Thursday, 14 January 2010

Temporary Identities in LTE/SAE - 2: RNTI's

Last year I covered some information on temporary identities but never got a chance to continue on it. Here is one on RNTI's

RNTI or Radio Network Temporary Identifier(s) are used primarily by eNB Physical Layer for scrambling the coded bits in each of the code words to be transmitted on the physical channel. This scrambling process in PHY happens before modulation. There is a sequence followed for scrambling, calculation of which depends on the RNTI(UE specific for channels like PDSCH,PUSCH) and cell specific (for broadcast channels like PBCH). Details could be found in [2].

The following table lists different kinds of RNTI's:

Lets look at some of these in slightly more detail:

P-RNTI (Paging RNTI): To receive paging messages from E-UTRAN, UEs in idle mode monitor the PDCCH channel for P-RNTI value used to indicate paging. If the terminal detects a group identity used for paging (the P-RNTI) when it wakes up, it will process the corresponding downlink paging message transmitted on the PCH.

SI-RNTI (System Information RNTI): The presence of system information on DL-SCH in a subframe is indicated by the transmission of a corresponding PDCCH marked with a special System Information RNTI (SI-RNTI). Similar to the PDCCH providing the scheduling assignment for ‘ normal ’ DL-SCH transmission, this PDCCH also indicates the transport format and physical resource (set of resource blocks) used for the system-information transmission.

M-RNTI (MBMS RNTI): Used in Rel-9 for MCCH Information change notification.

RA-RNTI (Random Access RNTI): The RA-RNTI is used on the PDCCH when Random Access Response (RAR) messages are transmitted. It unambiguously identifies which time-frequency resource was utilized by the UE to transmit the Random Access preamble. If multiple UEs had collided by selecting the same signature in the same preamble time-frequency resource, they would each receive the RAR.

C-RNTI (Cell RNTI): The C-RNTI to be used by a given UE while it is in a particular cell. C-RNTI allocation and details are too complex to explain in the blog so please refer to Nomor newsletter here.

TC-RNTI: When the UE does not have allocated C-RNTI then Temporaru C-RNTI is used. A temporary identity, the TC-RNTI, used for further communication between the terminal and the network. If the communication is successful then TC-RNTI is promoted eventually to C-RNTI in the case of UE not having a C-RNTI.

SPS-C-RNTI (Semi-Persistent Scheduling C-RNTI): For the configuration or reconfiguration of a persistent schedule, RRC signalling indicates the resource allocation interval at which the radio resources are periodically assigned. Specific transmission resource allocations in the frequency domain, and transmission attributes such as the modulation and coding scheme, are signalled using the PDCCH. The actual transmission timing of the PDCCH messages is used as the reference timing to which the resource allocation interval applies. When the PDCCH is used to configure or reconfigure a persistent schedule, it is necessary to distinguish the scheduling messages which apply to a persistent schedule from those used for dynamic scheduling. For this purpose, a special identity is used, known as the Semi-Persistent Scheduling C-RNTI (SPS-C-RNTI), which for each UE is different from C-RNTI used for dynamic scheduling messages. - Source: LTE, The UMTS Long Term Evolution: From Theory to Practice By Stefania Sesia, Issam Toufik, Matthew Baker

TPC-PUCCH-RNTI (Transmit Power Control-Physical Uplink Control Channel-RNTI) and TPC-PUSCH-RNTI (Transmit Power Control-Physical Uplink Shared Channel-RNTI): The power-control message is directed to a group of terminals using an RNTI specific for that group. Each terminal can be allocated two power-control RNTIs, one for PUCCH power control and the other for PUSCH power control. Although the power control RNTIs are common to a group of terminals, each terminal is informed through RRC signaling which bit(s) in the DCI message it should follow.

The following table lists the values that are assigned to different RNTI's in MAC:

[1] 3GPP TS 36.321 - Evolved Universal Terrestrial Radio Access (E-UTRA) Medium Access Control (MAC) protocol specification
[2] 3GPP TS 36.211 - Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation

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