In this chapter we will look into Rel-15 RACH procedure in brief. In upcoming chapters we shall have a look indetail about the message contents.
WKT UE will perform Cell Search and SSB decoding for downlink synchronization.
UE will perform RACH procedure for uplink synchronization and to obtain resource for MSG3 (RRC Connection Request). Refer previous chapter for the reasons to UE to perform RACH procedure.
There are 2 types of RACH procedure:
1. CBRA: Contention Based Rach Procedure
2. CFRA: Contention Free Rach Procedure.
1. Contention Based Rach Procedure:
In this type, a UE will select a preamle randomly. There is a chance of other UEs can also select the same preamble. So there are chances of conflict. So there will be an additional step to resolve the conflict.
MSG1: UE to gNB
After UE receive SSB and select a cell to latch on. It will check for RMSI to get the PRACH configuration. “RACH-ConfigCommon” parameter is sent by gNB in SIB1 to provide RACH related information to UE.
Then UE will select a Preamble and send Random Access Preamble Transmission over PRACH Physical Random Access Channel. This is also called as MSG1.
MSG2: Random Access Response from gNB. gNB to UE
Once gNB recieves a preamble, it will allocate uplink and downlink scheduling resources.
gNB then sends an RA-Preamble identifier, timing alignment information, initial uplink grant, and Temporary C-RNTI over PDSCH.
A single PDSCH can carry RA response to multiple UEs. UE will monitor PDCCH for RA response.
If a UE revieves a response it will check for RA-Preamble identifier, if it is the same as in transmitted RA Preamble, then RACH process is successful and then UE transmits uplink scheduling information. If 2 UEs sent the same RA-Preamble, both of the UE will send scheduling info to gNB.
In MSG4, actual contention resolution will occur.
If the UE did not receive a response within the RA response windlow, then the UE will start from step 1 again.
MSG3: Scheduled UL Transmission. UE to gNB
UE will send uplink scheduling information over PUSCH.
It can be sent in “InitialRRCConnectionSetup” or “RRC Connection Reestablishment”
MSG4: Contention Resolution. gNB to UE
If both of the UE will send the same preamble, gNB will resolve the conflit in this step.
After MSG3 is sent by UE, it will monitor PDCCH till the timer expires(4ms) and will stop the timer if it recieves below messages:
1. UE will get C-RNTI over PDCCH.
2. UE will get TC-RNTI and MAC PDU is successfully decoded.
If the contention resolution timer expiers, then it means RACH procedure has failed and UE need to perform RACH procedure again.
2. Contention Free Rach Procedure:
In non contention based Random Access, the preamble is allocated by gNB and those preamble are used only by gNB and is not usable by any UEs.
These dedicated preamble is provided to UE by RRC or PHY layer signalling. Hence there will be no conflict.