Saturday, May 18, 2013

RNTI (Radio Network Temporary Identifier)

RNTIs are always used to identify information dedicated to a particular subscriber on the radio interface, especially if common or shared channels are used for data transmission.

FDD RNTI Range Values FDD RNTI Type
0000 - 0009 RA-RNTI
000A - FFF2 C-RNTI, semi-persistent scheduling C-RNTI, temporary C-RNTI, TPC-PUCCH-RNTI and TPC-PUSCH-RNTI
FFF3 - FFFC Reserved

P-RNTI is the paging RNTI. It does not refer to a particular UE, but to a group of UEs. The P-RNTI is derived from the IMSI of the subscriber to be paged and constructed by the eNB. For this reason the IMSI is transmitted in a S1AP paging message from the MME to eNB, although in other S1AP signaling only the GUTI is used to mask the true identity of the subscriber.

The RA-RNTI is assigned by the eNB to a particular UE after this UE has sent a random access preamble on the Physical Random Access Channel (PRACH).

The C-RNTI format and encoding are specified in 36.321 (MAC). The C-RNTI is part of the MAC Logical Channel Group ID field (LCG ID). It defines which data sent in a DL direction.
C-RNTI comes in three different flavors: temp C-RNTI, semi-persistent scheduling C-RNTI, and permanent CRNTI.
The temp CRNTI is allocated to the UE during random access procedure (with a RRC Connection Setup message) and may turn into a permanent CRNTI depending on the result of a subsequently performed contention resolution procedure or in the case of contention free random access.
The semi-persistent scheduling C-RNTI is used if the UE is running services with a predictable unchanging QoS profile. A typical example is VOPI for which the required bit rate will not change during the entire session.

The SI-RNTI is send on the PDCCH. It signals to all mobiles in a cell where the broadcast System Information Blocks (SIBs) are found on the Physical Downlink Shared Channel (PDSCH).

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