The packet-switched domain of a radio communication network provides access for mobile terminals to external packet data networks such as the Internet; coverage is usually provided by means of a cellular structure. As mobile terminals move with the cellular coverage area the mobile terminals must be handed over from cell to cell if a session is to be preserved, each handover known as a horizontal handover. If the mobile terminals move from one radio communication network to another, handover can take place between the networks, known as a vertical handover. Handovers of both types should ideally take place with minimum disturbance of the user and of applications above the network layer (layer 3 OSI).
Current (and particularly future) radio communication environments comprise a number of different access technologies and different administrative domains in which the cellular coverage of one network overlays the cellular coverage of another; such an environment is herein referred to as a heterogeneous network environment. Mobile terminals, such as mobile phones, PDAs, and notebook computers, are being provided with the ability to connect to a number of different radio access networks to take advantage of the heterogeneous network environment. For example, a PDA may be provided with a WLAN interface for accessing computer networks, and a UMTS interface for making telephone calls and accessing the Internet. This functionality may be provided by a single re-configurable interface (e.g. with Software Defined Radio) or by physically separate interfaces. Such mobile terminals are referred to herein as multi-mode mobile terminals.
Each multi-mode mobile terminal (MMT) has a home network by which is meant that network providing a permanent point of contact for the MMT 13 e.g. by telephone number, network layer address (e.g. IP address) and may also be that network responsible for Authentication, Authorisation and Accounting (AAA), billing the user and storing user profiles for example. Usually the home network is also responsible for billing the user for access to the home network and any foreign network that the multi-mode mobile terminal uses.
Digital broadcast networks (such as American Television Standards Committee (ATSC), European Telecommunications Standards Institute Digital Video Broadcasting (DVB) and Digital Audio Broadcasting DAB, and Japanese Integrated Service Digital Broadcasting (ISDB)) are generally intended to offer point-to-multipoint unidirectional data transfer, although some schemes have been proposed for limited capacity data transfer from mobile terminals back to the broadcast network (for example DVB-Return Channel Terrestrial). Currently data is transmitted from a number of transmitters to provide coverage for a certain large geographical area (˜80 km radius). Digital broadcast networks are characterised by high data transfer rates on the downlink. For example a DVB network may broadcast multiplexed data transmission streams at a rate of the order of tens of Mbps. In contrast, mobile cellular networks offer a point-to-point bi-directional voice and limited data service between terminals (either mobile of fixed). Data transfer rates in mobile cellular networks are generally lower than digital broadcast networks. For example IMT-2000 (e.g. UMTS) networks will offer a bandwidth of approximately 2 Mbps.
Attention has recently been turned to use of digital audio and video broadcast networks for transmission of datagrams. For example the DVB-Handheld (DVB-H) standard has been proposed to permit mobile terminals to receive data (e.g. Web pages and e-mails) from broadcast networks. The present DVB-H draft (document A081) is available at www.dvb.org. In the future it is expected that the number of broadcast transmitters will increase, with each having a smaller area of coverage. Thus the digital broadcast network is and will be cellular insofar as the total geographical area covered by the network is divided into a number of cells, each delimited by the area of coverage of one (or a few) transmitter(s).
With increasing popularity of multi-mode mobile terminals, it will be important that the different network providers co-operate to provide a seamless service from the perspective of the user. Accordingly it is envisaged that different networks in the heterogeneous network environment should inter-work to this end, and this is subject of on-going research and development.
One aim of the inter-working of the networks is to offer “seamless roaming” to users. This can be defined as the ability to reduce the effect that changes at the network level have on the end-user's perception of a service. Ideally, the end-user would not notice, and would not need to be informed, when service is handed over vertically. Heterogeneous Roaming Agreements (HRAs) between service providers and network operators will offer the user the ability roam over different network types and technologies (e.g. GPRS, UMTS, WLAN, DVB) under different administrative domains whilst paying only a single invoice, undergoing one authentication process, etc.
It is envisaged that in a heterogeneous network environment different networks will be able to co-operate to provide improved services to the user since their areas of coverage will overlap. “Load-balancing” between networks may be administered, such that e-mails may be delivered to a user through his home network, whilst an attachment to the e-mail is delivered over a foreign network for example; also requests for multimedia services may be sent over a home network, but the content delivered over a foreign network. In this scenario it is important for the foreign network to know details of the multi-mode terminal such as its MAC address, whereabouts and in which foreign network cell the multi-mode mobile terminal resides so that datagrams can be addressed, routed and filtered correctly.
Accordingly there is a need for a method of discovering multi-mode mobile terminals in a heterogeneous network environment, and in particular for a way for the home network to discover details of those other foreign networks that each terminal can access, such as network type, network operator code and current cell location data. Preferably, such a method would be able to discover the foreign networks that the multi-mode mobile terminal can access at that point in time, rather than simply receiving a list of the interface types thereon. Such a method would facilitate the interworking of networks, which is desirable as described above.
A yet further problem with which the present invention is concerned is radio resource management. Many terminal-related functions are commonly treated by mobile networks on a per-terminal basis. This is because the circumstances that mobile terminals experience, and the requirements for their operation (i.e. allocated channels etc), are often assumed to dynamically vary independently for all terminals. However, in group mobility scenarios, such as public transportation in a train or coach, a large number of terminals commonly experience the same dynamic fluctuations in conditions. These might include radio conditions (i.e. large-scale shadowing), resource availabilities, radio-service availabilities, cell handover times and histories, amongst others. In such cases, it would be useful for the system to be able to deal with these terminals as a group for the sake of efficiency, and for functions pertaining to the whole group to be performed for the group as one. For example, through improved algorithms for radio resource control, the system would be able to predict that a large number of terminals are about to enter a cell (and provision resources in advance accordingly) based on the knowledge of another known member of the group entering it. This would greatly improve the efficiency, and likely the reliability, of radio resource control. Enabling the system to recognise whether terminals are moving in a group is a challenge for a number of reasons, as information available to the system about the exact locations of terminals—aside from their current cell or location area—is often sparse.
In one aspect of the present invention this problem is addressed by a method that only requires a grouping entity to be informed about a new cell promptly upon the handover of each terminal, to detect groups of terminals that move together. Hence the method is generically applicable to a range of cellular systems, requiring no hardware additions, and is also extremely computationally simple and efficient.
Group handover methods have been investigated previously in cellular radio networks. WO 00/74417 discloses a cellular communications system in which group handover of a number of mobile stations from one type of network, such as UMTS, to another type of network, such as GSM, can be performed to relieve congestion in an overloaded cell. In one aspect mobile stations are grouped according to where they are located in a cell, with position being defined by signal power measurements. The power transmission level used by the base station is measured for each mobile station. The power level together with an identifier of the mobile station is sent to a grouping module that clusters mobile stations by power level into two groups using a predetermined threshold. When the cell becomes overloaded, the grouping module generates a handover instruction to all mobile stations in the first group to switch communication either to another frequency or to a different network. Whilst this is operable in a cellular communications environment (albeit between different generations) it is not clear how such a technique might work in a heterogeneous environment where there is limited communication and co-operation between different network types and operators at best.