How to decode DL-SCH data when C-RNTI and DL-SCH Settings are unknown

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For SI-RNTI (65535) it is clear how to decode SIB1 info. This is done for every even SFN and enb.NSubframe=5. Following code snippet from Matlab SIB1 recovery example:
pdcch = struct('RNTI', 65535);
pdcch.ControlChannelType = 'PDCCH';
pdcch.EnableCarrierIndication = 'Off';
pdcch.SearchSpace = 'Common';
pdcch.EnableMultipleCSIRequest = 'Off';
pdcch.EnableSRSRequest = 'Off';
pdcch.NTxAnts = 1;
alldci = ltePDCCHSearch(enb, pdcch, dciBits);
[pdsch, trblklen] = hPDSCHConfiguration(enb, dci, pdcch.RNTI); % get this from Matlab helper func
pdsch.NTurboDecIts = 5;
[pdschIndices,pdschIndicesInfo] = ltePDSCHIndices(enb, pdsch, pdsch.PRBSet);
[pdschRx, pdschHest] = lteExtractResources(pdschIndices, rxsubframe, subfrHest);
[dlschBits,pdschSymbols] = ltePDSCHDecode(enb, pdsch, pdschRx, pdschHest, subfrNest);
[trkblkout, crc, decState] = lteDLSCHDecode(enb, pdsch, trblklen, dlschBits, decState);
sib1 = trkblkout; % if crc == 0
Now what is the procedure when we want to decode other subframes from 0 to 9. How can we do PDCCH blind search when we dont know C-RNTI? This can be UESpecific search, then we need to bruteforce from 0x000B to 0xFFF3 all possible RNTIs and see DCI candidates and try to DL-SCH decode subframe OFDM demod rxGrid with that, right? But this is too slow, like 17 mins per 1 subframe in 20MHz BW.
SIB1 is decoded with PDSCH config where Modulation is 'QPSK'. (set in hPDSCHConfiguration function explicitly). Does it mean that all other subframes should use the same PDSCH config as well?

Answers (1)

Kirsten Hart
Kirsten Hart on 3 Oct 2022
SIB1 is a piece of information with known modulation and periodicity, unlike any PDSCH associated with a particular UE. Decoding the control information for a UE is necessary to retrieve any PDSCH.
As you go through all possible RNTIs, you can use several techniques to validate whether a DCI for that RNTI makes sense such as decoding and reencoding the DCI.
There is an example that ships in LTE Toolbox that illustrates how to approach that complicated topic.
The UE Detection using Downlink Signals example is further explained here:
Please note that the 3GPP standard assumes that you know your own RNTI, which means that there is no standard way to look at information for other UEs, for which you do not know the RNTI.

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