All about SIB's in LTE

SIBs and their JOBS: (click on each of the SIB to know more about them)
SIB-1
Cell access related parameters and scheduling of other SIBs
SIB-2
Common and shared channel configuration, RACH related configuration are present
SIB-3
Parameters required for intra-frequency, inter-frequency and I-RAT cell re-selections
SIB-4
Information regarding INTRA-frequency neighboring cells (E-UTRA)
SIB-5
Information regarding INTER-frequency neighboring cells (E-UTRA)
SIB-6
Information for re-selection to INTER-RAT (UTRAN cells)
SIB-7
Information for re-selection to INTER-RAT (GERAN cells)
SIB-8
Information for re-selection to INTER-RAT (CDMA2000)
SIB-9
Information related to Home eNodeB  (FEMTOCELL)
SIB-10
ETWS (Earthquake and Tsunami Warning System) information (Primary notification)
SIB-11
ETWS (Earthquake and Tsunami Warning System) information (Secondary notification)
SIB-12
Commercial Mobile Alert Service (CMAS) information.
SIB-13
Contains the information required to acquire the MBMS control information associated with one or more MBSFN areas.


Query_1: Why SIBs are needed?
SIBs carry relevant information for the UE, which helps UE to access a cell, perform cell re-selection, information related to INTRA-frequency, INTER-frequency and INTER-RAT cell selections.

Query_2: How many types of SIBs are there?
In LTE there are 13 types of SIBs. Each SIB has its own job to do, which means, they are meant to carry information which are related to perform their assigned job.

Query_3: On which channels SIBs are transmitted?
All SIBs are transmitted on BCCH->DL-SCH->PDSCH.

Query_4: Minimum of how many SIBs are required for the UE to initiate Attach procedure?
In LTE, for a UE to access the eNB, at the most minimum 2 SIBs are required (SIB1 and SIB2).

Information regarding SIB2-SIB13 are carried in SI messages and are included in schedulingInfoList which is part of SIB1.

Scheduling of SIBs:
·         SIB1 uses a fixed schedule with a periodicity of 80 ms and repetitions made within 80 ms.
·         The first transmission of SIB1 is scheduled in subframe #5 of radio frames for which the SFN mod 8 = 0, and
·         Repetitions are scheduled in subframe #5 of all other radio frames for which SFN mod 2 = 0.
·         Scheduling of other SIB's i.e. SIB-2 to SIB-9 is defined by si-Periodicity and si-WindowLength. Both these IEs are explained below. For the rest of the SIBs, Paging is received by UE in RRC_IDLE or RRC_CONNECTED mode for notification of an in-coming SI message.

Scenarios where UE acquire SIBs:
UE will initiate the SIB acquisition procedure in any of the following criteria's:

·         UE is powered on (selecting a cell)
·         Cell re-selection
·         After HO completion
·         After entering E-UTRAN from another RAT
·         coming out of OUT OF COVERAGE situation
·         receiving a notification that SYSTEM INFORMATION has changed
·         receiving an indication about the presence of ETWS (Primary/Secondary), CMAS notification
·         receiving a request from CDMA 2000 upper layers
·         exceeding the maximum validity duration of SIBs

HOW and WHEN UE start acquiring SIBs (other than SIB1):
There are two IE's which help the UE to calculate when to read SI messages. They are:
a. schedulingInfoList
b. si-WindowLengthd

schedulingInfoList informs the UE regarding the presence of SIB type; other than SIB1. It carriers two more parameters:
i.  si_Periodicity
ii. sib_MappingInfo
· si_Periodicity tells about the period in terms of radio frames which helps the UE to read the required SIB. The value of this parameter is used for calculating the Radio frame where SI will be received.
· sib_MappingInfo carries the type of SIBs mapped
si-WindowLength specifies that a SIB should be transmitted somewhere within the specified window length. Value is in ms. This window starts at the starting sub-frame of the mentioned si_periodicity. SIBs can be received in any of the sub-frame as mentioned in the WindowLength.
Example:
SIB1 is received by UE with the following information:
schedulingInfoList[0]
SchedulingInfo
si_PeriodicitySchedulingInfosi_Periodicity_rf16
sib_MappingInfo[0]SIB_TypeSIB_Type_sibType3
schedulingInfoList[1]SchedulingInfo
si_PeriodicitySchedulingInfosi_Periodicity_rf32
sib_MappingInfo[0]SIB_TypeSIB_Type_sibType4
schedulingInfoList[2]SchedulingInfo
si_PeriodicitySchedulingInfosi_Periodicity_rf64
sib_MappingInfo[0]SIB_TypeSIB_Type_sibType5
si_WindowLength
ms20
(To look for the complete IE contents and explanation of SIB1 click here)

The above table states the following:
The first element of schedulingInfoList contains sib3Periodicity= 16 radio frames
or 160ms
Window length= 20ms
The seond element of schedulingInfoList contains sib4Periodicity= 32 radio frames
or 320ms
Window length= 20ms
The third element of schedulingInfoList contains sib5Periodicity= 64 radio frames
640ms
Window length= 20ms

Now, UE needs two information to read SI.
a. One is on which which Radio frame UE will receive the SIB. To determine which Radio frame, the formula is:

SFN mod T = FLOOR(x/10)

b. on which sub-frame will the SI window start. To determine the start of the SI window at sub-frame, the formula is:

a = x mod 10

Query: How to determine the value of x?

x = (n – 1)*w

where,
xInteger value
na number which corrosponds to the order of entry in schedulingInfoList.For sib3 n is 1,
for sib4 n is 2 and
for sib5 n is 3
wsi-WindowLength
Tsi-Periodicity of the concerned SI message


Let's start calulating the Radio frame and SI starting sub frame for SIB3, SIB4 and SIB5

Subframe from which the SI Window starts
For SIB-3x = (n – 1)*w
x=(1-1)*20
x=0
a = x mod 10
a= 0 mod 10
a=0
The SI Window will start at sub frame 0 of the Radio frame where SI will be acquired
For SIB-4x = (n – 1)*w
x=(2 - 1)*20
x=20
a = x mod 10
a= 20 mod 10
a=0
The SI Window will start at sub frame 0 of the Radio frame where SI will be acquired
For SIB-5x = (n – 1)*w
x=(3 - 1)*20
x=40
a = x mod 10
a= 40 mod 10
a=0
The SI Window will start at sub frame 0 of the Radio frame where SI will be acquired
Radio frame on which the UE will acquire designated SIB
For SIB-3SFN mod T = FLOOR(x/10)SFN mod 16 = FLOOR (0/10)
SFN mod 16 = 0
Whenever SFN mod 16 will be 0. UE is expecting SIB3 and it will start looking from sub frame 0 till the end of the mentioned si-WindowLength
For SIB-4SFN mod T = FLOOR(x/10)SFN mod 32 = FLOOR (20/10)
SFN mod 32 = 2
Whenever SFN mod 32 will be 2. UE is expecting SIB4 and it will start looking at sub frame 0 till the end of the mentioned si-WindowLength
For SIB-5SFN mod T = FLOOR(x/10)SFN mod 64 = FLOOR (40/10)
SFN mod 64 = 4
Whenever SFN mod 64 will be 4. UE is expecting SIB5 and it will start looking at sub frame 0 till the end of the mentioned si-WindowLength


SIBs and their JOBS: (click on each of the SIB to know more about them)
SIB-1
Cell access related parameters and scheduling of other SIBs
SIB-2
Common and shared channel configuration, RACH related configuration are present
SIB-3
Parameters required for intra-frequency, inter-frequency and I-RAT cell re-selections
SIB-4
Information regarding INTRA-frequency neighboring cells (E-UTRA)
SIB-5
Information regarding INTER-frequency neighboring cells (E-UTRA)
SIB-6
Information for re-selection to INTER-RAT (UTRAN cells)
SIB-7
Information for re-selection to INTER-RAT (GERAN cells)
SIB-8
Information for re-selection to INTER-RAT (CDMA2000)
SIB-9
Information related to Home eNodeB  (FEMTOCELL)
SIB-10
ETWS (Earthquake and Tsunami Warning System) information (Primary notification)
SIB-11
ETWS (Earthquake and Tsunami Warning System) information (Secondary notification)
SIB-12
Commercial Mobile Alert Service (CMAS) information.
SIB-13
Contains the information required to acquire the MBMS control information associated with one or more MBSFN areas.

9 comments:

  1. Why we need SIB2 to access the eNB. Isn't SIB1 Info sufficient? How many SIBs we need to have access the eNB in LTEAdv?

    ReplyDelete
    Replies
    1. SIB2 has the needed RACH parameters along with other PHY params.

      Not sure about LTEAdv. But both of them (SIB1, SIB2) should be mandatory.

      Delete
  2. very informative!

    ReplyDelete
  3. Best description I've seen. Thank you

    ReplyDelete
  4. Thanks for such a good explanation.

    ReplyDelete