LTE: Possible reason behind getting maximum of the ULSCH_HARQ_FB as DTX?

Question

Does anybody know about what could be the possible reason behind getting maximum of the ULSCH_HARQ_FB as DTX?

Answer 1

Suppose eNB is sending the data packet to UE in sfn x . UE will receive the packet and then UE would send HARQ response to eNB in x+4 sfn. If UE is not able to send HARQ response to eNB in this sfn(x+4) then eNB will assume DTX. Probable cause for getting DRX in UL may be the issue in packet scheduling at eNB or may be issue in sending HARQ response by UE.

Answer 2

For Every DL data packet transmitted by eNB to UE, eNB configures PHY with UL_CONFIG_REQ for HARQ reception in ULSCH. Now:possible usecases and causes for max ULSCH_HARQ as DTX are listed below:

-> eNB sends DL data packet to UE and UE receives it. Now PHY layer UE decode this DL data PDU, this may result in DTX in case PHY of UE is not able to decode the DL PDU correctly. It can happen if channel conditions are not good and DL data PDU got corrupted over the air.

-> Secondly lets say UE is able to decode the message correctly and UE reply back with HARQ ACK/NACK, then still in this case the PHY of eNB will have to decode the PHY_HARQ_IND message in uplink ULSCH channel, now it may result into DTX due to poor channel conditions in uplink ULSCH channel or otherwise if the UE encoding of ULSCH HARQ PDU was itself incorrect.

The max limit of ULSCH_HARQ will be reached in the scenarios where:

-> Either there is some issue in the scheduling/encoding of the DL data PDU at eNB end.
-> Scheduling/Encoding of PHY_HARQ response at UE end.
-> Channel conditions is bad and eNB is still transmitting DL PDUs at high MCS.

(Note: Channel conditions can be checked by observing UL_SINR in RX_ULSCH_IND, CRC in PHY_CRC_IND, DL CQI in PHY_CQI_IND femto APIs)