3GPP Rel-16: 2-step RACH Procedure

A random-access channel (RACH) is so named because it refers to a wireless channel (medium) that may be shared by multiple UEs and used by the UEs to (randomly) access the network for communications. Given below are some of the use cases of RACH in wireless communication system.

  1. RACH is used for call setup and to access the network for data transmissions.
  2. RACH is used for initial access to a network when the UE switches from a radio resource control (RRC) connected idle mode to active mode, or when handing over in RRC connected mode. Moreover, RACH may be used for downlink (DL) and/or uplink (UL) data arrival when the UE is in RRC idle or RRC inactive modes, and when re-establishing a connection with the network.
  3. RACH is used to request uplink scheduling if no dedicated scheduling-request resource has been assigned to UE.

3GPP release 15 defines a four step RACH procedure. Given below is an example four-step RACH procedure.

  1. A first message (MSG1) is sent from the UE to gNB on the physical random-access channel (PRACH). MSG1 includes a RACH preamble. This preamble is designed such that it can be detected even when there is lack of accurate timing information.
  2. gNB responds with a random-access response (RAR) message (MSG2) which includes the identifier (ID) of the RACH preamble, a timing advance (TA), an uplink grant, cell radio network temporary identifier (C-RNTI), and a back off indicator. MSG2 includes a PDCCH communication including control information for a following communication on the PDSCH.
  3. In response to MSG2, MSG3 is transmitted from the UE to gNB on the PUSCH. MSG3 includes a RRC connection request, a tracking area update, and a scheduling request.
  4. The gNB then responds with MSG4 which includes a contention resolution message.

The 3GPP in Release-16 has named legacy Random access procedure as 4-step RA type or Type-1 RA procedure and the new 2-step RA procedure as 2-step RA type or Type-2 RA procedure. As the name implies, the two-step RACH procedure effectively “collapses” the four messages of the four-step RACH procedure into two messages. Below given is an example two-step RACH procedure.

  1. A first enhanced message (msgA) is sent from the UE to gNB. In this case, msgA includes a RACH preamble for random access and a payload over PUSCH, which effectively combines MSG1 and MSG3 described above. The msgA payload, for example, includes the UE-ID and other signaling information (e.g., buffer status report (BSR)) or scheduling request (SR). This message is transmitted repeatedly with step-wise increased power until a response (msgB) is received.
  2. gNB responds with a random access response (RAR) message (msgB) which effectively combines MSG2 and MSG4 described above. For example, msgB includes the ID of the RACH preamble, a timing advance (TA), a back off indicator, a contention resolution messages, UL/DL grant, and a transmit power control (TPC) commands.

Given below are benefits of 2-step RACH procedure

  1. Benefit of 2-step RACH procedure is seen when msgA is detected by gNB quickly without repeated transmissions. otherwise there is an additional overhead (in comparison with 4-step procedure) of transmitting MsgA payload.
  2. Also, in case of operation in unlicensed spectrum, collapsed RACH procedure implies a reduced number of LBT (Listen Before Talk) operations with a corresponding reduction in overhead and delay.
  3. 2-step RACH requires less UE processing compared to 4-step RACH. Hence it has the benefit of power saving especially if a UE is under the scenario with small data traffic which requires the UE to wake up and transmit data intermittently.

Ref:

  1. Power Saving Techniques for 5G and Beyond: https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=9112193
  2. TWO-STEP RANDOM ACCESS CHANNEL (RACH) PROCEDURE TO FOUR-STEP RACH PROCEDURE FALLBACK United States Patent Application 2020012071

Cheers

~~Dheeraj