There are total 64 preambles which are initially divided into two groups, i.e. contention-free RA preambles and contention-
based RA preambles. The eNB reserves some preambles, Ncf for contention-free RA, and assigns distinct preambles
to different UEs. Rest of the preambles (64−Ncf) are used for contention-based RA, where each UE randomly generates
Contention free used where low latency is required, such as handover and resumption of downlink traffic for a UE.
There are total of 64 preambles available which are divided into two groups Group A and Group B. UE decides the preamble index from a group on the basis of parameters received in SIB2
NumberofRaPreambles: - eNodeB sends this value in SIB2 which denotes the total number of
preambles available for UE to send a Rach Request.
SizeOfRaPreamblesGroupA: -Total number of random access preambles sequences available
within Group A. Preambles are divided into Group A and Group B. Group A preambles are
intended for sending small packets and Group B preambles are intended for sending large
If sizeOfRA-PreamblesGroupA is equal to numberOfRA-Preambles then there is no Random Access Preambles
group B. The preambles in Random Access Preamble group A are the preambles 0 to sizeOfRA-PreamblesGroupA – 1
and, if it exists, the preambles in Random Access Preamble group B are the preambles sizeOfRA-PreamblesGroupA to
numberOfRA-Preambles – 1 from the set of 64 preambles .
You can check the group based on preambles receive in contention based or contention free.