Locating user equipment, i.e. defining the geographical location of user equipment, is an important function in cellular networks. In the United States, the Federal Communications Commission FCC requires that it must be possible to locate all user equipment making an emergency call to an accuracy of 50 meters. Location can also be utilized for commercial purposes, for instance for defining different tariff areas or for implementing a navigation service instructing a user. A location service (LCS) has until now been developed mainly for application to circuit-switched cellular networks, for instance the GSM system (Global System for Mobile Communications).
Different methods are used in implementing a location service. On the roughest level, the location of user equipment can be positioned on the basis of the identity of the cell serving the user equipment. This information is not very exact, since the diameter of a cell can be tens of kilometers.
A more accurate result is obtained by using as additional information the timing information of a radio link, for instance the timing advance (TA). In the GSM system, TA shows the location of the user equipment at an accuracy of approximately 550 meters. The problem is that if the cell in implemented by an omnidirectional antenna, the location of the user equipment is only known in relation to a base station on a circle drawn around it. A base station divided into three sectors, for instance, improves the situation somewhat, but even then, the location of the user equipment can only be positions to a 120-degree sector, in an area 550 meters deep, at a certain distance from the base station.
Even these inexact methods are enough for some applications, for instance for defining tariff areas. More accurate methods have also been developed. These methods are usually based on several different base stations taking measurements from a signal transmitted by the user equipment, the TOA method (Time of Arrival) is an example of these.
The user equipment, too, can take measurements from signals transmitted by several different base stations, one example of such a method is the E-OTD (Enhanced Observed Time Difference) method. In synchronized networks, the user equipment measures the inter-relations of the reception time-instances of signals received from different base stations. In non-synchronized networks, a location measurement unit (LMU) at a fixed, known measurement point also receives the signals transmitted by the base stations. The location of the user equipment is defined on the basis of geometrical components obtained from time delays.
Another location method is using a GPS (Global Positioning System) receiver in the user equipment. A GPS receiver receives a signal transmitted by at least four earth-orbiting satellites, on the basis of which it is possible to calculate the latitude, longitude and height of the location of the user equipment. The user equipment can position itself or it can be assisted in the positioning. The network part of the radio system can transmit an assisting message to the user equipment to speed up the positioning, i.e. the power consumption of the user equipment is reduced. The assisting message can contain the time, a list of visible satellites, a Doppler phase of a satellite signal and a search window of a code phase. The user equipment can transmit the received information to the network part which then calculates the location. The network part of the radio system refers herein to the fixed part of the radio system, i.e. the entire system excluding the user equipment.
So far, not very much attention has been paid to implementing a location service to packet-switched radio systems, such as GPRS (General Packet Radio Service) or EGPRS (Enhanced General Packet Radio Service). EGPRS is a GSM-based system utilizing packet-switched transmission. EGPRS uses EDGE (Enhanced Data Rates for GSM Evolution) technology to increase data transmission capacity. In addition to the GMSK (Gaussian Minimum-Shift Keying) modulation used normally in GSM, it is possible to use 8- PSK (8-Phase Shift Keying) modulation for packet data channels. The aim is mainly to implement non-real-time data transmission services, such as copying a file and using an Internet browser, but packet-switched real-time services for speech and video transmission can also be implemented.
Two different solutions have been defined for a location service in the GSM specifications: a base station system oriented solution and a network subsystem oriented solution. In the first solution, a serving mobile location center SMLC is connected to the base station controller, and in the second solution, to the mobile switching center. The UMTS specifications only define one solution: a radio network oriented solution. The GPRS specifications also define at least the radio network oriented solution.
Some of the above-mentioned location methods require communication between user equipment and the location center. In the GSM system, this communication is done using a third-layer radio resource location services protocol (RRLP).
In GPRS, the third-layer protocols only reside in the user equipment and core network, for instance in the support node SGSN, i.e. a radio network does not have third-layer protocols. It is, however, necessary to transmit RRLP-type information between the user equipment and the location center in the radio network. On the second layer, GPRS has a logical link control protocol which provides services for third-layer protocols.
Using location methods thus usually requires data transmission between the user equipment and the location center residing in the network part of the radio system. In circuit-switched radio systems, data transmission is implemented using the services of the third layer of the protocol stack. This is, however, not possible in packet-switched radio systems, because the serving mobile location center is in the radio network of the radio system, but the required third layers are in the core network of the radio system.
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