LTE troubleshoot case analysis (1)

1. Issue description:
Observed from eNB baseband log that at every subframe 9, eNB detected DTX for PDSCH transmission. And for subframe 3, eNB NACKed every PUSCH transmitted by UE.

2. Troubleshoot steps:
  (1) Since UL grant information is missed in baseband log, used Sanjole Intelijudge captured UL grant for 10secs.
  (2) From Intelijudge log, it showed that eNB gave UE UL grant at subframe 7 and subframe 9. The subframe 9 grant has CSI-requested bit set to 1; the subframe 7 grant has CSI-requested bit set to 0.
  (3) By comparing UE log, Intelijudge capture and eNB log, UE missed UL grant at subframe 9 (This is very hard to tell why).
  (4) With above missing, the whole process can be explained logically as follows:
      <1> At subframe 9, eNB sent UL grant with CSI-requested bit set to 1 (DL buffer size exceed threshold and a-periodic CQI report interval is 10ms). However, UE missed this grant.
      <2> At subframe 3, eNB was expecting UE to report CQI with PUSCH transmission but didn't get anything. Since this is interpreted as DTX, at subframe 7, eNB send ACK on PHICH and decided to use adaptive re-transmission. UE successfully decode this UL grant and at subframe 1, UE send PUSCH data, which is ACKed by eNB at subframe 5.

3. Conclusion:
The root cause is UE missed the DCI format 0 at subframe 9 and result in following consecutive consequences.