New mobile communication systems are currently being developed, which are to succeed current mobile communication systems, such as e.g. any 3GPP communication system beginning from second to fourth generations (2G, 3G, 4G), like UMTS, LTE, LTE-A, etc. Such new mobile communication systems are typically denoted as 5G communication systems. In 5G communication systems, it is generally envisioned to enable provision of new mobile low-latency and ultra-reliable services, and to provide other services like V2X communications more efficiently.
FIG. 1 shows a schematic diagram of a 5G communication system architecture, for which the present invention is applicable.
As shown in FIG. 1 (in the horizontal direction thereof), the 5G system architecture can be logically/functionally divided in a mobile access domain, a networking services domain and an application domain. As shown in FIG. 1 (in the vertical direction thereof), the 5G system architecture can also be logically/functionally divided in a control plane (CP) and a user plane (UP). As usual, the control plane is responsible for establishing and controlling connectivity of a terminal to the communication system, i.e. handling signaling for enabling service provision, while the user plane is responsible for realizing connectivity of a terminal to the communication system, i.e. handling data/traffic for service provision. The CP is handled by cMGW (in the NAS) and AP (in the AS), and the UP is handled by uGW (in the NAS) and AP (in the AS). While cMGW configures uGW to handle the user plane issues, tunnels established between uGW and AP are to deliver user traffic securely.
In the 5G architecture shown in FIG. 1, the AP can be regarded to constitute the radio access network (RAN), while the remaining entities can be regarded to constitute the core network (CN).
Herein, the structure and operation of the mobile access domain are specifically addressed.
In the 5G architecture, a RAN (or AS) mobility problem manifests (even more prominently than in current e.g. 4G architectures). This is essentially because of the large number of small cells and the possibility of terminals to access services using two or more service flows connected to two or more UP gateways, i.e. uGWs, offering those services, like e.g. Internet service from one UP gateway, voice service from another UP gateway, V2X service (e.g. over Ethernet) from still another UP gateway, and so on. Each of these services could be run over different radios. That is, 5G communication systems facilitate not only single-connectivity by plural terminals but also multi-connectivity by any single terminal.
Multi-connectivity generally refers to maintaining multiple radio links, and basically comprises                intra 5G multi-connectivity, wherein (one CP connection and) multiple UP connections are established and maintained over different radio interfaces of the same radio access technology (RAT) or system specification, such as parallel/simultaneous connections among the different radio interfaces of 5G, namely millimeter wave (mmWave), centimeter wave (cmWave) and WA (Wide Area, <6 GHz), and/or        inter RAT multi-connectivity, wherein (one or more CP connections and) multiple UP connections are established and maintained over different radio access technologies (RATs) or system specifications, such as parallel/simultaneous connections by 5G, LTE and or WLAN access types, not excluding a plural connections to 5G access type besides one or more connections to at least one other access type.        
Though solutions have been proposed for RAN mobility optimizations involving the CP, intra-RAT mobility events like handovers between 5G access points imply that the tunnel endpoints of the target access point have to be synchronized at the uGW during the mobility event in order to deliver user plane data seamlessly. Hence, to address the signaling issues and the number of RAN mobility events (e.g. intra-RAT mobility events), a logical entity called multicontroller is proposed in the 5G architecture. Such multicontroller acts as an aggregation node for multi-connectivity anchoring and management, i.e. an aggregator for 5G small cells and an anchor for multi-connectivity in 5G. A multicontroller serves to aggregate the S1*-C and S1*-U connections, thereby providing an abstraction layer for RAN (or AS) mobility with respect to the core network. Herein, the star (*) mark denotes an association to 5G systems. For example, S1 may denote S1 connection in LTE-A, whereas S1* denotes S1 connection in 5G.
FIG. 2 shows a schematic diagram of a multicontroller arrangement in a 5G communication system architecture, for which the present invention is applicable.
As shown in FIG. 2, a terminal (UE) is assumed to access two services from two different uGWs, while the terminal (UE) has single-connectivity to one AP at a time. Hence, the terminal (UE) has two service flows coming from uGW1 and uGW2, wherein e.g. uGW1 could offer Internet over IP access and uGW2 could offer V2X services over Ethernet. A multicontroller—considered as a RAN logical entity—aggregates/anchors CP and UP connections (and thus handles CP and UP issues, including e.g. UE context) for these two services. The CP connections (or their handling) are terminated by the RRC block in the multicontroller, while the UP connections (or their handling) are terminated by the NCS block in the multicontroller. The multicontroller in essence manages multiple radio interfaces (i.e. cmWave, mmWave, WA) and multiple APs of each radio interface.
In case of terminal mobility in the form of a handover between two APs served by different multicontrollers, as indicated by the rightward arrow in FIG. 2, all of the CP and UP connections (and associated CP and UP issues, including e.g. UE context) for both services are to be moved/reconfigured between the new multicontroller and the new AP, even though such handover would actually only affect one of the services. This is essentially because a multicontroller represents an entity with cohesive or collocated CP and UP functionality.
For example, when considering local breakout like in LTE (when the uGW is directly tunneling the data to the AP), a mobility event where the UE moves from the coverage of AP1 to coverage of AP2 (even when there is no change in the multicontroller serving AP2), there needs to be a signaling communication towards the correct uGW to inform the change in tunnel endpoint that it is transacting with AP2 representing the target AP. This has to happen via cMGW, since there is no direct interface (or signaling connection) to communicate between AP/multicontroller and uGW directly. This involves additional signaling and is not optimal, considering the number of small cells and the associated mobility events in 5G systems. The signaling to inform the UP gateway, i.e. uGW, about the change in the tunnel endpoint identifier is a longer procedure and always has to go through the cMGW.
Accordingly, a cohesive or collocated CP and UP functionality (in the multicontroller) means that any RAN mobility event resulting in the change of the serving multicontroller (i.e. handover to a target AP connected to a different multicontroller than the source AP) will imply the following:                All the service flows of the terminal need to be reconfigured to the new target multicontroller (aggregation node), irrespective of the need for reconfiguration of any respective service flow. That is, user mobility resulting in reconfiguration of one of multiple service flows causes that all of the multiple service flows (UP connections) as well as associated RRC signaling (CP connections) is also impacted.        This will also pose challenges to the dimensioning topology of multicontrollers (aggregation nodes), since the aggregation will be defined by the cohesive/collocated CP&UP unit and not the individual CP and UP parts.        Additional NAS signaling towards the CN is required to actuate RAN mobility, thereby polluting the NAS/CN with signaling overhead. This will involve the cMGW to update the associated uGW about the tunnel endpoint identifier (TEID) of the target AP.        
So, with the conventional 5G architecture, even with an aggregation node such as a multicontroller with cohesive or collocated CP and UP functionality, there is no optimized abstraction of RAN (or AS) mobility from the core network.
Accordingly, there is a demand for optimizing RAN (or AS) mobility in the network, especially in a communication system enabling multi-connectivity, in terms of the above-outlined considerations.