Query_1: Why SIBs are needed?
Query_2: How many types of SIBs are there?
Query_3: On which channels SIBs are transmitted?
Query_4: Minimum of how many SIBs are required for the UE to initiate Attach procedure?
Information regarding SIB2-SIB13 are carried in SI messages and are included in schedulingInfoList which is part of SIB1.
Scheduling of SIBs:
Scenarios where UE acquire SIBs:
UE will initiate the SIB acquisition procedure in any of the following criteria's:
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).
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
· 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_Periodicity | SchedulingInfosi_Periodicity_rf16 | |
| sib_MappingInfo[0] | SIB_TypeSIB_Type_sibType3 | |
| schedulingInfoList[1] | SchedulingInfo | |
| si_Periodicity | SchedulingInfosi_Periodicity_rf32 | |
| sib_MappingInfo[0] | SIB_TypeSIB_Type_sibType4 | |
| schedulingInfoList[2] | SchedulingInfo | |
| si_Periodicity | SchedulingInfosi_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 sib3 | Periodicity= 16 radio frames or 160ms | Window length= 20ms |
| The seond element of schedulingInfoList contains sib4 | Periodicity= 32 radio frames or 320ms | Window length= 20ms |
| The third element of schedulingInfoList contains sib5 | Periodicity= 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,
| x | Integer value | ||
| n | a 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 | ||
| w | si-WindowLength | ||
| T | si-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-3 | x = (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-4 | x = (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-5 | x = (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-3 | SFN 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-4 | SFN 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-5 | SFN 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 |
| LTE system information blocks | Description |
|---|---|
| MIB | Carries physical layer information of LTE cell which in turn help receive further SIs, i.e. system bandwidth |
| SIB1 | Contains information regarding whether or not UE is allowed to access the LTE cell. It also defines the scheduling of the other SIBs. carries cell ID, MCC, MNC, TAC, SIB mapping. |
| SIB2 | Carries common channel as well as shared channel information. It also carries RRC, uplink power control, preamble power ramping, uplink Cyclic Prefix Length, sub-frame hopping, uplink EARFCN |
| SIB3 | carries cell re-selection information as well as Intra frequency cell re-selection information |
| SIB4 | carries Intra Frequency Neighbors(on same frequency); carries serving cell and neighbor cell frequencies required for cell reselection as well handover between same RAT base stations(GSM BTS1 to GSM BTS2) and different RAT base stations(GSM to WCDMA or GSM to LTE or between WCDMA to LTE etc.) . Covers E-UTRA and other RATs as mentioned |
| SIB5 | Carries Inter Frequency Neighbors(on different frequency); carries E-UTRA LTE frequencies, other neighbor cell frequencies from other RATs. The purpose is cell reselection and handover. |
| SIB6 | carries WCDMA neighbors information i.e. carries serving UTRA and neighbor cell frequencies useful for cell re-selection |
| SIB7 | carries GSM neighbours information i.e. Carries GERAN frequencies as well as GERAN neighbor cell frequencies. It is used for cell re-selection as well as handover purpose. |
| SIB8 | carries CDMA-2000 EVDO frequencies, CDMA-2000 neighbor cell frequencies. |
| SIB9 | carries HNBID (Home eNodeB Identifier) |
| SIB10 | carries ETWS prim. notification |
| SIB11 | carries ETWS sec. notification |
Great article!
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