When the UE has obtained system information, it has to request an RRCconnection. Like it has no dedicated resources, the UE requests the connection using the Random Access Procedure using common uplink resources. 

  • At the end of the procedure, the UE is RRC connected
  • UE and eNodeB are able to exchange data using dedicated radio resources

This procedure is also used in case of: 

  • Initial access from RRC_IDLE
  • RRC Connection Re-establishment procedure
  • Handover
There are 4 steps to allow the UE to exchange signaling messages with the eNodeB
Random Access Response in LTE

  • PDSCH signaled through PDCCH with RNTI = PRACH subframe number
  • Contains the signature index of the detected RACH preamble (1rst contention resolution step)
  • Contains the timing alignment command to apply (2 ways transmission time)
  • Contains a temporary RNTI assigned to the UE
  • Contains the resource assignment for next PUSCH (acts as DCI0)
The Physical Random Access Channel (PRACH): 
  • is made up of 6 RBs anywhere in the spectrum. 
  • occupies between 1 and 3 sub-frames per frame. 
The preamble: 
  • is generated from the Zadoff-Chu sequence. 
  • is associated to an RA-RNTI. 
There are 64 preamble sequences available per cell.
The response conveys:
  • The RA-RNTI, to match the response with the preamble
  • Timing alignment information
  • The Temporary C-RNTI
  • The initial UL grant, allocation of resources for the temporary C-RNTI.
The C-RNTI identifies an RRC Connection.