In 3G UMTS the Radio Network Temporary Identifiers (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. Now, in LTE it is the rule that common channels and shared channels are used to transmit all UE-specific data, but also some network-specific data across the radio interface. For this reason the RNTI in LTE is not always related to a particular subscriber, but sometimes also used to distinguish broadcast network information from data streams of subscribers.
The RNTI is signaled in the MAC layer.
When MAC uses the Physical Downlink Control Channel (PDCCH) to indicate radio resource allocation, the RNTI that is mapped on the PDCCH depends on the logical channel type:
- C-RNTI, Temporary Cell Radio Network Temporary Identifier (temp C-RNTI), and Semi-Persistent Scheduling (SPS) C-RNTI for Dedicated Control Channel (DCCH) and DTCH;
- Paging Radio Network Temporary Identity (P-RNTI) for Paging Control Channel (PCCH);
- Random Access Radio Network Temporary Identifier (RA-RNTI) for Random Access Response (RAR) on DL-SCH;
- Temporary C-RNTI for Common Control Channel (CCCH) during the random access procedure;
- System Information Radio Network Temporary Identifier (SI-RNTI) for Broadcast Control Channel (BCCH).
The following values (given in Table 1) are defined for the different types of RNTI.
C-RNTI, semi-persistent scheduling C-RNTI, temporary C-RNTI, TPC-PUCCH-RNTI, and TPC-PUSCH-RNTI
Reserved for future use
The P-RNTI is the 4G complement of the paging indicator known from 3G UMTS. 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 S1 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). If this random access preamble is received by the eNB and network access granted, the base station sends an acquisition indication back to the mobile and this acquisition indication message contains the RA-RNTI. In turn the UE will use the RA-RNTI to send a RRC connection request message on the radio interface UL and the parameter will help to distinguish messages sent by different UEs on the Random Access Channel (RACH).
The C-RNTI is a 16-bit numeric value. Its format and encoding are specified in 3GPP 36.321 (MAC). The C-RNTI is part of the MAC Logical Channel Group ID field (LCG ID). It defines unambiguously which data sent in a DL direction within a particular LTE cell belongs to a particular subscriber. For instance, all RRC messages belonging to a single connection between a UE and the network are marked with the same C-RNTI value by the MAC entity that provided transport services to the RRC and NAS. Thus, C-RNTI is an important parameter for call tracing.
The C-RNTI comes in three different flavors: temp C-RNTI, semi-persistent scheduling C-RNTI, and permanent C-RNTI.
The temp C-RNTI is allocated to the UE during random access procedure (with a RRC connection setup message) and may turn into a permanent C-RNTI 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 subscriber is running services with a predictable unchanging QoS profile. A typical example is VoIP for which the required bit rate will not change during the entire connection. In such a case the dynamic (re)scheduling of radio resources, which is mandatory in the case of bursty payload traffic to ensure optimal usage of resource blocks, is not required. The SPS C-RNTI is used to indicate an area of resource blocks that will be used by the same UE for a longer time frame without any expected change.
The SI-RNTI is sent on the PDCCH. It does not stand for a particular UE identity. Instead it signals to all mobiles in a cell where the broadcast System Information Blocks (SIBs) are found on the Physical Downlink Shared Channel (PDSCH). This is necessary since the PDSCH is used to transport both broadcast system information for all UEs and signaling/payload for particular mobiles. In other words, the SI-RNTI indicates which DL resource blocks are used to carry SIBs that in 3G UMTS have been sent on the broadcast (transport) channel mapped onto the Primary Common Control Physical Channel (P-CCPCH). In LTE there is no CCPCH, only DL-SCH.