3GPP has specified the main deployment options for 5G development. These are designated as non-standalone (NSA) and standalone (SA) deployments. Initial 5G deployments will be based in Non-standalone (NSA) mode. LTE bands can initially be re-farmed to be used by NR in various combinations. Initial deployments will be based on LTE – NR Dual Connectivity, in NSA operation with NR as a capacity layer. Dual Connectivity always means two schedulers, for LTE and NR. In Non-standalone (NSA) there is option 1 & Option 3x specified by 3GPP, while in Standalone (SA) mode options 2, 4, 5 & 7x.
NR option 4
Option 4 is a critical SA configuration since this option allows 5G core connected customers to augment NR carriers with LTE carriers to ensure data rates remain competitive with those provided by NSA option 3x. However, Option 4 is not currently planned by some network or terminal suppliers. The use of Option 2 with DSS on all LTE frequency bands is a significant, however, costly alternative to Option 4.
LTE carrier aggregation and NR-LTE dual connectivity are already supported. Standalone NR carrier can be extended by adding an LTE carrier, using Dual Connectivity when needed. Option 4 allows NR SA users to access LTE carriers in addition to NR carriers, enabling a competitive user data rate to be supported compared to NSA, LTE only service.
Drawbacks for Option 4 are the extra development work that is needed for suppliers. Many procedures are not impacted, such as idle mode and RRC connection set-up. However, other functions like handover need minor adaptation. A couple of new processes are also required, e.g., secondary node addition and removal. Option 3x already includes support for PDCP UP anchor in NR, but adaptation to NG-u protocol is needed.
Additional configuration is also required, such as setting parameters controlling activation/deactivation of secondary LTE node and Xn-based LTE-NR neighbor relationship where Xn link is needed to support dual connectivity.
Alternatives to option 4
The first alternative is DSS on all carriers. The drawback here is the extra baseband resource needs to be dimensioned for both NR and LTE. There is also a non-negligible reduction in capacity & data rate from DSS, due to the need for control channels on both technologies. Another alternative is on-demand NSA fallback handover when no 5G core services are needed. However, in this case, 5G core services are not supported (for example, ATSSS for FMC). There is also service interruption during handover to LTE, extra delays to access higher rates, and increased signaling.
NR traffic experiences at least 20% user data rate and capacity reduction from methods to accommodate LTE CRS and PDCCH channels. LTE traffic experiences around 7.5% user data rate and capacity reduction from processes to accommodate NR SSB and CSI-RS channels. Furthermore, more baseband resources and licenses are used, while DSS carriers require baseband resources for both LTE and NR. Depending on the spare BBU capacity, this may or may not trigger BBU HW upgrade. Extra licenses apply even if no new hardware is needed.
On-demand NSA fallback handover is triggered when the user is transferring high volumes of data while using option 2 so that competitive data rates can be supported by aggregating LTE and NR spectrum in NSA operation. The procedure involves a handover from the master node on NR/5GC to LTE/EPC that will cause extra delay and interruption. The download may be complete before fallback is initiated unless the object is large. Special functions will be needed on LTE to retain user in NSA until the download is complete, and then redirect the user back to NR/5GC. With Option 4, the secondary node addition does not involve a change in either the UP or CP anchor, so the delay is shorter, and there should be no interruption.
Options 5 & 7
Option 5 connects the LTE base stations to the 5G core. The main benefit is that it enables the use of new 5G core services in areas not yet served by NR. Furthermore, it also allows NR SA to LTE handover without interworking between EPC and 5G core, when leaving the NR service.
However, hardware upgrades may be needed, especially in some older LTE base stations with extra development work for suppliers and extra configuration and testing. The LTE eNB needs to support a protocol conversion for both the NG-c and NG-u interfaces towards the 5G core.
Options 7/7x enable dual connectivity with NR carriers added to the LTE master. These options make use of high-bandwidth NR expansion carriers with 5G core without the need to deploy a low-frequency NR anchor layer. However, hardware upgrades are also needed in some older LTE base stations, with extra development, configuration, and testing. Furthermore, option 5 has an impact on the adaptation of Option 3x dual connectivity procedures to reflect new interfaces to the 5G core.
5G core to EPC Interworking
New Combo-core will support EPC capability embedded within. This means that it will support interworking when moving between NR/5GC and LTE/EPC. Option 5 is then not required if all LTE sites are connected to the new core network (CN). There are three different scenarios where such interworking might be required. Moving between an NR/LTE site and an LTE-only site, moving deep indoors from mid-band NR to low band LTE when NR is not yet deployed in low bands and for on-demand NSA selection (alternative 2 to option 4).
Options 5 & 7 (incl. 7x) may be considered as no longer prioritized since the chances of getting Option 4 developed are maximized by dropping the need for suppliers to develop Options 5 & 7. By the time option 5 or 7 would be developed, widespread deployment of NR would anyway be anticipated. This means that a lack of 5G core services would only apply to the shrinking number of LTE-only sites. Furthermore, a parallel low or mid-frequency band NR deployment is needed (e.g., using DSS in 2.1 GHz band), wherever NR in high bands is deployed, to provide an NR anchor layer for the SA scenario.
Alternative Options 6 & 8 are intentionally not defined by 3GPP as all operators saw the main 5G benefit of combining New Radio (NR) with the 5G core. According to RP-161266, options 6 & 8 would support a direct connection of NR to EPC without LTE dependence. For Rel-15, they were intentionally not defined, neither for Rel-16 or Rel-17. There are no 3GPP plans to standardize these options, although there have been some requests from India for Option 6. However, by the time these options would be theoretically deployable (if defined in Rel-18), SA option 2 will be a major standard in many parts of the world, and maybe even 6G will be on the horizon.
