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NR Cell Performance Evaluation with MIMO

This example models a 5G New Radio (NR) cell with multiple-input multiple-output (MIMO) antenna configuration and evaluates the network performance. You can customize the scheduling strategy to leverage the downlink MIMO capabilities and analyze the performance. The example performs measurement on multi-port channel state information reference signals (CSI-RS) which gNB uses to make MIMO scheduling decisions.

Introduction

MIMO improves network performance by improving the cell throughput and reliability. The example performs layer mapping and precoding to utilize MIMO in the downlink (DL) direction. The CSI report sent by UEs serves as an input to take MIMO scheduling decisions.

The example considers these operations within gNB and UEs that facilitate DL transmission and reception.

This example models:

  • Single-codeword DL spatial multiplexing to perform multi-layer transmission. Single-codeword limits the number of transmission layers to 4.

  • DL precoding to map the transmission layers to antenna ports. The example assumes one-to-one mapping from antenna ports to physical antennas.

  • DL channel quality measurement by UEs based on the multi-port CSI-RS received from the gNB. The same CSI-RS configuration applies to all the UEs.

  • Precoding matrix indicator (PMI) and channel quality indicator (CQI) reporting by UEs. The example does not support rank estimation and provides the rank to be reported by a UE as a configuration parameter. A UE reports the PMI and CQI with respect to the configured rank. The example supports Type-1 single-panel codebook for PMI.

  • PDSCH demodulation reference signal (DM-RS).

  • Free space path loss (FSPL), additive white Gaussian noise (AWGN), and clustered delay line (CDL) propagation channel model.

Nodes send the control packets (DL assignment, PDSCH feedback, and CSI report) out of band, without the need of resources for transmission and assured error-free reception.

DL MIMO

These are the key aspects of DL-MIMO.

Spatial multiplexing

Spatial multiplexing utilizes MIMO to perform multi-layer transmission. The minimum of number of transmit and receive antennas limits the number of layers (or maximum rank). The layer mapping process maps the modulated symbols of the codeword onto different layers. It maps every nth symbol of the codeword to nth layer. For instance, this figure shows the mapping of a codeword onto four layers.

Precoding

Precoding, which follows the layer mapping, maps the transmission layers to antenna ports. Precoding applies a precoding matrix to the transmission layers and outputs data streams to the CSI-RS antenna ports.

CSI reporting

CSI reporting is the process by which a UE, for DL transmissions, advises a suitable number of transmission layers (rank), PMI, and CQI values to the gNB. The UE estimates these values by performing channel measurements on its configured CSI-RS resources. For more details, see the 5G NR Downlink CSI Reporting example. The gNB scheduler uses this advice to decide the number of DL transmission layers, precoding matrix, modulation and coding scheme (MCS).

NR Protocol Stack

A node (gNB or UE) is a composition of NR stack layers. The helper classes hNRGNB.m and hNRUE.m create gNB and UE nodes, respectively, containing the radio link control (RLC), medium access protocol (MAC), and physical layer (PHY). For more details, see the NR Cell Performance Evaluation with Physical Layer Integration example.

Scenario Configuration

Configure simulation parameters in the simParameters structure.

rng('default'); % Reset the random number generator
simParameters = []; % Clear the simParameters variable
simParameters.NumFramesSim = 10; % Simulation time in terms of number of 10 ms frames
simParameters.SchedulingType = 0; % Set the value to 0 (slot based scheduling) or 1 (symbol based scheduling)

Specify the number of UEs in each cell, assuming that UEs have sequential radio network temporary identifiers (RNTIs) from 1 to simParameters.NumUEs. If you change the number of UEs, ensure that these simulation parameters are arrays of length equal to the value of simParameters.NumUEs: simParameters.UEDistance, simParameters.DLPacketPeriodicityUEs, simParameters.DLPacketSizesUEs, simParameters.UERxAnts.

simParameters.NumUEs = 4;
simParameters.UEDistance = [450; 600; 1000; 1500];
% Validate the distance of UEs from the gNB
validateattributes(simParameters.UEDistance, {'numeric'}, {'nonempty', 'vector', 'finite', 'numel', simParameters.NumUEs, '>', 0}, 'simParameters.UEDistance', 'UEDistance');

Specify the antenna counts at the gNB and UEs.

simParameters.GNBTxAnts = 16;
simParameters.UERxAnts = [2; 2; 2; 2];

Set the channel bandwidth to 10 MHz and the subcarrier spacing (SCS) to 15 kHz as defined in 3GPP TS 38.104 Section 5.3.2.

simParameters.NumRBs = 52;
simParameters.SCS = 15; % kHz
simParameters.DLBandwidth = 10e6; % Hz
simParameters.DLCarrierFreq = 2.635e9; % Hz

Specify the CSI-RS configuration.

simParameters.CSIRSRowNumber = 11; % CSI-RS row number as per 3GPP TS 38.211 Table 7.4.1.5.3-1
simParameters.CSIRSSubcarrierLocation = [1 3 5 7];
simParameters.CSIRSSymbolLocation = 0;
simParameters.CSIRSPeriod = [10 2];  % Slot periodicity and offset

Specify the CSI report configuration.

simParameters.PMIMode = 'Subband';  % 'Wideband' or 'Subband'
simParameters.CQIMode = 'Subband';  % 'Wideband' or 'Subband'
simParameters.PanelDimensions = [8 1]; % [N1 N2] as per 3GPP TS 38.214 Table 5.2.2.2.12
simParameters.SubbandSize = 4; % Refer TS 38.214 Table 5.2.1.4-2 for valid subband sizes
simParameters.CodebookMode = 1; % 1 or 2

Set the rank to be advised by UEs in CSI report. The example does not support rank estimation so the UEs use this rank to advice suitable PMI and CQI. For each UE, set a number less than or equal to the minimum of gNB's transmit antennas and UE's receive antennas. This is just the advised rank and you can customize the gNB scheduler to use a different rank for PDSCH transmission.

simParameters.RankIndicator = [2 2 2 2]; 

Specify the signal-to-interference-plus-noise ratio (SINR) to a CQI index mapping table for a block error rate (BLER) of 0.1.

simParameters.SINR90pc = [-7.46 -2.46 2.54 7.05 9.54 12.04 13.54 16.04 ...
   18.04 20.43 22.93 23.43 25.43 28.43 31.43];

Specify the transmit power.

simParameters.GNBTxPower = 35; % Tx power for gNB in dBm

Specify the scheduling strategy and the maximum limit on the RBs allotted for PDSCH. The transmission limit applies only to new PDSCH assignments, and not to the retransmissions.

simParameters.SchedulerStrategy = 'PF'; % Supported scheduling strategies: 'PF', 'RR', and 'BestCQI'
simParameters.RBAllocationLimitDL = 25; % For PDSCH

Logging and Visualization Configuration

The CQIVisualization and RBVisualization parameters control the display of the CQI visualization of RBs and the RB assignment visualization. To enable the RB visualization plot, set the RBVisualization field to true.

simParameters.CQIVisualization = true;
simParameters.RBVisualization = false;

The example updates the metrics plots periodically. Set the number of updates during the simulation.

simParameters.NumMetricsSteps = 20;

Write the logs to MAT-files. The example uses these logs for post-simulation analysis and visualization.

simParameters.ParametersLogFile = 'simParameters'; % For logging the simulation parameters
simParameters.SimulationLogFile = 'simulationLogs'; % For logging the simulation logs

Application traffic configuration

Set the periodic DL application traffic pattern for UEs.

simParameters.DLPacketPeriodicityUEs = [2; 2; 3; 3]; % Periodicity (in ms) at which gNB generates the DL packets
simParameters.DLPacketSizesUEs = [10000; 9000; 7500; 7500]; % Size of the generated DL packets (in bytes)
% Validate the DL traffic periodicity
validateattributes(simParameters.DLPacketPeriodicityUEs, {'numeric'}, {'nonempty', 'integer', 'vector', 'numel', simParameters.NumUEs, 'finite', '>', 0}, 'simParameters.DLPacketPeriodicityUEs', 'DLPacketPeriodicityUEs');
% Validate the size of DL packet generated by gNBs for UEs
validateattributes(simParameters.DLPacketSizesUEs, {'numeric'}, {'nonempty', 'integer', 'nrows', simParameters.NumUEs, 'finite', '>=', 0}, 'simParameters.DLPacketSizesUEs', 'DLPacketSizesUEs');

Derived Parameters

Compute the derived parameters based on the primary configuration parameters specified in the previous section and set some example-specific constants.

simParameters.DuplexMode = 0; % FDD (Value as 0) or TDD (Value as 1)
simParameters.NCellID = 1; % Physical cell ID
simParameters.Position = [0 0 0]; % Position of gNB in (x,y,z) coordinates
simParameters.ChannelModelType = 'CDL'; % To model CDL propagation channel
simParameters.ULBandwidth = simParameters.DLBandwidth; % Hz
simParameters.ULCarrierFreq = 2.6454e9; % Hz

Compute the slot duration for the selected SCS and the number of slots in a 10 ms frame.

slotDuration = 1/(simParameters.SCS/15); % In ms
numSlotsFrame = 10/slotDuration; % Number of slots in a 10 ms frame
numSlotsSim = simParameters.NumFramesSim * numSlotsFrame; % Number of slots in the simulation

Set the interval at which the example updates metrics visualization in terms of number of slots. Because this example uses a time granularity of one slot, the MetricsStepSize field must be an integer.

simParameters.MetricsStepSize = ceil(numSlotsSim / simParameters.NumMetricsSteps);
if mod(numSlotsSim, simParameters.NumMetricsSteps) ~= 0
    % Update the NumMetricsSteps parameter if NumSlotsSim is not
    % completely divisible by it
    simParameters.NumMetricsSteps = floor(numSlotsSim / simParameters.MetricsStepSize);
end

Specify one logical channel for each UE, and set the logical channel configuration for all nodes (UEs and gNBs) in the example.

numLogicalChannels = 1; 
simParameters.LCHConfig.LCID = 4;

Specify the RLC entity direction as 0 to specify DL only, 1 to specify UL only, or 2 to specify both UL and DL.

simParameters.RLCConfig.EntityDir = 0;

Create RLC channel configuration structure.

rlcChannelConfigStruct.LCGID = 1; % Mapping between logical channel and logical channel group ID
rlcChannelConfigStruct.Priority = 1; % Priority of each logical channel
rlcChannelConfigStruct.PBR = 8; % Prioritized bitrate (PBR), in kilobytes per second, of each logical channel
rlcChannelConfigStruct.BSD = 10; % Bucket size duration (BSD), in ms, of each logical channel
rlcChannelConfigStruct.EntityType = simParameters.RLCConfig.EntityDir;
rlcChannelConfigStruct.LogicalChannelID = simParameters.LCHConfig.LCID;

Set the maximum RLC service data unit (SDU) length, in bytes, as specified in 3GPP TS 38.323.

simParameters.maxRLCSDULength = 9000;

Set the simulation tick granularity (in terms of OFDM symbols) and the mapping type as per the configured scheduling type.

if ~isfield(simParameters, 'SchedulingType') || simParameters.SchedulingType == 0 % If no scheduling type is specified or slot based scheduling is specified
    rbAssignmentPlotPeriodicity = numSlotsFrame; % Update RB assignment visualization every frame (10 ms)
    simParameters.PDSCHMappingType = 'A';
else % Symbol based scheduling
    rbAssignmentPlotPeriodicity = 1; % Update RB assignment visualization every slot
    simParameters.PDSCHMappingType = 'B';
end

gNB and UEs Setup

Create the gNB and UE objects, initialize the channel quality information for UEs, and set up the logical channel at the gNB and UE. The helper classes hNRGNB.m and hNRUE.m create the gNB node and the UE node, respectively, each containing the RLC, MAC and PHY.

gNB = hNRGNB(simParameters); % Create gNB node
% Create scheduler
switch(simParameters.SchedulerStrategy)
    case 'RR' % Round robin scheduler
        scheduler = hNRSchedulerRoundRobin(simParameters);
    case 'PF' % Proportional fair scheduler
        scheduler = hNRSchedulerProportionalFair(simParameters);
    case 'BestCQI' % Best CQI scheduler
        scheduler = hNRSchedulerBestCQI(simParameters);
end
addScheduler(gNB, scheduler); % Add scheduler to gNB

gNB.PhyEntity = hNRGNBPhy(simParameters); % Create the PHY instance
configurePhy(gNB, simParameters); % Configure the PHY
setPhyInterface(gNB); % Set the interface to PHY

% Create the set of UE nodes
UEs = cell(simParameters.NumUEs, 1);
for ueIdx=1:simParameters.NumUEs
    simParameters.Position = [simParameters.UEDistance(ueIdx) 0 0]; % Position of UE
    UEs{ueIdx} = hNRUE(simParameters, ueIdx);
    ueParam = simParameters;
    ueParam.UERxAnts = simParameters.UERxAnts(ueIdx);
    ueParam.RankIndicator = simParameters.RankIndicator(ueIdx);
    UEs{ueIdx}.PhyEntity = hNRUEPhy(ueParam, ueIdx); % Create the PHY instance
    configurePhy(UEs{ueIdx}, ueParam); % Configure the PHY
    setPhyInterface(UEs{ueIdx}); % Set up the interface to PHY
    
    % Set up logical channel at gNB for the UE
    configureLogicalChannel(gNB, ueIdx, rlcChannelConfigStruct);
    % Set up logical channel at UE
    configureLogicalChannel(UEs{ueIdx}, ueIdx, rlcChannelConfigStruct);
    % Add DL data traffic pattern generators to gNB for the UEs
    dlPacketSize = simParameters.DLPacketSizesUEs(ueIdx);
    dlDataRate = ceil(1000/simParameters.DLPacketPeriodicityUEs(ueIdx)) * dlPacketSize *8e-3;
    if dlPacketSize > simParameters.maxRLCSDULength
        dlPacketSize = simParameters.maxRLCSDULength;
    end

    % Create an object for on-off network traffic pattern for the specified
    % UE and add it to the gNB. This object generates the downlink data
    % traffic on the gNB for the UE
    dlApp = networkTrafficOnOff('PacketSize', dlPacketSize, 'GeneratePacket', true, ...
        'OnTime', simParameters.NumFramesSim/100, 'OffTime', 0, 'DataRate', dlDataRate);
    gNB.addApplication(ueIdx, simParameters.LCHConfig.LCID, dlApp);
end

Set up the packet distribution mechanism.

simParameters.MaxReceivers = simParameters.NumUEs;
% Create DL packet distribution object
dlPacketDistributionObj = hNRPacketDistribution(simParameters, 0); % 0 for DL
% Create UL packet distribution object
ulPacketDistributionObj = hNRPacketDistribution(simParameters, 1); % 1 for UL
hNRSetUpPacketDistribution(simParameters, gNB, UEs, dlPacketDistributionObj, ulPacketDistributionObj);

Processing Loop

Run the simulation slot by slot. In each slot, execute these operations.

  • Run the gNB.

  • Run the UEs.

  • Log and visualize metrics for each layer.

  • Advance the timer for the nodes and send a trigger to application and RLC layers every millisecond. The application and RLC layers execute their scheduled operations based on a 1 ms timer trigger.

Create objects for MAC and PHY logging and visualization.

simSchedulingLogger = hNRSchedulingLogger(simParameters, 0); % 0 for DL
simPhyLogger = hNRPhyLogger(simParameters, 0); % 0 for DL
visualizer = hNRMetricsVisualizer(simParameters, 'MACLogger', simSchedulingLogger, 'PhyLogger', simPhyLogger, 'VisualizationFlag', 0);

Run the processing loop.

slotNum = 0;
numSymbolsSim = numSlotsSim * 14; % Simulation time in units of symbol duration
tickGranularity = 1;
% Execute all the symbols in the simulation
for symbolNum = 1 : tickGranularity : numSymbolsSim
    
    if mod(symbolNum - 1, 14) == 0
        slotNum = slotNum + 1;
    end
    % Run the gNB
    run(gNB);
    
    % Run the UEs
    for ueIdx = 1:simParameters.NumUEs
        run(UEs{ueIdx});
    end
    
    % MAC logging
    logCellSchedulingStats(simSchedulingLogger, symbolNum, gNB, UEs, 0); % For DL
    % PHY logging
    logCellPhyStats(simPhyLogger, symbolNum, gNB, UEs);
    
    % Visualization
    % Check slot boundary
    if symbolNum > 1 && ((simParameters.SchedulingType == 1 && mod(symbolNum, 14) == 0) || (simParameters.SchedulingType == 0 && mod(symbolNum-1, 14) == 0))
        % RB assignment visualization (if enabled)
        if simParameters.RBVisualization
            if mod(slotNum, rbAssignmentPlotPeriodicity) == 0
                % Plot at slot boundary, if the update periodicity is reached
                plotRBGrids(simSchedulingLogger);
            end
        end
        % CQI grid visualization (if enabled)
        if simParameters.CQIVisualization
            if mod(slotNum, numSlotsFrame) == 0 % Plot at frame boundary
                plotCQIRBGrids(simSchedulingLogger);
            end
        end
        % If the update periodicity is reached, plot scheduler metrics and PHY metrics visualization
        % at slot boundary
        if mod(slotNum, simParameters.MetricsStepSize) == 0
            plotMetrics(visualizer, slotNum);
        end
    end
    
    % Advance timer ticks for gNB and UEs
    advanceTimer(gNB, tickGranularity);
    for ueIdx = 1:simParameters.NumUEs
        advanceTimer(UEs{ueIdx}, tickGranularity);
    end
end

Figure Channel Quality Visualization contains an axes and other objects of type uicontrol. The axes with title Channel Quality Visualization for Cell ID - 1 contains 80 objects of type rectangle.

Simulation Visualization

The example shows four runtime visualization:

Simulation Logs

The example saves the simulation parameters and logs as MAT-files for post-simulation analysis and visualization. The example saves the simulation parameters in a MAT-file with the file name as the value of configuration parameter, simParameters.ParametersLogFile. It also saves the per-time-step logs, scheduling assignment logs, and BLER logs in a MAT-file named as value of simParameters.SimulationLogFile. After the simulation, open the file to load DLTimeStepLogs, SchedulingAssignmentLogs in the workspace.

Time step logs: For details of log format, see the "Simulation Logs" section of the NR PUSCH FDD Scheduling example.

Scheduling assignment logs: Information about all the scheduling assignments. For details about the log format, see the "Simulation Logs" section of the NR FDD Scheduling Performance Evaluation example.

Block Error Rate logs: Block error information observed in the DL direction. For details of log format, see the 'Simulation Logs' section in the NR Cell Performance Evaluation with Physical Layer Integration example.

At the end of the simulation, the example compares the achieved value for system performance indicators against their theoretical peak values (considering zero overheads). Performance indicators displayed are: achieved data rate, achieved spectral efficiency and BLER observed for UEs. The peak value calculations are as per 3GPP TR 37.910.

simulationLogs = cell(1,1);
if(simParameters.DuplexMode == 0) % FDD
    logInfo = struct('DLTimeStepLogs',[], 'SchedulingAssignmentLogs',[],'BLERLogs',[]);
else
    logInfo = struct('TimeStepLogs',[], 'SchedulingAssignmentLogs',[],'BLERLogs',[]);
end
dlStats = getPerformanceIndicators(simSchedulingLogger);
[logInfo.BLERLogs, logInfo.AvgBLERLogs] = getBLERLogs(simPhyLogger); % Block Error rate logs
fprintf('\nPeak DL throughput: %0.2f Mbps. Achieved average DL Throughput: %0.2f Mbps', dlStats(1, 1), dlStats(2, 1));
Peak DL throughput: 129.40 Mbps. Achieved average DL Throughput: 52.01 Mbps
fprintf('\nAchieved average DL Goodput: %0.2f Mbps', dlStats(5, 1));
Achieved average DL Goodput: 51.95 Mbps
fprintf('\nPeak DL spectral efficiency: %0.2f bits/s/Hz. Achieved average DL spectral efficiency: %0.2f bits/s/Hz', dlStats(3, 1), dlStats(4, 1));
Peak DL spectral efficiency: 12.94 bits/s/Hz. Achieved average DL spectral efficiency: 5.20 bits/s/Hz
disp(['Block error rate for each UE in the downlink direction: [' num2str(round(logInfo.AvgBLERLogs(:, 1)', 2)) ']']);
Block error rate for each UE in the downlink direction: [0.01           0           0           0]

You can get a post-simulation visualization of logs by running the script NRPostSimVisualization. For more details about the options to run this script, see the NR FDD Scheduling Performance Evaluation example.

% Read the logs and save them in MAT-files
if(simParameters.DuplexMode == 0) % FDD
    logInfo.DLTimeStepLogs = getSchedulingLogs(simSchedulingLogger);
else % TDD
    logInfo.TimeStepLogs = getSchedulingLogs(simSchedulingLogger);
end
logInfo.SchedulingAssignmentLogs = getGrantLogs(simSchedulingLogger); % Scheduling assignments log
simulationLogs{1} = logInfo;
save(simParameters.ParametersLogFile, 'simParameters'); % Save simulation parameters in a MAT-file
save(simParameters.SimulationLogFile, 'simulationLogs'); % Save simulation logs in a MAT-file

Further Exploration

You can use this example to further explore custom scheduling.

Custom scheduling

You can modify the existing scheduling strategy to implement a custom strategy. Follow the steps in 'Further Exploration' section of the NR FDD Scheduling Performance Evaluation example, which explains the steps to write a custom scheduling strategy for single-input single-output (SISO) configuration. Additionally, MIMO configuration appends more fields to the scheduling assignment structure. Populate the fields of scheduling assignments with values for precoding matrix, number of layers as per your custom scheduling strategy. For more information about the information fields of a scheduling assignment, see the description of the scheduleDLResourcesSlot function in the hNRScheduler.m helper file.

The scheduler in the example selects the rank and precoding matrix which a UE reports in the CSI. You can also customize this behavior to select any rank and precoding matrix by overriding the function selectRankAndPrecodingMatrix in your custom scheduler. For more details, see the description of the selectRankAndPrecodingMatrix function in the hNRScheduler.m helper file

References

[1] 3GPP TS 38.104. “NR; Base Station (BS) radio transmission and reception.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[2] 3GPP TS 38.214. “NR; Physical layer procedures for data.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[3] 3GPP TS 38.321. “NR; Medium Access Control (MAC) protocol specification.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[4] 3GPP TS 38.322. “NR; Radio Link Control (RLC) protocol specification.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[5] 3GPP TS 38.323. “NR; Packet Data Convergence Protocol (PDCP) specification.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[6] 3GPP TS 38.331. “NR; Radio Resource Control (RRC) protocol specification.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[7] 3GPP TR 37.910. “Study on self evaluation towards IMT-2020 submission.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

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