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LTE PDCCH decoding failure

Question asked by rrlagic on Oct 20, 2015
I am running 89600 v.18.7 on N9030A PXA analyzer to check LTE signal from the custom signal generator. The signal is downlink, it has primary and secondary sync signals, reference signals. In every subframe there is control format indicator, PDCCH and corresponding PDSCH. Mentioned signals have exactly same content in all subframes. But in some subframes PDCCH is not decoded properly. As a consequence, PDSCH is not decoded as well. I am watching OFDM Decoded Symbols window at different levels of PDCCH reverse processing. When PDCCH decoding is set to 'Descrambled', exactly same sequence is displayed for all subframes, and that is what I expect. However, if I select further processing, either PDCCH Deratematched or Decoded, then some subframes are decoded properly, but some display just no data. This happens when I use PDCCH decoding. When I set power based detection, it shows the actual transmission is present in expected place. I attach 3 screenshots to illustrate this situation. Analysis duration is 4 slots, with offset of 8 slots, i.e. subframes 4 and 5 are analyzed for clarity. 
Please suggest, why PDCCH may not be decoded properly in such circumstances. Will provide further details if necessary.
Thanks in advance.