All About RACH Contention Resolution Procedure

All About RACH Contention Resolution Procedure:

- RACH procedure can be Contention Based and Non-Contention Based.

Contention Based RACH Procedure:

-  In Contention based procedure UE selects a Random Access Resource i.e. UE selecting a PRACH resource, a Preamble Sequence and the next available Subframe for PRACH transmission.

- There are two types of contention based RACH procedure depending on MSG3 i.e. initial layer 3 message.

- For RRC Connection Establishment and RRC Connection Reestablishment the MSG3 is transferred on CCCH logical channel i.e. initial layer 3 message is a CCCH SDU. 
- Contention Resolution is based upon the reception of Contention Resolution Identity MAC CE. In this case a new CRNTI is allocated to the UE.
-  If ( CCCH SDU was included in Msg3 ) && If ( PDCCH transmission is addressed to its Temporary C-RNTI ) && If ( The MAC PDU is successfully decoded ) && If ( MAC PDU contains a UE Contention Resolution Identity MAC control element ) 
{
      If ( UE Contention Resolution Identity included in the MAC control element == CCCH SDU transmitted in Msg3) 
                    Then Consider Contention Resolution successful.
}

- For  Intra-System Hand Over and Uplink/Downlink data arrives while UE is in Non-synchronized RRC Connected State the MSG3 is transferred using the DCCH logical channel and contention resolution is based upon the reception of PDCCH on already allocated C-RNTI.

RACH Sequence

Non Contention Based RACH Procedure:
- ra-PreambleIndex (Random Access Preamble) and ra-PRACH-MaskIndex (PRACH Mask Index) explicitly signalled and and ra-PreambleIndex not equal to 000000.
- During Intra System HO(RRC Connection Reconfiguration with Mobility Control Info) or PDCCH order will provide the information about  ra-PreambleIndex and ra-PRACH-MaskIndex.