A reader Ramachandra C. asks , ” What is the Importance of SIBs and MIBs in LTE ? and how does it works? ”
It’s like asking, what is the importance of vitamins , minerals and nutrients in a human body. You don’t need multivitamins in huge quantities everyday like the calories. However, lack of essential nutrients may adversely affect your well being and quality of life.
I am not a nutritionist but I assume above analogy is appropriate :).
A picture is worth a thousand words. Let’s take a look at how much quantity of system information arrives in each radio frame. ?
Percentage of Air Interface occupied by MIB in case of 1.4 MHz bandwidth
As evident from the snapshot of radio frame above. It is clear that system information (Master Information Block (MIB)) in this case is not taking a lot of space in time and time frequency domain the air interface for this purpose.
How much of space does a MIB occupy in case of FDD is given below.
Overhead generated by MIB (PBCH) Physical broadcast channel in case of FDD
As evident even in worst case scenario i.e., 1.4 MHz it does not occupy more than 2.5% overhead for the case of extended cyclic prefix.
For the lovers of TDD. We have not ignored you. Don’t be sad there is the chart for the overhead generated by MIB in case of TDD. This chart is a little bit messy. In this case , depending upon the special sub-frame configuration , there are different flavors. Take look below.
MIB (PBCH) Overhead generated for Normal Cyclic Prefix in case of TDD
MIB (PBCH) Overhead generated in case of Extended cyclic prefix for TDD
In case of TDD, even in worst case scenario it occupies 9.8 and 10 % for normal and extended cyclic prefix respectively in case of 1.4 MHz spectrum.
We have discussed only the percentage overhead of MIB as of yet. But you get the idea of how small but effective system information is in order for the all function and operation of UE operations.
What is the importance of MIBs and SIBs ?
In simple words let’s say , ” UE reads system information in RRC Idle mode to acquire the parameters necessary to complete cell selection and cell reselection. System information also provides the parameters necessary to access the network and detect paging messages ”
System information is broadcast using a Master Information Block (MIB) and a series of System Information Blocks (SIB).
General importance of MIB and SIB 1 for LTE in pictorial format
UE starts by reading the MIB and this provides sufficient information to read SIB 1. SIB 1 provides scheduling information for the remaining SIB. The hierarchy of reading system information is shown below.
Hierarchy of reading System Information
A summary of MIB and each of the SIBs is provided in the table below.
Summary of System Information Content
Now only to start with, we will go a detail explanation of only MIB and SIB 1. For the nominal explanation how MIB and SIB 1 are used in Cell Search and cell selection procedure. You can read it over here.
Master information Block ( MIB)
Contents of MIB
The content of MIB is the same for both FDD and TDD deployments.
- The downlink channel bandwidth is signaled in terms of the number of Resource Blocks i.e., 6, 15, 25, 50, 75, or 100. This information allows the UE to proceed to decode the PCFICH and PDCCH, i.e., the UE can then identify Resource Blocks allocated to they System Information Blocks (SIB)
The PHICH configuration defines:
- the PHICH duration which can be either ‘normal’ or ‘extended’
- the PHICH resource (PHICH Group Scaling Factor) which can be 1/6, 1/2, 1, 2.
- The SFN field defines the 8 most significant bits of the SFN. The SFN requires 10 bits in total to provide a range from 0 to 1023. The 2 least significant bits are deduced from the 4 radio frame cycle used to transmit the complete BCH Transmission Time Interval (TTI), i.e. the first radio frame within the cycle corresponds to 00, the second to 01, the third to 10, and the fourth to 11.
System Information Block (SIB 1)
SIB 1 can be decoded from MIB once every 10 ms radio-frame. A SIB 1 is sent whenever SFN mod 8 = 0. Each transmission of SIB 1 is followed by 3 transmissions when SFN mod 2 = 0. This transmission timing for SIB 1 is shown below.
SIB1 Periodicity. Timing for the transmission and re-transmission of SIB 1
Contents of SIB 1
Th contents of SIB 1 is shown in the table below.
Contents of SIB 1
The TDD Configuration information in SIB 1 is only included by TDD networks. The IMS Emergency Support and Cell Selection Information at the end of SIB 1 were introduced within release 9 of 3GPP.
Detail Information on Contents of SIB 1:
- PLMN identity list can specify up to 6 PLMN identities, i.e., a single cell can be shared between PLMN. The first PLMN with in the list is the Primary PLMN. A flag is included for each each PLMN to indicate whether or not the cell is reserved for operator use.
Tracking Area Related
- The tracking area code (TAC) is applicable to all PLMN within the PLMN identity list, i.e. all PLMN share a common TAC. The TAC has a range of 0 to 65536.
Cell Identity Related
- The Cell Identity has a length of 28 bits and encapsulates the eNodeB identity. The eNodeB identity can have a length of either 20 or 28 bits, allowing either 256 cells or 1 cell per eNodeB respectively. The cell identity is applicable to all PLMN with in the PLMN identity list but can be concatenated with PLMN identity to generate a unique global cell identity.
Cell Barring Related
- The Cell Barred flag is included to inform the UE of whether or not the UE is allowed to select or reselect the cell in RRC Idle mode. It is applicable to all PLMN within the PLMN identity list.
- The Intra-Frequency Cell Reselection Allowed flag is used to indicate whether or not the UE is allowed to reselect other cells on the same RF carrier when the cell is barred.
Closed Subscriber Group Related
- If the Closed Subscriber Group (CSG) indication is set to TRUE then the UE is only allowed to access the cell if the subsequent CSG Identity matches an entry within the UE’s own list of allowed CSG. The CSG identity is applicable to the primary PLMN.
Cell Selection Related
- The minimum RSRP requirement for cell selection is defined in terms of Qrxlevmin. The actual value of Qrxlevmin = (signalled value *2) + 1
- Qrxlevmin Offset is subtracted from Qrxlevmin when the UE is camped on a Visited PLMN and cell selection is triggered due to a periodic search for a higher priority PLMN. The offset reduces the value of Qrxlevmin so allows cell selection to become easier. The actual value of the offset = (signalled value*2)
- Pmax defines the maximum allowed uplink transmit power for the cell. This also impacts the cell selection criteria if the UE transmit power capability is less than the maximum allowed for the cell.
Operating Band Related
- The operating band is specified by the Frequency Band Indicator because the downlink channel upon which the UE is receiving the system information does not uniquely identity the operating band, i.e. some operating bands overlap and have different duplex spacings.
Tentative Periodicity and Window used to schedule SIB 2 to SIB13
SIB 1 includes scheduling information for SIB 2 to SIB 13. Information is provided in terms of a System Information Periodicity and a System Information Window. The Periodicity is specific to groups of the SIB whereas the Window is common to all SIB.
Scheduling and Periodicity Window for SIB 2 to SIB 13
- The TDD Configuration specifies the Subframe Assignment and the Special Subframe Pattern. The Subframe Assignment represents the uplink-downlink subframe configuration, i.e. the number and pattern of subframes allocated to the uplink and downlink. The Special Subframe Pattern represents the special subframe configuration.
Remaining SIBs Related
- A System Information Value Tag is included within SIB 1 to provide an indication of whether or not the content of SIB 2 to SIB 9 or SIB 13 has changed and needs to be re-acquired. The value tag is not applicable to the MIB, SIB 1, SIB 10, SIB 11 nor SIB 12.
- The IMS Emergency Support flag was introduced within the release 9 version of the 3GPP specifications. It is used to indicate whether or not the cell supports IMS emergency bearer services for UE in Limited Service Mode (LSM). UE are in LSM when they are unable to authenticate with the MME.
- Qualmin and Qqualmin Offset were introduced the release 9 version of the 3GPP specifications. The release 8 version of the specifications defines its cell selection criteria based upon RSRP only. The release 9, and newer versions of the specifications use both RSRP and RSRQ as criteria for cell selection. The actual value equals the signalled value for both Qqualmin and Qqualmin Offset.
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