Improve cell search in UE
This MR further improves the cell search capability of OAI UE. The existing cell search option --ue-scan-carrier scans for SSB in all possible GSCN locations in the configured center frequency and bandwidth.
The option is now extended:
-
--ue-scan-carrier 1: existing scan -
--ue-scan-carrier 2: ignores the center frequency and performs SSB search in all GSCN locations in the configured NR band. This is done by changing the center frequency after one scan is finished till all GSCN locations are searched.
Additionally, there are improvements to SIB1 reception. When a MIB is decoded, the UE checks if the SIB1 is within the current bandwidth. If not, it changes the center frequency so that SIB1 is successfully received. After SIB1 reception, the UE again changes frequency to match the pointA of the gNB.
Limitations
The PHY only reports to MAC the first MIB detected in a scan and attempts to receive SIB1 of the corresponding cell. After the SIB1 is detected, the RA is initiated immediately.
IMO, the following procedure seems right.
- the cell search result reported to MAC should have all MIBs detected in the entire band.
- Then the MAC configures PHY to detect all SIB1s one at a time.
- After the SIB1s are detected, the MAC reports the cells to the higher layers (RRC, NAS?).
- The NAS should initiate the registration request for a specific cell. (Automatic if PLMN matches with IMSI or user initiated)
- Then the MAC upon receiving the UL NAS message, should initiate RA procedure with that cell.