To support seamless service continuity and network-controlled handover, the Mobility Management Entity (MME) and the new access and mobility management function (AMF) interact directly through the N26 reference point, which supports devices in single-registration mode (the device is either registered in the MME or in the AMF, but not in both simultaneously). The N26 reference point is used for both idle mode and connected mode mobility; the device initiates idle mode mobility (possibly triggered by the RAN), while connected mode is initiated by the RAN, and the device is informed when handover preparation has been completed. Furthermore, tight interworking also includes how to map protocol data unit (PDU) sessions in the 5GS to packet data network (PDN) connections in the EPS and vice versa.
The interworking architecture ensures that the new 5GC-capable devices are always connected to the UPF in the 5GC independently if they are connected though 4G or 5G access, which enables IP address preservation when devices move between accesses. Thus, service characteristics are maintained, since any colocated value-added services connected to the UPF are the same, and the policy control function (PCF) applies session policies for the device when connecting over 4G or 5G access.
Policy and subscription management need to be provided in a consistent way for a device using NR or LTE access. The interworking architecture provides several 5GC capabilities also over LTE/EPC, including support for network slicing. Operators can migrate from an existing EPC to a dual-mode EPC and 5GC network solution by migrating the packet gateway, the policy control and the subscription and data management, and by introducing new functionality.
Overcoming domain-specific migration challenges
A solid EPS to 5GS migration strategy will consider and address the challenges in devices and all four network domains: RAN, packet core, user data and policies, and services.
The RAN domain
In many markets, the new NR spectrum is first available in mid and high bands. Depending on the site grid, introducing the NG-RAN with NR SA only on these bands may lead to spotty NR coverage that is only suitable for local area services. When deploying NR SA for MBB, it is preferable to ensure continuous NR coverage within the targeted service area (a city, for example) to avoid frequent mobility between NR and LTE. Alternatively, intersystem mobility could be limited by conservative mobility thresholds.
Achieving higher capacity and continuous coverage requires a combination of NR on mid and high bands for capacity and NR FDD on sufficiently low bands for coverage . The NR FDD spectrum on low band can be either new, re-farmed or an existing LTE band that is shared between NR and LTE using dynamic spectrum sharing. NR bands can be combined using carrier aggregation (CA) or, in some cases, dual connectivity.
NR CA will be vital in enabling service providers to serve the growing number of 5G devices in the network while maintaining overall network performance and user experience. This is done by activating downlink CA (FDD+TDD) in the areas with low-band and mid-band NR. This not only boosts mid-band NR coverage, and consequently capacity gain, but also provides a further coverage boost by enabling some of the NR signaling to be moved to lower bands. Ericsson has shown that this can provide up to 3-7dB extra gain in link budget on the downlink .
NR SA with CA reduces complexity in the RAN and devices compared with dual connectivity as in NR NSA. The device does not need to transmit on two uplinks at the same time. The use of NR SA also reduces the time from a device being in inactive mode to full NR capacity, enabled by all control signaling being carried over NR instead of being dependent on LTE and the setup of dual connectivity. The benefit to the consumer is faster access to the full potential of the combined NR capacity when, for example, downloading a file or starting up a video.
To support existing and forthcoming services like voice and emergency when migrating MBB to the 5GS, the NR SA needs to support capabilities and coverage demands like intersystem handover, positioning and QoS. This can be a stepwise migration.
The user data and policies domain
The EPC-5GC tight interworking architecture shown in Figure 1 assumes common subscription management support regardless of the access technology used by a given user. Although a combined HSS/UDM (Home Subscriber Server/Unified Data Management) function is depicted, first deployments support subscription management for the EPC and 5GC by interworking between a separate HSS and UDM through an HTTP/REST (Hypertext Transfer Protocol/Representational State Transfer) interface.
Existing HSS functionality must be evolved to enable interworking with UDM and to support tight interworking between the EPC and 5GC. The evolution includes an upgrade of the HSS functionality offered to EPC serving nodes with, among other things, subscription parameters that enable user access to the 5GC to ensure IP session continuity and single registration across the EPC and 5GC.
Once the 5GC for MBB is introduced, sessions are anchored in the SMF+PGW-C function. The use of SMF+PGW-C allows the policy control and charging rules function (PCRF) used for policy control in the EPC to be replaced by a new dual-mode policy management system that supports 5G-enabled devices regardless of the access technology currently used.
HSS/UDM and PCF/PCRF business logic are also highly dependent on how subscription data and policy subscription data is managed (that is, provisioned, stored and accessed). The EPC allows the support of a data-layered architecture, where the HSS and PCRF make use of an external database to manage subscription data. These databases are known as the User Data Repository (4G-UDR) for HSS subscription data and the Subscription Profile Repository (SPR) for PCRF policy subscription data. The 5GC generalizes the concept into a Unified Data Repository (5G-UDR). The 5G-UDR stores subscription, policy subscription, application and exposure data.
During the introduction of the 5GC it is beneficial to deploy new dual-mode subscription, data and policy management systems that support the EPC-5GC tight interworking architecture and procedures as depicted in Figure 2.