Among the list of Base Station Architectures in 5G System. There are various architectures
- Standalone Base Station
- Non-Standalone Base Station
- Centralized Unit (CU)-Distributed Unit(DU) Split Base Station
- Control Plane (CP)-User Plane (UP) Separation
In this post, we are going to discuss only Non-Standalone Base Station.
Non-Standalone Base Station has three options
- Option 3
- Option 3a
- Option 3x
- Option 4
- Option 4a
- Option 7
- Option 7a
General Architecture
- Non-Standalone (NSA) Base Stations use Multi-RAT Dual Connectivity (MR-DC) to provide user plane throughput across both the 4G and 5G air-interfaces. It means Non-Standalone Base station architecture requires an eNodeB and gNodeB to operate together.
Both eNodeB and gNodeB can be connected using non-ideal backhaul --- which means a realistic transport connection. In terms of scheduling packets, both eNodeB and gNodeB have their own independent schedulers. - 4G UEs and 4G/5G Capable UEs in NSA Legacy 4G UE can continue to use the eNodeB in the normal way, while newer UE with a 4G/5G capability can take advantage of the Multi-RAT Dual Connectivity. Non-Standalone Base Stations can be used to provide
NON STANDALONE BASE STATION (OPTION 3)
Configuration for Option 3 architecture is shown in the figure below.
Figure 1: Non-Standalone Base Station Architectures using the 4G Core Network Option 3
- Master Node and Secondary Node: 4G eNodeB provides control plane connectivity towards the Core Network and acts as the Master Node (MN). 5G gNodeB has control plane connectivity across the X2 interface and acts as the Secondary Node (SN). These configuration require the gNodeB to support the X2 interface rather than Xn interface.
- Drawback of Option 3: Tunneling all user plane data through the legacy eNodeB is a drawback of Option 3, because it is likely that eNodeB hardware is estimated to support air-interface throughput offered by 4G and that hardware may not be capable of supporting higher throughput offered by 5G. PDCP layer within eNodeB (Master Node) dynamically splits the downlink data between the eNodeB and gNodeB. The data allocated to gNodeB is forwarded across the X2 interface.
NON STANDALONE BASE STATION (OPTION 3a)
The configuration for Option 3a is shown in figure below.
In case of 3a as well, control plane connectivity 4G eNodeB provides connectivity towards the Core Network and acts as Master node.
Figure 2: Non Standalone Base Station Architecture using 4G Core Network Option 3a
- Option 3a resolves the drawback of Option 3, by providing user plane connectivity between S-GW and gNodeB. The eNodeB remains the Master Node and is able to control the selection of the downlink data path from the S-GW, eNodeB can provide the MME with the IP address of the gNodeB for some EPS bearers, while it can provide the MME with its own IP address for other EPS Bearers.
Drawbacks of Option 3a
- Option 3a has the drawback of not supporting the transfer of application data across the X2 interface.
- Coverage provided by gNodeB may be smaller than the coverage provided by the eNodeB. As a result UE moving out of coverage of gNodeB has to switch all data transfer to the eNodeB using Path Switch request. Path switch request procedure is relatively slow and not necessarily a fast solution.
NON STANDALONE BASE STATION (OPTION 3x)
In order to overcome the drawbacks of Option 3 and 3a. Option 3x is developed.
In case of Option 3x, user plane paths are full meshed between eNodeB, gNodeB and SGW as shown in figure below.
Figure 3: Non-Standalone Base Station Architecture with 4G Core Network
In case of Option 3x, eNodeB remains the Master Node and is able to control the selection of downlink data path from S-GW, that is, eNodeB can provide the MME with the IP address of the gNodeB for some EPS Bearers, while it can provide the MME with its own IP address for other EPS Bearers.
If UE moves from gNodeB to eNodeB on account of poor coverage, gNodeB can dynamically forward data across the X2 interface towards the eNodeB.
OPTION 4 - NON STANDALONE BASE STATION
There is also a standalone base station architecture with configuration using 5G Core Network. In this case 5G gNodeB provides control plane connectivity towards the Core Network and acts as the Master node (MN). The Next Generation eNode B (ng-eNode B) has control plane connectivity across the Xn interface and acts as Secondary Node (SN). In case of Non-Standalone Base station for Option 4 and Option 4a configurations require the ng-eNodeB to support the Xn interface instead of X2 interface.
- For Option 4, the Master Node connects to the UPF and AMF as shown in Figure 4 below.
Figure 4: Non-Standalone Base Station Architectures using the 5G Core Network Option 4
OPTION 4a: NON STANDALONE BASE STATION
- Option 4a is a non-standalone base station configurations using 5G Core Network. 5G gNodeB provides control plane connectivity towards the Core Network and acts as Master node (MN). The next generation eNodeB (ng-eNodeB) has control plane connectivity across the Xn interface and acts as the Secondary Node (SN).
- For Option 4a, the ng-eNodeB supports the NG-U interface towards the User Plane Function (UPF). ng-eNodeB does not connect to the Access and Mobility Management function (AMF) as shown in Figure 5 below.
Figure 5: Non standalone base station archtiecture using 5G Core Network Option 4a
OPTION 7: NON-STANDALONE BASE STATION ARCHITECTURE
- Non-Standalone Base Station using 5G Core Network has another option which is labelled as Option 7.
- Option 7 Next Generation eNodeB (ng- eNodeB) connections to User Plane Function (UPF)
- Next Generation eNodeB (ng-eNodeB) provides control plane connectivity towards the Core Network and acts as the Master Node (MN). The gNodeB has control connectivity across the Xn interace and acts as the Secondary Node (SN).
- Option 7 require ng-eNodeB to support the NG-C, NG-U and Xn interfaces as shown in Figure 6
Figure 6: Non-Standalone base station architecture using 5G Core Network Option 7
OPTION 7a: NON-STANDALONE BASE STATION ARCHITECTURE
- In case of Option 7a, Non-standalone base stations using the 5G Core Network.
- In this case Next generation eNodeB (ng-eNodeB) and 5G node (gNodeB) both connects to User Plane Function (UPF)
References:
1. 3GPP TS 38.300
2. 3GPP TS 38.801
3. 5G NR in Bullets by Chris Johnson