HOW RA-RNTI Calculated in RACH Procedure

In cellular networks, RA-RNTI plays a crucial role in establishing communication between a UE and the eNB. Let's delve deeper into how RA-RNTI is calculated and utilized in the random-access procedure.

What is RA-RNTI?

?RA-RNTI is a temporary identifier assigned to a UE during the random-access process.

It facilitates the eNB to direct its response (Random Access Response - RAR) specifically to the UE that initiated the random-access request.

How is RA-RNTI Calculated?

The calculation of RA-RNTI relies on two key parameters (reference to TS36.321 (Section 5.1.4):

• t_id: Index of the first subframe within the designated PRACH occasion (0 ≤ t_id ≤ 9).
• f_id: Index of the specific PRACH within the subframe, based on ascending frequency domain order (0 ≤ f_id ≤ 5).

Here's the formula:

RA-RNTI = 1 + t_id + 10 * f_id

Important Points:

• In Frequency Division Duplex (FDD) mode, f_id is fixed at 0.
• This implies that the RA-RNTI value is essentially determined by the subframe number during which the UE transmits the Random-Access Preamble (Msg1).
• In simpler terms, the subframe number (between 0000 and 0009) incremented by 1 corresponds to the RA-RNTI.

I hope this explanation clarifies the concept of RA-RNTI in the RACH procedure. Feel free to discuss further or share your experiences in the comments below!

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