PRACH

By

Nagesh (nageshgowdahere@gmail.com)

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)

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

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:

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

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.

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:

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.

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:

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.

SIB2-RACH-PARAMS

http://lteuniversity.com/get_trained/expert_opinion1/b/lauroortigoza/archive/2012/03/21/rach-capacity-part-1-of-2.aspx)

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