【5G RRC】Introduction to 5G System Information SIB1

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I work for an internationally renowned terminal manufacturer and am responsible for the research and development of modem chips.
In the early days of 5G, he was responsible for the development of the terminal data service layer and the core network. Currently, he is leading the research on technical standards for 6G computing power networks.


The content of the blog mainly revolves around:
       5G/6G protocol explanation
       computing power network explanation (cloud computing, edge computing, end computing)
       advanced C language explanation
       Rust language explanation

1. Introduction to 5G system message SIB1

       In 5G NR, SIB1 carries some messages to evaluate whether the terminal is allowed to access the cell , as well as scheduling information of other system information . It also provides radio resource configuration information common to all terminals and restriction information required by unified access control (UAC, unified access control) .

Transfer details:

• Signaling radio bearer: not applicable;
• RLC-SAP: TM；
• Logical channel: BCCH;
• Transport channel: DL-SCH;
• Physical channel: PDSCH;
• Scrambled RNTI: SI-RNTI;
• Transmission direction: gNB --> UE;

2. SIB1 content classification

       The following is a list of content categories provided to UE by SIB1 message:

• cell selection information;
• Community access related information;
• Connection establishment failure control;
• SI scheduling information;
• General control information of the serving cell;
• IMS Emergency Support Logo;
• IMS-based emergency phone support sign;
• UE-related timers and constant parameters;
• UAC-related restriction information;
• Available Full Resume ID flags;

2.1 Cell selection information

       The cell selection information in SIB1 includes Q-RxLevMin , q-RxLevMinOffset , Q-RxLevMinSUL , Q-QualMin and q-QualMinOffset parameters. The value range of each parameter is shown in the figure below:

in,

• Q-RxLevMin : Used to indicate the dBm value of the minimum RSRP received during NR cell selection and reselection. Actual Q _rxlevmin = dBm field value * 2;
• q-RxLevMinOffset : is an offset (in dB) that needs to be added to the value of Q _rxlevmin (not the actual value). If this field does not exist, the UE applies a default value of 0 dB as the offset;
• Q-RxLevMinSUL : If the UE supports the SUL of the cell, then Q _rxlevmin (see explanation above) is obtained from this field, and its actual value Q _rxlevmin = field value * 2 [dBm];
• Q-QualMin : Used to indicate the minimum received RSRQ value (in dB) required for NR cell selection and reselection. If this field does not exist, the UE defaults the value of Q _qualmin to negative infinity, which means that the quality (RSRQ) check will always pass regardless of the quality of the received signal;
• q-QualMinOffset : is an offset (in dB) that needs to be added to the signal value for Q _qualmin . If this field does not exist, the UE will apply a default value of 0 dB as the offset;

2.2 Cell access related information

       The cell access related information in SIB1 includes PLMN information , TAC , RAC and Cell ID parameters. SIB1 can broadcast cell access information of up to 12 PLMNs . The value ranges of relevant parameters are as follows:

in:

• TAC (trackingAreaCode) : Indicates the TAC to which the cell specified by cellIdentity belongs. After obtaining SIB1, if the tracking area code (per PLMN) does not exist, the UE will consider this cell to be banned;
• ranac (RAN area code) : used to identify a RAN area within a TAC. The RAN area code will be used when configuring the RAN notification area;
• CellIdentity : used to identify a cell in a PLMN;
• cellReservedForOtherUse : This parameter is used to indicate whether the cell is reserved. This field applies to all configured PLMNs. If a cell is reserved, other UEs are not allowed to camp except for specific UEs;

2.3 Connection establishment failure control

        ConnEstFailurControl is used to configure parameters related to connection establishment failure control. When the terminal sends a RRCSetupRequest request, the timer T300 starts. If no response is received from the gNB before T300 expires, the terminal considers the connection establishment attempt to fail;

       If the timer T300 times out connEstFailCount times, the UE starts a timer given by connEstFailOffsetValidity , and reduces the RSRP or RSRQ semaphore through the connEstFailOffset parameter when performing cell selection or reselection . The value ranges of relevant parameters are as follows:

in,

• connEstFailCount : This parameter is the T300 timeout times of the service cell detected by the terminal before applying connEstFailOffset ;
• connEstFailOffset : is the parameter Qoffset _temp used in the cell selection or reselection process in 3GPP 38.304 . If this field does not exist, the value of Qoffset _temp will be set to infinity;
• connEstFailOffsetValidity : is the time for the UE to apply connEstFailOffset before deleting the offset of the evaluated cell , in seconds;

2.4 SI Scheduling Information

       The SI scheduling information of SIB1 provides information of other system information broadcasts (SIB2 to SIB9), their broadcast periods, SI window length and RACH parameters. The value range of its parameters is as follows:

2.5 General control information of the serving cell

       SIB1 Serving cell common information is used to configure the cell-specific parameters of a terminal's serving cell. These parameters include downlink frequency information , offset to point A , initial downlink BWP and paging channel configuration . It also includes RACH-ConfigCommon , PUSCH-ConfigCommon , *PUCCH-ConfigCommon and -timeAlignmentTimerCommon . The value range of its parameters is as follows:

2.6 IMS Emergency Support Flag

       The IMS emergency support flag of SIB1 indicates whether the cell supports the terminal IMS emergency bearer service in the restricted service mode. If not, the network in the cell does not support the IMS emergency call to the service-restricted terminal.

2.7 IMS-based emergency call support sign

The eCall Over IMS support flag        of SIB1 indicates whether the cell supports the IMS emergency call service.

2.8 UE-related timers and constant parameters

UE-TimerAndConstants        of SIB1 provides the timer and constant parameters used by UE in RRC_CONNECTED , RRC_INACTIVE and RRC_IDLE states.

2.9 UAC-related restriction information

The UAC-BarringInfo        of SIB1 contains cell barring information based on UAC (Unified Access Barring).

2.10 Available Full Resume ID flags

The useFullResumeID parameter        of SIB1 indicates which Resume identifier and Resume request message should be used when the UE wishes to transition from the inactive state to the connected state . If this field exists, UE uses full I-RNTI and RRCResumeRequest1 , if this field does not exist, uses short I-RNTI and RRCResumeRequest .

3. Summary

1. MIB and SIB1 are called the minimum system information (MSI, Minimum System Information) in 5G NR;

2. SIB1 can also be called Remaining Minimum System Information (RMSI, Remaining Minimum System Information);

3. In 5G NR, the MIB will provide the terminal with all the information needed to decode SIB1;

4. SIB1 is cell-specific information and is only valid for the serving cell;

5. SIB1 carries the key information required by the UE to access the cell;

6. SIB1 also includes information related to the availability and scheduling of other system messages, for example, the mapping of SIBs to system messages (SI), period, SI window size, etc.;

7. The transmission period of SIB1 is 160ms , and has a variable transmission repetition period within the 160ms transmission period;

8. When operating in ENDC or NSA mode, no SIB1 transmission is required;

9. The PDSCH resources of SIB1 are allocated using DCI Format 1_0 and transmitted in the common search space set;

10. In vRAN or Open RAN architecture (CU-DU gNB architecture), DU is responsible for generating the content of MIB and SIB1 ;

11. The MIB and SIB1 are forwarded to the CU through the gNB-DU system information parameters contained in the F1AP: F1 Setup Request or F1AP: gNB DU Configuration Update message sent by the DU;

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