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Uplink grant for UE while attempting random access procedure?

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I want to ask about the uplink grant for UE while attempting random access procedure. How the eNB decide the uplink grants of msg3? (Which subframe and which resource blocks). And the UE perform HARQ when transmitting and receiving the step 3 and step 4 messages. Some source said that, each step in HARQ is performed at 4 subframes intervals. If i get to the root of the problem, how many subframes ( or ms) does the random access procedure take?

posted Jul 23, 2016 by anonymous

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1 Answer

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Hi ,

Q 1 : Enb will decide the required grant based on your preamble transmission. there were spme size associated with the preamble index, so UE will select preamble based on its need.

Q2 : When UE perform HARQ process yes it will take 4ms to get the ACK/NACK, and one more 4ms to do newtx or retx .

Say examble RAR window is 8 Subframe(note : RAR window start 4 SF after rach subframe) and UE did rach in first subframe. ENB can send RAR within those 8 subframe. Say ENb send at 8th subframe (actually 12 subframe after RACH ( 8 + 4)), then UE will take 4 subframe to send uplink data (i.e msg 3 ) total 16 SF overed, then ENB will send contention resoultion message with in contention resoultion window timing, at the MAX 24 SF will take to finish rach process (only till msg3 transmission but actual rach procedure means till attach complete.).

answer Jul 25, 2016 by Jaganathan
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+1 vote

How many times UE can ask SR for uplink grant is there any limit for SR for uplink grant? When SR will not be available , because i heard UE will do rach if it don't have SR configuration so when this scenario will occur.

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I was thinking, Is it possible UE sends RRC connection reconfiguration complete message to target eNodeB without doing the RACH ? I meant to say, resource reserved by target eNodeB and send to UE within the handover command message ?

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In LTE random access procedure, if multiple UEs select the same preamble and send it to BS, there may have two conditions:
First, if these UEs have enough SNR, BS can detect a collision happen, then BS do not send RAR.
Otherwise, second, these UEs have a collision and BS can not detect all of them and send RAR, and all of these collision UEs can receive the same RAR.
My question is about the second one, there are multiple UEs use the same preamble, why BS do not know the collision ? why BS still send RAR to UEs ? why BS can still detect this preamble?

+1 vote

If my understanding is correct then as part of handover procedure, UE can do either contention based rach or non-contention based.
Now my question is that on what parameter basis target eNodeB decide for an UE to do contention/non-contention based rach during the handover procedure ?

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