1. Understanding contention based random access procedure.
If 2 UE choose the same preamble sequence and send this sequence to eNB at the same time, this leads to collision.
For example, if 2 UE send the same preamble sequence at the same time, there are 2 scenarios.
eNB will not be able to decode the preamble sent by UE.
In this case, both UE will wait till backoff-timer value for some random time and initiate the RACH procedure again.
eNB is able to decode RACH preamble from UE A but not from UE B.
In this scenario, eNB will send RAR with RA-RNTI for UE-A. But RA-RNTI will be same for UE-A and UE-B.
In this case both UE A, UE B will be receiving RAR.
But UE B is not knowing that eNB was not able to decode the RACH preamble.
So at that time, both UE will send “RRC Connection Request with a Random number” and start timer T300.
eNB will not be able to decode the message from UE B as UE B is using the timing advanced value that is intended for UE A.
eNB will send “RRC Connection Setup with Random NumberA” in downlink.
Both UE will decode the “RRC Connection Setup”. Then UE B will get to know, that the random number is of UE A.
Then UE B will start Random Access Procedure again.
2. Understanding non-contention based random access procedure.
During Handover and resumption of downlink traffic, non-contention based random access procedure is used.
The preamble sequences are allocated to UE by eNB.
As the sequences are allocated by eNB there are no collisions.
Hence this RACH procedure will be very fast.