- SIBs contain important system information that UEs need to access the cell. SIB1 schedules the transmission of other SIBs. - The types of SIBs include SIB1 to SIB13, with each SIB containing different parameters like cell access restrictions, neighbor cell info, etc. - UEs obtain SIB1 in every system information block to learn the scheduling of other SIBs, which are transmitted less frequently in other SI messages. This allows UEs to acquire the key system information needed to access the cell.

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Welcome
Warm Greetings
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SIB (System Information Block)
Types of SIB:
✓ SIB 1 : Cell Selection, Cell Access, SI Scheduling.
✓ SIB 2 : RACH, Access Barring, UL frequency Information, MBSFN Config.
✓ SIB 3 : Intra Frequency Cell Reselection.
✓ SIB 4 : Intra Frequency Neighbour Cell.
✓ SIB 5 : Inter Frequency Neighbour Cell.
✓ SIB 6 : UTRAN Neighbour Cell.
✓ SIB 7 : GERAN Neighbour Cell.
SIBs other than SIB1 are carried in SI messages and mapping
of SIBs to SI is done by “schedulingInfoList” List include in SIB1.

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SIB (System Information Block)
System Information Block Type1 (SIB 1)
✓ SIB1 : It contains information relevant when evaluating if a UE is allowed to access a cell and defines the scheduling
of other system information.
✓ Its periodicity is of 80 ms and repetitions made within 80 ms.
plmn-IdentityList : List of PLMN identities.
sib-MappingInfo : List of the SIBs mapped to this SI message. There is no mapping information of SIB2, it is always present in the first
SI message listed in schedulingInfoList list.
si-Periodicity : Periodicity of the SI-message in radio frames.
Si-WindowLength: Common SI scheduling window for all SI’s.
trackingAreaCode : A trackingAreaCode that is common for all the PLMNs listed
systemInfoValueTag : indicates if a change has occurred in the SI messages.
schedulingInfoList : The transmission cycles for other SIBs are determined
Other fields are also there like category0Allowed, cellBarred, cellReservedForOperatorUse, csg-Identity, csg-
Indication, freqBandIndicatorPriority, p-Max, q-QualMin etc.

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SIB1 Logs

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SIB (System Information Block)
System Information Block Type2 (SIB 2)
✓ There is no mapping information of SIB2
✓ It is always present in the first SystemInformation message listed in the schedulingInfoList list.
Parameter Description
ac-BarringInfo Access Class Barring configuration
radioResourceConfig used to specify common radio resource configurations in the system information and in the mobility control information
numberOfRA_Preambles Number of non-dedicated random access preambles
Preambleintialreceivedtargetpower
PowerRampingstep
preamblesGroupAConfig_exist Provides the configuration for preamble grouping. If the field is not signalled, the size of the random access preambles group A is equal to numberOfRA-Preambles
preambleTransMax Maximum number of preamble transmission
ra_ResponseWindowSize Duration of the RA response window. Value in subframes. Value sf2 corresponds to 2 subframes
mac_ContentionResolutionTimer Timer for contention resolution. Value in subframes. Value sf8 corresponds to 8 subframes
maxHARQ_Msg3Tx 4 Maximum number of Msg3 HARQ transmissions, used for contention based random access
prach_Config used to specify the PRACH configuration in the system information and in the mobility control information
prach_ConfigIndex Mentions Preamble format, SFN,subframe number

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SIB2 Logs

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SIB (System Information Block)
System Information Block Type3 (SIB 3)
Direction: E-UTRAN => UE
RLC Mode: TM
Logical Channel: BCCH
Transport Channel: DL-SCH
✓ The SIB3 contains cell re-selection information common for intra-frequency, inter-frequency and/or inter-RAT cell
re-selection.
✓ SIB3 also contains cell reselection priority information for the concerned carrier frequency or a set of frequencies

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SIB (System Information Block)
System Information Block Type3 (SIB 3)

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Cell Selection Criteria
UE would not start registration even though it successfully detected a cell and even decoded MIB and SIBs unless
the Srxleve > 0 and Squal > 0.

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Cell Selection Criteria
q-RxLevMin
The IE q-RxLevMin is used to indicate for cell re-selection the required minimum received RSRP level in the (EUTRA)
cell. Corresponds to parameter Qrxlevmin in 36.304 [4]. Actual value Qrxlevmin = IE value * 2 [dBm].
q-RxLevMinOffset
Parameter Qrxlevminoffset in 36.304 [4]. Actual value Qrxlevminoffset = IE value * 2 [dB]. If absent, apply the (default) value of 0 [dB] for Qrxlevminoffset.
Affects the minimum required Rx level in the cell.
q-QualMin
Parameter 'Qqualmin' in TS 36.304 [4]. If cellSelectionInfo-v920 is not present, the UE applies the (default) value of
negative infinity for Qqualmin.
q-QualMinOffset
Parameter 'Qqualminoffset' in TS 36.304 [4]. Actual value Qqualminoffset = IE value [dB]. If cellSelectionInfo-v920 is not present or the field is not present, the
UE applies the (default) value of 0 dB for Qqualminoffset. Affects the minimum required quality level in the cell.

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Cell Selection Criteria

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Random Access Procedure in LTE
RACH stands for Random Access Channel.
This is the first message from UE to eNB when you power it on.
✓To Achieve Uplink synchronization between UE and eNB.
✓ To Obtain the resource for RRC Connection Request (Message 3).
The next question comes in mind is “ When RACH “ ?
The answer is during below occasions as per 3gpp spec
(1) A UE in RRC_CONNECTED state, but not uplink-synchronized, needing to send new uplink data or control
information.
(2) A UE in RRC_CONNECTED state, but not uplink-synchronized, needing to receive new downlink data, and
therefore to transmit corresponding ACK/NACK in the uplink.
(3) A UE in RRC_CONNECTED state, handing over from its current serving cell to a target cell.
(4) A transition from RRC_IDLE state to RRC_CONNECTED, for example for initial access or tracking area updates.
(5) Recovering from radio link failure.
(6) When UE does not have any PUCCH resources available for SR(Scheduling Request)

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Random Access Procedure in LTE
There are two types of RACH Procedure :
1. Contention Based RACH Procedure (CBRA).
2. Non-Contention Based RACH Procedure /Contention free RACH Procedure (CFRA).
Contention Based RACH Procedure (CBRA)
Step 1: Preamble transmission.
Step 2: Random access response.
Step 3: Layer 2 / Layer 3 (L2/L3) message.
Step 4: Contention resolution message.
Non-Contention Based RACH Procedure /Contention free RACH Procedure (CFRA)
(applicable to only handover and DL data arrival)
✓Synchronization initiated by the eNB, using dedicated preambles during handovers
✓Up to 1 group of reserved random-access preambles
✓Unpredictable latency of the random access procedure can be circumvented for some use cases where low latency is
required, such as handover and resumption of downlink traffic for a UE, by allocating a dedicated signature to the UE on a
per-need basis.

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Random Access Procedure in LTE
RACH related parameter is obtained by UE by reading SIB-2 Parameters

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Random Access Procedure in LTE
RACH Preambles have 4 possible format
RACH Preambles
✓ There are total 64 preambles for each cell.
✓ Preamble available are divided into 2 groups.(Group A and Group B)
✓ Total 6PRBs are used to send RACH Preamble
https://youtu.be/nT0OfKc84Zw

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Random Access Procedure in LTE
Preamble transmission: The UE selects one of the 64 − Ncf available PRACH contention-based signatures, where Ncf is the
number of signatures reserved by the eNodeB for contention-free RACH. Signatures is further subdivided into two
subgroups.
numberofRaPreambles :- eNodeB sends this value in SIB2 which denotes the total number of preambles available for UE to
send a Rach Request.
sizeOfRaPreamblesGroupA :- It represents the number of preambles available within Group A.
So number of preamble in Group B = numberofRaPreambles - sizeOfRaPreamblesGroupA
MessageSizeGroupA :- It is used for selecting a preamble from a group. Its value is in bits.
Now UE needs to decide the group from which it needs the preamble. Group is decided on the basis of size of MSG3( RRC
connection request ).
If Msg3 size > messageSizeGroupA , preamble will be selected from GroupBelse preamble will be selected from Group A

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How to Generate 64 PRACH Preamble Sequences ?
The maximum number of PRACH Sequence a UE can use in a cell is 64.
SIB2 broadcast the parameters as follows.
a) rootSequenceindex
b) Highspeedflag
c) zeroCorrelationZoneConfig
✓ zero correlation zone configuration, which determines the number of cyclic shifts that the mobile can generate from a
single Zadoff-Chu root sequence.
(In a large cell the mobile is only allowed to use a few widely spaced cyclic shifts, because there would otherwise be a
risk of the base station confusing one cyclic shift from a nearby mobile with a slightly different cyclic shift from a distant
one)
✓ The root sequence index identifies the first Zadoff-Chu root sequence that the cell is actually using.
✓ The mobile generates as many cyclic shifts from this root sequence as the zero correlation zone configuration allows,
moves to the root sequences that follow and continues until it has generated all 64 preambles.
✓ The calculations are modified in cells that contain fast moving mobiles because there is a risk that the base station will
confuse one cyclic shift from a slow moving mobile with another from a fast moving one.
✓ The base station can deal with the problem by setting a high speed flag, which restricts the number of cyclic shifts that
are available in such cells.

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Resource Element Mapping
✓ In the frequency domain, a PRACH transmission has a bandwidth of six resource blocks.
✓ In the time domain, the transmission is usually one subframe long, but it can be longer or shorter.
✓ PRACH transmission comprises a cyclic prefix, a preamble sequence and a guard period.
✓ Preamble sequence contains one or two PRACH symbols, which are usually 800 μs long.

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PRACH Config Index
✓ The base station reserves specific resource blocks for the PRACH using
two parameters that it advertises in SIB 2, namely the PRACH
configuration index and the PRACH frequency offset.
✓ The table would give you at which frame and subframe that UE is
allowed to transmit a PRACH Preamble.
✓ PRACH configuration index specifies the preamble format and the
subframes in which random access transmissions can begin, while the
PRACH frequency offset specifies their location in the frequency
Domain.
✓ for example, if the PRACH configuration index is 5, which supports
PRACH transmissions in subframe 7 of every frame using preamble
format 0.
✓ And if the PRACH frequency offset is 7, so the transmissions
occupy resource blocks 7 to 12 in the frequency domain.

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LTE Random Access Procedure
The UE randomly selects an RA preamble
Which contain RA-RNTI and transmit on PRACH
How UE decides the Power used for Rach request Transmission:-
Now UE need to decide the power which will be used for RACH
Request transmission. Power is decided on the factors received in
SIB2 as:-
preambleInitialReceivedtargetPower:- Power factor which will be
used for first transmission of Rach Request.Value varies from -
120dBm to -90 dBm .
powerRampingStep:- This is mainly used when eNodeB is not able to
detect the Rach Request then UE will re transmit the RACH Request
by increasing the power to powerRampingStep factor.
power used for Rach Request transmission =
preambleInitialReceivedTargetPower + DELTA_PREAMBLE +
(PREAMBLE_TRANSMISSION_COUNTER – 1) * powerRampingStep
DELTA_PREAMBLE = This is preamble format based delta offset.
There are four formats available for preamble which are called as
preamble formats.
MSG 1

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✓ The eNodeB transmits the RA Response on the DL-SCH channel
✓ Derives RA-RNTI
✓ Calculate TC-RNTI Calculate Timing advance
✓ Uplink Resource Grant
✓ Hoping flag
✓ MCS
✓ back off indicator MAC header
Timing alignment to synchronize subsequent uplink transmissions from the UE.
An initial uplink resource grant for transmission of the Step 3 message.
An assignment of a Temporary Cell Radio Network Temporary Identifier (C-RNTI)
Backoff Indicator-which the eNodeB can set to instruct the UE to back off for a period of time before retrying a
random access attempt.
MSG 2

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✓ UE saves TC-RNTI from RAR.
✓ Channel –UL-SCH
✓ UE does not have a permanent identity till now, so it will have a random number as the UE
identity.
✓ The random UE identity is included in the RRC connection request.
✓ UE starts the T300 timer.
RRC Connection Request
MSG 3

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✓ UE receive the RRC Connection Setup message in MSG4.
✓ RRC Connection Setup message carries CRNTI for further communication with network.
✓ Channel –DL-SCH
✓ Contention resolution
MSG 4

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RRC Connection setup
RRC Connection setup

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UE will send the RRC Connection Setup message
to initiate further signaling
RRC Connection setup Complete
MSG 5

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Interview Question
1. What are the types of SIB?
2. What if MIB and SIB incorrectly set? - all the other SIBs will not be decoded by UE. And as a result, UE would not recognize the cell and
show "No Service" message.
3. Where we get cell barred and restrict information?
4. In brief describe (main parameters) what does UE get in SIB1?
5. In brief describe what does UE get in SIB?
6. Where is MIB and SIB1 mapped in frame?
7. Describe how do you get SIB2 to SIB13 info and periodicity?
8. Why RACH needed?
9. What is CBRA and CFRA?
10. How to select preamble whether in Group A or Group B? that depends on size of msg-3. If size of grpA given in sib-2 is greater than
msg3 bit size, grpB is used.Group B is used during HO..and Group A is during intial RACH process.
11. what is the triggering point for rach procedure..means how UE knows that it has to initiate RACH procedure ?
1.During initial attach. Attach request if UE is not registered to network 2.During Sr failure when sr crossed more than transmax(send by rrc)
3.pdcch order(enodeb initiate rach by sending dci when ue loss uplink sunchronization) 4.Handover(non contention based) 5. uplink
synchronization 6. TAU procedure if UE is attached to network and cell is changed 7. UE is in idle mode and wants to initiate some service(Data,
Call etc) 8.RACH procedure is triggered when UE is camped on to a cell and any of the below conditions happens: UE requires UL synchronization
with eNB for sending UL data UE is moving from idle to connected or Handover to other cell (Target eNB may/may not provide dedicated preamble
through rrc reconfiguration message of source eNB).
9. eNB receives DL data but finds UL synchronization with UE is lost (PDCCH order). eNB notifies UE to do PRACH by sending DCI 1A with/without
a dedicated preamble info. 10.UE doesn't have any grant to transmit UL Data and finds PUCCH resources are not configured/released for
transmission of SR eNB doesn't configure UE with any PUCCH resources for SR transmission or When max-SR transmission have reached, UE
releases PUCCH resources or Time alignment timer expiry, UE releases PUCCH resources

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Interview Question
12. UE fail to receive Rach response, how many times UE have to retry?
If UE did not get RAR then UE retry with pawer ramping with 2dbm And Max transmission is 6 But UE retry those 6 time retransmission and not get
RAR response Rach failure will happen and UE has to try again with new preamble. RACH failure may be due to,suppose when two UE's sent
PRACH ,if both ue ll rcv same T-CRNTI & Resources within same time n freq to nw then might be chance of collision or interference. So n/w could
on So n/w could only decode msg from only one ue & discard the other ue. After max_trans UE will trigger rach request on 2nd best cell if available
on same PLMN, if not than UE will go into FORBIDDEN plmn state.
13. What if when TWO UE having same preamble same location and sending same responses to Enb? What is the Enb
behavior?
14. RAR Failure Reasons?
If UE fails to receive RAR within RA-Response window or if received RAR doesn't contain the transmitted preamble index in any of the RAPID's,
RACH procedure is considered as unsuccessfulincrement PREAMBLE_TRANSMISSION_COUNT by 1 if PREAMBLE_TRANSMISSION_COUNT is
PreambleTransMax +1, indicate RACH problem to upper layers. if the preamble was selected by UE, UE will select a random back off time from 0
to back off parameter before going to retry RACH procedure.
Is eNB sending RACH MSG2 (if so check the RAPID correlate it with that of MSG1’s) also check if UE decoding SIB2 properly or not (some imp
params are raResponseWindowSize, PRACH Configuration Index.) .Please check RA Window Size, After sending the RACH MSG 1 UE monitors
PDCCH for certain duration i.e.,3 subframes (after transmission of MSG1 ) + raResponseWindowSize subframes (can be found from SIB2)

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Contact Details
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