Existing cellular network deployments are dominated by the 2G and 3G Third Generation Partnership Project (3GPP) standards. The process of rolling out so-called 4G networks has just begun, and it will be many years before 4G network coverage is sufficient to allow 2G and 3G networks to be withdrawn completely, if at all. A fundamental requirement for real-time service provision is the seamless handover of services for subscribers roaming across cell boundaries of the radio access network (RAN). Given the ongoing co-existence of 2G, 3G and 4G networks, it is particularly desirable to allow for the handover of real-time service connections such as circuit switched (CS) services between the different radio access technologies.
Considering further the 4G technology, this is being specified under the name LTE Long Term Evolution (LTE) and System Architecture Evolution (SAE) in 3GPP. The LTE RAN technology implements only a packet switched (PS) access, in contrast to 2G/3G, using GSM/EDGE Radio Access Network (GERAN) and Universal Terrestrial Radio Access Network (UTRAN) technologies respectively, which provide for both PS and CS access. In 2G and 3G networks PS connections are used to carry data traffic, whilst CS connections are used for real-time services such as voice or video. The target is that in 4G networks all services will be carried over PS connections.
However, during the roll-out phase of LTE RAN there will be cases that the LTE RAN or the user equipment (UE) are not yet capable of handling a voice call over PS connections. In this case the UE has to fallback to a corresponding CS service using 2G/3G via GERAN and UTRAN, if the user wants to initiate a voice call or wants to receive a voice call. This fallback is defined in 3GPP as CS-fallback (CSFB) and is defined in 3GPP TS 23.272 “Circuit Switched (CS) fallback in Evolved Packet System (EPS) Stage 2”.
Even if the UE and the LTE RAN is capable of handling a voice call over PS connections, due to the spotty LTE radio coverage during the roll-out phase of LTE RAN, the UE may move out of LTE RAN coverage. In this case it is necessary for the call to be transferred to a 2G or 3G radio access network, i.e. the call must be handed over from a PS access to a CS access.
In case a voice call is established using a PS access, the IP Multimedia Subsystem (IMS) network defined by 3GPP provides IP Multimedia services over mobile communication networks, which provides a common service control network for the PS and CS domains provided through the LTE, UTRAN, or GERAN radio accesses. In order to implement the access handover, media control must be transferred from the Evolved Packet Core (EPC) network of the 4G domain to an allocated Mobile Switching Centre (MSC) within the 2G/3G domain.
Interworking solutions for IMS Centralized Services (ICS) as specified in 3GPP TS 23.292, “IP Multimedia Subsystem (IMS) centralized services Stage 2”, allow IMS sessions using CS bearers to be treated as standard IMS sessions, which is required for the purpose of IMS Service Continuity. ICS defines signaling mechanisms between the UE and IMS for transport of information to centralize the service in the IMS, and TS 23.237 “IP Multimedia Subsystem (IMS) Service Continuity” defines the additional procedures needed for service continuity when using CS access for media transport. Within the context of TS 23.292 and TS 23.237, the further 3GPP document TS 23.216: “Single Radio Voice Call Continuity (SRVCC); Stage 2”, describes a mechanism for handing over a voice call from a PS to a CS access.
Consequently, there are cases where a UE is leaving the LTE RAN and is using 2G/3G RAN for the execution of a CS service. When the execution of the CS service has ended, the UE is still attached to the MSC via the 2G/3G RAN. However, 2G/3G RAN may not be a RAN preferred by the subscriber using the UE, as the data rates of the 2G/3G RAN may be limited. So it would be beneficial if the UE would return as quickly as possible to a preferred RAN after the CS service has terminated. The preferred network could be any kind of network which is preferred in the subscription of the subscriber using the UE, or is preferred by the operator of the communication network. As an example, LTE or any other high speed packet access (HSPA) network could be the preferred network for a UE.
Even without SRVCC there is need for a UE to move to its preferred network as soon as possible after the CS service has ended. An UE that starts a call using CS access (e.g. due to bad LTE coverage) and then moves into an area with good LTE coverage would benefit from moving as fast as possible to LTE after the provisioning of a CS service has been completed.
The 3GPP standards for SRVCC and CSFB describe some solutions for how to perform a fast return to LTE after CSFB or SRVCC respectively. However, these solutions imply changes in the implementation of the RAN, which is very difficult for an operator to deploy in practice. In a typical network there may be thousands of RAN nodes that would need to be upgraded, which is not feasible on top of the ongoing LTE RAN rollout. Other solutions are tailor-made for a specific UE radio chipset of a single vendor, and this solution then lacks support in UEs not being based on this particular radio chipset. These known solutions for fast return to LTE are also limited to a single LTE frequency as selection criteria.