2. SIB2 (SYSTEMINFORMATIONBLOCKTYPE2)
The SystemInformationBlockType2 contains radio resource
configuration information ( RadioResourceConfigCommonSIB )
which are used to specify the Random Access Parameters. (Sequence
shown bellow)
RACH-ConfigCommon is used to specify the generic ‘random
access parameters’, which has preambleInfo,
powerRampingParameters, ra-SupervisionInfo & maxHARQ-Msg3Tx.
PRACH( Physical Random Access Channel): is used to transfer the
Random Access Preamble, which is used to initiate the RAP.
PRACH-ConfigSIB includes rootSequenceIndex & prach-
ConfigIndex (Sequence shown bellow)
3. SIB2 (SYSTEMINFORMATIONBLOCKTYPE2) CONTINUED…
Before the Random Access Procedure is initiated, the following
info is to be available
The Available set of PRACH resources for the transmission of the Random
Access Preamble, prach-ConfigIndex ( to know more click)
The groups of Random Access Preambles and the set of available Random
Access Preambles in each group-A and group-B are calculated from the
parameters numberOfRA-Preambles and sizeOfRA-PreamblesGroupA.
RA response window size ra-ResponseWindowSize.
Power-ramping factor powerRampingStep
Initial preamble power preambleInitialReceivedTargetPower
Maximum number of Msg3 HARQ transmissions maxHARQ-Msg3Tx
Contention Resolution Timer mac-ContentionResolutionTimer
To see the SIB2-RACH-config Sequences Click-here
4. RACH Transport channel
The RACH Transport channel is used to transfer Random Access Preamble
control information between the MAC & Physical layers.
The RACH transport channel for does not transfer any higher layer
messages (Like RRC signalling/Application data), the UL-sch & DL-SCH are used to
transfer RRC signalling & application data.
The Physical layer is responsible for using the control information to
calculate the PRACH transmit power, selecting a preamble sequence and
transmitting the preamble using indicated PRACH resources.
The structure of the Random Access Preamble shown in fig:
5. WHEN RAP IS REQUIRED
The RAP is to initiate a call by setting up a connection from UE to
eNb,
following are 6 cases the RAP triggers.
Initial access from RRC_IDLE
RRC Connection Re-establishment procedure
Handover
DL data arrival during RRC_CONNECTED requiring random access
procedure, E.g. when UL synchronisation status is “non-synchronised”
UL data arrival during RRC_CONNECTED requiring random access
procedure, E.g. when UL synchronisation status is "non-synchronised" or
there are no PUCCH resources for SR available.
For positioning purpose during RRC_CONNECTED requiring random
access procedure, E.g. when timing advance is needed for UE positioning
6. Two types of RACH process
RAP can be either,
Contension based
Non-contension based
The Contension based involves the UE selecting Random
access resource and can be used for all Random access reasons,
whereas the Contension free involves the eNb allocating the
Random access resource and can be used for Intra-system
Handover.
7. Contension based RAP
Contension based procedure starts with the UE selecting a set of
resources for the PRACH in terms of a preamble sequence (The
preamble sequence is used to differentiate b/w multiple UE using the same subframe)
Contension based RACH process consist of 4-steps:
UE->eNb: Preamble Transmission (RA-RNTI, indication for Msg3 size)
UE<-eNb: RAR response (Timing Advance, T_C-RNTI, UL grant for Msg3)
UE->eNb: Msg3 Transmission (Message for early contention resolution)
UE<-eNb: Contension Resolution (Collision-free)
In this situation there is a probability of Preamble collisions, meaning
two or more UE’s are transmitting the same preamble sequence in
the same Frequency and Time resource.
Click here for fig:
8. Non-Contension based RAP
To initiate the Non-contension RACH process, UE should be in
RRC_Connected mode before the RACH process in Handover case.
In this case, eNb informs each of the UE of exactly when and which
Preamble sequence it has to use, so that it would not collide.
Non-Contension based RACH process consist of 3-steps:
UE<-eNb: RACH Preamble Assignment
UE->eNb: Preamble Transmission (RA-RNTI, indication for Msg3 size)
UE<-eNb: RAR (Timing Advance, T_C-RNTI, UL grant for Msg3)
This Procedure is simpler than Contension-based, less time
consuming, Preamble collisions are absent & contension resolution
is not needed.
Short delay, which is important for Handover & also when UE’s traffic
needs to be resumed.
9. RA Preamble
UE transmits the PRACH Preamble (RA-RNTI, indication for Msg3 size)
It occupies the number of subframes in time domain depends on the
Preamble format selected out of 4-formats (0,1,2 & 3) and 839
subcarriers in the frequency domain
the UE selects one of the preamble sequence out of 64-preamble
sequences which are seperated as GroupA & GroupB.
After transmitting the PRACH preamble, the UE searches for a
response during the time domain window defined by the RA
response window. The RA response window starts during the third
subframe after the preamble (response window size is collects from
SIB2).
Structure of Preamble shown bellow:
10. Contension Resolution
Contention Resolution is based on either C-RNTI on PDCCH or UE
Contention Resolution Identity (CRI) on DL-SCH
Once the Msg3 transmitted, UE starts mac-ContentionResolutionTimer
(broadcast by SIB2). If the UE does not receive a response from eNb before
the Timer expires then the UE returns to transmitting PRACH preamble.
If UE sent Msg3 using the CCCH then contention resolution is based upon
the eNb responding with a UE CRI MAC control element. The UE searches
for a PDCCH addressed to T-CRNTI.
If the UE manages to successfully decode the MAC PDU and identify own
Msg3 then contension resolution and RAP is successful and T-CRNTI
becomes the C-RNTI.
If the UE fails to decode the MAC PDU, then contention resolution fails and
UE returns to Transmitting PRACH preambles.
