LTE Attach Procedure

In this tutorial, we will discuss on the LTE Attach Procedure.

Master Information Block (MIB) (on BCH) periodicity 40ms:

It contains:

  • System Bandwidth
  • PHICH Configuration
  • SFN number if transmit antennas

System Information Block Type 1 (SIB1) (on DL-SCH) periodicity 80ms

It contains:

  • PLMN IDs
  • Tracking Area Code
  • Cell Identity
  • Access restrictions
  • Scheduling Information
LTE Attach Procedure

SIB2:

The important information on SIB2 is

  • RACH Configuration
  • BCCH, PCCH, PDSCH, PUSCH, PUCCH configuration
  • sounding RS Configuration
  • UE Timers

RACH related parameter in SIB2

RACH related parameter in SIB2

Random Access Procedure:

Here is the detail explanation of RA procedure.

Purpose of random access procedure is

  • To get uplink synchronisation
  • To get radio resources in uplink direction to send L3/L2 message

For Example, after boot up UE needs to search some suitable cell and send RRC connect request in uplink to trigger registration process.

UE gets all the random access related parameter of the cell by reading SIB2. Above I have highlighted all the random access parameter of SIB2.

UE triggers RACH process by sending RACH preamble to the enB on PRACH channel (Physical random access channel).

Here is the general structure of RACH preamble.


There are 64 preamble sequences available for each cell and UE generates these 64 preambles for the cell it wants to camp on. But remember all these 64 preamble sequences are not available of a UE.

Few preamble sequences are reserved by enB were non contention based random access.

numberOfRA_Preambles = denotes total number of preambles available for UE to choose from

64 – numberOfRA_Preambles = number of RACH preamble reserved by eNodeB for non-contention based RACH procedure

Preamble available to UE are divided into two groups (Group A and Group B).

sizeOfRA_PreamblesGroupA  = denotes the number of preambles available within Group A

numberOfRA_Preambles – sizeOfRA_PreamblesGroupA  = number of preambles in Group B

Group is decided based on size of L3/L2 message to be sent to eNodeB. For example, at the time of registration L3 message ‘RRC connection request’ is sent to the enodeB. If size of ‘RRC connection request is greater than SIB2 parameter , message size Group A preamble will be selected from Group B else preamble will be selected from Group A. From the selected group UE randomly choose the preamble.

SIB2 parameters used by UE to drive preamble sequences:

rootSequenceIndex    198     
      highSpeedFlag    0       
      zeroCorrelationZoneConfig    14      

Total 6 PRBs are used by UE to send RACH preamble. 

preambleInitialReceivedTargetPower = This will tell the power to be used for first transmission of RACH request. This value caries from -120dBm to -90dBm.

powerRampingStep = This is mainly used when eNodeB is not able to detect the RACH request.

In case of some failure UE will retransmit the RACH request by increasing the power to powerRampingStep factor.

preambleTransMax = It is necessary to have this parameter otherwise UE may all its battery if there are constant RACH failure.

Now we have understood the LTE Attach Procedure. Let’s take a quiz on LTE Testing.

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