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In LTE does UE performs contension based RACH or contension free RACH for UE uplink sync?

+4 votes
posted Feb 17 by Prabhavathi
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3 Answers

+3 votes

It is contention based RACH, In msg3 UE will send MAC-CRNTI control element to make contention resolve.

answer Feb 17 by Jaganathan
+3 votes

Both contention based and free rach are possible in case of UL unsynchronized.
If ue have UL unsynchronized and want to send data in UL then ue will perform contention based rach.

If ue is in UL unsynchronized and eNodeB want to schedule data in DL then eNodeB first send the pdcch order to ue. After getting pdcch order, ue will perform rach. This rach can be contention based or free. If eNodeB sends rach preamble in pdcch order then ue performs contention free rach otherwise ue performs contention based rach.

answer Feb 17 by Veer Pal Singh Yadav
+2 votes

UE performs both types of RACH but one at a time depending on the scenario. In handover case it performs contention free RACH if target enodeb instructs else it go ahead with contention based RACH.

answer Feb 17 by Vimal Kumar Mishra

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+1 vote

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Why do we need only contention free RACH in LAA cells?

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MAC notifies RLC of a transmission opportunity asking for data each TTI. So does RLC respond with multiple PDUs one for each UE (with different RLC SNs for each of the PDU). Or just sends down one PDU (with one RLC SN) having data of all the scheduled UEs?
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