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doc (RRC): document neighbour configuration, SIB3/4 and MeasGaps design

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Expand RRC usage and handover documentation to describe the two-level
neighbour configuration layout, lookup-key semantics,
and config-time SIB4 grouping behaviour. Expand the example config
to include a `frequency_list` block and per-neighbour SIB3/SIB4
offset fields. Add SIB2 config example.

Update handover-tutorial.md to describe the same nested model and
note that F1 and N2 handover share the same serving-cell keyed
mapping.

Add documentation about SIB3/SIB4 and MeasGaps implementation
in OAI with stress on the shared neighbour configuration data model.

Update FEATURE_SET.
This commit is contained in:
Guido Casati
2026-03-03 14:54:08 +01:00
committed by Guido Casati
parent c866078b21
commit 22783bb3a7
4 changed files with 263 additions and 11 deletions
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6
doc/FEATURE_SET.md
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@@ -154,7 +154,7 @@ These modes of operation are supported:
- RedCap SIB1 v17 IEs supported
- Coexistence of RedCap and Normal UEs
- Handling of RedCap capability for small PDCP/RLC SN size
- Scheduling of other SIBs (2, 19)
- Scheduling of other SIBs (2, 3, 4, 19)
- NTN
- Support downlinkHARQ-FeedbackDisabled-r17
- Support for 32 PDSCH and PUSCH HARQ processes per UE
@@ -194,7 +194,7 @@ These modes of operation are supported:
- NR RRC (38.331) Rel 17 messages using new [asn1c](https://github.com/mouse07410/asn1c)
- LTE RRC (36.331) also updated to Rel 15
- Generation of system information (SIB2)
- Generation of system information (SIB2, SIB3, SIB4)
- RRC can configure PDCP and SDAP (through E1), and RLC and MAC (through F1)
- Interface with GTP-U (tunnel creation/handling for S1-U (NSA), N3 (SA), F1 interfaces)
- Integration of RRC messages and procedures supporting UE 5G SA connection
@@ -380,7 +380,7 @@ These modes of operation are supported:
- MIB processing
- Scheduling of system information block 1 (SIB1) reception
* Other system information
- Scheduling of other system information blocks reception
- Scheduling of other system information blocks reception (SIB2, SIB3, SIB4)
* Random access procedure (needs improvement, there is still not a clear separation between MAC and PHY)
- Mapping SSBs to multiple ROs
- Scheduling of PRACH

92
doc/RRC/rrc-dev.md
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@@ -633,6 +633,98 @@ sequenceDiagram
Note over TargetCU: Then: encode Handover Command, send NGAP HANDOVER REQUEST ACKNOWLEDGE
```
### Neighbour cells
#### SIB3/SIB4 and measurement-gap implementation
The following section documents the implementation-level control flow
for SIB3/SIB4 and measurement-gap handling in current OAI, using the configured
neighbor cell list as a shared input model.
Briefly, the three procedures are:
- SIB3: CU derives and provides intra-frequency neighbour SI, which DU broadcasts
and UE uses for autonomous idle/inactive intra-frequency reselection.
- SIB4: CU derives and provides inter-frequency carrier/neighbour SI, which DU
broadcasts and UE uses for autonomous idle/inactive inter-frequency reselection.
- MeasGap: CU/DU coordinate dedicated `MeasGapConfig` in UE `MeasConfig`, DU
scheduler interrupts transmission (`nr_measgap_scheduling()`) for the UE to
apply the MeasGap configuration in connected mode for gap-based measurements that
are sent as `MeasurementReport` and processed at CU-CP (`rrc_gNB_process_MeasurementReport()`).
Event-driven reports (for example A3) are used by CU-side mobility logic and can
trigger handover procedures.
### MeasGap
```mermaid
sequenceDiagram
participant DM as Data model (neighbour_cell_configuration)
participant CU as CU-CP (RRC)
participant DU as DU (MAC/RRC)
participant UE as UE
CU-->>DM: Read serving + neighbour frequency/PCI/band data
DM-->>CU: Provide neighbour/frequency inputs for MeasConfig
CU->>CU: nr_rrc_get_measconfig
Note over CU: Build UE MeasConfig
CU->>CU: get_meas_timing_config (cell.mtc, ue.measConfig)
Note over CU: Check distinct ssbFrequency
alt one frequency only
Note over CU: returns NULL, no meas_timing_config sent to DU
else multiple frequencies
CU->>DU: F1AP UE Context Setup Request
Note over CU,DU: Includes cu_to_du_rrc_info.meas_timing_config
DU->>DU: create_measgap_config
DU->>DU: encode_measgap_config
DU-->>CU: F1AP UE Context Setup Response
Note over DU,CU: Includes du_to_cu_rrc_info.meas_gap_config
CU->>CU: get_meas_gap_config
Note over CU: Decode/store gap in UE MeasConfig (UE.measConfig.measGapConfig)
CU->>UE: RRCReconfiguration
Note over CU,UE: Includes MeasConfig.measGapConfig
rect rgba(210, 235, 255, 0.35)
Note over DU,UE: Measurement-gap window
DU->>DU: nr_measgap_scheduling
Note over DU: Interrupt transmission for meas gap
UE->>UE: nr_rrc_handle_meas_indication
Note over UE: Perform gap-based neighbour/inter-frequency measurements
UE-->>CU: MeasurementReport
end
Note over UE,CU: UL-DCCH MeasurementReport for configured events (e.g. A3)
CU->>CU: rrc_gNB_process_MeasurementReport
Note over CU: Run mobility decision logic (may trigger handover)
end
```
### SIB3/SIB4
```mermaid
sequenceDiagram
participant DM as Data model (neighbour_cell_configuration)
participant CU as CU-CP (RRC)
participant DU as DU
participant UE as UE
CU->>CU: cp_f1_served_cell_info_to_cell
CU->>DM: get_cell_neighbour_list
DM-->>CU: neighbour_cell_configuration for serving cell
CU->>CU: get_ssb_arfcn
Note over CU: Derive serving_ssb_arfcn from cell MTC
loop for each configured neighbour
Note over CU: Compare neighbour.absoluteFrequencySSB vs serving_ssb_arfcn
alt equal
CU->>CU: get_sib3_intra_freq_neighbors
else different
CU->>CU: get_sib4_inter_freq_neighbors
end
end
CU-->>DU: F1AP F1 Setup Response
DU-->>UE: BCCH-DL-SCH-Message
Note over DU,UE: Broadcast SystemInformation including configured SIB3/SIB4
UE->>UE: nr_rrc_ue_decode_NR_BCCH_DL_SCH_Message
Note over UE: Evaluate idle/inactive reselection criteria and timers
Note over UE,DU: After reselection, access/registration continues on selected cell DU as needed
```
## UE Context
UE context information is stored in `gNB_RRC_UE_t`, which includes:

164
doc/RRC/rrc-usage.md
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@@ -94,7 +94,81 @@ the [MAC configuration](../MAC/mac-usage.md) as well for SIB configuration.
deactivated, a transparent SDAP header is prepended to packets, but no
further processing is being done.
- `cu_sibs` (default: `[]`) list of SIBs to give to the DU for transmission.
Currently, SIB2 is supported.
Currently supported:
- SIB2: serving-cell reselection parameters (configured in `sib2_config`)
- SIB3: intra-frequency neighbour cell list (neighbours on the same SSB ARFCN as the serving cell)
- SIB4: inter-frequency carriers + neighbour lists, grouped per `(absoluteFrequencySSB, subcarrierSpacing)`. Per-frequency fields (e.g. `cellReselectionPriority`, `threshX_HighP`, `threshX_LowP`, `q_OffsetFreq`) come from `frequency_list`
Example activation:
```
cu_sibs = ( 2, 3, 4 );
```
SIB2 is configured per-gNB in `sib2_config` (see below). SIB3/SIB4 are derived from the neighbour configuration:
- `neighbour_list` / `neighbour_cell_configuration`: neighbour identity + per-neighbour offsets (`q_OffsetCell`, etc.)
- `frequency_list`: per-frequency SIB4 reselection parameters (priority/thresholds/`q_OffsetFreq`)
Example `gNBs.[0].sib2_config`:
```
cu_sibs = ( 2 );
sib2_config : {
q_Hyst = 0;
cellReselectionPriority = 0;
threshServingLowP = 0;
threshServingLowQ = 4;
s_NonIntraSearchP = 10;
s_NonIntraSearchQ = 8;
q_RxLevMin = -56;
q_QualMin = -18;
s_IntraSearchP = 22;
s_IntraSearchQ = 20;
t_ReselectionNR = 1;
deriveSSB_IndexFromCell = 1;
speed_t_Evaluation = 0;
speed_t_HystNormal = 0;
speed_n_CellChangeMedium = 1;
speed_n_CellChangeHigh = 2;
speed_sf_Medium = 1;
speed_sf_High = 0;
};
```
#### SIB3/SIB4 and measurement gaps
This section summarizes how SIB3/SIB4 and measurement gaps relate in NR and how OAI currently implements them.
From 3GPP TS 38.331:
- `SIB3` carries intra-frequency reselection information (`intraFreqNeighCellList`).
- `SIB4` carries inter-frequency reselection information (`interFreqCarrierFreqList`
and per-carrier neighbour lists).
- `MeasGapConfig` is part of dedicated `MeasConfig` (typically sent in
`RRCReconfiguration`) and controls measurement gaps in connected mode.
In other words:
- SIB3/SIB4 are broadcast SI for idle/inactive (`RRC_IDLE` / `RRC_INACTIVE`)
reselection behavior: UE performs autonomous cell reselection using broadcast
SI (SIB3 for intra-frequency, SIB4 for inter-frequency).
- `MeasGapConfig` is a dedicated UE measurement behavior and applies to
connected mode (`RRC_CONNECTED`): the network configures what the UE
measures and reports (periodic and event-based, e.g., A3), and those reports
are used by CU-CP mobility logic (including handover decisions).
MeasGap does not depend on SIB3/SIB4, however they share the same underlying
neighbour/frequency data model, which is the common source of serving +
neighbour frequency information (see also [Neighbor-gNB configuration](#neighbour-gnb-configuration)):
- SIB3/SIB4 generation is done on the CU-CP side (inside `rrc_gNB_du.c`) from
neighbour/frequency configuration and serving-cell MTC-derived ARFCN.
- Measurement-gap configuration also starts from the same neighbour/frequency
model: CU uses neighbour fields (frequency/PCI/band) to build UE
`MeasConfig` measurement objects, then DU derives/encodes gap parameters
from CU-provided timing (`meas_timing_config`) and returns `meas_gap_config`
for CU forwarding in dedicated `RRCReconfiguration`.
Detailed implementation flow and sequence diagrams are documented in [`rrc-dev.md`](./rrc-dev.md).
#### UE-specific configuration
@@ -105,12 +179,64 @@ the [MAC configuration](../MAC/mac-usage.md) as well for SIB configuration.
Refer to the [handover tutorial](../handover-tutorial.md) for detailed information about gNB neighbors and handover procedures.
##### Configuration structure and key semantics
The neighbour configuration is a 2-level structure:
- Outer list: `neighbour_list`
- Key: `nr_cellid` of the serving cell
- One entry per serving cell
- Inner list: `neighbour_cell_configuration`
- Actual neighbour cells for that serving cell
- Contains neighbour fields such as `gNB_ID`, neighbour `nr_cellid`, `physical_cellId`, frequency, PLMN, etc.
This same core configuration model is reused by multiple RRC procedures.
- SIB3/SIB4 generation: uses neighbour identity/frequency/offset fields to derive
intra/inter-frequency SI.
- Connected-mode measurement config: uses neighbour frequency/PCI/band fields to build
UE `MeasConfig` measurement objects.
- Handover-related procedures: reuse neighbour identity fields (e.g., cell ID/PCI/PLMN
TAC, gNB ID) for target selection and for populating target-cell information carried
in NGAP handover messages.
Conceptually, for each serving cell the RRC keeps:
- A per-frequency table (`inter_freqs`): one entry per `(absoluteFrequencySSB,
subcarrierSpacing)` used for SIB4, containing the SIB4 per-frequency fields
(priority, thresholds, `q_OffsetFreq`, `q_RxLevMin`, `t_ReselectionNR`).
- A per-neighbour list (`neighbour_cells`): one entry per neighbour, with
identity (cell ID, PCI, PLMN, TAC), frequency (`absoluteFrequencySSB`,
`subcarrierSpacing`, `band`), and SIB3/SIB4 per-neighbour offsets
(`q_OffsetCell`, `q_RxLevMinOffsetCell`, `q_QualMinOffsetCell`).
- A link from neighbours to frequencies: each neighbour implicitly points to
the matching `inter_freqs` entry via its `(absoluteFrequencySSB,
subcarrierSpacing)`; if no such frequency exists, it is treated as having no
SIB4 per-frequency configuration.
Notes:
- In `neighbour_list`, only `nr_cellid` is used as the key for lookup.
- `physical_cellId` belongs to neighbour-cell entries in `neighbour_cell_configuration` (inner list).
- `nr_cellid` entries in `neighbour_list` should be unique to avoid ambiguous lookup.
- Intra-frequency neighbours (SIB3) are derived only from the per-cell
`neighbour_cell_configuration` on the serving carrier.
At configuration time (`gnb_config.c`), neighbours are parsed into `neighbour_cells`,
per-neighbour SIB3/SIB4 offsets are validated, and a per-frequency array `inter_freqs`
is built by grouping neighbours by `(absoluteFrequencySSB, subcarrierSpacing)` and
in-range SIB4 per-frequency fields (`cellReselectionPriority`, `threshX_HighP/L`,
`q_OffsetFreq`) across neighbours on the same ARFCN. At SIB4 build time, the RRC uses
`inter_freqs` to create one `InterFreqCarrierFreqInfo` per ARFCN for inter-frequency
carriers (ARFCN different from the serving SSB ARFCN) and attaches all neighbours whose
`inter_freq_idx` points to that frequency entry.
##### Required configuration parameters
To define a neighbor cell in the configuration file, the following parameters are required:
To define a neighbour cell in the configuration file, the following parameters are required:
- `gNB_ID` - identifier of the neighbor gNB (e.g., `0xe01`)
- `nr_cellid` - cell identifier of the neighbor cell (e.g., `11111111`)
- `gNB_ID` - identifier of the neighbour gNB (e.g., `0xe01`)
- `nr_cellid` - cell identifier of the neighbour cell (e.g., `11111111`)
- `physical_cellId` - physical cell ID for radio identification (e.g., `1`)
- `absoluteFrequencySSB` - SSB frequency in ARFCN notation (e.g., `643296`)
- `subcarrierSpacing` - numerology index: 0=15kHz, 1=30kHz, 2=60kHz, 3=120kHz
@@ -123,19 +249,45 @@ To define a neighbor cell in the configuration file, the following parameters ar
Example configuration structure:
```
# Per-frequency SIB4 configuration (one entry per ARFCN), shared by all cells
frequency_list = (
{
absoluteFrequencySSB = 643296;
subcarrierSpacing = 1; # 30 kHz
band = 78;
frequency_config = (
{
cellReselectionPriority = 5;
threshX_HighP = 10;
threshX_LowP = 6;
q_OffsetFreq = 0;
# Optional: threshX_HighQ, threshX_LowQ, etc.
}
);
}
);
# Per-cell neighbour configuration; neighbours reference frequency_list via ARFCN/SCS
neighbour_list = (
{
nr_cellid = 12345678;
neighbour_cell_configuration = (
{
gNB_ID = 0xe01;
nr_cellid = 11111111;
physical_cellId = 1;
absoluteFrequencySSB = 643296;
subcarrierSpacing = 1; # 30 kHz
absoluteFrequencySSB = 643296; # ARFCN used to look up matching entry in frequency_list
subcarrierSpacing = 1; # 30 kHz
band = 78;
plmn = { mcc = 001; mnc = 01; mnc_length = 2 };
tracking_area_code = 1;
# Example per-neighbour offsets (SIB3/SIB4)
q_OffsetCell = 0;
q_RxLevMinOffsetCell = -1;
q_QualMinOffsetCell = -1;
}
);
}

12
doc/handover-tutorial.md
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@@ -512,8 +512,16 @@ configuration, e.g. [neighbour-config-rfsim.conf](../../ci-scripts/conf_files/ne
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 neighbor configuration is nested:
- `neighbour_list` outer entries are keyed by serving `nr_cellid`
- each outer entry contains `neighbour_cell_configuration`, i.e., the list of neighbor cells for that serving cell
In this model:
- outer `nr_cellid` entries should be unique
- neighbor `physical_cellId` values are defined in inner neighbor entries
- the same serving-cell keyed neighbor mapping is used for both F1 and N2 handover logic
See the example above for `neighbour-config-ho.conf`. The same configuration
is for both F1 and N2 handover.