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802.11 OFDM Beacon Receiver with Captured Data

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This example shows a receiver design that is able to recover 802.11™ OFDM beacon packets in non-HT format transmitted over the air from commercial 802.11 hardware. Beacon packets are typically transmitted in non-HT format, even for HT [1], VHT [1], and/or HE [2] capable hardware. Packet information such as SSID is printed to the command-line during recovery.

Introduction

This example illustrates the use of WLAN Toolbox™ to recover real-world signals. It demonstrates a receiver design including synchronization, transmission configuration recovery, and payload decoding for non-HT packets. The example recovers beacon packets from a file containing a captured baseband waveform.

Beacon Packet Recovery

The following steps happen sequentially to recover one non-HT packet:

  • Packet Detection: First a packet must be detected before any processing begins. This is accomplished by auto-correlating input symbols. Since the front of each 802.11 OFDM packet contains a repetitive structure called the L-STF, peaks will occur in the correlation when this packet is present. The L-STF field is then extracted and used for coarse frequency estimation.

  • Symbol Timing: Once a packet has been detected, future symbols will be collected and cross-correlated to locate the L-LTF. The resulting correlation peaks provide an accurate timing estimate. Once the full L-LTF is located, it is extracted and used for channel estimation and fine frequency estimation.

  • L-SIG Decoding: The first OFDM symbol after the L-LTF is the L-SIG field. This field must be recovered and decoded to determine the modulation, code rate, and length of the following payload. The information is used to capture the correct amount of data after the L-SIG for the complete payload and to decode that information.

  • Payload Decoding: All OFDM symbols after the L-SIG are buffered to a length determined by the L-SIG field. After all the symbols have been captured they are demodulated and decoded into their source bits. The source bits are then evaluated. This evaluation includes frame check sequence (FCS) validation and extraction of the header and body. If the packet is of subtype beacon, summary information such as SSID will be printed for the recovered packet.

Once a full packet is received or any failures occur during the processing chain, the receiver will return to packet detection to search for more packets. This process is repeated for the duration of the signal.

Streaming Process on Captured Data

In this example an off-the-air capture is processed to recover beacon frames. A Wi-Fi® signal was captured using an RF interface with one receive antenna at a sampling rate of 20 Msps. The captured waveform is stored in a binary baseband file. The file was created usingcomm.BasebandFileWriter.

The captured waveform is processed in a streaming fashion. A block of samples is pulled in for processing in each iteration. As many valid packets are retrieved as possible.comm.BasebandFileReaderis used to read blocks of samples from the binary baseband file.

% Create an object to stream the data from the filebasebandReader = comm.BasebandFileReader(...'Filename','nonHTBeaconRxData.bb',...'SamplesPerFrame', 80);% Number of samples in 1 OFDM symbol at 20 MHz

fr中心equency, sample rate and number of channels in the captured waveform are provided by the comm.BasebandFileReader object.

disp(['Center frequency: 'num2str(basebandReader.CenterFrequency/1e6)' MHz']) disp(['Sample rate: 'num2str(basebandReader.SampleRate/1e6)' Msps']) disp(['Number of receive antennas: 'num2str(basebandReader.NumChannels) newline])
Center frequency: 5785 MHz Sample rate: 20 Msps Number of receive antennas: 1

AnonHTFrontEnd对象执行前端处理和L-SIG 12月oding. The object is configured with a channel bandwidth of 20 MHz to process non-HT packets. Only one receive antenna is supported.

rxFrontEnd = nonHTFrontEnd('ChannelBandwidth','CBW20');

A while loop is used to process blocks of samples and recover beacon packets until no more data is available in the baseband file. In each iteration of the loop a block of samples is read from the baseband file and is processed byrxFrontEnd.rxFrontEndperforms front-end processing and buffers samples until a packet has been detected and the payload received. WhenpayloadFullis true, the full payload has been buffered andrxFrontEndreturns variables to allow the data within the packet to be recovered:

  • cfgNonHTcontains the recovered packet parameters from L-SIG.

  • rxNonHTDatais the time-domain non-HT data field signal.

  • chanEstcontains the channel estimates obtained from the L-LTF.

  • noiseVaris the fixed noise variance value.

The packet payload bits are recovered from the non-HT data field samples usingwlanNonHTDataRecover. The bits are then validated and decoded bywlanMPDUDecodeto recover the MAC frame parameters.wlanMPDUDecodereturns the following outputs that determine whether the received packet passed FCS check and whether the received packet is a beacon frame.

  • mpduCfgis an object of typewlanMACFrameConfigcontaining the recovered MAC frame parameters from the beacon frame.

  • statusis an enumeration of typestatuswhich is returned as 'Success' when MPDU passes FCS check and returned as 'FCSFailed' when the MPDU fails FCS check.

If a valid beacon is detected, the decoded SSID is displayed.

% Symbol-by-symbol流过程numValidPackets = 0;while~isDone(basebandReader)% Pull in one OFDM symbol, i.e. 80 samplesdata = basebandReader();% Perform front-end processing and payload buffering[payloadFull, cfgNonHT, rxNonHTData, chanEst, noiseVar] =...rxFrontEnd(data);ifpayloadFull% Recover payload bits with zero-forcing equalizationrecBits = wlanNonHTDataRecover(rxNonHTData, chanEst,...noiseVar, cfgNonHT,'EqualizationMethod','ZF');% Decode and evaluate recovered bits[mpduCfg, ~, status] = wlanMPDUDecode(recBits, cfgNonHT);ifstrcmp(status,'Success') && strcmp(mpduCfg.FrameType,'Beacon') frameBody = mpduCfg.ManagementConfig;% Display SSIDdisp(['SSID: ', frameBody.SSID]) numValidPackets = numValidPackets + 1;endendenddisp([num2str(numValidPackets),' Valid Beacon Packets Found']) release(basebandReader); release(rxFrontEnd);
SSID: MathWorks-SDR SSID: MathWorks-SDR SSID: MathWorks-SDR 3 Valid Beacon Packets Found

Further Exploration

See802.11 OFDM Beacon Receiver with USRP Hardwarefor an example of processing live signals with USRP.

Appendix

This example uses the following helper functions and objects:

Selected Bibliography

  1. IEEE Std 802.11™-2020. 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.

  2. IEEE Std 802.11ax™-2021. 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.