the BTSs or implement them as stand-alone units. If the measurement units are implemented as stand-alone units, the communication between the measurement units and the network is preferably carried out over the air interface. The stand-alone units can have separate antennas or share antennas with an existing BTS.
Similar to AOA, TDOA is a purely network-based solution and will support legacy handsets. The drawback, of course, is the installation of equipment in almost every base station, a potentially much more expensive proposition for the mobile operator. Another challenge with TDOA is that it requires that the mobile device be in range of at least three base stations. This is often not the case in rural or even some suburban areas.
A commercially available solution from Trueposition (http://www.trueposition.com) supports both AOA and TDOA.