802.11ax Downlink OFDMA Multinode System-Level Simulation
This example shows how to simulate a WLAN multinode downlink (DL) orthogonal frequency-division multiple access (OFDMA) network consisting of an IEEE® 802.11ax™  access point (AP) and four stations (STAs), by using WLAN Toolbox™ and the Communications Toolbox™ Wireless Network Simulation Library.
Using this example, you can:
Simulate DL OFDMA communication from the AP to STAs.
Visualize the time spent by each node in Idle, Contention, Transmission, and Reception state.
Capture the application layer (APP), medium access control layer (MAC) and physical layer (PHY) statistics for each node.
The simulation results show performance metrics such as throughput, packet latency, and packet loss captured at each node.
Additionally, you can further explore the example by performing these tasks.
OFDM and OFDMA
The IEEE 802.11ax standard introduces significant enhancements over the existing 802.11ac standard . One of the key improvements is OFDMA, which is an extension of OFDM digital modulation technology into a multiuser (MU) environment. The fundamental objective of OFDMA is to efficiently use the available frequency space. OFDMA partitions the channel bandwidth into multiple mutually exclusive sub-bands, called resource units (RUs). By partitioning the channel bandwidth, multiple users can access the channel simultaneously. As a result, 802.11ax supports concurrent transmissions of packets to multiple users.
For example, a conventional 20 MHz channel can be partitioned into a maximum of nine subchannels. An 802.11ax AP can simultaneously transmit packets to nine 802.11ax STAs by using OFDMA. The simultaneous transmission of frames reduces excessive contention overhead at the MAC layer and minimizes the PHY layer preamble overhead. In OFDMA, the AP controls the allocation of RUs.
The 802.11ax standard specifies two types of OFDMA:
DL OFDMA - The AP transmits packets to multiple STAs simultaneously using a different RU for each STA.
Uplink (UL) OFDMA - Multiple STAs transmit packets to an AP simultaneously, with each STA using a different RU.
In this figure, the 802.11n/ac/ax AP transmits DL frames to four OFDM STAs independently over time. The AP uses the entire channel bandwidth to communicate with a single OFDM STA. Similarly, an OFDM STA uses the entire channel bandwidth to communicate with an 802.11n/ac/ax AP in an UL OFDM transmission.
This figure shows an OFDMA transmission. The 802.11ax AP partitions the channel bandwidth into RUs for four OFDMA STAs on a continuous basis for simultaneous DL transmissions.
For information about how to simulate a UL OFDMA multinode simulation, see the 802.11ax Uplink OFDMA Multinode System-Level Simulation example.
System-Level Simulation Scenario
The example creates, configures, and simulates this 802.11ax system-level scenario, consisting of one AP and four associated STAs.
In the preceding figure:
AP transmits DL data to all the STAs simultaneously.
STAs respond with a UL acknowledgment frame upon receiving the DL data frames from the AP.
Check for Support Package Installation
Check if the Communications Toolbox™ Wireless Network Simulation Library support package is installed. If the support package is not installed, MATLAB® returns an error with a link to download and install the support package.
Configure Simulation Parameters
Set the seed for the random number generator to
1. The seed value controls the pattern of random number generation. The random number generated by the seed value impacts several processes within the simulation, including backoff counter selection at the MAC layer and predicting packet reception success at the physical layer. To improve the accuracy of your simulation results after running the simulation, you can change the seed value, run the simulation again, and average the results over multiple simulations.
Specify the simulation time in seconds. To visualize a live state transition plot for all of the nodes, set the
enablePacketVisualization variable to
true. To view the node performance visualization, set the
enableNodePerformancePlot variable to
simulationTime = 0.1; enablePacketVisualization = true; enableNodePerformancePlot = true;
This example uses abstracted MAC and PHY at each WLAN node because OFDMA is only supported for abstracted MAC and PHY. For more information about abstracted MAC and PHY, see the Get Started with WLAN System-Level Simulation in MATLAB example.
PHYAbstractionMethod = "tgax-evaluation-methodology";
Configure WLAN Scenario
Initialize the wireless network simulator by using the
networkSimulator = wirelessNetworkSimulator.init;
Specify the number of nodes in the network. The example scenario consists of one AP and four associated STAs.
numNodes = 5; numSTAs = numNodes-1;
Specify the positions of the AP and STA nodes. The
staPositions vectors specify the -, -, and - Cartesian coordinates of the AP and STAs, respectively. Units are in meters.
apPosition = [0 0 0]; staPositions = [((30/numSTAs).*(1:numSTAs))' ((30/numSTAs).*(numSTAs:-1:1))' zeros(numSTAs,1)];
To model the scenario, create an AP and four STAs using
wlanDeviceConfig objects. The
wlanDeviceConfig object enables you to specify the configuration parameters for the AP and STAs. Create two WLAN device configuration objects, one for the AP and the other for the STAs. You can use the same device configuration object to configure all the STAs. To configure an AP and a STA, set the
Mode property of the
wlanDeviceConfig object to "AP" and "STA" respectively. Configure the
TransmitPower properties of AP device configuration object. Note that AP and STA device configuration objects must have the same value of
The example simulates these DL OFDMA frame exchange sequences.
Enable DL MU frames followed by MU block ack request (BAR) trigger frame to simultaneously solicit UL acknowledgment frames from multiple scheduled STAs.
Optionally enables MU request to send request to send (RTS)/clear to send (CTS) exchanges by specifying the
DisableRTSproperty of the
This example uses the round-robin scheduling strategy to select STAs for each transmission. The assignment of RUs is fixed for a given number of users and bandwidth.
accessPointCfg = wlanDeviceConfig(Mode="AP",TransmissionFormat="HE-MU-OFDMA", ... % AP device configuration MCS=8,DisableRTS=true, ... MPDUAggregationLimit=5, ... TransmitPower=20); stationCfg = wlanDeviceConfig(Mode="STA",MPDUAggregationLimit=5); % STAs device configuration
To create one AP and four STA nodes from the specified configuration, use the
wlanNode objects. Specify the node names, node positions, and device configuration objects. You can use the
wlanNode object to create multiple nodes at a time by providing multiple node positions. Specify the PHY abstraction method used by the AP and STAs. Configure the
MACFrameAbstraction property to indicate that the MAC frames are abstracted by the nodes.
accessPoint = wlanNode(Name="AP", ... Position=apPosition, ... DeviceConfig=accessPointCfg,... PHYAbstractionMethod=PHYAbstractionMethod, ... MACFrameAbstraction=true); stations = wlanNode(Name="STA"+(1:numSTAs), ... Position=staPositions, ... DeviceConfig=stationCfg, ... PHYAbstractionMethod=PHYAbstractionMethod, ... MACFrameAbstraction=true);
Create a WLAN network consisting of an AP and four STAs.
nodes = [accessPoint stations];
To ensure all the nodes are configured properly, use the
hCheckWLANNodesConfiguration helper function.
Associate all the STAs to the AP by using the
associateStations object function of the
networkTrafficOnOff objects to generate an On-Off application traffic pattern. Configure the On-Off application traffic by specifying the application data rate and packet size. The number of objects created is equal to the number of AP-STA pairs that have downlink traffic exchange in the example. Add application traffic from the AP node to the corresponding STA node by using the
addTrafficSource object function and specifying the associated STA as the destination.
% Create four networkTrafficOnOff objects, one for each AP-STA pair in the scenario for staID=1:numNodes-1 trafficSource(staID) = networkTrafficOnOff(DataRate=100000,PacketSize=100); %#ok<SAGROW> addTrafficSource(accessPoint,trafficSource(staID),DestinationNode=stations(staID),AccessCategory=0); end
To model a random TGax fading channel between each node, this example uses
hSLSTGaxMultiFrequencySystemChannel helper function. When you model a fading channel, the probability of packet transmission success depends on these factors.
Quality of the channel between an AP and STA for the selected RU
Modulation and coding scheme (MCS)
In this example, the RU allocation does not consider the quality of the channel between the AP and each STA. Therefore, if the channel quality between an AP and a STA is poor and the selected MCS is high, packet loss is likely to occur.
Add the channel model to the wireless network simulator by using the
addChannelModel object function of the
channel = hSLSTGaxMultiFrequencySystemChannel(nodes); addChannelModel(networkSimulator,channel.ChannelFcn)
Simulation and Results
Add the nodes to the wireless network simulator.
To view the state transition plot, use the
hPlotPacketTransitions helper object. To disable the packet communication over frequency subplot, set the
FrequencyPlotFlag property of
hPlotPacketTransitions helper object to
if enablePacketVisualization packetVisObj = hPlotPacketTransitions(nodes,simulationTime,FrequencyPlotFlag=false); end
To view the node performance visualization, use the
hVisualizePerformance helper object.
if enableNodePerformancePlot performancePlotObj = hVisualizePerformance(nodes,simulationTime); end
Run the network simulation for the specified simulation time. The runtime visualization shows the time spent by the AP and the STA in Idle, Contention, Transmission, and Reception state.
The preceding figure displays these simulation plots.
Throughput (in Mbps) at each transmitter node (AP).
Packet loss ratio (ratio of unsuccessful data transmissions to the total data transmissions) at each transmitter node (AP).
Average packet latency incurred at each receiver node (STAs). The average packet latency shows the average latency that each STA incurs to receive the downlink traffic from the AP.
Retrieve the APP, MAC, and PHY statistics at each node by using the
statistics object function of the
stats = statistics(nodes);
You can use this example to further explore these functionalities.
Throughput Comparison of OFDM and OFDMA
Generate throughput results of these OFDM and OFDMA transmission scenarios by using the
hCompareOFDMvsOFDMAThroughputs helper function.
An AP serving a maximum of 140 STAs with OFDM configuration.
An AP serving a maximum of 140 STAs in an OFDMA configuration.
Plot the throughput results as a function of the number of DL STAs for OFDM and OFDMA configurations. By default, the
hCompareOFDMvsOFDMAThroughputs helper function plots the static throughput values of OFDM and OFDMA transmissions. To reproduce the results, set the
plotStaticThroughputValues flag to
plotStaticThroughputValues = true;
The preceding plot shows the throughput comparison of OFDM and OFDMA. Because of the simultaneous transmissions to multiple users, the throughput obtained by using OFDMA transmission is greater than the throughput obtained by using OFDM transmission. Increasing the number of DL STAs has minimal impact on the OFDM throughput.
Faster Execution Using Parallel Simulation Runs
If you want to run multiple simulations, you can speed up the simulations by enabling parallel computing using the
parfor loop. The parfor loop is an alternative to the
for loop. The parfor loop enables you to execute multiple simulation runs in parallel, thereby reducing the total execution time. To use parfor, you need to install the Parallel Computing Toolbox™. For more information about running multiple simulations by using parfor loop, see
hCompareOFDMvsOFDMAThroughputs helper function.
The example uses these helpers:
hCheckWLANNodesConfiguration— Check if the node parameters are configured correctly
hSLSTGaxMultiFrequencySystemChannel— Return a system channel object and set the path loss model
hSLSTGaxAbstractSystemChannel— Return a channel object for abstracted PHY layer
hSLSTGaxSystemChannel— Return a channel object for full PHY layer
hSLSTGaxSystemChannelBase— Return the base channel object
hCompareOFDMvsOFDMAThroughputs— Retrieve throughput results for OFDM and OFDMA transmission scenarios
hPlotPacketTransitions— Plot the state transition figure
1. Institute of Electrical and Electronics Engineers (IEEE). IEEE Standard for Information Technology--Telecommunications and Information Exchange between Systems Local and Metropolitan Area Networks--Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications Amendment 1: Enhancements for High-Efficiency WLAN. IEEE 802.11ax-2021. IEEE, May 19, 2021. https://doi.org/10.1109/IEEESTD.2021.9442429.
2. Institute of Electrical and Electronics Engineers (IEEE). IEEE Standard for Information Technology--Telecommunications and Information Exchange between Systems Local and Metropolitan Area Networks--Specific Requirements Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. IEEE 802.11-2020. IEEE, February 26, 2021. https://doi.org/10.1109/IEEESTD.2021.9363693.