In this chapter we shall see where the UE can get the resources for PRACH transmission.
Before UE can send a PRACH, it needs to select an SSB then UE will selects an SSB with SS-RSRP above rsrp-ThresholdSSB for PRACH transmission.
For normal RACH procedure, UE will get the configuration from SIB1. But there are special scenarios like beam failure recovery and On Demand SI that needs to be looked into.
Different scenarios for Random Access Resource selection:
1. Random Access resource selection for BFR.
2. Random Access resource selection for SI request (On-Demand SI).
3. Random Access Resource Selection for Reconfiguration with Sync.
4. Random Access resource selection for PDCCH-Order.
In further chapters we shall see in detail on the messages sent and received.
RACH can be triggered by MAC/ RRC/ PDCCH Order. Below table shows the trigger.
UE to initiate RACH procedure, it needs to know the following:
1. The RACH occasions available per SSB
2. Number of available Contention based preambles
3. Preamble format to use
4. Frequency Domain Resources
5. Time Domain Resources
6. Initial Power for PRACH transmission.
“rach-ConfigCommon” IE is used to provide RACH related information through SIB1.
1. Random Access resource selection for BFR. (MAC layer trigger)
1. For CBRA, it will take the RACH config from SIB1.
2. For CFRA, “BeamFailureRecoveryConfig” IE from RRC is used:
rach-ConfigBFR: It provides the config for contention free random access occasions for BFR.
rsrp-ThresholdSSBL1-RSRP: It gives the threshold to be used for determining whether a candidate beam may be used by the UE to attempt contention free random access to recover from beam failure (see TS 38.213 [13], clause 6).
candidateBeamRSList: UE may be configured with a set of a candidate beams with this list.
2. Random Access resource selection for SI request (On-Demand SI). (RRC Layer Trigger)
UE in RRC_IDLE and RRC_INACTIVE, a request for Other SI triggers an RA procedure.
If the network configured the UE with PRACH resources for SI request, CFRA is used, else CBRA is used.
Network can configure UE with dedicated RACH resources for SI request with “SI-RequestConfig” that is transmitted in SI-SchedulingInfo IE (in SIB1).
3. Random Access Resource Selection for Reconfiguration with Sync.
For recinfigurationwithsync, RA configuration will be provided in “rach-ConfigDedicated” of “ReconfigurationWithSync” IE.
cfra: This field provides the required parameters for CFRA to a given target cell.
4. Random Access resource selection for PDCCH-Order.
For uplink out-of-synchronization, RACH will be initiated by PDCCH order.
When the gNB has downlink data to send, it will use PDCCH Order to force the UR to initiate RACH and to complete uplink time synchronization.
PDCCH Order uses PDCCH DCI 1_0 to configure to UE
TS 38.212 defines the below:
If the CRC of the DCI format 1_0 is scrambled by C-RNTI and the “Frequency domain resource assignment” field are of all ones, the DCI format 1_0 is for random access procedure initiated by a PDCCH order, with all remaining fields set as follows
-> Random Access Preamble index – 6 bits
-> UL / SUL indicator – 1 bit
-> SS/PBCH index – 6 bits
-> PRACH Mask index – 4 bits
-> Reserved bits – 10 bits
If RA preamble index provided by PDCCH order is not equal to 0b000000, the UE performs CFRA on the SSB signalled by PDCCH order, otherwise (RA preamble index = 0b000000), the UE performs CBRA.
Brief understanding of RACH config parameters:
rach-ConfigCommon setup :
rach-ConfigGeneric
prach-ConfigurationIndex 75,
msg1-FDM one,
msg1-FrequencyStart 0,
zeroCorrelationZoneConfig 0,
preambleReceivedTargetPower -109,
preambleTransMax n10,
powerRampingStep dB4,
ra-ResponseWindow sl20
,
totalNumberOfRA-Preambles 8,
ssb-perRACH-OccasionAndCB-PreamblesPerSSB one : n8,
ra-ContentionResolutionTimer sf16,
prach-RootSequenceIndex l139 : 91,
msg1-SubcarrierSpacing kHz120,
restrictedSetConfig unrestrictedSet
prach-ConfigurationIndex: It points to the table TS 38.211 prach-ConfigurationIndex. This defines which PRACH format to be used and when to send the PRACH in Time domain and number of RACH occasions available.
msg1-FDM: The number of PRACH transmission occasion FDMed in one time instance.
msg1-FrequencyStart: This value refers to the Offset of lowest PRACH transmission occasion in frequency domain with respective to PRB 0.
preambleTransMax: Max number of RA preamble transmission performed before declaring a failure
powerRampingStep: Power ramping steps for PRACH. This value define the amount of power UE needs to increase for re attempting the RACH procedure due to failure.
ra-ResponseWindow: Msg2 (RAR) window length in number of slots. If the UE did not receive the response within the window, it will consider it as failure and start Msg1 transmission again.
totalNumberOfRA-Preambles: Total number of preambles used for contention based and contention free 4-step or 2-step random access in the RACH resources defined in RACH-ConfigCommon, excluding preambles used for other purposes.
ra-ContentionResolutionTimer: The initial value for the contention resolution timer.
msg1-SubcarrierSpacing: Subcarrier spacing of PRACH.
ssb-perRACH-OccasionAndCB-PreamblesPerSSB: The meaning of this field is twofold: the CHOICE conveys the information about the number of SSBs per RACH occasion and CB-PreamblesPerSSB indicates the number of Contention Based preambles available per SSB.