Drawbacks of LTE
System Information (SI) design
LTE
MIB, SIB1 and SIB2 are mandatory for the UE to access a
cell and all system information is periodically transmitted by the network.
This
approach of broadcasting SI periodically over entire cell area is energy inefficient in cases for example when there are no UEs in the cell.
Moreover, this approach in 5G could lead to excessive signaling overhead in
cases where several beams are used to broadcast SI to be able to reach the entire
coverage area of the cell.
A
different approach is adopted in NR which is explained below.
NR System
Information
System
Information (SI) in NR consists of a MIB and a number of SIBs, which are
divided into Minimum SI and Other SI.
Minimum
SI carries basic information required for initial access
and for acquiring any other SI. Minimum SI consists of MIB and SIB1.
For a UE to be allowed to camp on a cell, it
must have acquired the contents of the Minimum SI from that cell.
Other
SI consists of all SIBs not broadcast in the Minimum
SI. The UE does not need to receive these SIBs before accessing the cell. Other
SI is also known as On-Demand SI because gNB transmits/broadcasts these
SIBs when explicitly requested by UE(s).
This
approach enhances network energy performance and reduces singalling overhead in
the cell by transmitting SI only when explicitly requested by UE(s). This (for
example) implies the network can completely avoid transmitting ‘Other SI’ when
there is no UE in the cell.
System
Information Summary
The
following table summarises system information contents; MIB/SIBs are discussed
thoroughly in future posts. Click on corresponding link in the table below to get
redirected to the respective post.
SIB Type
|
SIB Contents
|
| SFN, critical information for the reception of SIB1, Cell barred flag, Intra frequency reselection allowed flag | |
| Cell selection info., PLMN list, Cell ID, tracking area code, RAN area code, cell reserved flag, connection establishment failure control info., SI scheduling info., serving cell’s common uplink and downlink configuration (RACH, paging etc…), SUL configuration, SSB scheduling information, cell-specific TDD UL/DL configuration, Cell’s IMS emergency bearer support flag (for UE’s in limited service), emergency call over IMS support flag, UE’s timers and constants, Access control information etc… | |
| Cell re-selection information, mainly related to the serving cell | |
| Information about the serving frequency and intra-frequency neighbouring cells relevant for cell re-selection (including cell re-selection parameters common for a frequency as well as cell specific re- selection parameters) | |
| Information about other NR frequencies and inter-frequency neighbouring cells relevant for cell re-selection (including cell re-selection parameters common for a frequency as well as cell specific re- selection parameters) | |
| Information about E-UTRA frequencies and E-UTRA neighbouring cells relevant for cell re- selection (including cell re-selection parameters common for a frequency as well as cell specific re-selection parameters) | |
| ETWS primary notification | |
| ETWS secondary notification | |
| CMAS warning notification | |
| Information related to GPS time and Coordinated Universal Time (UTC) |
System
Information Acquisition
The
UE shall acquire SI upon cell selection (e.g. upon power on), cell-reselection,
return from out of coverage, after reconfiguration with sync completion, after
entering the network from another RAT, upon receiving an indication that the SI
has changed, upon receiving a PWS (Public Warning System) notification and
whenever the UE does not have a valid version of a stored SIB.
The
figure below summarizes system information provisioning in general;
Master
Information Block (MIB)
MIB
is mapped on to BCCH logical channel and is carried on BCH transport channel.
BCH is then mapped on to PBCH.
MIB
is transmitted with a periodicity of 80 ms and is repeated (according to
SSB periodicity) within the 80 ms. MIB contents are same over 80 ms period
and the same MIB is transmitted over all SSBs within the SS burst set.
MIB
provides the UE with parameters (e.g. CORESET#0 configuration) required
to acquire SIB1, more specifically, information required for monitoring
of PDCCH for scheduling PDSCH that carries SIB1.
PBCH/MIB
is thoroughly discussed in the post 5G NR: PBCH and MasterInformation Block (MIB).
System
Information Block 1 (SIB1)
As
MIB and SIB1 are called as ‘minimum SI’, SIB1 alone
is known as ‘Remaining Minimum System Information’ (RMSI).
SIB1
carries the most critical information required for the UE to access the cell
e.g., random access parameters.
SIB1
includes information regarding the availability and scheduling of other SIBs e.g.
mapping of SIBs to SI message, periodicity, SI-window size etc…
SIB1
also indicates whether one or more SIBs are only provided on-demand, in which case,
it may also provide PRACH configuration needed by the UE to request for the
required SI.
SIB1
is transmitted on the DL-SCH (logical channel: BCCH) with a periodicity of 160
ms and variable transmission repetition periodicity within 160 ms.
SIB1
is cell-specific SIB.
Other SI
Other
SI encompasses all SIBs not broadcast in the Minimum
SI. It is not mandatory for the UE to receive these SIBs before accessing
the cell.
Other
SI messages are mapped to BCCH logical channel and either periodically broadcast
on DL-SCH or broadcast on-demand on DL-SCH (i.e. upon request from UEs in
RRC_IDLE or RRC_INACTIVE) or sent in a dedicated manner on DL-SCH to UEs in
RRC_CONNECTED.
Other SI is also known as On-Demand
SI because gNB transmits these SIBs when explicitly requested by UE(s).
SI-messages
Other SI are carried in SystemInformation (SI)
messages, which are transmitted on the DL-SCH. SIB1 contains scheduling
information for all SI messages.
Only SIBs
having the same periodicity can be mapped to the same SI message.
Each SI
message is sent within periodically occurring time domain windows called
SI-windows and only one window length is defined for all SI messages.
Each SI
message is sent within the corresponding SI-widow and the SI-windows of
different SI messages do not overlap.
An SI
message may be transmitted a number of times within the SI-window.
Any SIB
except SIB1 can be configured to be either cell specific or area
specific, using an indication in SIB1. More details in ‘SIB-Validity’
section in this post.
‘Other SI’
Scheduling
As
discussed already, Other SI can be transmitted in the following ways;
-
Periodically
broadcast on DL-SCH or
-
Broadcast
on-demand on DL-SCH (i.e. upon explicit request from UEs)
-
Sent
in a dedicated manner on DL-SCH to UEs in RRC_CONNECTED
Procedure for requesting On-demand SI
For
UEs in RRC_IDLE and RRC_INACTIVE, a request for Other SI triggers a random
access procedure (see: 5G NR: Random AccessProcedure).
After
reading SI scheduling information from SIB1, the UE first determines broadcast
status (via si-BroadcastStatus)
of an SI message. This field indicates if one or several required SIBs within
the SI-message are being broadcasted or not.
If
si-BroadcastStatus is set ‘broadcasting’,
the UE would acquire the concerned SIB(s) normally.
If
si-BroadcastStatus is set ‘notbroadcasting’,
the UE would proceed with RA procedure to acquire those SIB(s).
- For
this purpose, if the network configures the UE with PRACH resources for SI
request, contention-free random access procedure (CFRA) is used, otherwise contention-based
random access (CBRA) procedure is used.
SI request using CFRA procedure:
-
The
network may configure the UE with dedicated RA resources for SI request purpose
in SI-RequestConfig within SI-SchedulingInfo IE in SIB1.
-
Msg1 is used to indicate the requested Other SI. SI-RequestConfig is
shown below;
-
As
can be seen from the above figure, si-RequestResources can be configured
for each SI-message.
-
In
CFRA, the minimum granularity of the request is one SI message (i.e. a set of
SIBs).
-
UE
selects appropriate RA preamble and transmits Msg1.
-
After
receiving Msg1, based on the RA resource used for Msg1, the gNB
knows which SI-message the UE is requesting for.
-
The
gNB acknowledges the SI request in Msg2.
-
The
gNB broadcasts/transmits the required SI-message.
- To receive the concerned SI message, the UE follows
the procedure explained in the section ‘Acquisition of an SI message’.
-
The
procedural flow is illustrated in the figure below.
SI request using CBRA procedure:
-
If
SI-RequestConfig is excluded from SIB1, the UE relies on CBRA
procedure to request for SI and sends RRCSystemInfoRequest message
in Msg3.
-
The gNB acknowledges the request in Msg4.
-
The
gNB broadcasts/transmits the required SI-message.
-
To
receive the concerned SI message, the UE follows the procedure explained in the
section ‘Acquisition of an SI message’.
-
The
procedural flow is illustrated in the figure below.
-
The
RRCSystemInfoRequest is used to request for SI message(s). UE
transmits it using SRB0, Transparent Mode and CCCH logical channel (on Msg3).
-
The
contents of RRCSystemInfoRequest are given in the figure below;
-
The
UE indicates the requested SI message(s) using requested-SI-List.
The first/leftmost bit corresponds to the first SI-message configured by schedulingInfoList
in si-SchedulingInfo in SIB1, second bit corresponds to the second
SI message, and so on.
After UE’s SI request has been acknowledged by the gNB (Msg2 in CFRA and
Msg4 in CBRA), it broadcasts/transmits the required SI-message. To receive the
SIBs, the UE starts monitoring PDCCH whose CRC is scrambled with SI-RNTI by
following the procedure explained below.
Acquisition of an SI message
For
acquiring an SI message, UE shall determine start of SI window (radio frame and
slot number) for the concerned SI message. The procedure is as follows;
1. 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 within si-SchedulingInfo in SIB1.
2. Determine the integer value ‘x’ such that x = (n – 1) × w,
where w is the si-WindowLength.
3. The start SFN of the SI window determined by 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.
4. The starting slot of the SI window in the SFN (determined
above) is given by x mod
N.
Acquisition
of SI message is explained below with an example. In this example, assume that
there is one SIB in each SI message. The first entry in schedulingInfoList
(in SIB1) is SIB2, the second entry is SIB3 and so on up to
SIB5. A subcarrier spacing of 30 kHz is assumed, which means that number
of slots (N) in a radio frame is 20. SI window length (w) of 20
slots is assumed. Different periodicities (second column in the below table) are
assumed for different SI messages.
Entry number
(n)
|
SI Periodicity (T)
(in radio frames)
|
x = (n – 1) × w
|
Slot Number (a)
a = x mod N
|
Radio frame Number
SFN mod T = FLOOR(x/N)
|
1 (SIB2)
|
8
|
0
|
0
|
SFN mod 8 = 0
|
2 (SIB3)
|
8
|
20
|
0
|
SFN mod 8 = 1
|
3 (SIB4)
|
16
|
40
|
0
|
SFN mod 16 = 2
|
4 (SIB5)
|
16
|
60
|
0
|
SFN mod 16 = 3
|
| Additional Assumptions: Numerology µ = 1 (SCS = 30 kHz) => Number of slots in a radio frame (N) = 20 w = si-WindowLength = s20 = 20 slots |
||||
...
|
...
|
|||||||||||||||||||||
SFN
|
0
|
1
|
2
|
3
|
4
|
5
|
6
|
7
|
8
|
9
|
10
|
11
|
12
|
13
|
14
|
15
|
16
|
17
|
18
|
19
|
20
|
The
UE shall receive the PDCCH scrambled with SI-RNTI from the start of the SI-window
and continue until the end of the SI-window (given by si-WindowLength)
or until the SI message was received.
If
the SI message was not received by the end of the SI-window, repeat
reception at the next SI-window occasion for the concerned SI.
System
Information Transfer via Dedicated Signalling
For
a UE in RRC_CONNECTED, the network can provide system information through
dedicated signalling using the RRCReconfiguration message, e.g. if the
UE has an active BWP with no common search space configured to monitor SI or
paging.
Also,
for PSCell and SCells, the network provides the required SI by dedicated
signalling, i.e. within RRCReconfiguration message.
SIB-validity
The
UE stores the acquired SIB after acquiring a MIB or a SIB1 or
an SI message in a serving cell.
Whenever
the UE receives SIB1 from a serving cell, it compares received and
stored parameters such as first PLMN-Identity included in the PLMN-IdentityInfoList,
the systemInformationAreaID and the valueTag. If received and
stored parameters are identical, UE considers the stored SIBs as valid for the
cell.
The
UE shall delete any stored version of a SIB after 3 hours from the moment it
was successfully confirmed as valid.
The
UE may use a valid stored version of the SI except MIB, SIB1, SIB6,
SIB7 or SIB8 e.g. after cell re-selection, upon return from out
of coverage or after the reception of SI change indication.
Area scope of a SIB
-
As
discussed already, any SIB except SIB1 can be configured to be either
cell specific or area specific, using an indication in SIB1.
- The cell specific SIB is applicable only within a cell
that provides the SIB while the area specific SIB is applicable within an area
referred to as SI area, which consists of one or several cells and is
identified by SI area ID (systemInformationAreaID).
-
The
scheduling information in SIB1 may contain ‘area scope’ (areaScope) for an individual SIB. If
this field is present, the concerned SIB is area specific otherwise, it
is cell specific.
-
If
the SIB is area specific, the UE compares the received SI area ID to that of
the stored one for the concerned SIB. The UE re-acquires the concerned SIB if
the SI area ID is different.
SI change
indication and PWS Notification
The
SI modification and indication procedure almost similar to that of LTE.
Change
of SI (other than for ETWS and CMAS) only occurs at specific radio frames, i.e.
the concept of a modification period is used.
SI
may be transmitted a number of times with the same content within a
modification period.
The
modification period boundaries are defined by SFN values for which SFN mod m= 0, where m
is the number of radio frames comprising the modification period. The
modification period is configured by SIB1 via modificationPeriodCoeff.
When
the network changes (some of the) SI, it first notifies the UEs about this
change, i.e. this may be done throughout a modification period. In the next
modification period, the network transmits the updated SI.
These
general principles are illustrated in figure below, in which different colours
indicate different SI.
Upon
receiving a change notification, the UE acquires the new SI immediately from
the start of the next modification period. The UE applies the previously
acquired SI until the UE acquires the new SI.
The
UE receives indications about SI modifications and/or PWS (Public Warning
System) notifications using Short Message transmitted with P-RNTI over DCI. DCI
format 1_0 scrambled with P-RNTI is used for this purpose.
The
‘Short Message’ received within DCI is a 8-bit field and is interpreted as
follows.
Bit
|
Short Message
|
Bit 1 (MSB)
|
systemInfoModification If set to 1: indication of a BCCH modification other than SIB6, SIB7 and SIB8 |
Bit 2
|
etwsAndCmasIndication If set to 1: indication of an ETWS primary and/or secondary notification and/or a CMAS notification |
Bit 3-8
|
Not used in 3GPP release 15.6 version, and shall be ignored by UE if received |
If
the received ‘short message’ indicates ETWS/CMAS notification, the UE should
immediately re-acquire SIB1 and based on scheduling information, the UE
shall acquire SIB6/SIB7/SIB8.
If
the received ‘short message’ indicates SI modification, the UE shall try to re-acquire
SI from the start of next modification period.
SI-RNTI
As
discussed already, SI-RNTI is used for identification of Broadcast
and System Information in the downlink.
It
is a common RNTI meaning that, it is not allocated to any UE explicitly. SI-RNTI’s
value is fixed/standardized to be 65535 (0xFFFF).
PDCCH DCI
format 1_0 is used for SI purpose.
Misc.
Information
The
physical layer imposes a limit to the maximum size a SIB can take. The
maximum SIB1 or SI message size is 2976 bits.
If
the UE cannot determine the full contents of the minimum SI of a cell by
receiving from that cell, the UE shall consider that cell as barred.
Reference:
3GPP TS 38.300 and 38.331
