10.1.5 Random Access Procedure

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

10.1.5.0 General

The random access procedure is characterized by:

– Common procedure for FDD and TDD;

– One procedure irrespective of cell size and the number of serving cells when CA is configured;

The random access procedure is performed for the following events related to the PCell:

– Initial access from RRC_IDLE;

– RRC Connection Re-establishment procedure, as defined in TS 24.301 [20];

– Handover, except for NB-IoT or when RACH-less HO is configured;

– 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.

The random access procedure is also performed on a SCell to establish time alignment for the corresponding sTAG.

For E-UTRA connected to 5GC, the random access procedure is also performed for the transition from RRC_INACTIVE.

In DC, the random access procedure is also performed on at least PSCell upon SCG addition/modification, if instructed, or upon DL/UL data arrival during RRC_CONNECTED requiring random access procedure. The UE initiated random access procedure is performed only on PSCell for SCG.

Furthermore, the random access procedure takes two distinct forms:

– Contention based (applicable to all six events, but the sixth event for positioning is applicable for NB-IoT only);

– Non-contention based (applicable to only handover, DL data arrival, positioning and obtaining timing advance alignment for a sTAG).

Normal DL/UL transmission can take place after the random access procedure.

An RN supports both contention-based and non-contention-based random access. When an RN performs the random access procedure, it suspends any current RN subframe configuration, meaning it temporarily disregards the RN subframe configuration. The RN subframe configuration is resumed at successful random access procedure completion.

For NB-IoT, the random access procedure is performed on the anchor carrier or on a non-anchor carrier based on system information.

10.1.5.1 Contention based random access procedure

The contention based random access procedure is outlined on Figure 10.1.5.1-1 below:

Figure 10.1.5.1-1: Contention based Random Access Procedure

The four steps of the contention based random access procedures are:

1) Random Access Preamble on RACH in uplink:

– There are two possible groups defined and one is optional. If both groups are configured the size of message 3 and the pathloss are used to determine which group a preamble is selected from. The group to which a preamble belongs provides an indication of the size of the message 3 and the radio conditions at the UE. The preamble group information along with the necessary thresholds are broadcast on system information.

2) Random Access Response generated by MAC on DL-SCH:

– Semi-synchronous (within a flexible window of which the size is one or more TTI) with message 1;

– No HARQ;

– Addressed to RA-RNTI on PDCCH;

– Conveys at least RA-preamble identifier, Timing Alignment information for the pTAG, initial UL grant and assignment of Temporary C-RNTI (which may or may not be made permanent upon Contention Resolution);

– Intended for a variable number of UEs in one DL-SCH message.

3) First scheduled UL transmission on UL-SCH:

– Uses HARQ;

– Size of the transport blocks depends on the UL grant conveyed in step 2.

– For initial access:

– Conveys the RRC Connection Request generated by the RRC layer and transmitted via CCCH;

– Conveys at least NAS UE identifier but no NAS message;

– RLC TM: no segmentation.

– For RRC Connection Re-establishment procedure:

– Conveys the RRC Connection Re-establishment Request generated by the RRC layer and transmitted via CCCH;

– RLC TM: no segmentation;

– Does not contain any NAS message.

– After handover, in the target cell:

– Conveys the ciphered and integrity protected RRC Handover Confirm generated by the RRC layer and transmitted via DCCH;

– Conveys the C-RNTI of the UE (which was allocated via the Handover Command);

– Includes an uplink Buffer Status Report when possible.

– For other events:

– Conveys at least the C-RNTI of the UE;

– In the procedure to resume the RRC connection or in the EDT procedure for User Plane CIoT EPS/5GS Optimisations:

– Conveys the RRC Connection Resume Request generated by the RRC layer and transmitted via CCCH;

– Conveys a Resume ID (for EPS) or I-RNTI (for 5GS) to resume the RRC connection;

– For the MO-EDT procedure for User Plane CIoT EPS/5GS Optimisations:

– Conveys ciphered user data transmitted via DTCH;

– RLC UM/AM: no segmentation;

– Does not contain any NAS message.

– For NB-IoT:

– In the procedure to setup the RRC connection:

– An indication of the amount of data for subsequent transmission(s) on SRB or DRB can be indicated.

– For EDT for Control Plane CIoT EPS/5GS Optimisations:

– Conveys the RRC Early Data Request generated by the RRC layer and transmitted via CCCH;

– Conveys NAS UE identifier;

– For the MO-EDT procedure for Control Plane CIoT EPS/5GS Optimisations:

– Conveys user data concatenated in a NAS message;

– RLC TM: no segmentation.

4) Contention Resolution on DL:

– Early contention resolution shall be used i.e. eNB does not wait for NAS reply before resolving contention;

– For NB-IoT, for initial access, RRC connection resume procedure and RRC Connection Re-establishment procedure, eNB may transmit MAC PDU containing the UE contention resolution identity MAC control element without RRC response message;

NOTE: In Release 13, NB-IoT UEs do not support the MAC PDU containing the UE contention resolution identity MAC control element without RRC response message for initial access, RRC connection resume procedure and RRC Connection Re-establishment procedure.

– Not synchronised with message 3;

– HARQ is supported;

– Addressed to:

– The Temporary C-RNTI on PDCCH for initial access and after radio link failure;

– The C-RNTI on PDCCH for UE in RRC_CONNECTED.

– HARQ feedback is transmitted only by the UE which detects its own UE identity, as provided in message 3, echoed in the Contention Resolution message;

– For initial access, RRC Connection Re-establishment procedure and EDT for Control Plane CIoT EPS/5GS Optimisations, no segmentation is used (RLC-TM).

The Temporary C-RNTI is promoted to C-RNTI for a UE which detects RA success and does not already have a C-RNTI; it is dropped by others. A UE which detects RA success and already has a C-RNTI, resumes using its C-RNTI.

When CA is configured, the first three steps of the contention based random access procedures occur on the PCell while contention resolution (step 4) can be cross-scheduled by the PCell.

When DC is configured, the first three steps of the contention based random access procedures occur on the PCell in MCG and PSCell in SCG. When CA is configured in SCG, the first three steps of the contention based random access procedures occur on the PSCell while contention resolution (step 4) can be cross-scheduled by the PSCell.

10.1.5.2 Non-contention based random access procedure

The non-contention based random access procedure is outlined on Figure 10.1.5.2-1 below:

Figure 10.1.5.2-1: Non-contention based Random Access Procedure

The three steps of the non-contention based random access procedures are:

0) Random Access Preamble assignment via dedicated signalling in DL:

– eNB assigns to UE a non-contention Random Access Preamble (a Random Access Preamble not within the set sent in broadcast signalling).

– Signalled via:

– HO command generated by target eNB and sent via source eNB for handover;

– PDCCH in case of DL data arrival or positioning;

– PDCCH for initial UL time alignment for a sTAG.

1) Random Access Preamble on RACH in uplink:

– UE transmits the assigned non-contention Random Access Preamble.

2) Random Access Response on DL-SCH:

– Semi-synchronous (within a flexible window of which the size is two or more TTIs) with message 1;

– No HARQ;

– Addressed to RA-RNTI on PDCCH;

– Conveys at least:

– Timing Alignment information and initial UL grant for handover;

– Timing Alignment information for DL data arrival;

– RA-preamble identifier;

– Intended for one or multiple UEs in one DL-SCH message.

When performing non-contention based random access on the PCell while CA is configured, the Random Access Preamble assignment via PDCCH of step 0, step 1 and 2 of the non-contention based random access procedure occur on the PCell. In order to establish timing advance for a sTAG, the eNB may initiate a non-contention based random access procedure with a PDCCH order (step 0) that is sent on a scheduling cell of activated SCell of the sTAG. Preamble transmission (step 1) is on the indicated SCell and Random Access Response (step 2) takes place on PCell.

When performing non-contention based random access on the PCell or PSCell while DC is configured, the Random Access Preamble assignment via PDCCH of step 0, step 1 and 2 of the non-contention based random access procedure occur on the corresponding cell. In order to establish timing advance for a sTAG, the eNB may initiate a non-contention based random access procedure with a PDCCH order (step 0) that is sent on a scheduling cell of activated SCell of the sTAG not including PSCell. Preamble transmission (step 1) is on the indicated SCell and Random Access Response (step 2) takes place on PCell for MCG and PSCell for SCG.

10.1.5.3 Interaction model between L1 and L2/3 for Random Access Procedure

Random access procedure described above is modelled in Figure 10.1.5.3-1 below from L1 and L2/3 interaction point of view. L2/L3 receives indication from L1 whether ACK is received or DTX is detected after indication of Random Access Preamble transmission to L1. L2/3 indicates L1 to transmit first scheduled UL transmission (RRC Connection Request in case of initial access) if necessary or Random Access Preamble based on the indication from L1.

Figure 10.1.5.3-1: Interaction model between L1 and L2/3 for Random Access Procedure