@@ -99,6 +99,26 @@ the [MAC configuration](../MAC/mac-usage.md) as well for SIB configuration.
...
@@ -99,6 +99,26 @@ the [MAC configuration](../MAC/mac-usage.md) as well for SIB configuration.
### Neighbor-gNB configuration
### Neighbor-gNB configuration
TBD
#### What is a gNB neighbor?
Network continuity is a key aspect of 5G. In the 5G architecture, gNB neighbors play a central role in maintaining service continuity through mechanisms such as handover and load balancing. By definition, a gNB neighbor is another gNB that can be measured and linked by the UE. If the current serving gNB is no longer optimal, the UE may connect to a neighbor gNB.
To support this behavior, the network configuration specifies additional frequencies and cells that the UE should measure. The UE reports these measurements to the network, which then decides whether or not to initiate a handover.
Neighbor types include:
-**Intra-gNB neighbors** - cells belonging to the same gNB
-**Inter-gNB neighbors** - cells belonging to different gNBs
-**Inter-RAT neighbors** - cells belonging to another RAT (e.g., LTE)
#### Required configuration parameters
To define a neighbor cell in the configuration file, the following parameters are typically needed:
-**gNB ID** - unique identifier of the gNB
-**Cell ID** - identifier of the cell within the gNB
-**Physical Cell ID** - identifier of the cell’s synchronization signal (PCI)
-**Absolute Frequency** - frequency used by the SSB (absoluteFrequencySSB)
An N2 handover involves the transfer of a UE from one gNB to another via the 5G core network. Unlike F1 handover, where the CU handles the process internally between its DUs, N2 handover requires signaling through the AMF, making it a core-network-based handover.
We assume:
* Two independent gNBs connected to the same 5GC via N2 interface.
* A UE initially connected to gNB-PCI0, which will be handed over to gNB-PCI1.
* Handover is triggered by either by decision based measurement event (e.g. A3) or telnet command.
## Steps to run N2 handover with OAI UE
1. Similarly to F1 handover, UE does not support any measurement reporting and handover is triggered by
telnet command. Therefore, ensure that both gNBs and UE are built with telnet support:
Ensure the UE successfully registers with the network.
4. Start the target gNB (gNB PCI1) e.g.
```sh
sudo ./nr-softmodem -O ../../../targets/PROJECTS/GENERIC-NR-5GC/CONF/gnb.sa.band78.fr1.106PRB.pci1.rfsim.conf --sa--rfsim--telnetsrv--telnetsrv.shrmod ci --gNBs.[0].min_rxtxtime 6 --rfsimulator.serveraddr 127.0.0.1
```
5. Trigger the N2 handover, e.g.
From gNB-PCI0, trigger handover on target gNB with PCI 1 for UE ID 1:
```sh
echo ci trigger_n2_ho 1,1 | nc 127.0.0.1 9090 &&echo
```
where the input parameters correspond to the PCI of the neighbor call and the RRC ID of the UE.
This will initiate the N2 handover on the source gNB.
## Neighbour list and measurement configuration
Make sure the configuration file contains a neighbour list and measurement configuration, e.g. [neighbour-config-rfsim.conf](../../ci-scripts/conf_files/neighbour-config.conf). This configuration can also be present in a different file and included in the gNB configuration file with `@include "neighbour-config-rfsim.conf"`.
For each gNB there is a `neighbour_cell_configuration` linked to its serving cell ID.
The measurement configuration is based on A2 and A3 measurement events in 5G NR. These events are used by the UE to report radio conditions to the gNB. The A2 Measurement Event indicates that the serving cell’s signal quality has degraded below a defined threshold and the UE shall initiate measurement of neighboring cells. The A3 Measurement Event indicates that a neighboring cell’s signal quality is better than that of the serving cell by a certain offset and the UE shall trigger handover to a stronger neighboring cell
This is an example with comments on how to use the configuration file:
