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

Question

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?

Answer 1

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.

Comments

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?

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This is a old thread, please ask fresh question so that it can reach to right audiences.

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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 .....