All About Random Access Procedure In LTE

When UE Initiate Random Access Procedure In LTE:
- Transition from RRC Idle to RRC Connected mode.
- Completing an Intra-System Hand Over.
- Uplink data arrives while UE is in Non-synchronized RRC Connected State(Time alignment Timer Expiry).
- Downlink data arrives while UE is in Non-synchronized RRC Connected State(Time alignment Timer Expiry).
- RRC Connection Re-establishment.

PRACH : Physical Random Access Channel:
- Used to transfer RANDOM ACCESS PREAMBLES to initiate RANDOM ACCESS PROCEDURE.
- Do not transfer RRC Signaling Messages or Application Data.

RACH - Config Common Information: 

RACH-ConfigCommon IE

RACH-ConfigDedicated IE

RACH Procedure:
- RACH Procedure can be
             - Contention Based.
             - Non Contention Based.
- Contention Based procedure involves the UE selecting a Random Access Resource i.e. UE selecting a PRACH resource, a Preamble Sequence and the next available Subframe for PRACH transmission.
- Non-Contention Based procedure involves the eNodeB allocating the Random Access Resource i.e. eNodeB allocating ra-PreambleIndex and ra-PRACH-MaskIndex.
- Contention Based RACH Procedure can be applicable for all RACH reasons but Non Contention Based RACH Procedure can be applicable for :
             - Completing an Intra-System Hand Over.
             - Downlink data arrives while UE is in Non-synchronized RRC Connected State.

Random Access Resource Selection:

Random Access Group Selection:
If (ra-PreambleIndex == Allocated By eNB && ra-PRACH-MaskIndex == Allocated By eNB)
      {
      If (ra-PreambleIndex != 000000)
           {
            Random Access Preamble && PRACH Mask Index = Allocated By eNB.
            }
       }
Elseif
      {
      If (MSG3 == Not Transmitted Yet)
           {
            If (Random Access Preambles group B == present)
                 {
                 If ( sizeof(MSG3) > messageSizeGroupA && pathloss < (P-CMAX,c– preambleInitialReceivedTargetPower – deltaPreambleMsg3 – messagePowerOffsetGroupB))
                      {
                       select the Random Access Preamble from  Random Access Preambles group B
                       }
                  Else
                      {
                       select the Random Access Preamble from  Random Access Preambles group A
                       }
                  }
              Else
                  {
                   select the Random Access Preamble from  Random Access Preambles group A
                   }
             }
       Elseif (MSG3 == Re-transmitted)
              {
               select the Same Group of Random Access Preambles = used for the preamble transmission attempt corresponding to the first transmission of Msg3
               }
       }

Random Access Preamble Selection Within the Group:
- Randomly select a Random Access Preamble within the selected group.
- The random function shall be such that each of the allowed preamble will have equal probability.
- set PRACH Mask Index to 0.

PRACH Resource Selection:
- Subframe for PRACH transmission selected using restrictions given by the prach-ConfigIndex and the PRACH Mask Index.
- physical layer timing requirements i.e. UE may take into account the possible occurrence of measurement gaps when determining the next available PRACH subframe.

Random Access Preamble Power : 
- PREAMBLE_RECEIVED_TARGET_POWER = preambleInitialReceivedTargetPower (RACH-ConfigCommon) + deltaPreambleMsg3 (UplinkPowerControlCommon) + (PREAMBLE_TRANSMISSION_COUNTER – 1) * powerRampingStep (RACH-ConfigCommon)
- Physical layer transmit a preamble using the selected PRACH, corresponding RA-RNTI, preamble index and PREAMBLE_RECEIVED_TARGET_POWER

Random Access Response (RAR) Reception:
- Once RACH is transmitted UE shall monitor PDCCH regardless of the possible occurrence of a measurement gap.
- RAR is identified by RA-RNTI.
- RA Response window starts at subframe that contains the end of the preamble transmission + three subframes.
- RA Response window has a length ra-ResponseWindowSize(SIB2) subframes
- The RA-RNTI is calculates as :
          RA-RNTI= 1 + t_id+10*f_id
          where t_id = Subframe within which start of preamble was transmitted(0<= t_id<=9).
                    f_id = Frequency domain index of the PRACH within the subframe.(0<= f_id<=5).
- For FDD there can be maximum one set of RBs allocated to PRACH within a subframe so f_id is always 0.
- So for FDD RA-RNTI= 1 + t_id.