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contention free preambles lies under which preamble group ?

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contention free preambles lies under which preamble group ?
posted Mar 19, 2018 by Himani

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1 Answer

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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
one preamble.
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.

answer Mar 19, 2018 by Mohit Mohan
But numberOfRA-Preambles parameter is only for  non-dedicated preambles.
In this enb divides Group A and Group can check in 36.321 for condition.
In this they are comparing sizeOfRA-PreamblesGroupA  and  numberOfRA-Preambles  for selection of groups.(64 - numberOfRA-Preambles) are dedicated preambles. As of my knowledge  it does not belongs to any group .
Let me know any correction needed!!!!
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+2 votes

Say e.g ENB given some random preamble to UE1 to do RACH, but in -case same preamble is selected by UE2(preamble given to UE 1) and doing RACH .
In the above scenario ENB will not give any CRM message to UE's so how contention will be resolved for UE2?

+1 vote

If HO fails the UE will initiate contention based reestablishment and connect to some ENB, But target ENB have provided the contention free Rach preamble , at failure case how long ENB will hold this preamble because it has to release for other UE?

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