“5.1.4 Random Access Response reception" in "TS36.321” says how to calculate RA_RNTI as follows.
The RA-RNTI associated with the PRACH in which the Random Access Preamble is transmitted, is computed as:
RA-RNTI = 1 + t_id + 10 * f_id
Where t_id is the index of the first subframe of the specified PRACH (0 <≤ t_id <10), and f_id is the index of the specified PRACH within that subframe, in ascending order of frequency domain (0≤ f_id< 6).
For FDD, f_id is fixed as 0.
Therefore, RA_RNTI is decided by the sending timing (SubFrame) of PRACH Preamble by UE. It means that (the subframe number (number between 0000~0009) of PRACH transmission + 1) is RA-RNTI.
It means that UE specifies RA_RNTI by the sending timing (SubFrame) of PRACH Preamble.
转载自：http://www.sharetechnote.com/html/RACH_LTE.html#How_can_we_get_RA_RNTI