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802.11 OFDM Beacon Frame Generation

This example shows how to generate packets containing MAC beacon frames suitable for baseband simulation or over-the-air transmission using an SDR platform.

Introduction

This example creates an IEEE® 802.11™ beacon frame as described in section 8.3.3.2 of [ 1 ]. A WiFi device can be used to view the beacon packet transmitted using SDR hardware as shown in the figure below.

The beacon frame is a type of management frame, it identifies a basic service set (BSS) formed by a number of 802.11 devices. The access point of a BSS periodically transmits the beacon frame to establish and maintain the network. The beacon frame consists of a MAC header, a beacon frame body and a valid frame check sequence (FCS). The beacon frame body contains the information fields which allows stations to associate with the network as shown in the figure below. A WLAN beacon packet is created using the wlanWaveformGenerator function. This function consumes MAC information bits and processes them to form a baseband beacon packet. In this example the generated waveform can be:

  • Stored in a baseband file format. The file format can be used with the example 802.11 OFDM Beacon Receiver with Live Data, which performs beacon packet decoding and describes the receiver processing.

  • Transmitted over-the-air. The beacon packet is upconverted for RF transmission using Xilinx® Zynq-Based Radio SDR hardware. The radio hardware allows a waveform to be transmitted over-the-air.

To transmit the beacon over-the-air, the Xilinx Zynq-based radio support package is required. This can be installed using the Add-On Explorer. More information about SDR platforms can be found here.

Example Setup

The beacon packet can be written to a baseband file and transmitted using an SDR platform. To transmit the beacon using the SDR platform set useSDR to true. To write to a baseband file set saveToFile to true.

useSDR = false;
saveToFile = false;

Create IEEE 802.11 Beacon Frame

The beacon packets are periodically transmitted as specified by the Target Beacon Transmission Time (TBTT) in the beacon interval field. The beacon interval represents the number of Time Units (TUs) between TBTT, where 1 TU represents 1024 microseconds. A beacon interval of 100 TU results in a 102.4 milliseconds time interval between successive beacons. The MAC frame bits for the beacon frames are generated using the helper function helperGenerateBeaconFrame.

SSID = 'TEST_BEACON'; % Network SSID
beaconInterval = 100; % In Time units (TU)
band = 5;             % Band, 5 or 2.4 GHz
chNum = 52;           % Channel number, corresponds to 5260MHz

% Generate Beacon frame
[mpduBits,fc] = helperGenerateBeaconFrame(chNum, band, beaconInterval, SSID);

Create IEEE 802.11 Beacon Packet

A beacon packet is synthesized using wlanWaveformGenerator with a non-HT format configuration object. In this example an object is configured to generate a beacon packet of 20 MHz bandwidth, 1 transmit antenna and BPSK rate 1/2 (MCS 1).

cfgNonHT = wlanNonHTConfig;              % Create a wlanNonHTConfig object
cfgNonHT.PSDULength = numel(mpduBits)/8; % Set the PSDU length in bits

% The idle time is the length in seconds of an idle period after each
% generated packet. The idle time is set to the beacon interval.
txWaveform = wlanWaveformGenerator(mpduBits, cfgNonHT, 'IdleTime', beaconInterval*1024e-6);
Rs = wlanSampleRate(cfgNonHT);           % Get the input sampling rate

Save Waveform to File

This section saves the waveform in a baseband file using BasebandFileWriter.

if saveToFile
    % The waveform is stored in a baseband file
    BBW = comm.BasebandFileWriter('nonHTBeaconPacket.bb', Rs, fc); %#ok<UNRCH>
    BBW(txWaveform);
    release(BBW);
end

For information about automatically detecting and synchronizing the waveform stored in the baseband file format see 802.11 OFDM Beacon Receiver with Live Data.

Transmission with an SDR Device

This section demonstrates the transmission of the beacon packet using an SDR device. The modulated signal is sent over-the-air using an SDR platform. By default, the example is configured to run with ZedBoard™ and ADI FMCOMMS2/3/4 hardware. You can replace the named hardware 'ZedBoard and FMCOMMS2/3/4' with 'ZC706 and FMCOMMS2/3/4' or 'PicoZed SDR' in the SDR transmit object, tx, to run with ZC706 and ADI FMCOMMS2, FMCOMMS3, FMCOMMS4 hardware or PicoZed™ SDR.

if useSDR
    tx = sdrtx('ZedBoard and FMCOMMS2/3/4'); %#ok<UNRCH>
    tx.ShowAdvancedProperties = true;
    tx.BypassUserLogic = true;
    osf = 2; % OverSampling factor
    tx.BasebandSampleRate = Rs*osf;
    % The center frequency is set to the corresponding channel number
    tx.CenterFrequency = fc;
end

The transmitter gain tx.Gain parameter drives the power amplifier in 'ZedBoard and FMCOMMS2/3/4' radio. This parameter is used to impair the quality of the waveform, you can change this parameter to reduce transmission quality, and impair the signal. These are suggested values, depending on your antenna configuration, you may have to tweak these values. The suggested values are:

  1. Set to 0 for increased gain (0dB)

  2. Set to -10 for default gain (-10dB)

  3. Set to -20 for reduced gain (-20dB)

The transmitRepeat function transfers the baseband waveform to the SDR platform, and stores the signal samples in hardware memory. The example then repeatedly transmits this waveform over-the-air until the release method of the transmit object is called. Messages are displayed in the command window to confirm that transmission has started successfully.

if useSDR
    % Set transmit gain
    tx.Gain = 0;  %#ok<UNRCH>
    % Resample transmit waveform
    txWaveform = resample(txWaveform, osf, 1);
    % Transmit over-the-air
    transmitRepeat(tx, txWaveform);
end

Conclusion and Further Exploration

This example has demonstrated how to generate a beacon packet for the IEEE 802.11 standard. A WiFi device can be used to view the beacon packet transmitted using SDR hardware. Alternatively, the stored baseband beacon packet can be processed to recover the transmitted information using the example 802.11 OFDM Beacon Receiver with Live Data.

Appendix

This example uses the following helper functions:

Selected Bibliography

  1. IEEE Std 802.11ac™-2013 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 4: Enhancements for Very High Throughput for Operation in Bands below 6 GHz.

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