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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
packets.
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 B.you 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?

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Handover RACH will be initiated as Contention free, RNTI will be included in Mobility info. But while doing RACH What will be the RNTI value included in RAR? Will this be same as Mobility RNTI?

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