Highlights of Random Access Procedure in LTE:
- Contention based access and Non-contention based
- Function of the MAC Layer
- Provide Timing Advance for UL transmission to the UE
- Resource Assignment for transmission of AS message on UL-SCH
Contention Resolution:
To avoid two or more UEs trying to access the RACH channel, the LTE design provides upto 64 different RACH configurations which the UEs in a cell can use. The configuration ID to be used to transmit the RACH Preamble is determined by the higher layer. The algorithm which computes the configuration index to be used in upcoming RACH transmission should be random enough to minimise the chances of collision.Timing Advance for UL transmission to the UE
The BCH provides the Frame and Slot synchronisation but does not provide the round trip delay. The UE must know how much early it should transmit in UL to compensate the trip delay, so that the transmission arrives at the slot boundary at eNB. This timing advance is provided by the eNB in the Random Access Response.The UE must do a Random Access when in RRC Connected Mode, the time for synchronisation expires.
Resource Assignment for transmission of AS message on UL-SCH
The Random Access Response message provides the UL-grant for transmission of the AS message. The UE utilises the UL-grant to send a AS message.Random Access Procedure - Contention based
Figure: Contention based RA Procedure
(1) UE selects a P-RACH configuration ID(out of 64) to be used in P-RACH transmission of the RA Preamble. The RA Preamble is computed and transmitted for the corresponding RA-RNTI.
If there is no response from the network, the UE re attempts with higher transmission power.
Note: the Random Access RNTI(RA-RNTI) is used by the UE to identify the DL-SCH transmission destined for it on the DL-SCH channel.
(2) eNB sends a Radom Access Response on the DL-SCH, using the RA-RNTI sent by the UE earlier. It sends the Timing Advance Info, UL grant for the UE on UL-SCH and a temporary C-RNTI. temp C-RNTI is used by the UE for UL-SCH scheduled tx.
(3) The UE uses the UL grant on UL-SCH to transmit the Access Stratum Message(e.g. RRC Connection Setup Request) along with the temp C-RNTI to identify itself.
The UE can indicate if it requires more resources in the UL using the MAC-Buffer Status Report(BSR).
(4) The eNB sends the Contention Resolution message on the DL-SCH using the temp C-RNTI. This message contains the C-RNTI for the UE. After the UE receives the contention Reslution, it replaces temp-CRNTI with this CRNTI and uses it for all future UL transmissions.
The contention based RA Procedure is used for:
- transition from RRC_IDLE to RRC Connected(RRC Connection Establishment)
- in RRC_CONNECTED when UL data has to be sent and UL Synchronisation state is “non-synchronised”
Note: the UL synchronisation state is changed to non-synchronised after the expiry of the TimeAlignmentTimer. At this point a UL Time Alignment must be done using RA Procedure.
Random Access Procedure - Non Contention based
The non-contention based Random Access procedure is initiated by the network.
Figure: Non-Contention based RA Procedure
(1) The network sends the RA Preamble Assignment Info to the UE on DL-SCH(using the C-RNTI)
(2) UE sends the assgined RA Preamble on the assigned P-RACH configuration ID
(3) Network sends the Random Access Response with Timing Advance Info.
The non-contention based RA Procedure is used by the network for:
- When timing advance is required for UE positioning.
- in RRC_CONNECTED when DL data has to be sent and RA Procedure is required.
- Inter system or intra LTE Handover