One of the other numbers which you would very frequently come accross is 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. For example, your driver's license number let you identify yourself from all other drivers. Social Security number do the same thing as well.
Getting more specifically into LTE, this RNTI is used to indentify one specific radio channel from other radio channel and one user from another user. As you may recall, in WCDMA is a RNTI concept which is carried as part of MAC header to deferentiate one user to another while in communication state. and in WCDMA case it used special channelization code to deferentiate one radio channel from the other.
Putting it very simply, you can think of RNTI as a kind of UE ID for the traffic between UE and eNB lower layer. But it would be more accurate to think of it as '[UE ID] + DCI Type ID' since each of DCI message (more precisely the CRC bits of each DCI) is scrambled by a specific RNTI Value. [UE ID] (UE ID in bracket) means that some RNTI is dedicated for a specific UE and some RNTI is not.
Then you may notice that there are not so many different DCI types but there are a huge set of RNTI types and ask how a few DCI types can be mapped to such a big number of RNTI types. Actually this is one of the thing you have to (or will) figure out as you read along this page.
Followings are the list of topics that will be described in this page.
How/Who determines each types of RNTI ?
How RNTI value is informed ?
How each of RNTI is used ?
What kind of RNTIs are there in LTE ?
The answer is A LOT -:). Followings are the brief summary of RNTIs being used in LTE. More detailed explanation will be updated continuously later.
P-RNTI : It stands for Paging RNTI. Used for Paging Message.
RA-RNTI : It stands for Random Access RNTI. Used for PRACH Response.
C-RNTI : It stands for Cell RNTI. Used for the transmission to a specific UE after RACH.
Who issues these 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 be used". 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.
There are a lot of tables related to RNTI Type and Specific Physical/MAC layer operation. It would not be easy to make any sense out of these tables, but just look into these tables as often as possible. As your understanding / knoledge of LTE PHY procedure, you would make more sense out of these tables.
Following table from 36.321 shows the range of values which is allocated for each RNTI types.
Following table from 36.321 shows the types of RNTIs and it's usage.
RA-RNTI : UE determines RA-RNTI as explained in here in the RACH page.
T-CRNTI : Network allocate T-CRNTI via RAR as shown in here in the RACH page.
C-RNTI : T-CRNTI becomes C-RNTI once Contention Resolution gets passed.
How a RNTI value is informed to the other party of the communication ? If you are mainly working on higher layer or in WCDMA, you may expect there is a specific signaling message or MAC field that carries this ID. But in LTE, this ID is embedded and carried to the other party in much lower layer. Also, there is no specific field to carry RNTI value itself. It is combined with CRC field of each Physical layer data as shown below (This is only one example for PDCCH case).
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.
A DCI transports downlink or uplink scheduling information, or uplink power control commands for one RNTI. The RNTI is implicitly encoded in the CRC.
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.
Following is some examples of 3GPP description related to RNTI. Just to focus which type of DCI is scrambled by which RNTI Type.
δ_PUCCH is a UE specific correction value, also referred to as a TPC command, included in a PDCCH with DCI format 1A/1B/1D/1/2A/2 or sent jointly coded with other UE specific PUCCH correction values on a PDCCH with DCI format 3/3A whose CRC parity bits are scrambled with TPC-PUCCH-RNTI.
The UE attempts to decode a PDCCH of DCI format 3/3A with the UE�s TPC-PUCCH-RNTI and oneor several PDCCHs of DCI format 1A/1B/1D/1/2A/2 with the UE�s C-RNTI or SPS C-RNTI on every subframe except when in DRX.
If the UE decodes a PDCCH with DCI format 1A/1B/1D/1/2A/2 and the corresponding detected RNTI equals the C-RNTI or SPS C-RNTI of the UE, the UE shall use the δ PUCCH provided in that PDCCH.
A preamble index, a target preamble received power (PREAMBLE_RECEIVED_TARGET_POWER), a corresponding RA-RNTI and a PRACH resource are indicated by higher layers as part of the request.
If a UE is configured by higher layers to decode PDCCH with CRC scrambled by the SI-RNTI, the UE shall decode the PDCCH and the corresponding PDSCH according to any of the combinations defined in table 7.1-1. The scrambling initialization of PDSCH corresponding to these PDCCHs is by SI-RNTI. // SI-RNTI scrambles the DCI for SIB messages.
If a UE is configured by higher layers to decode PDCCH with CRC scrambled by the P-RNTI, the UE shall decode the PDCCH and the corresponding PDSCH according to any of the combinations defined in table 7.1-2. The scrambling initialization of PDSCH corresponding to these PDCCHs is by P-RNTI. // P-RNTI scrambles the DCI for Paging message.
If a UE is configured by higher layers to decode PDCCH with CRC scrambled by the C-RNTI, the UE shall decode the PDCCH and any corresponding PDSCH according to the respective combinations defined in table 7.1-5. The scrambling initialization of PDSCH corresponding to these PDCCHs is by C-RNTI.
If a UE is configured by higher layers to decode PDCCH with CRC scrambled by the Temporary C-RNTI and is not configured to decode PDCCH with CRC scrambled by the C-RNTI, the UE shall decode the PDCCH and the corresponding PDSCH according to the combination defined in table 7.1-7. The scrambling initialization of PDSCHcorresponding to these PDCCHs is by Temporary C-RNTI.