1. Field of the Invention
This invention relates to handover in communication systems, and especially to handovers that can assist in enabling the positioning of devices operating in such systems.
(2) Description of Related Art
Positioning or locating services (LCS) are becoming an important aspect of communication systems such as mobile telephony systems. These services allow the location of a user terminal or user equipment (UE) to be estimated. Knowledge of the location may be of interest to the user of the UE, and can also be used to offer the user additional services based on his location. One valuable advantage is that if the user uses his UE to make a call to the emergency services, knowledge of the location of the user could help the emergency services to offer assistance rapidly.
There are a number of ways in which the location of the UE can be estimated, but in one typical way timing differences are calculated for communications between the UE and a number of base stations. Using the timing differences the distance between the UE and each of the base stations can be estimated, and then with knowledge of the locations of the base stations the position of the UE can be triangulated.
One example of the types of system in which such services can be provided is a mobile telephony system, for example a cellular telephony system.
On the implementation of the proposed 3G (Third Generation) or UMTS (Universal Mobile Telephony System) system, it is planned that LCS will be supported by a number of cellular network systems. Examples include older GSM (Global System for Mobile Communications) standard networks operating according to the 3GPP standard for LCS, together with full 3G/UMTS standard networks, also standardized by 3GPP.
When a user is communicating in a region where older GSM networks and UMTS networks fully or partially overlap, it is intended that the networks and the user's terminal will be capable of operating so that the terminal can switch from communicating in one network to communicating in another. For example, in the situation where the networks' coverage partially overlaps, a user may move from a location where there is coverage from one of the networks to an adjaCent location where there is coverage from another of the networks. It is intended that as he does so, his terminal will be handed over from the first network to the second so that the user can continue to communicate. Even when the networks overlap entirely, a user's terminal may be handed over from network one to the other, for example to satisfy capacity constraints.
It can be expected that there. will be differences between the LCS capabilities of networks of different standards. For example, the methods that they support for determining the position of a terminal may be different. As a result, even though their standards are commonly set, there may be differences in the LCS capabilities between the networks. Due to those differences one of the networks may be better suited than another to provide LCS for a particular purpose. As a result, when a user's terminal is in a location where there is overlapping coverage from two or more interoperable systems and a need to locate the user arises, there can be situations in which a particular one of the networks that provides coverage there best fulfils the positioning requirements in that situation. However, at the time when the need arises to locate the user, the user's terminal might be connected to another network than the preferred one that is best able to fulfil the positioning requirements.
There may also be situations where due to the capabilities of the user's terminal it is more preferable to determine its location using one network than another. For example, the terminal might support the E-OTD method in GSM but might not support any of the positioning methods standardized for WCDMA (such as IPDL-OTDOA and A-GPS). This situation may be considered very likely to happen, because location services standards for GSM have been finalized before those of UMTS, and so terminals that support the GSM methods can be expected to be produced first.
The current requirements specified by the U.S. FCC Phase II Mandate set out strict requirements for positioning of a user's terminal. The FCC's Third Report and Order (FCC 99-245) in the matter of revision of the commission's rules to ensure compatibility with Enhanced 911 Emergency Calling Systems (CC Docket No. 94-102 RM-81.43), adopted Sep. 15, 1999, states: “We adopt the following revised standards for Phase II location accuracy and reliability: For network-based solutions: 100 meters for 67% of calls, 300 meters for 95 percent of calls; For handset-based solutions: 50, meters for 67% of calls, 150 meters for 95 percent of calls”. Meeting these requirements will require a very high level of performance from positioning systems.
There is therefore a need for a means by which the positioning of a terminal can be improved when it is communicating in a network other than the one that can best estimate its position.