Once the RACH configuration and preambles are selected, it is to be sent out to the available PRACH occation.
For that RA-RNTI is to be calculated according to the PRACH occation.
In this chapter we shall see:
1. What is the power required to send Msg1 preamble ?
2. How to calculate RA-RNTI that is unique per UE ?
1. What is the power required to send Msg1 preamble ?
TS 38 321 gives the below formula to calculate PRACH:
PPRACH=min{PCMAX, PPRACHtarget+PL} [dBm]
Here:
P_{PRACH,target} = PREAMBLE_RECEIVED_TARGET_POWER
According to TS 38 321, PPRACHtarget can be calculated by:
PPRACHtarget= preambleReceivedTargetPower + Delta_Preamble + (Preamble_Transmission_Counter – 1) * powerRampingStep
Here:
-> preambleReceivedTargetPower is obtained by parameter’preambleReceivedTargetPower’
-> DELTA_PREAMBLE can be obtained from TS 38 321 tables 7.3-1 and 7.3-2, according to different preamble format to look up the table
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-> PREAMBLE_POWER_RAMPING_COUNTER is 1 for initial transmission, and the retransmission is incremented by one each time.
-> PREAMBLE_POWER_RAMPING_STEP is derived from the parameter’powerRampingStep’
Then we calculate PathLoss-PL by:
PL = referenceSignalPower – higherlayerfilteredRSRP
Example:
Consider the below config:
rach-ConfigGeneric
{
dust-ConfigurationIndex 2,
msg1-FDM one,
msg1-FrequencyStart 2,
zeroCorrelationZoneConfig 6,
preambleReceivedTargetPower -100,
preambleTransMax n10,
powerRampingStep dB4,
ra-ResponseWindow sl20
},
ss-PBCH-BlockPower -8
Assume that the RSRP is -94dBm
Then the initial transmit power of Msg1 = -100+(0)+(1-1)*4+[-8-(-94)]= -14dBm
2. How to calculate RA-RNTI that is unique per UE ?
When a UE sends Msg1, it will calcualte RA-RNTI by using below formula and save it.
After that gNB will also calculate RA-RNTI and use the RA-RNTI to Msg2 PDCCCH CRC fo DCI format 1_0 is scrambled.
Hence, only the UE with corrent RA-RNTI is able to decode the DCI of PDCCH.
RA-RNTI = 1 + s_id + 14 × t_id + 14 × 80 × f_id + 14 × 80 × 8 × ul_carrier_id
-> s_id is the index of the first OFDM symbol of the PRACH occasion (0 ≤ s_id < 14)
-> t_id is the index of the first slot of the PRACH occasion in a system frame (0 ≤ t_id < 80)
-> f_id is the index of the PRACH occasion in the frequency domain (0 ≤ f_id < 8)
-> ul_carrier_id is the UL carrier used for RA Preamble transmission (0 for NUL carrier, and 1 for SUL carrier).