802.11ax OFDMA

The IEEE 802.11ax introduced significant enhancements over the existing 802.11ac standard [ 2 ]. One of the key improvements is the OFDMA, which is an extension of orthogonal frequency division multiplexing (OFDM) digital modulation technology into a multiuser environment. The principle of OFDMA is to efficiently use the available frequency space. OFDMA partitions the channel bandwidth into multiple mutually exclusive subbands, called RUs. By partitioning the channel bandwidth, multiple users can access the air interface simultaneously. As a result, concurrent transmissions of small frames to multiple users is possible at the same time. For example, a conventional 20 MHz channel can be partitioned into a maximum of nine subchannels. Subsequently, using OFDMA, an 802.11ax AP can simultaneously transmit small frames to nine 802.11ax stations. The simultaneous transmission of frames not only cuts down on excessive overhead at the MAC but also minimizes the contention overhead. In OFDMA, the allocation of RUs is completely controlled by the AP. The 802.11ax standard specifies two types of OFDMA transmissions, namely downlink (DL) and uplink (UL) OFDMA.

Comparison of OFDM and OFDMA

This section illustrates the difference between OFDM and OFDMA. In this figure, the 802.11n/ac/ax AP transmits DL to 4 OFDM stations independently over time. The entire channel bandwidth is used for the DL communication between the AP and a single OFDMA station. The same holds true for any uplink transmission from a single 802.11n/ac/ax client to the 802.11n/ac/ax AP.

This figure shows that when OFDMA is used, the 802.11ax AP partitions the channel bandwidth into RUs for multiple OFDMA stations on a continuous basis for simultaneous DL transmissions. By partitioning the channel bandwidth, OFDMA makes efficient use of the available frequency spectrum, resulting in reduced MAC contention and PHY preamble overhead.

An 802.11ax AP can also coordinate with 802.11ax OFDMA stations for simultaneous uplink transmissions.

Model 802.11ax Network with OFDMA

This example demonstrates DL OFDMA communication in an 802.11ax network with one AP and four stations. These stations are linearly placed with AP located at the start. These stations implement carrier-sense multiple access with collision avoidance (CSMA/CA) with physical carrier sense. The physical carrier sensing uses the clear channel assessment (CCA) mechanism to determine whether the medium is busy before transmitting. This example is an enhancement over the 802.11 MAC and Application Throughput Measurement example. The enhancements are related to adding 802.11ax DL OFDMA support in the MAC and PHY library blocks. However, the Node Position Allocator (NPA), Visualizer and the Application Traffic Generator blocks are the same as in the 802.11 MAC and Application Throughput Measurement example.

The components of a WLAN node are shown in this figure. The information is retrieved by pressing the arrow button for each node in the above figure.

Application Configuration

This example uses the same application layer blocks (traffic generator and traffic receiver) as used in the 802.11 MAC and Application Throughput Measurement example. You can add or remove any number of application blocks with different configuration options like Packet Size, Packet Interval, Destination Name, and Access Category (AC). Multiple applications with same AC can be enabled simultaneously with different Destination Names. To add or remove applications, enter the Applications subsystem block of a node by double clicking on it. This figure shows the applications inside the Applications subsystem block.

This figure shows the configuration options for an application. Double click on the Application Traffic Generator block to retrieve these options.

MAC Configuration

The AP can be configured to transmit DL OFDMA multiuser format frames by setting PHY Tx Format to HE-MU-OFDMA in MAC configuration parameters. You can also limit the number of users in DL OFDMA transmission using the Max Downlink Stations property.

Model Limitations with HE-MU-OFDMA Format:

When the PHY Tx Format is HE-MU-OFDMA, the Ack Policy, RTS Threshold, and Number of Transmit Chains options are disabled.

Simulation Results

To measure the network throughput, simulations are carried out for these two scenarios:

The throughput results obtained through these simulations are plotted as a function of the number of DL stations for both the OFDM (HE-SU) and the OFDMA (HE-MU-OFDMA) configurations.

Simulation Configuration

Create an 802.11ax network with nine stations and one AP. At the AP, add nine Application Traffic Generator blocks generating data to the nine stations. To enable the stations as transmitters, configure AppState of an Application Traffic Generator block to On. Configure the AP/station(s) with the values shown in this table.

Simulate the network for HE-SU and HE-MU-OFDMA formats by varying the number of applications enabled as 1, 2, 4, 6, 8, and 9 for each simulation. At the end of each simulation run, the throughput values for each node are retrieved from the statistics.mat file and summed to obtain the total throughput.

Plot throughput results for OFDM and OFDMA configurations.

AP as transmitter

This code plots OFDM and OFDMA configurations using only AP as transmitters:

figure;

% Number of DL stations
numStations = [1 2 4 6 8 9];

% Throughput results for OFDMA configuration (Mbps)
throughputOFDMA = [32.64 46.08 59.52 55.68 72.96 82.08];

% Throughput results for OFDM configuration (Mbps)
throughputOFDM = [33.76 33.76 33.76 33.76 33.76 33.76];

% Plot throughput obtained from OFDM simulations
plot(numStations, throughputOFDM,'-o');
% Retain OFDM throughput plot
hold on;
% Plot throughput obtained from OFDMA simulations
plot(numStations, throughputOFDMA,'-x');
grid on;
xlabel('Number of DL Stations');
ylabel('Throughput (Mbps)');
legend('OFDM', 'OFDMA', 'Location', 'northeastoutside');
title('Downlink Throughput at AP');

AP and stations as transmitters

This code plots OFDM and OFDMA configurations using both APs and stations as transmitters:

figure;

% Number of DL stations
numStations = [1 2 4 6 8 9];

% Throughput results for OFDMA configuration (Mbps)
throughputOFDMA = [20.8 27.52 32.64 29.76 40.96 38.88];
% Throughput results for OFDM configuration (Mbps)
throughputOFDM = [21.44 16.64 10.72 8.16 8 7.2];

% Plot throughput obtained from OFDM simulations
plot(numStations, throughputOFDM,'-o');
% Retain OFDM throughput plot
hold on;
% Plot throughput obtained from OFDMA simulations
plot(numStations, throughputOFDMA,'-x');
grid on;
xlabel('Number of DL Stations');
ylabel('Throughput (Mbps)');
legend('OFDM', 'OFDMA', 'Location', 'northeastoutside');
title('Downlink Throughput at AP');

The above plots show the 802.11ax throughput comparison of OFDM and OFDMA. As OFDMA reduces the MAC contention and the PHY preamble overhead, the throughput obtained with OFDMA is greater than that obtained using OFDM. When only AP is configured as the transmitter, the DL throughput shows no change for OFDM. However, when both the AP and the stations are configured as the transmitters, the DL throughput shows a steady decline. This is because the AP gets fewer opportunities to transmit as the number of transmitting stations increase in the network.

This example enables you to model a DL OFDMA communication in a multinode IEEE 802.11ax network. The DL OFDMA support is added to the PHY and MAC library blocks. The application layer configuration allows you to enable multiple applications with same AC using different destination stations. Round-robin scheduling strategy is used to select stations for next transmission. The assignment of RUs is fixed based on the number of users. The plots confirm that the throughput at AP with OFDMA is greater than that obtained using OFDM.

Further Exploration

Scheduling Stations for Transmission

This example uses round-robin scheduling algorithm to select stations for next transmission.

RU allocation

In a transmission, the allocation index defines the assignment of RUs. The allocation index is defined in Table 27-24 of [ 1 ]. For each 20 MHz subchannel, an 8-bit index describes the number of RUs, the size of RUs, and the number of users transmitted on each RU. In this example, the allocation of RUs to the DL stations is fixed based on the number of users as shown in this table. In both the above plots, the throughput drop for OFDMA from four stations to six stations can be affiliated to the RU size. Transmission time is shorter in a 52-tone RU as compared to 26-tone RU. When an MU frame is transmitted, all the PSDUs in the MU frame are padded to align at the same transmission time. The presence of 26-tone RU for six users results in a longer transmission time, thereby causing the throughput drop between four and six users.

The model used in this example implements four stations (allocation index 112). You can modify the station scheduling algorithm and RU allocation by updating the helper function roundRobinScheduler used by the EDCA MAC block.

Appendix

The example uses these helpers:

References