5g NR system messages

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 1.  News briefing

         System messages are divided into MIB And a series of SIB news ：

        MIB stay BCH Up transfer , The period is 80 ms, Repeat in 80 ms Finish in , It includes obtaining... From the cell SIB1 Required parameters . MIB Carry the necessary basic information ：SFN,hich-config And dl Bandwidth information (band-width).

        SIB1 stay DL-SCH In order to 160ms The period and time of 160ms Variable transmission repetition period transmission within .SIB1 The default transmission repetition period is 20ms However, the actual transmission repetition period depends on the network implementation . about SSB and CORESET Reuse mode 1,SIB1 The repeat transmission period is 20ms. about SSB and CORESET Reuse mode 2/3,SIB1 Transmission repetition period and SSB Same period . SIB1 Including about other SIB Availability and scheduling （ for example ,SIB To SI Mapping of messages , periodic ,SI Window size ） Information about , It indicates whether only one or more... Are available on demand SIB, And in this case , It needs to be configured by UE perform SI request .

        SIB1 Outside of the SIB stay SystemInformation（SI） Carry in the message , The message is in DL-SCH Up transfer . Only those with the same periodicity SIB To map to the same SI news . Every SI The message is sent in the time domain window that appears periodically （ Called all SI Messages with the same length SI window ）. Every SI News and SI Associated with the window , And different SI News SI Windows do not overlap . in other words , In a SI Only the corresponding... Is sent in the window SI news . Can be in SI Send multiple times in the window SI news . except SIB1 Anything other than SIB You can use SIB1 The indication in is configured as a specific cell or a specific area . Community specific SIB Only applicable to the provision of SIB My community , And region specific SIB Apply to what is called SI Area of area , The area consists of one or more cells , And by the systemInformationAreaID identification ;

         -  For being in RRC_CONNECTED Of UE, The Internet can use RRCReconfiguration Messages provide system information through dedicated signaling , for example , If UE With activity BWP, There is no public search space configured to monitor system information or paging .

         -  about PSCell and SCell, The network provides the required services through dedicated signaling SI, That is to say RRCRe Configuration message . However ,UE Will get PSCell Of MIB In order to obtain SCG Of SFN timing （ It can be associated with MCG Different ）. Changing SCell Correlation SI when , Network release and add related SCell. about PSCell, The required configuration can only be changed by using synchronous reconfiguration SI.

         Be careful ： Physical layer pair SIB Limit the maximum size that can be used . Maximum SIB1 or SI The message size is 2976 position .

attach ：

MCG：Master Cell group, Main cell group

SCG：Secondary Cell group, Auxiliary community group

MCG and SCG It's a double link （DC,Dual connectivity） The next concept , It can be simply understood as UE First, initiate random access （RACH） Of Cell Where Group Namely MCG. If there is no double link , There is no MCG and SCG The concept of . Or it can be understood as , If there is no double link , Then the community group corresponds to MCG.

 PCell：Primary Cell, The main area

SCell：Secondary Cell, Auxiliary community

PSCell：Primary Secondary Cell, Main and auxiliary communities

stay MCG Next , There may be many Cell, There is a cell for initiating initial access , This community is called PCell. seeing the name of a thing one thinks of its function ,PCell yes MCG The most “ The main ” My community .MCG Under the PCell and MCG Under the SCell By carrier aggregation （CA,Carrier aggregation） Technology is united .

similarly , stay SCG There will also be a major community in the next , That is to say PSCell, It can also be simply understood as in SCG The cell that initiates the initial access .SCG Under the PSCell and SCG Under the SCell through CA Technology is united .

2.  System message acquisition

2.1  Access to the process

chart 2  System message acquisition

        UE Use SI Get process to get AS and NAS Information , This process is applicable to RRC_IDLE,RRC_INACTIVE and RRC_CONNECTED All in state UE.AS Layer information includes common channel information , some UE Required timer , Community selection / Reselection information and adjacent area information ;NAS Layer information includes operator information, etc .UE Get this information through system messages , To determine the UE Resident in the cell 、 Reselection and the behavior of initiating a call .

         be in RRC_IDLE and RRC_INACTIVE State of UE It shall be ensured that （ At least ）MIB,SIB1 to SIB4 and SIB5 A valid version of （ If UE Support E-UTRA）.

        UE Select... In the community （ Such as power on ）, Cell reselection , Switching within the system is completed , After completing the reconfiguration from synchronization , From other RAT Get into 5G RAT, And returning to the coverage area from the non coverage area , The system message acquisition process will begin .

         When UE Obtain... In the service cell MIB or SIB1 or SI When the news , And if the UE Store the acquired SIB, be UE Relevant information should be stored areaScope, If there is , Then store PLMN- PLMN-IdentityInfoList Logo in ,cellIdentity,systemInformationAreaID（ If there is ） and valueTag（ If there is ）, Such as SIB Of si-SchedulingInfo As shown in . UE You can use except MIB,SIB1,SIB6,SIB7 or SIB8 In addition to the SI Valid stored version of , for example , After the cell is re selected , Return from coverage or when SI After changing the instruction .

         When UE After getting the system message correctly , System messages will not be read repeatedly , The system message will only be re read when any of the following conditions are met ：

• received gNB paging , Indicates that the system message has changed .

• received gNB paging , instructions ETWS or CMAS Message broadcast .

• System message received correctly since last time 3 Hours later, .（38.331 5.2.2.2.1 delete any stored version of a SIB after 3 hours from the moment it was successfully confirmed as valid）

2.2  System message change indication

         Use modification cycle , That is, sending SI Broadcast the updated information in the modification cycle after changing the indicated modification cycle SI. Modify the cycle boundary by SFN Value definition ,SFN mod m = 0, among m Is the number of wireless frames including the modification period . The modification cycle is configured by the system information . UE Use pass DCI And P-RNTI Send a short message together to receive information about SI Modification or PWS Instructions for notification . SI The repetition of the change indication can occur in the previous modification cycle .

         be in RRC_IDLE or RRC_INACTIVE Medium UE Will be in each DRX Monitor in its own paging timing during the cycle SI Change instructions . If you are active BWP Up for UE Provide public search space to monitor paging , be RRC_CONNECTED Medium UE During each modification cycle, the... In any paging opportunity will be monitored SI Change the instruction at least once , Such as TS 38.213 The first 13 As specified in section .

         stay RRC_IDLE or RRC_INACTIVE Medium ETWS Or have CMAS The ability of UE Will be in each DRX Monitor about in its own paging timing during the cycle PWS Instructions for notification . If you are active BWP On the UE Provide public search space to monitor paging , be RRC_CONNECTED Medium ETWS Or have CMAS The ability of UE Should be in every defaultPagingCycle Monitor for... In any paging opportunity PWS Instructions for notification .

         For short message reception in paging opportunity ,UE Monitor for paging PDCCH Monitoring timing .

3.  System message channel model

The picture above clearly shows MIB and SIB Channel transmission path from top to bottom ,MIB the BCH transmission ,SIB the DL-SCH transmission .

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