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Why we use two groups of preambles A and B in RACH?

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In the process of random access resource selection preambles are selected from group A and B with certain criteria.

Now my question is why we have to use this two sets instead of one set of 64 preambles?

posted Jun 23, 2014 by anonymous

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1 Answer

+1 vote

Group A preamble is randomly selected, when data to be transmitted is relatively low and when UE is is poor coverage. Group B preamble is randomly selected, when data to be transmitted is relatively high and UE is in good coverage.

answer Sep 6, 2014 by anonymous
1.) Why are contention based preambles divided in 2 groups A and B and why can't we send larger packets using group A preamble?
2.) What is the role of messagePowerOffsetGroupB apart from determining the preamble group?

3.) In poor coverage areas, how does it help if a preamble is sent using a Group A preamble?
This is a old thread, please ask fresh question so that it can reach to right audiences.
UE's upper layer requests MAC to initiate rach procedure, if L3 message size exceeds (MessageSizeGroupA - recieved in SIB2), MAC will use preable from group B. (L3 msg size and UE's unknown distance from eNB and cell radius - are 3 main factors for different preable format and preable groups).
Hope this answer's you query .....
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