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RNTI's in LTE:

RNTI stands for Radio Network Temporary Identifier. RNTIs are used to differentiate/identify a connected mode UE in the cell, a specific radio channel, a group of UEs in case of paging, a group of UEs for which power control is issued by the eNB, system information transmitted for all the UEs by the eNB etc…

There are a several RNTI types in LTE such as SI-RNTI, P-RNTI, C-RNTI, Temporary C-RNTI, SPS-CRNTI, TPC-PUCCH-RNTI, TPC-PUSCH-RNTI, RA-RNTI, and M-RNTI. Each RNTI’s usage, its value range etc…are discussed in detail below

SI-RNTI (System Information RNTI)
SI-RNTI is used for broadcast of system information.
It is a common RNTI meaning that, it is not allocated to any UE explicitly.
SI-RNTI is of 16-bit in length and its value is fixed to 65535 (0xFFFF). A single SI-RNTI is used to address SystemInformationBlockType1 as well as all SI messages
Broadcast of System Information uses BCCH logical channel which is then mapped to DL-SCH transport channel which intern mapped to PDSCH physical channel. The UEs should know the scheduling information for PDSCH which is carrying System Information. The required scheduling information is contained in DCI (Downlink Control Information) whose CRC is scrambled by SI-RNTI
DCI Formats which carries scheduling information for System Information are DCI Format 1A and DCI Format 1C in common search space
The UE starts decoding PDCCH scrambled with SI-RNTI at the start of SI-Window (for the concerned SI message) until the end of the SI-window, or until the SI message was received excluding the following subframes.
- subframe #5 in radio frames for which SFN mod 2 = 0
- any MBSFN subframes
- any uplink subframes in TDD

If the SI message was not received by the end of the SI-window, the UE repeats reception at the next SI-window occasion for the concerned SI message

P-RNTI (Paging RNTI)
P-RNTI is used by the UEs for the reception of paging.
It is a common RNTI meaning that it is not allocated to any UE explicitly.
P-RNTI is of 16-bit in length and its value is fixed to 65534 (0xFFFE).
Paging message is carried by PCCH logical channel which is mapped to PCH transport channel. The PCH transport channel is mapped to PDSCH physical channel. The eNB scrambles PDCCH’s CRC with P-RNTI for transmission of PDSCH that carries paging information
DCI Formats which carries scheduling information for paging are DCI Format 1A and DCI Format 1C in common search space

RA-RNTI (Random Access RNTI)
As part of Random Access procedure, the eNB’s MAC generates Random Access Response (RAR) as a response to the Random Access Preamble transmitted by the UE. RAR is transmitted on DL-SCH transport channel which intern is mapped to PDSCH. The eNB scrambles PDCCH’s CRC with RA-RNTI for transmission of PDSCH that carries RAR(s).
RA-RNTI can be addressed to multiple UEs, i.e., multiple UEs might decode PDCCH scrambled by the same RA-RNTI.
RA-RNTI unambiguously identifies which time-frequency resource was utilized by the UE to transmit the Random Access preamble (explained below)
RA-RNTI is of 16-bit in length and its value is derived from the below equation where t_id is the index of the first subframe of the specified PRACH (0≤ t_id