LTE Redirection to NR SA
This tutorial is about how to configure for redirection from LTE to NR SA and how to verify it.
Cell Redirection a mechanism where UE can change the cell in Idle mode. The idle mode cell change happens in several different situations as listed below.
- UE changes cell to another cell which has higher cell power with the same PLMN as the current cell. This is called Cell Reselection.
- UE changes cell to another cell which has higher priorities. This priorities may be configured in SIB or RRC Release message. This is called Cell Reselection.
- UE lost signal from the current cell and change to another cell with selectable cell. Strictly speaking this is more like a cell selection, but it can be considered as a type of reselection.
- UE first attached to a visiting PLMN and changes to another cell with Home PLMN and vice versa. . Strictly speaking this is called Roaming.
- UE attached to a cell and Network (eNB) explcitely direct UE to change to another cell via RRCConnectionRelease message. This is called Cell Redirection.
We suppot a few different use cases with a little bit different configurations as summarized below.
- case 1: rrc_redirect. this is used when triggering a manual RRC release' using the rrc_cnx_release remote API or when the core network triggers a redirection procedure for emergency fallback
- case 2: meas_config_desc: { nr_redirect:{} or eutra_redirect:{} }. Redirection is triggered when the UE sends the event based measurement report
- case 3: meas_config_desc: { nr_redirect:{} or eutra_redirect:{} } + eps_fallback_fast_return_preferred_method: "redirection". This is applied when the eNB is aware of the source NR cell during the EPS fallback (thanks to NGAP/S1AP singalling) and the source NR cell is in the EUTRA neighbor cell list, then the RRC release message is automatically populated with the redirection to NR
Table of Contents
- LTE Redirection to NR SA
Test Setup
Test setup for this tutorial is as shown below. This is just for low layer testing, you may not need any complicated IP layer setup.
- SIM Card used in this tutorial is the one delivered with the system as it is.
- If you want to change the configuration, The tutorial Configuration Guide would help
Key Configuration Parameters
Followings are important configuration parameters for this tutorial. You may click on the items for the descriptions from Amarisoft documents.
- ncell_list : In this link, you would get the descriptions for all the items listed below
- rat
- cell_id
- meas_config_desc : In this link, you would get the descriptions for all the items listed below
- a1_report_type
- a1_rsrp
- a1_hysteresis
- a1_time_to_trigger
- a2_report_type
- a2_hysteresis
- a2_time_to_trigger
- a3_report_type
- a3_offset
- a3_hysteresis
- a3_time_to_trigger
- rsrp_filter_foeff
- nr_b1_report_type
- nr_b1_rsrp
- nr_b1_hysteresis
- nr_b1_time_to_trigger
- nr_rsrp_filter_coeff
- nr_cell_redirect : In this link, you would get the descriptions for all the items listed below
- b1_threshold_rsrp
- hysteresis
- time_to_trigger
- meas_gap_config
Configuration
I have used enb-redir-sa.cfg which is copied and modified from gnb-nsa.cfg.
I am using the default mme, ims config as shown below.
Following is the configuration in enb-redir-sa.cfg.
The important setting in this part is to set ALLOW_SA to 1 that allow NR cell the idle mode operation like redirection. Regarding other configuration like MIMO (N_ANTENNA), Bandwidth(N_RB_DL) etc, you can configure them in any way you like.
You need to add a NR cell to LTE cell as a neighbour cell. You can do this by adding ncell_list within cell_list, which contains a nr cell like {rat: "nr", cell_id: 2,} as in this test.
For NR cell, you don't need any specific configuraton. Just configure it like default NR cell configuration. In this test, I configured NR cell to FDD and band to n5.
I increased inactivity_timer to a large value to give eNB/gNB enough time for you to tweaking cell power without releasing RRC. The important part for this test is to add nr_cell_redirect parameter to meas_config_desc setting. This is to allow UE to redirect from LTE cell to NR cell if the measurement criteria is met.
Perform the Test
Check the cell configuration and see if it is configured as intended.
Power on UE and get it attached to LTE Cell. When the power condition is met, you see UE redirect to NR cell and camp on to it, but the detailed operation cannot be confirmed in 't' command. You need to verify it in the captured log.
Log Analysis
Confirm that both LTE cell and NR cell are broadcasting SIBs as expected. This ensures that both cells are ready for idle mode operation.
Check out SIB1 of the NR cell to make it sure some important configurations (e.g, ssb-PositionsInBurst, ssb-PeriodicityServingCell etc) are set as you intended.
Make it sure that eNB sets measurement configuration in RRC Connection Reconfiguration to detect NR cell. You can check it out with measConfig IE.
For the details, first check out the contents of measObjectToAddModList and make it sure that the target NR cell is added with right configuration.
Then check out reportConfigToAddModList IE and all the events (especially eventB1-NR) are properly configured as intended.
Lastly check out measIdToAddModList and make it sure that measObjectId and reportConfigId for NR cell is added.
Make it sure that measGapConfig is enabled.
If all the measurement configuration is properly set and cell power is within the expected range, you should get the measurementReport for NR cell. If you don't get this, you need to do troubleshoot (e.g, tweaking cell power) first until you get the report.
Once the expected measurement report is recieved, eNB will send RRCconnectionRelease with redirectedCarrierInfo that specifies the target NR cell.
Ensure that UE trigger RRC setup to the target cell (NR cell)
Then it is expected that UE initiate Registration to core network of the target cell.
UE would trigger Tracking Area Update Request that will proceed to all the rest of registration process to the target cell.
RRC / NAS Signaling
RrcConnectionRelease
: This is the RrcConnectionRelease message sent by eNB to trigger the redirection. (
{
message c1: rrcConnectionRelease: {
rrc-TransactionIdentifier 0,
criticalExtensions c1: rrcConnectionRelease-r8: {
releaseCause other,
redirectedCarrierInfo nr-r15: {
carrierFreq-r15 632544,
subcarrierSpacingSSB-r15 kHz30,
smtc-r15 {
periodicityAndOffset-r15 sf20-r15: 0,
ssb-Duration-r15 sf1
}
}
}
}