【5G RRC】Introduction to 5G System Information SIB2

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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 SIB2

       In 5G NR, SIB2 provides general information related to cell reselection of the same frequency, different frequency and different systems (NR and LTE) . It also provides information specific to intra-frequency cell reselection .

       In 5G, cell reselection can be either cell-based or beam-based measurements . When the gNB transmits multiple beams, the UE needs to generate cell measurement information from one or more beam measurements.

1.1 Transmission Details

• Signaling Radio Bearer: not used;
• RLC-SAP：TM；
• Logical channel: BCCH;
• Transport channel: DL-SCH;
• Physical channel: PDSCH;
• RNTI所码：SI-RNTI；
• Direction: gNB–>UE;

2. SIB2 message content category

Information provided by SIB2 includes:

• General cell reselection information;
• Service frequency point information for cell reselection;
• Same-frequency cell reselection information;

2.1 General cell reselection information

The cellReselectionInfoCommon structure of SIB2 provides common information for cell reselection between intra-frequency, inter-frequency and inter-system. The relevant structure is as follows:

• nrofSS-BlocksToAverage and absThreshSS-BlocksConsolidation
  This parameter is only applicable to intra-frequency cell reselection. 5G NR cell operation is a multi-beam operation, so for cell reselection, the cell quality measurement is based on the SSBs of multiple beams in the same cell. The relevant process is as follows:

  • If the above two fields are not configured in SIB2, the UE will use the strongest SSB beam as the measurement of each cell;
  • If SIB2 is configured with these two fields, the UE derives the measurement quantity of each cell according to the linear average value of a group of SSB beams. Beams are selected such that the measured value of the beam is above a given threshold absThreshSS-BlocksConsolidation and the maximum number of beams used to calculate the average does not exceed nrofSS-BlocksToAverage ;

• rangeToBestCell
  This parameter is used for inter-frequency cell reselection procedures of the same frequency and the same priority. R-criteriaThe UE first ranks the cells as specified in Section 5.2.4.6 of the 3GPP specification 38.304 , and then uses the parameter rangeToBestCell to decide the candidate cell:

  • If rangeToBestCell is not configured , the terminal will reselect the cell with the highest ranking;
  • If rangeToBestCell is configured , the UE will perform cell reselection on the cell with the largest number of beams above the threshold (for example, absThreshSS-BlocksConsolidation ), and the R value of this cell should be within the rangeToBestCell range of the R value of the highest ranked cell;
  • If there are multiple such cells, the UE will perform cell reselection on the cell with the highest rank among them;
  • Networks should only configure non-negative (in dB) values ​​for this parameter;

• q-Hyst
  is used to indicate the hysteresis value of the ranking criterion. _{It is the parameter Q hyst} used in the serving cell R standard , which is used for cell reselection between different frequencies of the same frequency and equal priority;

• speedStateReselectionPar
  This parameter is used to determine the mobility state of the UE;

• q-HystSF
  is the scaling factor according to the terminal moving speed, sf-Medium and sf-High are the additional hysteresis applied to Q _hyst defined in TS 38.304 in the medium-speed and high-speed moving states, respectively . For example, dB-6 corresponds to -6dB, dB-4 corresponds to -4dB, and so on;

2.2 Service frequency information for cell reselection

The parameter details for each IE are summarized below:

• s-NonIntraSearchP is S _{nonIntraSearchP} in TS38.304 , and s-NonIntraSearchQ is S _{nonIntraSearchQ} in TS38.304 . These parameters specify the measurement thresholds of S _rxlev (RSRP) and S _{qual (RSRQ) between the same frequency and different systems respectively;}

  • For s-NonIntraSearchP, the actual value is equal to the value of the field *2 (dB), if the field does not exist, the default value is used, which is infinity;
  • For s-NonIntraSearchQ, the actual value is equal to the value of this field, if the field does not exist, the default value of 0 dB is used;
  • For the inter-frequency and inter-system frequencies provided by the system information, the UE uses these parameters according to the following rules according to the priority information provided in the system information:
    • Higher priority : For inter-frequency or inter-system frequencies whose reselection priority is higher than that of the serving NR frequency reselection priority, UE will measure the inter-frequency or inter-system frequency with higher priority, regardless of s- NonIntraSearchP and s-NonIntraSearchQ ;
    • Equal or Lower Priority : For reselection priorityequal to or lower thanInter-frequency of NR service frequency reselection priority, and for reselection prioritylower thanNR service frequency reselection priority between different systems. Then the serving cell will judge whether to start measurement according to these parameters;
  • If the serving cell satisfies S _rxlev > S _{nonIntraSearchP} and S _qual > S _{nonIntraSearchQ} , the UE does not need to perform cell measurement of NR inter-frequency or inter-system frequency with the same or lower priority;
  • Otherwise, the UE shall measure the NR inter-frequency or inter-system frequency of equal or lower priority;

• The threshServingLowP and threshServingLowQ parameters, and specify the RSRP and RSRQ thresholds respectively, are used by the UE on the serving cell when reselecting to a RAT or frequency with a lower priority than the current serving cell:

  • The actual value of threshServingLowP is equal to the field value*2 (dB);
  • The actual value of threshServingLowQ is equal to the field value;
  • If threshServingLowQ is configured in SIB2 , when the quality of the UE serving cell is lower than the threshold threshServingLowQ and the quality of the target cell is higher than the given threshold, the UE will perform cell reselection to a rat or frequency with a lower priority;
  • If in SIB2NoConfigure threshServingLowQ , then the UE will judge whether to perform cell reselection to a rat or frequency with a lower priority according to the parameter threshServingLowP . Only when the RSRP of the serving cell is lower than the threshold given by threshServingLowP , and the RSRP of the target cell is higher than a certain threshold, the UE will reselect;

• The CellReselectionPriority parameter provides information about the carrier frequencyabsolute priority, and used in the cell reselection process. The highest priority is 7 and the lowest priority is 0. This is a required field;

• The CellReselectionSubPriority parameter indicates a fractional value which is added to the value of cellReselectionPriority to obtain the absolute priority of the associated carrier frequency in E-UTRA and NR. The value oDot2 corresponds to 0.2, oDot4 to 0.4, and so on. This is an optional field;

• absolute priority = cellReselectionPriority + CellReselectionSubPriority (if configured);

2.3 Same-frequency cell reselection information

The intraFreqCellReselectionInfo structure of SIB2 contains general information about intra-frequency cell reselection. The parameters of intraFreqCellReselectionInfo are detailed as follows:

• Q-RxLevMin is used to indicate the minimum received RSRP value (unit: dBm) required for NR cell reselection. Actual value Q _rxlevmin = field value * 2dbm;

• Q-RxLevMinSUL If the UE supports the SUL frequency under this cell, get Q _rxlevmin from this field . Actual value Q _rxlevmin = field value * 2dbm;

• Q-QualMin is used to indicate the minimum RSRQ value (in dB) required for NR cell reselection. If this field does not exist, the UE will assume that the value of Q _qualmin is negative infinity, which means that regardless of the quality of the received signal, the RSRQ check will always pass;

• s-IntraSearchP corresponds to the parameter S _IntraSearchP , and s-IntraSearchQ corresponds to the parameter S _IntraSearchQ . These parameters specify the S _rxlev (RSRP) threshold and S _qual (RSRQ) threshold (in dB) for co-frequency measurements, respectively;

  • For s-IntraSearchP, the actual value is equal to the field value *2 (dB), if the field does not exist, the default value is infinity. For s-IntraSearchQ, the actual value is equal to the value of this field, if the field does not exist, the default value is 0 dB;
  • If the service unit satisfies S _rxlev >S _IntraSearchP and S _qual >S _IntraSearchQ , the terminal does not need to perform intra-frequency measurement. Otherwise, the UE shall perform co-frequency measurement;

• The t-ReselectionNR parameter defines the cell reselection timer value of the relevant NR frequency;

• frequencyBandList defines a series of frequency band information that can be used for NR cell reselection;

• p-Max indicates the maximum transmission power allowed by adjacent cells of the same frequency, in dBm;

• smtc defines the measurement timing configuration of co-frequency measurement, that is, the measurement timing for UE to measure SSB. If this field does not exist, the UE assumes that the SSB cycle of the same-frequency cell is 5ms. It contains the following two IEs:

  • duration configures the duration of receiving SSB in the subframe of the measurement window;
  • periodicityAndOffset configures the period and offset of the measurement window that receives the SSB;

• ss-RSSI-Measurement is used to configure RSSI measurement based on synchronization reference signal. ss-RSSI-Measurement includes measurementslots and endsymbol information, as follows:

  • measurementSlots indicates the slot information where the terminal can perform RSSI measurement.BIT STRINGThe length of is equal to the slot number of the configured SMTC window (determined by duration and SCS). The first (leftmost) bit in the bitmap corresponds to the first slot in the SMTC window, the second bit in the bitmap corresponds to the second slot in the SMTC window, and so on. The slot where the UE can perform measurement is set to 1 in the corresponding bitmap;
  • endSymbol indicates that in a slot configured with RSSI measurement, the UE performs RSSI measurement from symbol 0 to the given endSymbol symbol;

• ssb-ToMeasure configures a set of SSBs to be measured within the SMTC measurement time. When this field is absent, the UE will measure all SSBs. The ssb-ToMeasure parameter is represented by a bitmap as follows:

  • shortBitmap : The length is 4 bits, suitable for f _c ≤ 3 GHz, and the maximum number of SSBs in the SSB burst set is 4;
  • mediumBitmap : The length is 8 bits, suitable for 3 GHz < f _c ≤ 6 GHz, and the maximum number of SSBs in the SSB burst set is 8;
  • longBitmap : The length is 64 bits, suitable for f _c > 6 GHz, and the maximum number of SSBs in the SSB burst set is 64;

• deriveSSB-IndexFromCell indicates whether the UE can use the time of the serving cell to derive the SSB index transmitted by the neighboring cell. If this field is set to true, the UE shall assume that the SFN and frame boundaries are aligned with neighboring cells on the serving frequency;

• t-ReselectionNR-SF is a speed-based scaling factor for Treselection _NR in TS38.304 :

  • _{Multiply Treselection NR} with sf-High or sf-Medium according to the current movement state (high or medium speed) ;
  • _{The value of Treselection NR} after applying the corresponding scaling factor will always beless than or equal to_{Treselection NR} before scaling ;
  • This usually means allowing reselection to happen sooner (based on the scaling factor) in high or medium movement states;

3. Summary

• Three kinds of system information in 5G, namely MSI, RMSI and OSI;
• SIB2 is the third type of system information, namely other system information (OSI);
• SIB2 provides general information related to cell reselection of the same frequency, different frequency and different systems (NR and LTE);
• The scheduling information of SIB2 is provided by SIB1;
• SIB2 can be broadcast periodically or on demand, and can also be sent to the UE in a dedicated manner;
• SIB2 can be configured as cell specific or area specific ;
• SIB2 is mapped to BCCH logical channel, DL-SCH transmission channel and PDSCH physical channel;

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