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
- Calculating Radnom value
- s_id = 3, t_id = 4, f_id = 2,ul_carrier_id=0
- RA-RNTI= 1 + 3 + 14 × 4 + 14 × 80 × 1 + 14 × 80 × 8 × 0
- RA-RNTI = 4 + 56 + 1120 + 0 = 1180
Reference
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