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Enhanced Physical Downlink Control Channel (EPDCCH) Generation

This example shows how to generate an Enhanced Physical Downlink Control Channel (EPDCCH) transmission using the LTE System Toolbox™.

Introduction

This example shows how to generate a downlink transmission including EPDCCH, EPDCCH Demodulation Reference Signal (DMRS), Cell-Specific Reference Signal (CRS) and Channel State Information Reference Signal (CSI-RS). Beamforming of the EPDCCH is included for both Localized and Distributed transmission. The output of the example is a resource grid populated with the transmitted channels and an OFDM modulated time-domain waveform which transmits that resource grid. A plot is also produced which details the Resource Element (RE) usage for each of the channels.

eNodeB Configuration

Cell-wide settings are configured with a structure enb. This structure contains parameters that belong to the eNodeB.

% Number of downlink resource blocks corresponding to 5MHz bandwidth
enb.NDLRB = 25;

% Duplexing mode: 'FDD' or 'TDD'
enb.DuplexMode = 'FDD';

% Number of Cell-specific Reference Signal (CRS) antenna ports
enb.CellRefP = 1;

% Subframe number
enb.NSubframe = 0;

% Cyclic prefix length: 'Normal' or 'Extended'
enb.CyclicPrefix = 'Normal';

% Frame number
enb.NFrame = 0;

% Cell identity
enb.NCellID = 0;

% Channel State Information Reference Signal (CSI-RS) subframe schedule
enb.CSIRSPeriod = 'On';

% Configuration index of the CSI-RS
enb.CSIRSConfig = 1;

% Number of CSI-RS antenna ports in use with this configuration
enb.CSIRefP = 2;

% Zero-Power CSI-RS subframe schedule
enb.ZeroPowerCSIRSPeriod = 'Off';

EPDCCH Configuration

A structure chs is created, containing parameters relevant to the transmission of a DCI message which will be coded and modulated on the EPDCCH channel. Unlike the structure enb above which configures cell-wide settings, the structure here configures channel-specific settings for the transmission of a particular channel, in this case the EPDCCH. (The structure name chs is an abbreviation of "channel-specific".)

% DCI format to send on the EPDCCH
chs.DCIFormat = 'Format1A';

% Radio Network Temporary Identifier (RNTI)
chs.RNTI = 1;

% Transmission type: 'Localized' or 'Distributed'
chs.EPDCCHType = 'Localized';

% Zero-based indices of PRB pair set associated with EPDCCH search space
chs.EPDCCHPRBSet = 4:5;

% Initial transmission symbol for EPDCCH transmission
chs.EPDCCHStart = 2;

% Scrambling identity for the EPDCCH
chs.EPDCCHNID = 0;

% EPDCCH format
chs.EPDCCHFormat = 1;

Subframe Resource Grid Creation

An empty resource grid subframe is created for one subframe. In this example, the 3rd dimension (planes) of this resource grid are intended to represent physical antennas. The mapping between antenna ports and physical antennas for the various channels and signals used in this example will be described when the channels and signals are mapped to this resource grid. The empty subframe resource grid is created using the lteDLResourceGrid function. The optional second input argument allows the 3rd dimension size (the number of planes) to be specified explicitly (by default the value of enb.CellRefP, the number of CRS ports, is used to determine the number of planes). Note that for distributed EPDCCH transmission, two antenna ports are used and therefore a minimum of two physical antennas are required; if the number of CRS ports or CSI-RS ports is greater than 2 then the largest of these values is used, this allows the resource grid to contain all configured CRS or CSI-RS ports.

maxEpdcchPorts = 2;
nTxAnts = max([enb.CSIRefP enb.CellRefP maxEpdcchPorts]);
subframe = lteDLResourceGrid(enb,nTxAnts);

DCI Message Creation

A DCI message of the format indicated by chs.DCIFormat is created using the function lteDCI. The output structure dci represents the DCI message as described in TS36.212 Section 5.3.3.1 [ 3 ]. The bit vector dciBits contains the actual message bits to be encoded.

% Create a DCI message
[dci,dciBits] = lteDCI(enb,chs);

EPDCCH Candidate Selection

The EPDCCH is transmitted in an "EPDCCH candidate", a set of Enhanced Control Channel Elements (ECCEs). Each ECCE maps onto a set of Enhanced Resource Element Groups (EREGs), which in turn map onto particular resource elements in the subframe resource grid. In order to create the resource element indices for a particular EPDCCH transmission, an EPDCCH candidate must be chosen. The function lteEPDCCHSpace creates a matrix of valid EPDCCH candidates for the given configuration, with each row of the matrix giving the inclusive [begin,end] indices of a single EPDCCH candidate. Any row of this matrix can therefore be extracted and assigned to the chs.EPDCCHECCE parameter field which will be used by the function lteEPDCCHIndices to determine the resource element indices for the EPDCCH transmission.

candidates = lteEPDCCHSpace(enb,chs);

chs.EPDCCHECCE = candidates(3,:);

EPDCCH Data Bit Capacity

In order to determine the EPDCCH data bit capacity, the function lteEPDCCHIndices is used, which creates the resource element indices for the EPDCCH, following TS36.211 Section 6.8A.5 [ 3 ]; these indices will be used later for mapping the EPDCCH transmission to the subframe resource grid. This function also returns a structure containing useful EPDCCH "dimensionality information" values including epdcchInfo.EPDCCHG, the EPDCCH data bit capacity, which will be used to configure the rate matching when encoding the DCI message.

% Calculate EPDCCH resource element indices and associated dimensionality
% information including the EPDCCH data bit capacity epdcchInfo.EPDCCHG
[epdcchIndices,epdcchInfo] = lteEPDCCHIndices(enb,chs);

DCI Message Encoding

Next, the bit vector dciBits is passed to the function lteDCIEncode which performs CRC insertion, tail-biting convolutional coding and rate matching, following TS36.212 Sections 5.3.3.2 to 5.3.3.4 [ 3 ]. Note that the third argument to lteDCIEncode specifies the rate matching capacity (for non-enhanced PDCCH transmissions, this third argument can be omitted and the rate matching capacity is derived internally from the PDCCH format).

% Perform DCI message encoding with a rate matching output size equal to
% the EPDCCH data bit capacity
codedDciBits = lteDCIEncode(chs,dciBits,epdcchInfo.EPDCCHG);

EPDCCH Modulation

The EPDCCH modulation is performed using the function lteEPDCCH, following TS36.211 Sections 6.8A-2 to 6.8A-4 [ 2 ]. The resulting symbols epdcchSymbols will be mapped to the subframe resource grid after appropriate beamforming. The EPDCCH is transmitted on a subset of the antenna ports 107...110:

  • For chs.EPDCCHType='Localized', the EPDCCH is transmitted on a single antenna port chosen from 107...110 as a function of a number of parameters including the RNTI.

  • For chs.EPDCCHType='Distributed', the EPDCCH is transmitted on two antenna ports, either {107,109} for normal cyclic prefix or {107,108} for extended cyclic prefix.

The indices produced by lteEPDCCHIndices map antenna ports 107...110 (0-based) to planes 1...4 (1-based) of a subframe resource grid. A plane is the third dimension of a subframe resource grid as described in the Data Structures documentation. Typically LTE System Toolbox functions separate the symbols and indices for different antenna ports into separate columns. However for the EPDCCH the symbols and their indices are presented in a single column and the indices therein correspond to the appropriate antenna port for each resource element. This approach is taken for two reasons:

  • The number of EPDCCH symbols mapped to each antenna port may be different.

  • The mapping between symbols and antenna ports is dependent on many parameters and to represent this in lteEPDCCH would result in a very significant increase in the number of parameters required by this function.

Beamforming of the EPDCCH for transmission upon physical antennas will be described later.

epdcchSymbols = lteEPDCCH(enb,chs,codedDciBits);

EPDCCH DMRS Modulation

The DMRS associated with the EPDCCH, epdcchDmrsSymbols, is created using the function lteEPDCCHDMRS, following TS36.211 Section 6.10.3A.1 [ 2 ]. The associated resource element indices, epdcchDmrsIndices, are also created using the function lteEPDCCHDMRSIndices, following TS36.211 Section 6.10.2.A.2 [ 2 ]. The arrangement of DMRS symbols and their indices for different antenna ports mirrors that described for the EPDCCH symbols and indices above. Beamforming of the EPDCCH DMRS for transmission upon physical antennas will be described later.

epdcchDmrsSymbols = lteEPDCCHDMRS(enb,chs);
epdcchDmrsIndices = lteEPDCCHDMRSIndices(enb,chs);

Generate CRS and CSI-RS

The CRS and CSI-RS signals and their corresponding resource element indices are created, and the signals are mapped into the subframe resource grid:

  • The indices produced by lteCellRSIndices map antennas ports 0...3 (0-based) to planes 1...4 (1-based) of the subframe resource grid. In this example, a single CRS port (enb.CellRefP=1) is configured and therefore the CRS will be mapped to the first plane of the subframe resource grid.

  • The indices produced by lteCSIRSIndices map antenna ports 15...22 (0-based) to planes 1...8 (1-based) of the subframe resource grid. In this example, two CSI-RS ports (enb.CSIRefP=2) are configured and therefore the CSI-RS will be mapped to the first two planes of the subframe resource grid.

This mapping matches the generic beamforming model described in TS36.101 Annex B.4.3 [ 1 ].

% Create CSI-RS and map to the subframe resource grid
csirsIndices = lteCSIRSIndices(enb);
csirsSymbols = lteCSIRS(enb);
subframe(csirsIndices) = csirsSymbols;

% Create CRS and map to the subframe resource grid
crsIndices = lteCellRSIndices(enb);
crsSymbols = lteCellRS(enb);
subframe(crsIndices) = crsSymbols;

Beamforming of EPDCCH Transmission

The EPDCCH and its DMRS must now be beamformed and mapped to physical antennas for transmission. The beamforming vectors here are chosen in accordance with TS36.101 Annex B.4.4 [ 1 ] for distributed transmission and TS36.101 Annex B.4.5 [ 1 ] for localized transmission.

In preparation for beamforming, the EPDCCH symbols are concatenated with the EPDCCH DMRS symbols and the corresponding indices are also concatenated. The EPDCCH and its DMRS must undergo the same beamforming, therefore they can be processed together when applying the beamforming.

% Concatenate EPDCCH symbols/indices with EPDCCH DMRS symbols/indices to
% facilitate beamforming
allSymbols = [epdcchSymbols; epdcchDmrsSymbols];
allIndices = [epdcchIndices; epdcchDmrsIndices];

The complete set of indices allIndices is then converted to subscripts and the subscripts are processed to obtain a list of active EPDCCH antenna ports ports and active EPDCCH resource blocks rbs. ports and rbs will be used to control two loops which carry out the beamforming on a per port and per resource block basis.

% Determine the set of EPDCCH antenna ports 'ports' and resource blocks
% 'rbs' used by the EPDCCH and its DMRS; the value of 4 below is the
% number of EPDCCH antenna ports (107...110) on which an EPDCCH might be
% transmitted (i.e. 'allIndices' may generally contain indices for any of
% the 4 EPDCCH antenna ports).
[K,L,~] = size(subframe);
[resubs,~,portsubs] = ind2sub([K L 4],allIndices);
rbsubs = floor((resubs-1)/12)+1;
rbs = unique(rbsubs);
ports = unique(portsubs.');

For localized EPDCCH transmission, the beamforming described in TS36.101 Annex B.4.5 [ 1 ] uses a single random beamforming vector across all resource blocks.

% For localized transmission, a single beamforming vector 'W' is used for
% the single EPDCCH port and across all resource blocks
if (strcmpi(chs.EPDCCHType,'Localized'))
    codebookIdx = randi([0 3],1);
    W = lteDLPrecode(1,nTxAnts,'SpatialMux',codebookIdx);
end

For distributed EPDCCH transmission, the beamforming described in TS36.101 Annex B.4.4 [ 1 ] uses a different beamforming vector W for each resource block and each of the two antenna ports used. Therefore the beamforming vector selection is carried out in loops across the set of resource blocks and ports. Note that the loops below are applicable for both distributed and localized transmission: for localized transmission the beamforming vector selected above is applied to each resource block (for the single port used), whereas for distributed transmission the beamforming vector is both selected and applied for each resource block and antenna port. The code is structured as follows:

For each antenna port and for each Resource Block (RB):

  • the vector of logical values thisport is true for EPDCCH/DMRS symbols which apply to the current antenna port.

  • the vector of logical values thisrb is true for EPDCCH/DMRS symbols which apply to the current RB.

  • For distributed transmission, select a beamforming vector W

  • Apply beamforming to the EPDCCH/DMRS symbols and EPDCCH/DMRS indices for the current antenna port and current RB. To assist with the beamforming of indices, the function lteExtractResources is used. The call to this function takes the resource elements indices for a given antenna port as the input. The first output of the function, which is skipped here, is the set of REs for all physical antennas in the subframe resource grid subframe in the same time/frequency locations as the input indices. The second output, which is used as our beamformed indices, contains the corresponding indices for the REs in the first output, essentially a projection of the input indices onto all physical antennas. The symbols and indices for the current antenna port and current RB are obtained by using the logical vectors thisport and thisrb to extract the appropriate elements of the EPDCCH/DMRS symbols allSymbols and EPDCCH/DMRS indices allIndices.

  • Map the beamformed EPDCCH/DMRS to the subframe resource grid.

% Matrix to store the codebook index used for each resource block and
% antenna port
codebookIdxs = zeros(length(rbs),length(ports));

% For each EPDCCH antenna port in use:
for p=1:length(ports)
    thisport = (portsubs==ports(p));

    % For each RB in use:
    for r = 1:length(rbs)
        thisrb = (rbsubs==rbs(r));

        % For distributed transmission, a beamforming vector 'W' is
        % selected for each EPDCCH port and each resource block; the
        % beamforming vector chosen for a particular resource block must be
        % different on each of the antenna ports
        if (strcmpi(chs.EPDCCHType,'Distributed'))
            unusedIdxs = setxor(0:3,codebookIdxs(r,1:p-1));
            codebookIdx = unusedIdxs(randi(length(unusedIdxs),1));
            W = lteDLPrecode(1,nTxAnts,'SpatialMux',codebookIdx);
        end

        % Record the codebook index used for this resource block and
        % antenna port
        codebookIdxs(r,p) = codebookIdx;

        % Apply beamforming to the symbols and indices
        bfSymbols = allSymbols(thisport & thisrb) * W;
        [~,bfIndices] = ...
            lteExtractResources(allIndices(thisport & thisrb),subframe);

        % Map the beamformed symbols to the physical antennas. The symbols
        % must be added to the existing subframe resource grid because
        % different ports may share the same time/frequency locations and
        % therefore the beamformed symbols must be superposed.
        subframe(bfIndices) = subframe(bfIndices) + bfSymbols;

    end

end

Plot Resource Element Assignments

The subframe resource grid is plotted to indicate the locations of the EPDCCH, EPDCCH DMRS, CRS and CSI-RS. If chs.EPDCCHStart>0, the location of the legacy control region is also shown.

hEPDCCHGenerationPlot(enb,chs);

OFDM Modulation

Finally the subframe resource grid is OFDM modulated; the resulting matrix has two columns; each column contains the complex baseband time-domain waveform samples for each physical antenna.

waveform = lteOFDMModulate(enb,subframe);

Appendix

This example uses the helper function:

Selected Bibliography

  1. 3GPP TS 36.101 "User Equipment (UE) radio transmission and reception"

  2. 3GPP TS 36.211 "Physical channels and modulation"

  3. 3GPP TS 36.212 "Multiplexing and channel coding"

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