UL out of Sync

4G LTE
1

Hi Experts,
Can anyone tell what action enb performs whenever UE is in UL out of Sync and DL data is scheduled for UE.

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2

Hi @Saurabh63, Here could be 2 possible scenario during DL scheduling from eNB.

  1. Do you mean UE is insync DL and out of sync in UL?
  2. Do you mean ue is in idle state?
3

Hi @vikram ,

  1. Do you mean UE is insync DL and out of sync in UL? — YES UE is in-sync in DL & out-of-sync in UL only.
  2. Do you mean ue is in idle state? - UE is in connected mode.
4

Scenario 1 Answer: In this scenario eNB will force ue to do contention free RACH and its a part of PDCCH order procedure.

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5

Contention free RACH?? How enb will provide this info to UE.

6

This scenario will happen when eNB has some DL data in its buffer. Then eNB will send PDCCH order DCI1A. Once ue will decode this it will perform contention free RACH followed by resumption of DL data.

7

DCI -1A contents below - Which field shows contention free RACH?

image
image989×653 41.6 KB

8

Yes @Saurabh63 , Image shared by you is also correct but for PDCCH order Format 1A is used for random access procedure initiated by a PDCCH order only if format 1A CRC is scrambled
with C-RNTI and all the remaining fields are set as follows:

  • Localized/Distributed VRB assignment flag – 1 bit is set to ‘0’
  • Resource block assignment –   log )2/)1(( DL
    RB
    DL
    2 RB NN + bits, where all bits shall be set to 1
  • Preamble Index – 6 bits
  • PRACH Mask Index – 4 bits, [5]
  • All the remaining bits in format 1A for compact scheduling assignment of one PDSCH codeword are set to
    zero

For more information on this please refer 36.212 - 5.3.3.1.3

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9

Thanks for the information, I referred to spec’s mentioned by you and got the required info, also I looked at below spec having details regarding PRACH Mask Index, doc states which one to use at what time.