It is known from the state of the art in a communication system providing different radio access technologies (RAT) to support an inter-system handover of a mobile terminal between these technologies. Such a handover may be performed for instance between a 3 G (3rd generation) radio access network and a 2 G (2nd generation) radio access network, or within a 3 G system between a UTRAN (universal mobile telecommunication services terrestrial radio access network) employing WCDMA (wideband code division multiple access) and a GSM (global system for mobile communications) radio access network (RAN). As a prerequisite for an inter-system handover, the mobile terminal in question is operable with the communication system via all concerned types of radio access networks (RANs). Such mobile terminals are referred to as multi-mode terminals. One example of a multi-mode terminal is a dual-mode terminal which is operable in 2 G and 3 G networks.
The network signalling employed for performing an inter-system handover between UMTS Universal Mobile Telecommunication Services) and GSM is described for example in the technical specification 3GPP TS 23.009 V3.14.0 (2003/06): “Handover procedures” (Release 1999)”.
There are several reasons for which a change, i.e. an inter-system handover, of the radio access technology is enabled.
The most important reason for an inter-system handover is differences of coverage and connection/service quality in the communication system. In some situations, a mobile terminal may move out of the coverage area of the first radio access system. Moreover, the radio connection quality provided by this first radio access system may degrade below a given threshold value. If at the same time, another radio access system still provides coverage and or a better radio connection quality, an inter-system handover can guarantee a continued and satisfactory supply of the mobile terminal. The first radio access system is typically a GSM/GPRS system and the second radio access system a WCDMA system.
Another essential reason for an inter-system handover is load, i.e. the current amount of traffic in different radio access systems. When the load in a first radio access system exceeds a pre-defined threshold value, an overflow of mobile terminals can be handed over to another radio access system. In this case, the first radio access system is typically GSM/GPRS and the second radio access system WCDMA.
A third reason for an inter-system handover is QoS (Quality of Service) requirements of requested services. The operator of the communication system defines service based handover criteria according to its preferences. These criteria are then stored in a service priority table in the core network, which initiates the handover. The same table resides additionally in the Radio Network Controller (RNC) of an UTRAN of the communication system. The table in the RNC can be used in case the RNC receives no handover information from the core network. In an exemplary assignment of services to different radio access technologies, GSM may be preferred for speech and WCDMA for packet data, while circuit switched data has to be transmitted using WCDMA. An additional criterion for a service-based handover is load, load and service based handover thus being a more accurate term than service-based handover.
As specified for example in the technical specification 3GPP TS 25.413 V3.4.0 (2000-12): “UTRAN Iu Interface RANAP Signalling (Release 1999)”, a service based handover from a 3 G UTRAN to a 2 G GSM radio access network is implemented by an optional parameter called “Service Handover” in the RANAP (radio access network application part) messages “RAB Assignment Request” and “Relocation Request” on the Iu interface between the core network and an UTRAN. Three values are defined for this parameter, “HO to GSM should be performed”, “HO to GSM should not be performed” and “HO to GSM shall not be performed”. Thus, the core network is not able to force the RNC to carry out a handover from a 3 G to a 2 G radio access network, but only to propose a handover. The final decision is taken by the RNC based on additional criteria like load, coverage and radio connection quality. Furthermore, in load and service based handovers, an RNC hands mobile terminals over to a base station subsystem (BSS) periodically and in groups, not immediately and on an individual basis.
All known inter-system handovers are decided by the network, which prevents a communication system to make use of possible advantages of an inter-system handover in several situations.
The current 3GPP specifications treat UTRAN, GSM and GERAN radio access cells equally, i.e. there are no strong means for the network operator or the user to guide a mobile terminal towards the most suitable radio access technology. It is known to favour one or another public land mobile network (PLMN), location area (LA), routing area (RA) or cell based on defined criteria. However, cells of different radio access technologies may be mixed in a single PLMN, LA or RA, and no method has been proposed so far for forcing the mobile terminal to a specific radio access technology, and not even for favouring a certain radio access technology. Only if a requested service cannot be provided in the current serving cell, the cell may be handed over to another cell by the network, or the cell may be cleared.