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

+4 votes
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posted 6 days ago by Prabha

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3 Answers

+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 5 days ago by Veer Pal Singh Yadav
+2 votes

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

answer 6 days ago by Jaganathan
+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 6 days ago by Vimal Kumar Mishra
Similar Questions
+1 vote

I don't know the complete use of CQI at eNodeB. Does it impact only downlink scheduling parameters or also considered while granting uplink resources ?

+2 votes

This is not a usual case. Assume an eNodeB has multiple bearers (GBR and non-GBR) with the UE, eNodeB enforces subscribed downlink data rate by using the different scheduling algorithm. Can eNodeB reject the uplink grant assuming UE is crossing its subscribed limit ?

+1 vote

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?
This same question could be put this way too: Are the RLC SNs assigned independently for each UE or are they assigned by incrementing by 1 every TTI?

+4 votes

Actually there are three resource allocation types being used to allocate resources to UE(s) in downlink and uplink. For downlink all three (Type 0, 1 and 2) used but for uplink only resource allocation type 2 is being used.


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