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Detecting/Decoding PDSCH

 

Main purpose of this page is to show how to decode PDSCH data that is contained in resource Grid of one subframe. The important steps are as follows :

    i) Create a physical layer symbols for all the channels (RS, PSS, SSS, PDCCH, PDSCH)

    ii) Put all the channel data into a resource grid

    iii) Take out all the resource element for PDSCH from the resource grid

    iv) Decode the data (Convert the PDSCH to codeword bit stream)

Main purpose of this page is step iv), but most of the portions in the following source code is used for step i)~iii)

 

< SISO - Decoding PDSCH of one Subframe Resource Grid >

    % First you have to define properites of a eNodeB.  

    % NDLRB indicate System Bandwith in the unit of RBs.

    % NDLRB 6 = 1.4 Mhz, NDLRB 15 = 3.0 Mhz, NDLRB 25 = 5.0 Mhz,

    % NDLRB 50 = 10 Mhz, NDLRB 75 = 15 Mhz, NDLRB 100 = 20 Mhz

    % CellRefP indicate number of downlink Antenna. CellRefP = 1 means 1 transmission antenna (SISO)

    % NCellID indicate PCI (Physical Channel Identity) of the Cell

    % NSubframe indicate the subframe number.

    enb.CyclicPrefix = 'Normal';

    enb.PHICHDuration = 'Normal';

    enb.Ng = 'Sixth';

    enb.NDLRB = 6;

    enb.CellRefP = 1;

    enb.DuplexMode = 'FDD';

     

    enb.NCellID = 0;

    enb.NSubframe = 0;

    enb.CFI = 1;

     

    % Now populate all the information in DCI field as you like. Understanding details of DCI is also pretty huge topics.

    % You would need separate page for DCI for the details.

     

    dci.NDLRB = enb.NDLRB;

    dci.DCIFormat = 'Format1A';

    dci.AllocationType = 0;

    dci.Allocation.RIV = 18;

    dci.ModCoding = 10;

    dci.HARQNo = 0;

    dci.NewData = 0;

    dci.TPCPUCCH = 0;

    dci.DuplexMode = 'FDD';

    dci.NTxAnts = 1;

     

    % once you defined all the detailed fields of DCI, just pass it to lteDCI() function with eNB info as follows,

    % then you will get the bit stream for the DCI.

     

    [dciMessage,dciMessageBits] = lteDCI(enb,dci);

     

    % for this step, you need to set a couple of additional parameters as shown below. C_RNTI will be XORed to CRC bits

    % PDCCHFormat will determined Aggregation Level.

    %          PDCCHFormat 0 indicate Aggregation Level 1

    %          PDCCHFormat 1 indicate Aggregation Level 2

    %          PDCCHFormat 2 indicate Aggregation Level 4

    %          PDCCHFormat 3 indicate Aggregation Level 8

     

    C_RNTI = 100;                         

    pdcchConfig.RNTI = C_RNTI;            

    pdcchConfig.PDCCHFormat = 0;          

     

    % then pass dciMessageBits and pdcchConfig to lteDCIEncode, the you would get the encoded bitstream.

     

    codedDciBits = lteDCIEncode(pdcchConfig, dciMessageBits);

     

    % If you pass the enb into ltePDCCHInfo() function, it will give you the amount of resources that can be allocated

    % for PDCCH allocation. This is not the amount of resource for only one DCI. It will give you the total/maximum

    % amount of the resources that can be allocated for PDCCH.

     

    pdcchDims = ltePDCCHInfo(enb);

     

    % With ltePDCCHSpace, you can get the list of all the possible spaces that can carry PDCCH. In this example,

    % the space were shown in the unit of bits.

     

    pdcchBits = -1*ones(pdcchDims.MTot, 1);

     

    % generate an array with the length that can accommodate all the possible PDCCH bits.

     

    candidates = ltePDCCHSpace(enb, pdcchConfig, {'bits', '1based'});

     

    % select one of the candidate bitSection and assign the codedDcitBits. You can select any candidate bit section,

    % but in this example, I selected the first candidate section.

     

    pdcchBits ( candidates(1, 1) : candidates(1, 2) ) = codedDciBits;

     

    % if pass the encodedBits into ltePDCCH(), it will generate the modulated physical layer symbols.

     

    pdcch_sym = ltePDCCH(enb, pdcchBits);

    pdcch_sym_ind = ltePDCCHIndices(enb,{'1based','re'});

    pdcch_sym_arrayIndex = 0:length(pdcch_sym)-1;

     

    % Now we set various parameters defining PDSCH channel. (In real transmission, you would need to create

    % a dci that is corresponding the configuration here. But in this example, I will go without defining DCI)

     

    pdsch.NTxAnts = 1;

    pdsch.NLayers = 1;

    pdsch.TxScheme = 'Port0';

    pdsch.Modulation = {'16QAM'};

    pdsch.RV = 0;

    pdsch.RNTI = C_RNTI;

     

    % Now I have to create a vector carrying the number of PRB indexes that will be used to carry this PDSCH.

    % for example, pdsch_prbs in following section would create a vector [0 1 2 3]

     

    START_RB = 0;

    N_RB = 4;

     

    pdsch_prbs = (START_RB:(START_RB+N_RB-1)).';

     

    % Now we have to generate a bit sequence which would exactly fit to the number of resource elements that are

    % allocated for PDSCH for this specific subframe. To figure out exact Resource Element information, unlike in other

    % channel processing, I would run ltePDSCHIndices() first. As you see in the following code, ltePDSCHIndices()

    % returns the information that would give you the size of transport block size in the unit of bits.

     

    [pdsch_sym_ind,pdschIndInfo] = ltePDSCHIndices(enb,pdsch,pdsch_prbs,{'1based','re'});

    codedTrBlkSize = pdschIndInfo.G;

     

    % now I would create a bit array that carries the user data. In this example, I generated randomly but in real

    % situation, this would carry your user data (e.g, image, movie, files etc)

     

    UncodedTrBlkSize = 300; % Must be less than coded transport size

    dlschTransportBlk = round(rand(1,UncodedTrBlkSize));

    dlschTransportBlk = round(rand(1,codedTrBlkSize));

     

    % now if you pass all the information to lteDLSCH), it will generate the encoded codeword data for the transport

    % block you defined.

     

    codeword = lteDLSCH(enb,pdsch,codedTrBlkSize,dlschTransportBlk);

     

    % now if you pass the encoded data (codeword) with eNB and pdsch config to ltePDSCH(), you can generate

    % physical layer symbols for the encoded data.

     

    pdsch_sym = ltePDSCH(enb,pdsch,codeword);

    pdsch_sym_arrayIndex = 0:length(pdsch_sym)-1;

     

    % Following is to create an empty resource grid for one subframe.

     

    resourceGrid = lteDLResourceGrid(enb);

     

    % Following is to create symbols for Cell Specific Reference Signal and make a list of resource index for the

    % reference signal.

     

    rsAnt0 = lteCellRS(enb,0);

    indAnt0 = lteCellRSIndices(enb,0);

    resourceGrid(indAnt0) = rsAnt0;

     

    % Following is to create symbols for PBCH and make a list of resource index for the signal (channel)

     

    mib_bits = lteMIB(enb);

    bch_cw = lteBCH(enb,mib_bits);

     

    pbch_sym = ltePBCH(enb,bch_cw);

    pbch_sym_arrayIndex = 0:length(pbch_sym)-1;

    pbch_sym_ind = ltePBCHIndices(enb,{'1based','re'});

     

    % Following is to create symbols for PSS and make a list of resource index for the signal

     

    pss = ltePSS(enb);

    pss_arrayIndex = 0:length(pss)-1;

    pss_sym_ind = ltePSSIndices(enb,0,{'1based','re'});

     

    % Following is to create symbols for SSS and make a list of resource index for the signal

     

    sss = lteSSS(enb);

    sss_arrayIndex = 0:length(sss)-1;

    sss_sym_ind = lteSSSIndices(enb,0,{'1based','re'});

     

    % Following is to create symbols for PCFICH and make a list of resource index for the signal

     

    cfi_cw = lteCFI(enb);

    pcfich_sym = ltePCFICH(enb,cfi_cw);

    pcfich_sym_arrayIndex = 0:length(pcfich_sym)-1;

    pcfich_sym_ind = ltePCFICHIndices(enb,{'1based','re'});

     

    % Following is to create symbols for PHICH and make a list of resource index for the signal

     

    PHICH_Group_Index = 0;

    PHICH_Sequence_Index = 1;

    HARQ_Indicator_Value = 0; % 0 = NACK, 1 = ACK

     

    phich_sym = ltePHICH(enb,[PHICH_Group_Index,PHICH_Sequence_Index,HARQ_Indicator_Value]);

    phich_sym_arrayIndex = 0:length(phich_sym)-1;

    phich_sym_ind = ltePHICHIndices(enb,{'1based','re'});

     

    % Following part is filling the resource grid with each of the signal..

     

    pss_scale = 1.0;

    sss_scale = 1.0;

    phich_scale = 1.0;

    pcfich_scale = 1.0;

    pbch_scale = 1.0;

    pdcch_scale = 1.0;

    pdsch_scale = 1.0;

     

    resourceGrid(pss_sym_ind) = pss_scale .* pss;

    resourceGrid(sss_sym_ind) = sss_scale .* sss;

    resourceGrid(pcfich_sym_ind) = pcfich_scale .* pcfich_sym;

    resourceGrid(phich_sym_ind) = phich_scale .* phich_sym;

    resourceGrid(pbch_sym_ind) = pbch_scale .* pbch_sym(1:length(pbch_sym_ind));

    resourceGrid(pdcch_sym_ind) = pdcch_scale .* pdcch_sym;

    resourceGrid(pdsch_sym_ind) = pdsch_scale .* pdsch_sym;

     

    % This is not a required step, but I tried to change the name resourceGrid to a more meaningful name.

     

    txSubframe = resourceGrid;

     

    % This is the step to retrieve PDSCH physical layer symbol from the resourceGrid. In this example, I used the

    % pdsch_sym_ind that I used to generate PDSCH. But in real situration, the reciever should figure out the indice

    % based on enb parameter and DCI parameters.

     

    RxPdschSym = txSubframe(pdsch_sym_ind);

     

    % Once you get PDSCH physical layer symbols (I/Q) , just pass it into ltePDSCHDecode() and it will return the

    % codeword data. As you see, I used the same variable enb, pdsch that I used when I was generating PDSCH.

    % But in real situration, the reciever should figure out the indice based on enb parameter and DCI parameters.

     

    [rxCW,RxSymbols] = ltePDSCHDecode(enb,pdsch,RxPdschSym);

     

    % rxCW that was obtained in previous step is a 'cell' object. It is not in the form of ordinary array or vector format.

    % To convert the cell object into a vector, you can use cell2mat() function as follows.

     

    rxCWarray=cell2mat(rxCW);

     

    % if you print out rxCWarray (e.g, rxCWarray(1:10)), you will notice it is not a bit sequence (binary data).

    % they are real numbers. To convert the real number to binary number corresponding to 0 or 1, you can use

    % 'rxCWarray>0'. And use isequal() to compare the decoded PDSCH and original PDSCH codeword to verify

    % if the decoding processing was performed properly.

     

    isequal(int8(rxCWarray>0),double(codeword))

     

    decState = [];

     

    chanConfig.NLayers = pdsch.NLayers;

    chanConfig.TxScheme = pdsch.TxScheme;

    chanConfig.Modulation = pdsch.Modulation;

    chanConfig.RV = pdsch.RV;

    chanConfig.NTurboDecIts = 5;

     

    [rxTrBlk,blockCrc,decState] = lteDLSCHDecode(enb,chanConfig,UncodedTrBlkSize,rxCWarray,decState);

    isequal(int8(rxTrBlk'),int8(dlschTransportBlk))

     

     

 

Disclaimer ! :

 

This page is only to show you the overall logics and visualization for various LTE physical layer channels. I haven't investigated much about verifying about the accuracy.

If you think the code is not so efficient, it is 100% my fault. I haven't made any effort for effiecient code. I just tried to create code as simple as possible for the readers. As you know, easy-to-read code is not always efficient for a specific chipset.

If you find any mistake in terms of accuracy, it is also very highly likely be my fault. Not the problem of Matlab tool box itself.

Any comment and corrections if you find any mistake will be welcome and appreciated.