# 5G NR RA-RNTI Calculation

A 5G UE picks a random preamble for RACH procedure to get the uplink synchronisation. The preamble is referenced with an ID know as  RAPID (Random Access Preamble ID). 5G preamble is also use Zadoff-Chu sequence. Each preamble transmission is associated with an RA-RNTI.

Random Access RNTI (RA-RNTI) is used during Random Access procedure, the gNB’s MAC generates Random Access Response (RAR) as a response to the Random Access Preamble transmitted by the UE. RAR is transmitted on DL-SCH transport channel which intern is mapped to PDSCH. The gNB scrambles PDCCH’s CRC with RA-RNTI for transmission of PDSCH that carries RAR(s). RA-RNTI can be addressed to multiple UEs, i.e., multiple UEs might decode PDCCH scrambled by the same.

The RA-RNTI associated with the PRACH where Random Access Preamble is transmitted can be computed follows:

• RA-RNTI= 1 + s_id + 14 × t_id + 14 × 80 × f_id + 14 × 80 × 8 × ul_carrier_id
Variable Detail
s_id Index of the first OFDM symbol of the specified PRACH (0 ≤ s_id < 14)
t_id Index of the first slot of the specified PRACH in a system frame (0 ≤ t_id < 80)
f_id Index of the specified PRACH in the frequency domain (0 ≤ f_id < 8)
ul_carrier_id Uplink carrier used for Msg1 transmission (0 for NUL carrier, and 1 for SUL carrier)

Calculating RN-RNTI

• Calculating Min value – 0x0001
• s_id = 0 , t_id = 0, f_id = 0 , ul_carrier_id=0
• RA-RNTI= 1 + 0 + 14 × 0 + 14 × 80 × 0 + 14 × 80 × 8 × 0 = 1
• Calculating Max value
• s_id = 14 , t_id = 79, f_id = 7 , ul_carrier_id=1
• RA-RNTI= 1 + 13 + 14 × 79 + 14 × 80 × 7 + 14 × 80 × 8 × 1
• RA-RNTI= 1 +13 +1106 + 7840 + 8960 = 17920