参考资料：3GPP 38.300、3GPP 38.331

1. 名词缩写

缩写	全称	描述
SI	System Information	系统信息
MIB	Master Information Block	主信息块
SIB	System Information Block	系统信息块
RMSI	Remaining Minimum SI	剩余最小SI，即除MIB外的最小SI
SSB	Synchronization Signal Block	同步信息块
PSCell	Primary SCG Cell
BWP	Bandwidth Part

2. 系统消息

SI包括一个MIB和多个SIB。SI分为最小SI 和其它SI。

（1）最小SI ：包括了初始接入和如何获取其它SI的基本信息。

• MIB：包含了如何获取SIB1的信息，以及其它。
  ★★ 携带的消息：systemFrameNumber、subCarrierSpacingCommon、 ssb-SubcarrierOffset、dmrs-TypeA-Position、pdcch-ConfigSIB1、cellBarred 、 intraFreqReselection（是否允许同频重选）。
  ★★ MIB消息在BCH上周期性重复广播，周期为80ms。重复次数由SSB的周期决定。
  
• SIB1：包括了初始接入需要的信息以及其它SI的调度信息。
  ★★ 携带的消息： cellSelectionInfo 、cellAccessRelatedInfo 、connEstFailureControl、si-SchedulingInfo、 servingCellConfigCommon、ims-EmergencySupport、eCallOverIMS-Support 、ue-TimersAndConstants 、uac-BarringInfo、useFullResumeID。
  ★★ SIB1也叫RMSI，剩余最小SI，即除MIB外的最小SI。
  ★★ SIB1在DL-SCH上周期性重复广播，周期为160ms。在每个周期内重复广播的次数是不固定的，其取决于具体实现。默认是8次。
  ★★ 当UE处于RRC_CONNECTED态时，可通过周期广播，或者通过RRCReconfiguration消息获取SIB1。
  ★★ SIB1指示其它SI是周期性广播，还是按需提供。若按需提供其它SI，则SIB1还需包括UE执行SI请求的信息。

（2）其它SI：不包括在最小SI的其余所有系统消息。

• 获取其它SI的方式：
  ① 周期性广播，其广播信息由SIB1中携带的si-SchedulingInfo决定；
  ② 按需广播（ RRC_IDLE、RRC_INACTIVE），或者通过RRCReconfiguration发送给特定UE（ RRC_CONNECTED）；
• SIB2：包括了小区重选公共信息（同频、异频、RAT间），服务小区小区重选信息、同频小区重选信息。

SIB2 ::=                            SEQUENCE {
    cellReselectionInfoCommon           SEQUENCE {
        nrofSS-BlocksToAverage              INTEGER (2..maxNrofSS-BlocksToAverage)          OPTIONAL,       -- Need R
        absThreshSS-BlocksConsolidation     ThresholdNR                                     OPTIONAL,       -- Need R
        rangeToBestCell                     RangeToBestCell                                 OPTIONAL,       -- Need R
        q-Hyst                              ENUMERATED {
                                                dB0, dB1, dB2, dB3, dB4, dB5, dB6, dB8, dB10,
                                                dB12, dB14, dB16, dB18, dB20, dB22, dB24},
        speedStateReselectionPars           SEQUENCE {
            mobilityStateParameters             MobilityStateParameters,
            q-HystSF                        SEQUENCE {
                sf-Medium                       ENUMERATED {dB-6, dB-4, dB-2, dB0},
                sf-High                         ENUMERATED {dB-6, dB-4, dB-2, dB0}
            }
        }                                                                                   OPTIONAL,       -- Need R
    ...
    },
    cellReselectionServingFreqInfo      SEQUENCE {
        s-NonIntraSearchP                   ReselectionThreshold                            OPTIONAL,       -- Need R
        s-NonIntraSearchQ                   ReselectionThresholdQ                           OPTIONAL,       -- Need R
        threshServingLowP                   ReselectionThreshold,
        threshServingLowQ                   ReselectionThresholdQ                           OPTIONAL,       -- Need R
        cellReselectionPriority             CellReselectionPriority,
        cellReselectionSubPriority          CellReselectionSubPriority                      OPTIONAL,       -- Need R
        ...
    },
    intraFreqCellReselectionInfo        SEQUENCE {
        q-RxLevMin                          Q-RxLevMin,
        q-RxLevMinSUL                       Q-RxLevMin                                      OPTIONAL,       -- Need R
        q-QualMin                           Q-QualMin                                       OPTIONAL,       -- Need S
        s-IntraSearchP                      ReselectionThreshold,
        s-IntraSearchQ                      ReselectionThresholdQ                           OPTIONAL,       -- Cond RSRQ
        t-ReselectionNR                     T-Reselection,
        frequencyBandList                   MultiFrequencyBandListNR-SIB                    OPTIONAL,       -- Need S
        frequencyBandListSUL                MultiFrequencyBandListNR-SIB                    OPTIONAL,       -- Need R
        p-Max                               P-Max                                           OPTIONAL,       -- Need R
        smtc                                SSB-MTC                                         OPTIONAL,       -- Need R
        ss-RSSI-Measurement                 SS-RSSI-Measurement                             OPTIONAL,       -- Need R
        ssb-ToMeasure                       SSB-ToMeasure                                   OPTIONAL,       -- Need R
        deriveSSB-IndexFromCell             BOOLEAN,
        ...,
        [[
        t-ReselectionNR-SF                  SpeedStateScaleFactors                          OPTIONAL        -- Need N
        ]]
    },
    ...
}

RangeToBestCell    ::= Q-OffsetRange

• SIB3：包括了同频邻区信息和同频黑名单小区信息，用于同频小区重选。

SIB3 ::=                            SEQUENCE {
    intraFreqNeighCellList              IntraFreqNeighCellList      OPTIONAL,   -- Need R
    intraFreqBlackCellList              IntraFreqBlackCellList      OPTIONAL,   -- Need R
    lateNonCriticalExtension            OCTET STRING                OPTIONAL,
    ...
}

IntraFreqNeighCellList ::=          SEQUENCE (SIZE (1..maxCellIntra)) OF IntraFreqNeighCellInfo

IntraFreqNeighCellInfo ::=          SEQUENCE {
    physCellId                          PhysCellId,
    q-OffsetCell                        Q-OffsetRange,
    q-RxLevMinOffsetCell                INTEGER (1..8)              OPTIONAL,   -- Need R
    q-RxLevMinOffsetCellSUL             INTEGER (1..8)              OPTIONAL,   -- Need R
    q-QualMinOffsetCell                 INTEGER (1..8)              OPTIONAL,   -- Need R
    ...
}

IntraFreqBlackCellList ::=          SEQUENCE (SIZE (1..maxCellBlack)) OF PCI-Range

• SIB4：包括了异频小区重选信息，异频邻区信息，以及异频黑名单小区信息，用于异频小区重选。

SIB4 ::=                            SEQUENCE {
    interFreqCarrierFreqList            InterFreqCarrierFreqList,
    lateNonCriticalExtension            OCTET STRING                                OPTIONAL,
    ...
}

InterFreqCarrierFreqList ::=        SEQUENCE (SIZE (1..maxFreq)) OF InterFreqCarrierFreqInfo

InterFreqCarrierFreqInfo ::=        SEQUENCE {
    dl-CarrierFreq                      ARFCN-ValueNR,
    frequencyBandList                   MultiFrequencyBandListNR-SIB                OPTIONAL,   -- Cond Mandatory
    frequencyBandListSUL                MultiFrequencyBandListNR-SIB                OPTIONAL,   -- Need R
    nrofSS-BlocksToAverage              INTEGER (2..maxNrofSS-BlocksToAverage)      OPTIONAL,   -- Need R
    absThreshSS-BlocksConsolidation     ThresholdNR                                 OPTIONAL,   -- Need R
    smtc                                SSB-MTC                                     OPTIONAL,   -- Need R
    ssbSubcarrierSpacing                SubcarrierSpacing,
    ssb-ToMeasure                       SSB-ToMeasure                               OPTIONAL,   -- Need R
    deriveSSB-IndexFromCell             BOOLEAN,
    ss-RSSI-Measurement                 SS-RSSI-Measurement                         OPTIONAL,
    q-RxLevMin                          Q-RxLevMin,
    q-RxLevMinSUL                       Q-RxLevMin                                  OPTIONAL,   -- Need R
    q-QualMin                           Q-QualMin                                   OPTIONAL,   -- Need S,
    p-Max                               P-Max                                       OPTIONAL,   -- Need R
    t-ReselectionNR                     T-Reselection,
    t-ReselectionNR-SF                  SpeedStateScaleFactors                      OPTIONAL,    -- Need S
    threshX-HighP                       ReselectionThreshold,
    threshX-LowP                        ReselectionThreshold,
    threshX-Q                           SEQUENCE {
        threshX-HighQ                       ReselectionThresholdQ,
        threshX-LowQ                        ReselectionThresholdQ
    }                                                                               OPTIONAL,   -- Cond RSRQ
    cellReselectionPriority             CellReselectionPriority                     OPTIONAL,   -- Need R
    cellReselectionSubPriority          CellReselectionSubPriority                  OPTIONAL,   -- Need R
    q-OffsetFreq                        Q-OffsetRange                               DEFAULT dB0,
    interFreqNeighCellList              InterFreqNeighCellList                      OPTIONAL,   -- Need R
    interFreqBlackCellList              InterFreqBlackCellList                      OPTIONAL,   -- Need R
    ...
}

InterFreqNeighCellList ::=          SEQUENCE (SIZE (1..maxCellInter)) OF InterFreqNeighCellInfo

InterFreqNeighCellInfo ::=          SEQUENCE {
    physCellId                          PhysCellId,
    q-OffsetCell                        Q-OffsetRange,
    q-RxLevMinOffsetCell                INTEGER (1..8)                              OPTIONAL,   -- Need R
    q-RxLevMinOffsetCellSUL             INTEGER (1..8)                              OPTIONAL,   -- Need R
    q-QualMinOffsetCell                 INTEGER (1..8)                              OPTIONAL,   -- Need R
    ...

}

InterFreqBlackCellList ::=          SEQUENCE (SIZE (1..maxCellBlack)) OF PCI-Range

• SIB5：包括了E-UTRA小区重选信息、E-UTRA小区信息，E-UTRA黑名单小区信息，用于RAT间小区重选。

SIB5 ::=                            SEQUENCE {
    carrierFreqListEUTRA                CarrierFreqListEUTRA                        OPTIONAL,       -- Need R
    t-ReselectionEUTRA                  T-Reselection,
    t-ReselectionEUTRA-SF               SpeedStateScaleFactors                      OPTIONAL,       -- Need S
    lateNonCriticalExtension            OCTET STRING                                OPTIONAL,
    ...
}

CarrierFreqListEUTRA ::=            SEQUENCE (SIZE (1..maxEUTRA-Carrier)) OF CarrierFreqEUTRA

CarrierFreqEUTRA ::=                SEQUENCE {
    carrierFreq                         ARFCN-ValueEUTRA,
    eutra-multiBandInfoList             EUTRA-MultiBandInfoList                     OPTIONAL,       -- Need R
    eutra-FreqNeighCellList             EUTRA-FreqNeighCellList                     OPTIONAL,       -- Need R
    eutra-BlackCellList                 EUTRA-FreqBlackCellList                     OPTIONAL,       -- Need R
    allowedMeasBandwidth                EUTRA-AllowedMeasBandwidth,
    presenceAntennaPort1                EUTRA-PresenceAntennaPort1,
    cellReselectionPriority             CellReselectionPriority                     OPTIONAL,       -- Need R
    cellReselectionSubPriority          CellReselectionSubPriority                  OPTIONAL,       -- Need R
    threshX-High                        ReselectionThreshold,
    threshX-Low                         ReselectionThreshold,
    q-RxLevMin                          INTEGER (-70..-22),
    q-QualMin                           INTEGER (-34..-3),
    p-MaxEUTRA                          INTEGER (-30..33),
    threshX-Q                           SEQUENCE {
        threshX-HighQ                       ReselectionThresholdQ,
        threshX-LowQ                        ReselectionThresholdQ
    }                                                                               OPTIONAL        -- Cond RSRQ
}

EUTRA-FreqBlackCellList ::=         SEQUENCE (SIZE (1..maxEUTRA-CellBlack)) OF EUTRA-PhysCellIdRange

EUTRA-FreqNeighCellList ::=         SEQUENCE (SIZE (1..maxCellEUTRA)) OF EUTRA-FreqNeighCellInfo

EUTRA-FreqNeighCellInfo ::=         SEQUENCE {
    physCellId                          EUTRA-PhysCellId,
    q-OffsetCell                        EUTRA-Q-OffsetRange,
    q-RxLevMinOffsetCell                INTEGER (1..8)                              OPTIONAL,       -- Need R
    q-QualMinOffsetCell                 INTEGER (1..8)                              OPTIONAL        -- Need R
}

• SIB6：ETWS主通知。
• SIB7：ETWS辅通知
• SIB8：CMAS告警通知
• SIB9：GPS和UTC相关信息。UE可根据SIB9获取GPS、UTC以及本地时间。
   

3. UE获取系统消息

（1）UE何时需要获取系统消息？

• 小区选择（刚开机）、小区重选、从未覆盖的区域到有覆盖的区域、从其它RAT进入到当前RAT、收到系统消息变更的指示、收到PWS通知、存储的SI已过期、after reconfiguration with sync completion。

（2）UE如何获取系统消息？

• 获取MIB和SIB1：
  ★★ 当UE获取MIB和SIB1消息失败时，则认为该小区禁止接入。
  ★★ 处于RRC_IDLE或 RRC_INACTIVE的UE，根据38.213协议所述，在BCCH信道上获取MIB消息；
  ★★ 处于RRC_IDLE、 RRC_INACTIVE，或已激活BWP，searchSpaceSIB1 和pagingSearchSpace配置了公共搜索空间且被指示系统消息已变更RRC_CONNECTED的UE，根据MIB消息，在DL-SCH信道上获取SIB1。

• 获取其它SI：

（3）UE存储系统消息

• 
   

4. 系统消息的有效性