UE Identity's in LTE

• Globally Unique Temporary Identity (GUTI)
  
  GUTI is allocated to the UE by the MME and has two components GUMMEI (Globally Unique MME ID) and the M-TMSI (MME-TMSI). The GUMMEI identifies the MME.When contacting the network, the mobile sends the GUTI to the base station which then uses the parameter to identify the MME to which it will send the request to re-establish the communication session. The Globally Unique MME Identifier (GUMMEI) is constructed from the MCC, MNC and MME Identifier (MMEI).

• Temporary Mobile Subscribe Identity (M-TMSI)
  
  The M-TMSI identifies the UE within the MME. An M-TMSI identifies a user between the UE and the MME. The relationship between M-TMSI and IMSI is known only in the UE and in the MME.This value is allocated by MME.

• Temporary Mobile Subscriber Identity (S-TMSI)
  
  For paging purposes, the mobile is paged with the S-TMSI. The S-TMSI is constructed from the MMEC and the M-TMSI. S-TMSI = MMEC + M-TMSI. It uniquely identify's the UE within an MME group. It is also included in RRC Connection Request.

• International Mobile Subscriber Identity (IMSI)
  
  IMSI is used for subscriber identification and stored in the Subscriber Identity Module (SIM). IMSI is usually 15 digits long. The first 3 digits are the Mobile Country Code (MCC), and is followed by the Mobile Network Code (MNC), either 2 digits (Europeanstandard) or 3 digits (North American standard). The remaining digits are the mobile station identification number (MSIN) within the network's customer base.

• International Mobile Equipment Identity (IMEI)
  
  The IMEI number is used by the network/operators to identify valid devices and therefore can be used for stopping a stolen phone from accessing the network. It is usually found printed on the phone.

Radio Network Temporary Identifier (RNTI) is used as UE identifiers within E-UTRAN and in signalling messages betweeen UE and E-UTRAN.

• Cell RNTI (C-RNTI)
  
  The C-RNTI provides a unique UE identification at the cell level identifying RRC Connection. Each RRC connection is associated with C-RNTI.
• Random Access RNTI (RA-RNTI)
  
  The RA-RNTI is assigned by the eNB to a particular UE after this UE has sent a random access preamble on the Physical Access Channel (PRACH). If this random acccess preamble is received by the eNB and network 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 RRC connection request message on the radio interfac UL and the parameter will help to distinguish messages sent by differnt UEs on the Random Access Channle (RACH). This procedure is called as contention based random access procedure.

• System Information RNTI (SI-RNTI)
  
  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 singaling/payload for particular mobiles. In other words, the SI-RNTI indicated which DL resource blocks are used to carry SIBs.
• Paging RNTI (P-RNTI)The P-RNTI is derived from the IMSI of the subscribed to be paged and constructed by the eNB. For this reason IMSI is transmitted in a S1AP paging message fromk the MME to eNB. To receive paging messages from E-UTRAN, UEs in idle mode monitor the PDCCH channel for P-RNTI value used to indicate paging.
• Temporary Cell RNTI (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.