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NR TDD Symbol Based Scheduling Performance Evaluation

This example shows how to implement a symbol based scheduling scheme in time division duplexing (TDD) mode to assign uplink (UL) and downlink (DL) resources and evaluates the network performance. Symbol based scheduling of resources allows shorter transmission durations spanning a few symbols in the slot. You can also switch to slot based scheduling. In TDD mode, physical uplink shared channel (PUSCH) and physical downlink shared channel (PDSCH) transmissions are scheduled in the same frequency band with separation in time domain. Scheduling strategy used in the example can be customized to evaluate the performance. A passthrough physical (PHY) layer without any physical layer processing is used, which adopts a probability-based approach to model packet reception failures. The performance of the scheduling strategy is evaluated in terms of achieved throughput and the fairness in resource sharing.

Introduction

The example considers the following operations within the gNB and UEs that facilitate UL and DL transmissions and receptions.

The complete PUSCH or PDSCH packet is transmitted in the first symbol of its allocated symbol set. Receiver processes the packet in the symbol just after the last symbol in the allocated symbol set.

This example models:

  • Configurable TDD DL-UL pattern.

  • Slot based and symbol based DL and UL scheduling. UL scheduler ensures that the UEs get the required PUSCH preparation time.

  • Noncontiguous allocation of frequency domain resources in terms of resource block groups (RBGs).

  • Configurable subcarrier spacing resulting in different slot durations.

  • Asynchronous hybrid automatic repeat request (HARQ) mechanism.

  • Periodic DL and UL application traffic pattern.

  • RLC operating in UM mode.

  • Single bandwidth part covering the entire carrier bandwidth.

Following control packets are assumed to be sent out of band i.e. without the need of resources for transmission and assured error-free reception: UL assignment, DL assignment, buffer status report (BSR), PDSCH feedback, and CQI report. These control packets follow the TDD DL and UL timings. For example, BSR and PDSCH feedback are sent in UL time while resource assignments are sent in DL time.

TDD DL-UL Pattern Configuration

NR provides a flexible way of configuring the DL and UL resources. The parameters used to define a custom TDD configuration are:

  1. DL-UL transmission periodicity in ms.

  2. Reference subcarrier spacing to calculate the number of slots in the DL-UL pattern. In this example, it is assumed to be same as actual subcarrier spacing used for transmission.

  3. Number of consecutive full DL slots at the beginning of each DL-UL pattern.

  4. Number of consecutive DL symbols in the beginning of the slot following the last full DL slot.

  5. Number of consecutive full UL slots at the end of each DL-UL pattern.

  6. Number of consecutive UL symbols in the end of the slot preceding the first full UL slot.

The example does not model flexible symbols, so the symbols with unspecified symbol type are assumed for guard period. Here is an example of resulting TDD DL-UL pattern based on these parameters. This DL-UL pattern repeats itself in the timeline.

  • reference_scs = 15 kHz (i.e. 1 ms slot), DLULPeriodicity = 5 ms, numDLSlots = 3, numDLSyms = 7, numULSlots = 1, numULSyms = 5

Number of slots in DL-UL periodicity with respect to reference SCS of 15 kHz, NumSlotsDLULPeriodicity = 5

NumberOfGuardSymbols = TotalSymbolsInPattern - TotalSymbolsWithTypeSpecified

= (14 * NumSlotsDLULPeriodicity) - (numDLSlots*14 + numDLSyms + numULSyms + numULSlots*14)

= 2 symbols

Scheduler

UL and DL schedulers distribute the UL and DL resources respectively among the UEs. You can choose any one of the implemented scheduling strategies: proportional fair (PF), best CQI, or round robin (RR). The various supported inputs to the schedulers are shown along with the scheduling strategies that consider them.

Both UL and DL schedulers run at the start of a slot when the first symbol is a DL symbol. Schedulers run in DL time so that assignments (UL and DL) can be instantly sent to UEs in the DL direction. The run time of the scheduler algorithm as well as the propagation delay is assumed to be zero. The output of scheduling operations is an array of assignments. Each assignment contains the information fields to fully define a PUSCH or PDSCH transmission.

  • UL Scheduler

The UL scheduling operation follows these two steps.

  1. Select slots to be scheduled: The criteria used in this example selects all the upcoming slots (including the current one) containing unscheduled UL symbols that must be scheduled now. Such slots must be scheduled now as they cannot be scheduled in the next slot with DL symbols, depending on the value of PUSCH preparation time capability of the UEs. It ensures that the UL resources are scheduled as close as possible to the actual transmission time.

Below are 2 examples to explain how UL slots are selected in this example for scheduling, based on PUSCH preparation time.

(i) Assuming that the UEs require PUSCH preparation time equivalent to 10 symbols, when the UL scheduler runs in Slot-A, it does not select any slot for scheduling. Because scheduling in the next slot (i.e. Slot-B) provides enough PUSCH preparation time (14 symbols) for the UL transmission in Slot-C. Later, when the UL scheduler runs in Slot-B, it selects both Slot-C and Slot-D for scheduling. Slot-D is scheduled in Slot-B itself (and not in Slot-C) because Slot-C is a full UL slot and hence does not have any DL symbols for sending the assignments in the DL direction.

(ii) Assuming that the UEs require PUSCH preparation time equivalent to 16 symbols, Slot-C is scheduled in Slot-A itself. Because scheduling in Slot-B would only provide 14 symbols of PUSCH preparation time to start transmission in Slot-C. Slot-D is scheduled in Slot-B.

2. Resource scheduling: If any slots are selected in the first step, assign the UL resources of those slots to the UEs.

  • DL scheduler

The DL scheduler runs at each slot beginning with a DL symbol and assign resources of the first upcoming slot containing DL symbols. So, when the DL scheduler runs at the start of Slot-A, it schedules DL resources of Slot-B.

Symbol based scheduling

NR allows the TTI to start at any symbol position in the slot and with TTI granularity in symbols. The figure shows the way UL scheduler operates in this example to schedule UL symbols of a slot with TTI granularity of two symbols. The slot shown contains six UL symbols. Scheduler completes the iteration of the UL symbols in three iterations with each iteration distributing the frequency resources of two symbols. The DL scheduler also follows a similar approach for DL scheduling.

Scenario Configuration

Set the parameters for simulation.

rng('default'); % Reset the random number generator
simParameters = []; % Clear simParameters variable
simParameters.NumFramesSim = 100; % Simulation time in terms of number of 10 ms frames

% Number of UEs in the simulation. UEs are assumed to have sequential radio
% network temporary identifiers (RNTIs) from 1 to NumUEs. If you change the
% number of UEs, ensure that the following simulation parameters are array
% of length equal to NumUEs: simParameters.UEDistance,
% simParameters.ULPacketPeriodicityUEs, simParameters.ULPacketSizesUEs,
% simParameters.DLPacketPeriodicityUEs, simParameters.DLPacketSizesUEs
simParameters.NumUEs = 4;
simParameters.UEDistance = [100; 150; 300; 400]; % Distance of UEs from gNB (in meters)

% Set the value of scheduling type to 0 (slot based scheduling) or 1
% (symbol based scheduling). If value is not set, the default value is 0
simParameters.SchedulingType = 1;

% Set the minimum time-domain assignment in terms of symbol duration.
% Applicable only for symbol based scheduling. For slot based scheduling,
% it is always full slot (14 symbols)
simParameters.TTIGranularity = 4;

% Define the TDD DL-UL pattern. The reference subcarrier spacing used for
% calculating slot duration for the pattern is assumed to be same as actual
% subcarrier spacing used for transmission as defined by simParameters.SCS.
% Keep only the symbols intended for guard period during DLULPeriodicity
% with type (DL or UL) unspecified
simParameters.DLULPeriodicity = 5; % Duration of the DL-UL pattern in ms
simParameters.NumDLSlots = 2; % Number of consecutive full DL slots at the beginning of each DL-UL pattern
% Define the number of consecutive DL symbols following the full DL slots
% and the number of consecutive UL symbols preceding the full UL slots. In
% the case of slot based scheduling, only simParameters.NumDLSyms is
% applicable and it defines the number of DL symbols at the start of the
% slot, while the remaining symbols of the slot are assumed to be allotted
% for guard period
simParameters.NumDLSyms = 8; % Number of consecutive DL symbols in the beginning of the slot following the last full DL slot
simParameters.NumULSyms = 4; % Number of consecutive UL symbols in the end of the slot preceding the first full UL slot
simParameters.NumULSlots = 2; % Number of consecutive full UL slots at the end of each DL-UL pattern

% Mapping between distance from gNB (first column in meters) and maximum
% achievable UL CQI value (second column). For example, if a UE is 700
% meters away from the gNB, it can achieve a maximum CQI value of 8 as the
% distance falls within the [501, 1000] meters range, as per the mapping.
% Set the distance in increasing order and the maximum achievable CQI value
% in decreasing order
simParameters.CQIvsDistance = [
    200  15;
    300  12;
    500  10;
    1000  8;
    1200  7];

% Set the periodic UL and DL application traffic pattern for UEs
simParameters.ULPacketPeriodicityUEs = [20; 30; 20; 30]; % Periodicity (in ms) at which the UL packets are generated by UEs
simParameters.ULPacketSizesUEs = [4000; 6000; 5500; 4000]; % Size of the UL packets (in bytes) generated by UEs
simParameters.DLPacketPeriodicityUEs = [20; 15; 15; 20]; % Periodicity (in ms) at which the DL packets are generated for UEs at gNB
simParameters.DLPacketSizesUEs = [6000; 5000; 10000; 8000]; % Size of the DL packets generated (in bytes) for UEs at gNB

% Medium access control (MAC) configuration
simParameters.SchedulerStrategy = 'PF'; % Supported scheduling strategies: 'PF', 'RR' and 'BestCQI'
% PUSCH preparation time. gNB ensures that PUSCH assignment is received at
% UEs PUSCHPrepTime ahead of the transmission time
simParameters.PUSCHPrepTime = 200; % In microseconds
% Maximum RBs allotted to a UE for PUSCH and PDSCH transmissions (limit is
% applicable for new transmission assignments and not for the retransmissions)
simParameters.RBAllocationLimitUL = 15; % For PUSCH
simParameters.RBAllocationLimitDL = 15; % For PDSCH
simParameters.BSRPeriodicity = 1; % Buffer status report transmission periodicity (in ms)

% PHY layer and channel configuration
% RB count for 5 MHz band with 15 kHz subcarrier spacing (SCS). The complete
% bandwidth is assumed to be allotted for PUSCH or PDSCH
simParameters.NumRBs = 25;
simParameters.SCS = 15; % kHz
simParameters.DLBandwidth = 5e6; % Hz
simParameters.ULBandwidth = 5e6; % Hz
simParameters.DLCarrierFreq = 2.595e9; % Hz
simParameters.ULCarrierFreq = 2.595e9; % Hz

% Configure parameters to update channel conditions for the UEs. Channel
% quality is periodically improved or deteriorated by CQIDelta every
% channelUpdatePeriodicity seconds for all RBs of a UE. Whether channel
% conditions for a particular UE improve or deteriorate is randomly
% determined: RB_CQI = RB_CQI +/- CQIDelta
simParameters.ChannelUpdatePeriodicity = 0.2; % In sec
simParameters.CQIDelta = 1;

% Logging and visualization configuration
% Flag to enable or disable runtime CQI visualization
simParameters.CQIVisualization = true;
% Flag to enable or disable run time visualization of RB assignment. If enabled,
% then for slot based scheduling it updates every frame (10 ms) to show RB
% allocation to the UEs for different slots of the last frame. For
% symbol based scheduling, it updates every slot to show RB allocation to
% the UEs over different symbols of the last slot
simParameters.RBVisualization = false;
% The output metrics plots are updated periodically NumMetricsSteps times within the
% simulation duration
simParameters.NumMetricsSteps = 20;
% MAT-files to write the logs into. They are used for post simulation analysis and visualization
simParameters.ParametersLogFile = 'simParameters'; % For logging the simulation parameters
simParameters.SimulationLogFile = 'simulationLogs'; % For logging the simulation logs

hNRSchedulingTDDValidateConfig(simParameters); % Validate the simulation configuration

Derived Parameters

Based on the primary configuration parameters, compute the derived parameters. Additionally, set some example specific constants.

simParameters.DuplexMode = 1; % TDD (example specific constant)
simParameters.NCellID = 1; % Physical cell ID
simParameters.GNBPosition = [0 0 0]; % Position of gNB in (x,y,z) coordinates
simParameters.NumLogicalChannels = 1; % Only 1 logical channel is assumed in each UE (Example specific constant)
slotDuration = 1/(simParameters.SCS/15); % Slot duration in ms
numSlotsFrame = 10/slotDuration; % Number of slots in 10 ms frame
numSlotsSim = simParameters.NumFramesSim * numSlotsFrame; % Simulation time in units of slot duration
numSymbolsSim = numSlotsSim * 14; % Simulation time in units of symbol duration
% Interval at which metrics visualization updates in terms of number of
% slots. Make sure that MetricsStepSize is 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

% DL and UL packet periodicities for UEs in terms of number of slots
appPeriodicityUEsSlotsUL = simParameters.ULPacketPeriodicityUEs ./ slotDuration;
appPeriodicityUEsSlotsDL = simParameters.DLPacketPeriodicityUEs ./ slotDuration;

% Mapping between logical channel and logical channel group identifier (LCGID)
simParameters.LCHConfig.LCGID = ones(simParameters.NumUEs, simParameters.NumLogicalChannels);
% Priority of each logical channel
simParameters.LCHConfig.Priority = ones(simParameters.NumUEs, simParameters.NumLogicalChannels);
% Prioritized bitrate (PBR) of each logical channel (in kilo bytes per second)
simParameters.LCHConfig.PBR = 8*ones(simParameters.NumUEs, simParameters.NumLogicalChannels);
% Bucket size duration (BSD) of each logical channel (in ms). However, the priority,
% PBR and BSD for logical channel is not relevant with single logical
% channel assumed in this example
simParameters.LCHConfig.BSD = 10*ones(simParameters.NumUEs, simParameters.NumLogicalChannels);
% Logical channel id (logical channel ID of data radio bearers starts from 4) 
simParameters.LCHConfig.LCID = ones(simParameters.NumUEs, simParameters.NumLogicalChannels) .* (4:4+simParameters.NumLogicalChannels-1);

% RLC entity direction. Value 0 represents DL only, 1
% represents UL only and 2 represents both UL and DL
% directions. Setting entity direction to have both UL and DL
simParameters.RLCConfig.EntityDir = 2*ones(simParameters.NumUEs, simParameters.NumLogicalChannels);
% Construct information for RLC logger
lchInfo = repmat(struct('LCID',[],'EntityDir',[]), [simParameters.NumUEs 1]);
for idx = 1:simParameters.NumUEs
    lchInfo(idx).LCID = simParameters.LCHConfig.LCID(idx, :);
    lchInfo(idx).EntityDir = simParameters.RLCConfig.EntityDir(idx, :);
end

% Maximum RLC SDU length (in bytes) as per 3GPP TS 38.323
simParameters.maxRLCSDULength = 9000;

% Calculate maximum achievable CQI value for the UEs based on their distance
% from the gNB
maxUECQIs = zeros(simParameters.NumUEs, 1); % To store the maximum achievable CQI value for UEs
for ueIdx = 1:simParameters.NumUEs
    % Based on the distance of the UE from gNB, find matching row in CQIvsDistance mapping
    matchingRowIdx = find(simParameters.CQIvsDistance(:, 1) > simParameters.UEDistance(ueIdx));
    if isempty(matchingRowIdx)
        maxUECQIs(ueIdx) = simParameters.CQIvsDistance(end, 2);
    else
        maxUECQIs(ueIdx) = simParameters.CQIvsDistance(matchingRowIdx(1), 2);
    end
end

% Define initial UL and DL channel quality as an N-by-P matrix,
% where 'N' is the number of UEs and 'P' is the number of RBs in the carrier
% bandwidth. The initial value of CQI for each RB, for each UE, is given
% randomly and is limited by the maximum achievable CQI value corresponding
% to the distance of the UE from gNB
simParameters.InitialChannelQualityUL = zeros(simParameters.NumUEs, simParameters.NumRBs); % To store current UL CQI values on the RBs for different UEs
simParameters.InitialChannelQualityDL = zeros(simParameters.NumUEs, simParameters.NumRBs); % To store current DL CQI values on the RBs for different UEs
for ueIdx = 1:simParameters.NumUEs
    % Assign random CQI values for the RBs, limited by the maximum achievable CQI value
    simParameters.InitialChannelQualityUL(ueIdx, :) = randi([1 maxUECQIs(ueIdx)], 1, simParameters.NumRBs);
    % Initially, DL and UL CQI values are assumed to be equal
    simParameters.InitialChannelQualityDL(ueIdx, :) = simParameters.InitialChannelQualityUL(ueIdx, :);
end

% Update periodicity of RB assignment visualization in terms of number of slots
if ~isfield(simParameters, 'SchedulingType') || simParameters.SchedulingType == 0 % If no scheduling type is specified or slot based scheduling is specified
    rbAssignmentPlotPeriodicity = numSlotsFrame; % Update every frame (10 ms)
else % Symbol based scheduling
    rbAssignmentPlotPeriodicity = 1; % Update every slot
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 gNB and UE. The helper classes hNRGNB.m and hNRUE.m create gNB and UE node respectively, containing the RLC and MAC layer. For MAC layer, hNRGNB.m uses the helper class hNRGNBMAC.m to implement the gNB MAC functionality and hNRUE.m uses hNRUEMAC.m to implement the UE MAC functionality. Schedulers are implemented in hNRSchedulerRoundRobin.m (RR), hNRSchedulerProportionalFair.m (PF), hNRSchedulerBestCQI.m (Best CQI) . All the schedulers inherit from the base class hNRScheduler.m which contains the core scheduling functionality. For RLC layer, both hNRGNB.m and hNRUE.m use hNRUMTransmitter.m to implement the functionality of the RLC transmitter and hNRUMReceiver.m to implement the functionality of the RLC receiver. Passthrough PHY layer for UE and gNB is implemented in hNRUEPassThroughPhy.m and hNRGNBPassThroughPhy.m, respectively.

simParameters.Position = simParameters.GNBPosition;
gNB = hNRGNB(simParameters); % Create the gNB node
% Create and add 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 = hNRGNBPassThroughPhy(simParameters); % Add passthrough PHY
configurePhy(gNB, simParameters);
setPhyInterface(gNB); % Set the interface to PHY layer

% 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);
    UEs{ueIdx}.PhyEntity = hNRUEPassThroughPhy(simParameters, ueIdx); % Add passthrough PHY
    configurePhy(UEs{ueIdx}, simParameters);
    setPhyInterface(UEs{ueIdx}); % Set the interface to PHY layer
    
    % Initialize the UL CQI values at gNB
    updateChannelQuality(gNB, simParameters.InitialChannelQualityUL(ueIdx, :), 1, ueIdx); % 1 for UL
    % Initialize the DL CQI values at gNB and UE. The DL CQI values
    % help gNB in scheduling, and UE in packet error probability estimation
    updateChannelQuality(gNB, simParameters.InitialChannelQualityDL(ueIdx, :), 0, ueIdx); % 0 for DL
    updateChannelQuality(UEs{ueIdx}, simParameters.InitialChannelQualityDL(ueIdx, :));
    
    % Setup the logical channel and add application traffic
    for lcIdx = 1:simParameters.NumLogicalChannels
        % Create RLC channel configuration structure
        rlcChannelConfigStruct.EntityType = simParameters.RLCConfig.EntityDir(ueIdx, lcIdx);
        rlcChannelConfigStruct.LogicalChannelID = simParameters.LCHConfig.LCID(ueIdx, lcIdx);
        rlcChannelConfigStruct.LCGID = simParameters.LCHConfig.LCGID(ueIdx, lcIdx);
        rlcChannelConfigStruct.Priority = simParameters.LCHConfig.Priority(ueIdx, lcIdx);
        rlcChannelConfigStruct.PBR = simParameters.LCHConfig.PBR(ueIdx, lcIdx);
        rlcChannelConfigStruct.BSD = simParameters.LCHConfig.BSD(ueIdx, lcIdx);
        % Setup logical channel at gNB for the UE
        configureLogicalChannel(gNB, ueIdx, rlcChannelConfigStruct);
        % Setup logical channel at UE
        configureLogicalChannel(UEs{ueIdx}, ueIdx, rlcChannelConfigStruct);
        
        % Add data traffic pattern generators to gNB and UE nodes
        ulPacketSize = simParameters.ULPacketSizesUEs(ueIdx);
        % Calculate the data rate (in kbps) of On-Off traffic pattern using
        % packet size (in bytes) and packet interval (in ms)
        ulDataRate = ceil(1000/simParameters.ULPacketPeriodicityUEs(ueIdx)) * ulPacketSize * 8e-3;
        % Limit the size of the generated application packet to the maximum RLC
        % SDU size. The maximum supported RLC SDU size is 9000 bytes
        if ulPacketSize > simParameters.maxRLCSDULength
            ulPacketSize = simParameters.maxRLCSDULength;
        end
        % Create an object for On-Off network traffic pattern and add it to the
        % specified UE. This object generates the uplink (UL) data traffic on the UE
        ulApp = networkTrafficOnOff('PacketSize', ulPacketSize, 'GeneratePacket', true, ...
            'OnTime', simParameters.NumFramesSim/100, 'OffTime', 0, 'DataRate', ulDataRate);
        UEs{ueIdx}.addApplication(ueIdx, simParameters.LCHConfig.LCID(ueIdx, lcIdx), ulApp);
        
        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 (DL) 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(ueIdx, lcIdx), dlApp);
    end
end

% Setup the UL and DL 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

Simulation is run symbol by symbol to execute corresponding operations. If slot based scheduling is chosen, the execution jumps from the current slot boundary to the next slot boundary. The operations executed are:

  • Run the MAC and PHY layers of gNB

  • Run the MAC and PHY layers of UEs

  • Layer specific logging and visualization

  • Advance the timer for the nodes. Every 1 ms it also sends trigger to application and RLC layers. Application layer and RLC layer execute their scheduled operations based on 1 ms timer trigger.

% To store the following UE metrics for each symbol: throughput bytes
% transmitted, goodput bytes transmitted, and pending buffer amount bytes
% (for logging purpose). The number of goodput bytes is calculated by
% excluding the retransmissions from the total transmissions. The complete
% transmission is assumed to be done in the first symbol of transmission.
% Metrics are for DL direction if symbol type is DL (likewise, for UL
% symbol)
UEMetrics = zeros(simParameters.NumUEs, 3);

% To store current CQI values on the RBs for the UEs for logging
channelQuality = zeros(simParameters.NumUEs, simParameters.NumRBs);

% To store the current value of last received NDI flag for HARQ processes
% of UEs for logging. NDI values are for DL HARQ processes, if symbol type is DL
% (likewise, for UL symbol)
HARQProcessStatus = zeros(simParameters.NumUEs, 16);

% To store the RLC statistics of a slot for logging
ueRLCStats = cell(simParameters.NumUEs, 1);
gNBRLCStats = cell(simParameters.NumUEs, 1);

% Create logging and visualization objects for MAC scheduling and RLC
simSchedulingLogger = hNRSchedulingLogger(simParameters);
simRLCLogger = hNRRLCLogger(simParameters, lchInfo);
if ~isfield(simParameters, 'SchedulingType') || simParameters.SchedulingType == 0 % If no scheduling type is specified or slot based scheduling is specified
    tickGranularity = 14; % Operations are only at slot boundary. Simulation jumps from one slot boundary to next
else % Symbol based scheduling
    tickGranularity = 1; % Operations can be at symbol boundary. Simulation moves symbol-by-symbol
end

slotNum = 0;
% Execute all the symbols in the simulation
for symbolNum = 1 : tickGranularity : numSymbolsSim
    UEMetrics(:) = 0;
    HARQProcessStatus(:) = 0;
    symbolType = currentSymbolType(gNB); % Get current symbol type: DL/UL/Guard
    if mod(symbolNum - 1, 14) == 0
        slotNum = slotNum + 1;
    end
    
    % Run MAC and PHY layers of gNB
    run(gNB.MACEntity);
    run(gNB.PhyEntity);
    
    % Run MAC and PHY layers of UEs
    for ueIdx = 1:simParameters.NumUEs
        % Read the last received NDI flags for HARQ processes for
        % logging (Reading it before it gets overwritten by run function of MAC)
        if symbolType == 0 % DL
            HARQProcessStatus(ueIdx, :) = getLastNDIFlagHarq(UEs{ueIdx}.MACEntity, 0); % 0 for DL
        else
            HARQProcessStatus(ueIdx, :) = getLastNDIFlagHarq(UEs{ueIdx}.MACEntity, 1); % 1 for UL
        end
        run(UEs{ueIdx}.MACEntity);
        run(UEs{ueIdx}.PhyEntity);
    end
    
    % RLC logging (only at slot boundary)
    if mod(symbolNum - 1, 14) == 0 %
        for ueIdx = 1:simParameters.NumUEs % For all UEs
            % Get RLC statistics
            ueRLCStats{ueIdx} = getRLCStatistics(UEs{ueIdx}, ueIdx);
            gNBRLCStats{ueIdx} = getRLCStatistics(gNB, ueIdx);
        end
        logRLCStats(simRLCLogger, ueRLCStats, gNBRLCStats); % Update RLC statistics logs
    end
    
    % MAC logging
    % Read UL and DL assignments done by gNB MAC scheduler
    % at current time. Resource assignments returned by a scheduler (either
    % UL or DL) is empty, if either scheduler was not scheduled to run at
    % the current time or no resources got scheduled
    [resourceAssignmentsUL, resourceAssignmentsDL] = getCurrentSchedulingAssignments(gNB.MACEntity);
    % Read throughput and goodput bytes sent for each UE
    if symbolType == 0 % DL
        [UEMetrics(:, 1), UEMetrics(:, 2)] = getTTIBytes(gNB);
        UEMetrics(:, 3) = getBufferStatus(gNB); % Read pending buffer (in bytes) on gNB, for all the UEs
    end
    for ueIdx = 1:simParameters.NumUEs
        if symbolType == 0 % DL
            % Read the DL channel quality for the UE
            channelQuality(ueIdx,:) = getChannelQuality(gNB, 0, ueIdx); % 0 for DL
        else
            % Read the UL channel quality for the UE
            channelQuality(ueIdx,:) = getChannelQuality(gNB, 1, ueIdx); % 1 for UL
            % Read throughput and goodput bytes transmitted by the UE in the current TTI
            [UEMetrics(ueIdx, 1), UEMetrics(ueIdx, 2)] = getTTIBytes(UEs{ueIdx});
            UEMetrics(ueIdx, 3) = getBufferStatus(UEs{ueIdx}); % Read pending buffer (in bytes) on UE
        end
    end
    % Update logs based on the current symbol run of UEs and gNB
    logScheduling(simSchedulingLogger, symbolNum, [resourceAssignmentsUL resourceAssignmentsDL], UEMetrics, channelQuality, HARQProcessStatus, symbolType);
    
    % Visualization
    % RB assignment visualization (if enabled)
    if simParameters.RBVisualization
        if symbolNum > 1 && (simParameters.SchedulingType == 1 && (mod(symbolNum, 14) == 0)) || (simParameters.SchedulingType == 0 && (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 symbolNum > 1 && mod(symbolNum-1, 14) == 0 && mod(slotNum, numSlotsFrame) == 0 % Plot at frame boundary
            plotCQIRBGrids(simSchedulingLogger);
        end
    end
    % Plot scheduler metrics and RLC metrics visualization at slot
    % boundary, if the update periodicity is reached
    if symbolNum > 1 && mod(symbolNum-1, 14) == 0 && mod(slotNum, simParameters.MetricsStepSize) == 0
        plotMetrics(simSchedulingLogger);
        plotMetrics(simRLCLogger);
    end
    
    % Advance timer ticks for gNB and UEs by 'tickGranularity' symbols
    advanceTimer(gNB, tickGranularity);
    for ueIdx = 1:simParameters.NumUEs % For all UEs
        advanceTimer(UEs{ueIdx}, tickGranularity);
    end
end

Simulation Visualization

The five types of runtime visualization shown are:

  • Display of CQI values for UEs over the PUSCH or PDSCH bandwidth: For details, see the 'Channel Quality Visualization' figure.

  • Display of resource grid assignment to UEs: The 2-D time-frequency grid shows the resource allocation to the UEs. For slot based scheduling, it updates every 10 ms (frame length), and shows the RB allocation to the UEs in the previous frame. For symbol based scheduling, it updates every slot, and shows the RB allocation of symbols of the previous slot. For details, see the 'Resource Grid Allocation' figure.

  • Display of UL scheduling metrics plots: The 'Uplink Scheduler Performance Metrics' figure includes plots of the: UL throughput (per UE and cell), UL goodput (per UE and cell), resource share percentage among UEs (out of the total UL resources) to convey the fairness of scheduling, and pending UL buffer status of the UEs to show whether UEs are getting sufficient resources. The maximum achievable data rate value for UL throughput is metricsStepSize slots. shown with a dashed line in throughput and goodput plots. The performance metrics plots update every

  • Display of DL scheduling metrics plots: Like uplink metrics plots, the 'Downlink Scheduler Performance Metrics' displays corresponding subplots for DL direction. The performance metrics plots update every metricsStepSize slots

  • Display of RLC metrics plot: The 'RLC Throughput Visualization' figure shows the throughput of RLC logical channel for the selected UE. The RLC metrics plot updates every metricsStepSize slots.

Simulation Logs

The parameters used for simulation and the simulation logs are saved in MAT-files for post simulation analysis and visualization. The simulation parameters are saved in MAT-file with filename as the value of configuration parameter simParameters.ParametersLogFile. The per time step logs, scheduling assignment logs, and RLC logs are saved in the MAT-file simParameters.SimulationLogFile. After the simulation, open it to load TimeStepLogs, SchedulingAssignmentLogs, and RLCLogs in the workspace.

Time step logs: The table shows a sample time step entry. Each row of the table represents a symbol or a slot, based on the chosen scheduling type (symbol based or slot based). The information in a row is for DL, if the type of symbol (or slot) is DL. Likewise, for UL symbol (or slot).

Each row contains the following information:

  • Frame: Frame number.

  • Slot: Slot number in the frame.

  • Symbol: Symbol number in the slot (Only for symbol based scheduling).

  • Type: Symbol (or Slot) type as 'DL', 'UL', or 'Guard'. For slot based scheduling, type can only be DL/UL. As the slot containing the guard symbols is assumed to be a DL slot with guard symbols at the end of the slot.

  • RBG Allocation Bitmap: N-by-P bitmap matrix, where N is the number of UEs and P is the number of RBGs in the bandwidth. If an RBG is assigned to a particular UE, the corresponding bit is set to 1. For example, [ 0 0 1 1 0 1 0 1 0 1 0 0 0; 1 1 0 0 0 0 0 0 0 0 1 0 0; 0 0 0 0 1 0 1 0 1 0 0 1 1; 0 0 0 0 0 0 0 0 0 0 0 0 0] means, that the bandwidth has 13 RBGs and UE-1 is assigned the RBG indices 2, 3, 5, 7, and 9; UE-2 is assigned the RBG indices: 0, 1, and 10; UE-3 is assigned the RBG indices: 4, 6, 8, 11, and 12; and UE-4 is not assigned any RBG.

  • MCS: Row vector of length N where N is the number of UEs. Each value corresponds to the modulation and coding scheme (MCS) index for the PUSCH or PDSCH transmission. For example, [10 12 8 -1] means that only UE-1, UE-2, and UE-3 are assigned UL resources (symbol type is 'UL') for this symbol and use MCS values 10, 12, and 8, respectively.

  • HARQ Process: Row vector of length N, where N is the number of UEs. The value is the HARQ process ID used by UE for the PUSCH transmission or used by gNB for PDSCH transmission. For example, [0 3 6 -1] means that only UE-1, UE-2, and UE-3 are assigned UL resources (symbol type is 'UL') for this symbol and use the HARQ process IDs 0, 3, and 6, respectively.

  • NDI: Row vector of length N, where N is the number of UEs. The value is the NDI flag value in the assignment for PUSCH or PDSCH transmission. For example, [0 0 1 -1] means that only UE-1, UE-2, and UE-3 are assigned UL resources for this symbol and use the NDI flag values (which determine whether a new transmission or a retransmission is done) are 0, 0, and 1, respectively.

  • Tx Type: Tx Type specifies the transmission type (new transmission or retransmission). Row vector of length N, where N is the number of UEs. Possible values are either 'newTx', 'reTx', or 'noTx'. 'noTx' means that the UE is not allotted PUSCH or PDSCH resources. For example: ['newTx' 'newTx' 'reTx' 'noTx'] means that only UE-1, UE-2, and UE-3 are assigned UL resources for this symbol. UE-1 and UE-2 transmit a new packet from the specified HARQ process, while UE-3 retransmits the packet in the buffer of the specified HARQ process.

  • CQI for UEs: N-by-P matrix, where N is the number of UEs and P is the number of RBs in the bandwidth. A matrix element at position (i, j) corresponds to the CQI value for UE with RNTI i at RB j.

  • HARQ NDI Status: N-by-P matrix, where N is the number of UEs and P is the number of HARQ processes. A matrix element at position (i, j) is the last received NDI flag at UE i for DL or UL HARQ process ID j. For new transmissions, this value and the NDI flag in the PUSCH or PDSCH assignment must toggle for the HARQ process in the assignment.

  • Throughput Bytes: Row vector of length N, where N is the number of UEs. The values represent UL or DL MAC bytes transmitted by or for the UEs in this symbol. Note that the total throughput bytes for the complete PUSCH or PDSCH transmission are shown in the row corresponding to the first symbol of the transmission.

  • Goodput Bytes: Row vector of length N, where N is the number of UEs. The values represent new UL or DL transmission MAC bytes transmitted by or for the UEs in this symbol. Like throughput, all the goodput bytes for the complete PUSCH or PDSCH are shown in the row corresponding to first symbol of the transmission.

  • Buffer Status of UEs: Row vector of length N, where N is the number of UEs. The values represent the amount of UL direction pending buffers at UEs (or DL direction pending buffers for UEs at gNB).

Scheduling assignment logs: Information of all the scheduling assignments and related information is logged in this table. Each row is one UL or DL assignment. For details of log format, see the 'Simulation Logs' section of NR FDD Scheduling Performance Evaluation example.

RLC logs: For more information on the RLC log format, see NR PUSCH FDD Scheduling.

You can run the script NRPostSimVisualization to get a post simulation visualization of logs. In the post simulation script, you are provided with variable isLogReplay, which provides these options to visualize 'Resource Grid Allocation' and 'Channel Quality Visualization' figures.

  • Set isLogReplay to true for a replay of the simulation logs.

  • Set isLogReplay to false to analyze the details of a particular frame or a particular slot of a frame. In the 'Resource Grid Allocation' window, input the frame number and slot number to visualize the resource assignment of the particular slot, if scheduling type is symbol based. For slot based scheduling, enter the frame number to visualize the resource assignment for the entire frame. The frame number entered here controls the frame number for 'Channel Quality Visualization' figure too.

% Read the logs and write them to MAT-files
% Get the logs
simulationLogs = cell(1,1);
logInfo = struct('TimeStepLogs',[], 'SchedulingAssignmentLogs',[] ,'RLCLogs',[]);
[logInfo.TimeStepLogs] = getSchedulingLogs(simSchedulingLogger);
logInfo.SchedulingAssignmentLogs = getGrantLogs(simSchedulingLogger); % Scheduling assignments log
logInfo.RLCLogs = getRLCLogs(simRLCLogger); % RLC statistics logs
simulationLogs{1} = logInfo;

save(simParameters.SimulationLogFile, 'simulationLogs'); % Save simulation logs in a MAT-file
save(simParameters.ParametersLogFile, 'simParameters'); % Save simulation parameters in a MAT-file

Further Exploration

You can use this example to further explore these options.

Custom scheduling

You can modify the existing scheduling strategy to implement a custom one. Refer to 'Further Exploration' section of NR FDD Scheduling Performance Evaluation example to see the steps involved.

Use 5G Toolbox™ physical layer

You can also switch from passthrough PHY layer to 5G Toolbox™ physical layer processing by creating PHY objects using hNRGNBPhy.m and hNRUEPhy.m. For more details, see 'gNB and UEs Setup' section of NR Cell Performance Evaluation with Physical Layer Integration. Set the simulation parameter simParameters.SchedulingType to 0 as hNRGNBPhy.m and hNRUEPhy.m only support slot based scheduling.

Based on the chosen scheduling strategy, this example demonstrates the assignment of UL and DL resources to multiple UEs by the gNB. UL and DL scheduling performance is analyzed based on runtime plots of throughput, goodput, resource share fairness, and pending buffer status of the UEs. A more thorough post simulation analysis by using the saved logs gives a detailed picture of the operations happening on a per symbol or per slot basis.

Use RLC AM

You can also switch the operating mode of an RLC entity from UM to acknowledged mode (AM) by modifying the input structure fields EntityType and SeqNumFieldLength in the configureLogicalChannel function of hNRNode.m. For more details, see 'Further Exploration' section of NR FDD Scheduling Performance Evaluation.

Appendix

The example uses these helper functions and classes:

References

[1] 3GPP TS 38.214. “NR; Physical layer procedures for data.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

[2] 3GPP TS 38.321. “NR; Medium Access Control (MAC) protocol specification.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

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

[4] 3GPP TS 38.331. “NR; Radio Resource Control (RRC) protocol specification.” 3rd Generation Partnership Project; Technical Specification Group Radio Access Network.

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