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How downlink shared channel data used for scrambling the Radio Network Temporary Identifier?

+2 votes
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I'm a little confused about the description of RNTI values used for scrambling DL-SCH data in Section 7.1 of 36.213. My questions are the following,

a) From the Transmitter(eNB) perspective, how do we pick the right RNTI value for scrambling DL-SCH data as part of the PDSCH processing ?. To elaborate, Is the choice between C-RNTI, SI-RNTI, P-RNTI for c_init made based on Transmission Scheme ? How ?

b) What is the range ( in decimal values) corresponding to each of the above mentioned RNTI choices ?

posted Apr 3, 2014 by Ankur Athari

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1 Answer

+2 votes

Lets start from scratch:

When a UE first turns on, the first RNTI value that it will search for is the System Information (SI) - RNTI. This has a hex value of xFFFF. All SIBs will be tagged with this RNTI in the control channel.

Upon processing SIB information and determining the operator configuration for parameters such as the PRACH, the UE will transmit a Random Access Preamble ID (RAPID) on the PRACH.

Within a window specified in SIB2, the tower will respond with a Random Access Response to UE who transmitted during this PRACH opportunity. The Random Access (RA)-RNTI will tag this message in the PDCCH. The RA-RNTI can range from x0001 to x003C. The RA-RNTI is calculated from the actual PRACH used to transmit on. Note that with FDD there is only 1 possible PRACH opportunity per subframe, so only 10 of those RA-RNTI values are ever used.

The RA-response message contains a Temporary Cell (C)-RNTI, which does not become permanent until the UE passes the contention resolution process. The RA-Response also contains an uplink grant. The UE must transmit the next message in the attach procedure (RRC Connection Request) with ULSCH data scrambled with the Temporary C-RNTI.

The next message from the tower will be the RRC Connection setup message, and the contention resolution happens with the information contained in this message. This message is scrambled by the tower with the Temporary C-RNTI. Assuming the UE passes contention resolution, the Temporary C-RNTI becomes the C-RNTI and is used by the UE for the duration of that session. If it does no pass contention resolution, it must RACH again. The range of C-RNTI is "everything not used by the other fixed, RNTI values".

Paging is also used to address UEs. Note that pages may be intended for multiple or individual UEs. It may add clarity to state right up front that there is only 1 value of the Paging RNTI (P-RNTI) and that is xFFFE. Because there is only value, the paging message itself, contains what's referred to as "record list" that lists the TMSI of individual UEs that are being uniquely addressed. (IMSI can be used in rare circumstances). The structure of the paging message makes it clear whether information is intended for multiple and/or individual UEs. Closer inspection of the paging message reveals that individually targeted pages only indicate to the UE that there is incoming circuit or packet switched data. In the latter case the UE will RACH to connect to the network to get that data.

So it's not so much the transmission mode as it is the UE state that determines what RNTI that it is
looking for.

I hope that it will help you.

answer Apr 4, 2014 by Vikram Luthra
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