Friday 3 October 2014

22:28 - No comments

RNTI

RNTI stands for Radio Network Temporary Identifier. As the name implies, it is a kind of Identification number. Normally we use indentification number to differntiate one thing from all other similar things.
Followings are the brief summary of RNTIs being used in LTE. i hope this diagram will clear you about RNTI what kind of RNTI? what type of RNTI? what are the value for RNTI? what is the mapping of RNTI?



Who issues these RNTI ?
Network issues RNTI.

Exactly what does RNTI do for each of those radio channel ? The detailed process differs with the types of RNTIs, but generally speaking all of these RNTI is used to scramble the CRC part of the radio channel messages. It implies that if UE does not know the exact RNTI values for each of the cases, it cannot decode the radio channel messages even though the message reaches the UE intact.



One of the most common questions that I got about RNTI is "There are a lot of different types of RNTI and I don't see any RNTI information on DCI or Higher layer signaling message. Then how can PHY layer know which RNTI it has to use to decode a data ?". The answer is "MAC or Layer 1 controller would instruct PHY on which RNTI it has to use". Then a next questions comes out. "How MAC or Layer 1 controller would know which RNTI to be used ?". There is no explicit algorithm for this, MAC/L1 controller needs to figure it out "based on context". For example, if it is at the subframe where SIB is transmitted, it would instruct PHY to use SI-RNTI. if UE is in connected mode, it may instruct to use C-RNTI, TPC RNTI etc.

 How each of RNTI is used ?
Following is the quotes from 3GPP specification showing how RNTI is used for various cases.. for the exact details, you should see the specification but this partial quote would give you a rough idea of the usage of RNTI.

From 3GPP TS:36.212

5.3.3 Downlink control information
  • A DCI transports downlink or uplink scheduling information, or uplink power control commands for one RNTI. The RNTI is implicitly encoded in the CRC. 
5.3.3.1.3 Format 1A
  • Format 1A is used for random access procedure initiated by a PDCCH order only if format 1A CRC is scrambledwith C-RNTI
  • For distributed VRB: .. if the format 1A CRC is scrambled by RA-RNTI, P-RNTI, or SI-RNTI
5.3.3.2 CRC attachment
  • This section explain in detail on how CRC is scrambled by RNTI. Following is the summary of this process. As you see, RNTI is used to scramble CRC bits of PDCCH.

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