SIB1

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Hi Guys today we are learning SIB1 in detail for LTE.

SIB1

Why SIB1 is required for initial attach process?

Once uplink synchronization between the UE and the network has been established and the UE has read the MIB, the UE waits for System Information Block Type 1 (SIB1) which carries cell access related information and supplies the UE with the scheduling of other system information blocks.

Reading SIB1 is mandatory for the UE to gain access to the network as the UE cannot know about the sequences it has to transmit in PRACH or the transmit power at which eNodeB is transmitting or the subframe/UL-DL pattern in case of TDD without decoding the SIB. If the UE is unable to decode SIB it will simply abandon that network

Using which channel to transmit SIB1?

The channel mapping of SIB1 is BCCH --> DL-SCH --> PDSCH channels.

Which sub-frame carries SIB1 info?

SIB1 is transmitted in subframe 5 of every even frame, with four different Redundancy Versions (RVs) transmitted consecutively giving an overall period of 80msec or 8 frames.

What is the periodicity of transmission SIB1?

A new SIB1 is transmitted every 80ms and repetitions are made within 80ms.

How are SIB1 repetitions schedule?

The first transmission of SIB1 is scheduled in subframe 5 for which the SFN mod8 = 0, and repetitions are scheduled in subframe 5 of all other radio frames for which SFN mod2 = 0.

What are the important Information Elements that UE reads in SIB?

The UE, upon receiving SIB1, checks the IE freqBandIndicator to see if the frequency band indicated in it is a part of the frequency bands supported by the UE. If not, the UE considers the cell as barred.

The UE also checks if the cell is reserved for operator use.

Another IE is q_RxLevMin which is the minimum required RSRP level for cell selection.

How is SIB transmitted by the eNodeB? 

The eNodeB uses PDCCH and PDSCH to transmit SIB.

The DCI (usually a format 1A) is scrambled with the SI-RNTI (whose value is 65535 or 0xFFFF) and is carried in the PDCCH. In the same subframe a SIB is encoded by the MAC/L2 sd s physical layer transport block which is taken by the PDSCH encoder on the L1 and the L1 adds a 24 bit TB CRC and CB CRCs (if applicable). The SIB TB undergoes the same process as other TBs but the SIB is always transmitted using single antenna or transmission diversity and never in MIMO mode.

SIB1 IE description:

Here is the list of information elements carried in SIB1:

PLMN Identity List: Can contain up to 6 PLMNs with the first one being the primary PLMN.
PLMN Identity: It is a combination of MCC and MNC which identifies the operator globally.
Cell Reserved For Operator Use: It can be Reserved or notReserved. If multiple PLMNs are indicated in SIB1, this field is specified per PLMN inverse of Cell Barred.
Tracking Area Code: It identifies a tracking area for paging the users.
Cell Identity: It identifies a cell within PLMN.

Cell Barred: It identifies whether the cell is barred or not. If it is barred then the user is not allowed to access the cell. In case of multiple PLMNs this field is common for all PLMNs list in SIB1.
Intra Frequency Reselection: It identifies whether intra-frequency reselection is allowed or not.
CSG Indication: It identifies whether the cell belongs to a Closed Subscriber Group.
CSG Identity: A cell belongs to a CSG if a limited set of users are allowed access to it.
q_RxLevMin: It is the minimum RSRP level required for cell selection. Multiply by 2 to obtain the actual value in dBm.
q_RxLevMinOffset: It indicates the offset to the minimum required signal level.
p-Max: It defines the maximum uplink transmit power allowed for the UE.
Freq Band Indicator: It indicates LTE frequency band.
SI Periodicity: It is the periodicity of System Information Blocks except than SIB1.
SIB Mapping Info: It is a list of mapped SIBs.
SIB Type: It indicates the type of transmitted SIB message in the list.
SI Window Length: It defines the length of the window within which multiple transmissions of the SI message are allowed. The Window Length can be set to 1, 2, 5 10, 15, 20 or 40 ms.
System Info Value Tag: UEs can use System Info Value Tag to verify if the previously stored SI messages are still valid or a change has occurred in the SI messages.

ims-EmergencySupport: Indicates whether the cell supports IMS emergency bearer services for UEs in limited service mode. If this IE is absent from the SIB, this feature is not supported by the network.

How does UE know when to decode SIB? 

SIB1 is always transmitted on subframe-5 with the periodicity of 20ms with a new SIB1 transmitted every 80ms and repetitions made within 80m0. This is known to the UE and it looks for PDCCH with SI-RNTI on that particular radio frame-subframe combination. Once it has decoded SIB1, it knows the periodicities of other SIB messages as this scheduling information is part of SIB1 message.

Note: Once the content of SIB1 gets changed then it has to remain same for next 80ms and the change can happen only on a radio frame (modulo) 80 boundaries.

SIB1 Recovery (Decoding).

The following steps are performed by the UE in this process:

·         Demodulation of Physical Control Format Indicator Channel (PCFICH), decoding of CFI

·         Decoding of PDCCH

·         Blind search  of PDCCH

·         Recovery of SIB bits: Demodulation  of PDSCH and decoding  of DL-SCH

·         Buffering of the DL-SCH HARQ state followed by resetting

After recovery the SIB CRC should be 0.

The UE performs these decoding steps in a loop for each occurrence of a subframe carrying SIB1 in the received signal. As already mentioned, SIB1 is transmitted in subframe 5 of every even frame, so the UE checks the input signal to establish that at least one occurrence of SIB1 is present. For each SIB1 subframe, the UE plots the channel estimate magnitude response and the constellation of the received PDCCH.