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Uplink Waveform Modeling Using SRS and PUCCH

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This example demonstrates how to configure User Equipment (UE) and cell-specific Sounding Reference Signals (SRS) transmission. Physical Uplink Control Channel (PUCCH) is also configured for transmission.

Introduction

The SRS configuration is split into 2 parts - UE-specific and cell-specific. The UE-specific part describes the schedule and content of actual SRS transmissions for this UE. The cell-specific part describes the time schedule when any UE in the cell can transmit - the UE-specific schedule must be a subset of this schedule.

In this example the cell-specific SRS configuration has 5ms periodicity with an offset of 0 (signaled by srs.SubframeConfig = 3 as indicated in TS36.211, Table 5.5.3.3-1 [ 1 ]). The UE-specific SRS configuration has 10ms periodicity with an offset of 0 (signaled by srs.ConfigIdx = 7 as indicated in TS36.213, Table 8.2-1 [ 2 ]). The cell-specific configuration means that for this cell, two opportunities for SRS transmission exist within each frame, subframes 0 and 5. All UEs in the cell must shorten their Physical Uplink Control Channel (PUCCH) transmissions during these subframes to allow for SRS reception without interference, even if they are not transmitting SRS themselves. The UE-specific configuration means that this UE is configured to generate SRS only in subframe 0.

The output at the MATLAB® command window when running this example shows PUCCH transmission in all 10 subframes, with shortening in subframes 0 and 5, and an SRS transmission in subframe 0.

UE Configuration

ue = struct;
ue.NULRB = 15;                  % Number of resource blocks
ue.NCellID = 10;                % Physical layer cell identity
ue.Hopping = 'Off';             % Disable frequency hopping
ue.CyclicPrefixUL = 'Normal';   % Normal cyclic prefix
ue.DuplexMode = 'FDD';          % Frequency Division Duplex (FDD)
ue.NTxAnts = 1;                 % Number of transmit antennas
ue.NFrame = 0;                  % Frame number

PUCCH Configuration

pucch = struct;
% Vector of PUCCH resource indices, one per transmission antenna
pucch.ResourceIdx = 0:ue.NTxAnts-1;
pucch.DeltaShift = 1;               % PUCCH delta shift parameter
pucch.CyclicShifts = 0;             % PUCCH delta offset parameter
pucch.ResourceSize = 0;             % Size of resources allocated to PUCCH

SRS Configuration

srs = struct;
srs.NTxAnts = 1;        % Number of transmit antennas
srs.SubframeConfig = 3; % Cell-specific SRS period = 5ms, offset = 0
srs.BWConfig = 6;       % Cell-specific SRS bandwidth configuration
srs.BW = 0;             % UE-specific SRS bandwidth configuration
srs.HoppingBW = 0;      % SRS frequency hopping configuration
srs.TxComb = 0;         % Even indices for comb transmission
srs.FreqPosition = 0;   % Frequency domain position
srs.ConfigIdx = 7;      % UE-specific SRS period = 10ms, offset = 0
srs.CyclicShift = 0;    % UE-cyclic shift

Subframe Loop

The processing loop generates a subframe at a time. These are all concatenated to create the resource grid for a frame (10 subframes). The loop performs the following operations:

  • SRS Information: By calling lteSRSInfo we can get information related to SRS for a given subframe. The IsSRSSubframe field of the structure srsInfo returned from the lteSRSInfo call indicates if the current subframe (given by ue.NSubframe) is a cell-specific SRS subframe (IsSRSSubframe = 1) or not (IsSRSSubframe = 0). The value of this field can be copied into the ue.Shortened field. This ensures that the subsequent PUCCH generation will correctly respect the cell-specific SRS configuration for all subframes, omitting the last symbol of the PUCCH in the cell-specific SRS subframes.

  • PUCCH 1 Demodulation Reference Signal (DRS) Generation and Mapping: The DRS signal is located in the 3rd, 4th and 5th symbols of each slot and therefore never has the potential to collide with the SRS.

  • PUCCH 1 Generation and Mapping: Unlike the DRS, the PUCCH 1 transmission can occupy the last symbol of the subframe unless ue.Shortened = 1. In this case the last symbol of the subframe will be left empty.

  • SRS Generation and Mapping: Here we generate and map the SRS according to the UE-specific SRS configuration. Both the lteSRSIndices and lteSRS functions use the fields ue.NSubframe and srs.ConfigIdx to determine if the current subframe is configured for SRS transmission; if not, the output of both functions is empty.

txGrid = [];    % Create empty resource grid

for i = 1:10    % Process 10 subframes

        % Configure subframe number (0-based)
        ue.NSubframe = i-1;
        fprintf('Subframe %d:\n',ue.NSubframe);

        % Establish if this subframe is a cell-specific SRS subframe,
        % and if so configure the PUCCH for shortened transmission
        srsInfo = lteSRSInfo(ue, srs);
        ue.Shortened = srsInfo.IsSRSSubframe; % Copy SRS info to ue struct

        % Create empty uplink subframe
        txSubframe = lteULResourceGrid(ue);

        % Generate and map PUCCH1 DRS to resource grid
        drsIndices = ltePUCCH1DRSIndices(ue, pucch);% DRS indices
        drsSymbols = ltePUCCH1DRS(ue, pucch);       % DRS sequence
        txSubframe(drsIndices) = drsSymbols;        % Map to resource grid

        % Generate and map PUCCH1 to resource grid
        pucchIndices = ltePUCCH1Indices(ue, pucch); % PUCCH1 indices
        ACK = [0; 1];                               % HARQ indicator values
        pucchSymbols = ltePUCCH1(ue, pucch, ACK);   % PUCCH1 sequence
        txSubframe(pucchIndices) = pucchSymbols;    % Map to resource grid
        if (ue.Shortened)
            disp('Transmitting shortened PUCCH');
        else
            disp('Transmitting full-length PUCCH');
        end

        % Configure the SRS sequence group number (u) according to TS
        % 36.211 Section 5.5.1.3 with group hopping disabled
        srs.SeqGroup = mod(ue.NCellID,30);

        % Configure the SRS base sequence number (v) according to TS 36.211
        % Section 5.5.1.4 with sequence hopping disabled
        srs.SeqIdx = 0;

        % Generate and map SRS to resource grid
        % (if active under UE-specific SRS configuration)
        [srsIndices, srsIndicesInfo] = lteSRSIndices(ue, srs);% SRS indices
        srsSymbols = lteSRS(ue, srs);                         % SRS seq.
        if (srs.NTxAnts == 1 && ue.NTxAnts > 1) % Map to resource grid
            % Select antenna for multiple antenna selection diversity
            txSubframe( ...
                hSRSOffsetIndices(ue, srsIndices, srsIndicesInfo.Port)) = ...
                srsSymbols;
        else
            txSubframe(srsIndices) = srsSymbols;
        end
        % Message to console indicating when SRS is mapped to the resource
        % grid.
        if(~isempty(srsIndices))
            disp('Transmitting SRS');
        end

        % Concatenate subframes to form frame
        txGrid = [txGrid txSubframe]; %#ok
end

Subframe 0:
Transmitting shortened PUCCH
Transmitting SRS
Subframe 1:
Transmitting full-length PUCCH
Subframe 2:
Transmitting full-length PUCCH
Subframe 3:
Transmitting full-length PUCCH
Subframe 4:
Transmitting full-length PUCCH
Subframe 5:
Transmitting shortened PUCCH
Subframe 6:
Transmitting full-length PUCCH
Subframe 7:
Transmitting full-length PUCCH
Subframe 8:
Transmitting full-length PUCCH
Subframe 9:
Transmitting full-length PUCCH

Results

The figure produced shows the number of active subcarriers in each SC-FDMA symbol across the 140 symbols in txGrid. All SC-FDMA symbols contain 12 active subcarriers corresponding to the single resource block bandwidth of the PUCCH except:

  • symbol 13, the last symbol of subframe 0 which has 48 active subcarriers corresponding to an 8 resource block SRS transmission

  • symbol 83, the last symbol of subframe 5 which has 0 active subcarriers corresponding to the shortened PUCCH (last symbol empty) to allow for potential SRS transmission by another UE in this cell.

figure;
for i = 1:ue.NTxAnts
    subplot(ue.NTxAnts,1,i);
    plot(0:size(txGrid,2)-1,sum(abs(txGrid(:,:,i)) ~= 0),'r:o')
    xlabel('symbol number');
    ylabel('active subcarriers');
    title(sprintf('Antenna %d',i-1));
end

Plot the resource grid with the PUCCH at the band edges and the SRS comb transmission in subframe 0.

figure;
pcolor(abs(txGrid));
colormap([1 1 1; 0 0 0.5])
shading flat;
xlabel('SC-FDMA symbol'); ylabel('subcarrier')

Further Exploration

SRS transmit antenna selection can be demonstrated by setting ue.NTxAnts = 2 and examining the subplots produced for each antenna; the SRS is transmitted on antenna 0 while the PUCCH is shortened on both (all) antennas. A pattern of antenna selection across this one-frame run can be shown by further configuring srs.SubframeConfig = 0 and srs.ConfigIdx = 0. This configures a cell-specific SRS configuration of 2ms periodicity with an offset of 0 (signaled by srs.SubframeConfig = 0) and also a UE-specific SRS configuration of 2ms periodicity with an offset of 0 (signaled by srs.ConfigIdx = 0). In this case an SRS is transmitted by this UE on even subframes, and the transmit antenna alternates with each transmission.

SRS transmission on multiple antennas using resource diversity can be shown by setting ue.NTxAnts = 2 and srs.NTxAnts = 2. In this case the SRS is always transmitted on both (all) antennas with orthogonal resources on each antenna.

Appendix

This example uses this helper function.

Selected Bibliography

  1. 3GPP TS 36.211 "Physical channels and modulation"

  2. 3GPP TS 36.213 "Physical layer procedures"