When UE is switched on it will acquire system information for cell selection, cell-reselection.
MIB and SIB1 are 2 important SI that UE requires to select and latch on to a cell.
MIB and SIB1 are called as “MSI” “Minimum System Information”.
Standalone SIB1 is called as “RMSI” “Remaining Minimum System Information”.
All other SI fro SIB2 to SIB9 is called as OSI “Other System Information”
MIB is periodically broadcasted on BCH.
SIB1 is periodically broadcasted on DL-SCH or unicast on DL-SCH.
Other SI:
SIB2 through SIB9 can be periodically broadcast on DL-SCH or Broadcast on-demand on DL-SCH or unicast on DL-SCH.
SIB1 contains the scheduling information for all SI messages.
The SIB that have the same periodicity can be mapped to the same SI message.
SI_window is the time domain window that each SI message is send and only one window length is defined for all SI messages.
Scheduling Other SI:
OSI can be sent in below ways and we shall look at the ways in details:
1. Periodically broadcasted on DL-SCH or
2. Broadcasted on-demand on DL-SCH or
3. Sent in dedicated manner on DL-SCH to UEs in RRC-CONNECTED mode.
1. Broadcasted on-demand by UE on DL-SCH
When UE is in RRC_IDLE or RRC_INACTIVE, request for OSI will trigger a random access procedure.
UE will read SI scheduling information from SIB1 and UE will determine the broadcast status from “si-BroadcastStatus” for an SI message.
If “si-BroadcastStatus” is “broadcasting”, the UE would acquire the required SIB normally.
If “si-BroadcastStatus” is “notbroadcasting”, then UE would proceed with RA procedure to acquire required SIB.
If network configures UE with PRACH resource for SI request, then CFRA is used else CBRA will be used.
On Demand SI request using CBRA procedure:
If “SI-RequestConfig” is not present in SIB1, then CBRA is used to request SI using RRCSystemInfoRequest message in Msg3.
gNB will respond using MSG4.
After that gNB will broadcast the required SI.
–RRCSystemInfoRequest
The RRCSystemInfoRequest message is used to request SI message(s) required by the UE as specified in clause 5.2.2.3.3.
It is transmitted using SRB0 -> TM mode -> CCCH logical channel on MSG3.
Below are the contents of “RRCSystemInfoRequest”
“requested-SI-List” it contains the list of requested SI messages.
According to the order of entry in the list of SI messages configured by schedulingInfoList in si-SchedulingInfo in SIB1, first bit corresponds to first/leftmost listed SI message, second bit corresponds to second listed SI message, and so on.
After gNB acknowledge this in MSG2 in CFRA and MSG4 in CBRA, it will broadcast the required SI messages.
-- ASN1START
-- TAG-RRCSYSTEMINFOREQUEST-START
RRCSystemInfoRequest-IEs ::= SEQUENCE {
requested-SI-List
BIT STRING (SIZE (maxSI-Message)), --32bits
spare BIT STRING (SIZE (12))
}
RRC-PosSystemInfoRequest-r16-IEs ::= SEQUENCE {
requestedPosSI-List
BIT STRING (SIZE (maxSI-Message)), --32bits
spare BIT STRING (SIZE (11))
}
-- TAG-RRCSYSTEMINFOREQUEST-STOP
-- ASN1STOP
On Demand SI request using CFRA procedure:
gNB may configure the UE with dedicated RA resources for SI in SI-RequestConfig within SI-SchedulingInfo IE in SIB1.
–SI-RequestConfig
The IE SI-RequestConfig contains configuration for Msg1 based SI request.
-- ASN1START
-- TAG–SI-REQUESTCONFIG-START
SI-RequestConfig::= SEQUENCE {
rach-OccasionsSI
SEQUENCE {
rach-ConfigSI RACH-ConfigGeneric,
ssb-perRACH-Occasion ENUMERATED {oneEighth, oneFourth, oneHalf, one, two, four, eight, sixteen}
} OPTIONAL, -- Need R
si-RequestPeriod
ENUMERATED {one, two, four, six, eight, ten, twelve, sixteen} OPTIONAL, -- Need R
si-RequestResources
SEQUENCE (SIZE (1..maxSI-Message)) OF SI-RequestResources
}
SI-RequestResources ::= SEQUENCE {
ra-PreambleStartIndex INTEGER (0..63),
ra-AssociationPeriodIndex INTEGER (0..15) OPTIONAL, -- Need R
ra-ssb-OccasionMaskIndex INTEGER (0..15) OPTIONAL -- Need R
}
-- ASN1STOP
-- TAG–SI-REQUESTCONFIG-STOP
“SI-RequestConfig” can be used to configure for each SI-message.
UE selects appropriate RA preamble and transmits MSG1.
gNB acknowledges the SI request in MSG2.
Then gNB wil transmits the required SI message.
Acquisition of an SI messages
Below steps are followed for an UE to acquire an SI message:
1. UE shall determine the start of SI window that includes radio frame and slot number.
2. for the concerned SI message, determine the number n which corresponds to the order of entry in the list of SI messages configured by schedulingInfoList in si-SchedulingInfo in SIB1;
3. determine the integer value x = (n – 1) × w, where w is the si-WindowLength
4. the SI-window starts at the slot #a, where a = x mod N, in the radio frame for which SFN mod T = FLOOR(x/N), where T is the si-Periodicity of the concerned SI message and N is the number of slots in a radio frame
Example:
Consider below scenario:
1. There is one SIB in each SI message.
2. The first entry in schedulingInfoList of SIB1 is SIB2, the second entry is SIB3 and so on up to SIB5.
3. Sub carrier spacing of 30KHz and the number of slots is 20.
4. Different periodicity for different SI is given in 2nd column. For example, SIB2 periodicity is 8 in radio frame. It will be transmitted in 8 radio frame.
TS 38.300 and 38.331
