System information is downlink broadcast information transmitted periodically by base station. it is very critical information for a mobile (UE) to connection the Base station (gNB in 5G) in any technology. In 5G, UE reads system information for cell camping when it is powered on, for cell selection and re-selection when it is in RRC IDLE mode. System information basically provides all necessary details like System frame number, System bandwidth, PLMN, cell selection and re-selection thresholds etc. to access network.
Classification of System information:
At high level in 5G New Radio, this system information can be divided into three categories
- Minimum System Information (MSI)
- Remaining Minimum System Information (RMSI)
- Other System Information (OSI)
These categories include Master Information Block (MIB) and 9 System Information Blocks (SIBs). The MIB information is transmitted via BCH and PBCH channels while SIBs are transmitted via DL-SCH and PDSCH channels.
This mapping of system information and different channels is shown in below figure.
As shown in picture the Minimum Information block includes of MIB and Remaining System information. RMSI includes SIB1 and Other System Information (OSI) includes SIB2 to SIB9. MIB and SIB1 has is own RRC Message “MasterInformationBlock” and “SystemInformationBlock1” respectively where as SIB2 to SIB9 are wrapped within a generic RRC message known as “SystemInformation”. Minimum system information is broadcast periodically, while other system information may either be broadcast, or provisioned in a dedicated manner, either triggered by the network or upon request from the UE.
When other System Information is required by the UE, before the UE sends the OSI request, UE needs to know whether it is available in the cell and whether it is broadcast or not. The UE in RRC_IDLE or RRC_INACTIVE should be able to request the OSI without requiring a state transition. For the UE in RRC_CONNECTED, dedicated RRC signaling can be used for the request and delivery of the OSI. The other SI may be broadcast at configurable periodicity and for certain duration. It is network decision whether the OSI is broadcast or delivered through dedicated UE specific RRC signaling
How UE utilize the Minimum System Information:
When a UE is powered on, it performs a cell search procedure and decode the PSS and SSS information to get the Physical cell ID. Then UE scan for the MIB at set GSCN and acquires a suitable SS/PBCH block.
MIB information is directly available on PBCH channel. After successful MIB decode UE get the information regarding Control Resource Set (CORSET) and PDCCH Search space required for SIB1 decoding. SIB1 provide network access parameters along with scheduling information about all other system information. The system information is scrambled with SI-RNTI and DCI format 1_0 is used during allocated PDSCH resources for transmission at gNB.
Following is the list of System Information defined as part of 3GPP release 15 for 5G.
- Master Information Block (MIB) : Provides SFN, SCS, SIB1 PDCCH resource, cell barred information
- System Information Block 1 (SIB1): Cell selection Information, PLMN, TAC, Cell Identity, RAN Notification Information, SI scheduling info for OSI, Serving Cell Information
- System Information Block 2 (SIB2): Cell re-selection information common for intra-frequency, inter-frequency and/ or inter-RAT
- System Information Block 3 (SIB3): Intra frequency cell re-selection information e.g. PCI, q-Offset, q-RxLev, q-Qual, Black cell list
- System Information Block 4 (SIB4): Inter frequency cell re-selection information e.g. NR-ARFCN
- System Information Block 5 (SIB5): Inter system cell re-selection toward LTE e.g. EARFCN
- System Information Block 6 (SIB6): Earth quake and Tsunami Warning System primary notifications
- System Information Block 7 (SIB7): Earth quake and Tsunami Warning System secondary notifications
- System Information Block 8 (SIB8): Commercial Mobile Alert services (CMAS) notification
- System Information Block 9 (SIB9): Timing information for UTC, GPS and local time
- 3GPP TS 38.331 5G; NR; Radio Resource Control (RRC)
- 3GPP TS 38.211: NR; Physical channels and modulation
- TR 38 912 5G; Study on New Radio (NR) access technology