802.11ac Waveform Generation with MAC Frames
This example shows how to generate an IEEE® 802.11ac™ transmission containing MAC frames suitable for performing radio packet error rate (PER) receiver tests.
WLAN System Toolbox™ can be used to generate standard compliant waveforms for performing receiver tests. A basic WLAN receiver test scenario is shown in the diagram below.
The device under test (DUT) is stimulated with RF test vectors, usually through a wired link. The packet error rate (PER) is a metric used to test the performance of a receiver at a given receive signal power in the presence of noise, interference, or other impairments. The PER is defined as the number of incorrectly decoded packets divided by the total number of transmitted packets.
The frame check sum (FCS) within a MAC frame is used to determine whether a MAC frame has been decoded correctly by the receiver, and therefore whether the packet has been received in error. The general MAC frame for IEEE 802.11ac contains the following fields:
- MAC header
- Frame body
The data to transmit from a higher layer is contained within the frame body of the MAC frame. The transmitter uses a cyclic redundancy check over the MAC header and frame body field to generate the FCS value. The receiver calculates the CRC and compares this to the received FCS field to determine if an error has occurred during transmission.
In this example an IEEE 802.11ac waveform consisting of multiple VHT format packets is generated. The function wlanWaveformGenerator can be used to generate a waveform containing one or more packets. wlanWaveformGenerator consumes physical layer service data units (PSDUs) for each packet and performs the appropriate physical layer processing to create the waveform. In this example a multi-packet baseband waveform with a MAC header and valid FCS is synthesized. This waveform may be downloaded to a signal generator for RF transmission and used for receiver PER testing. Source code is provided to download and play the waveform using a Keysight Technologies™ N5172B signal generator. The example processing is illustrated in the following diagram:
IEEE 802.11ac VHT Format Configuration
The format specific configuration of a VHT waveform synthesized with wlanWaveformGenerator is described by a VHT format configuration object. The object is created using the wlanVHTConfig function. The properties of the object contain the configuration. In this example an object is configured for a 160 MHz bandwidth, 1 transmit antenna, 1 space-time stream and QPSK rate 1/2 (MCS 1).
vhtCfg = wlanVHTConfig; % Create packet configuration vhtCfg.ChannelBandwidth = 'CBW160'; % 160 MHz channel bandwidth vhtCfg.NumTransmitAntennas = 1; % 1 transmit antenna vhtCfg.NumSpaceTimeStreams = 1; % 1 space-time stream vhtCfg.MCS = 1; % Modulation: QPSK Rate: 1/2
Waveform Generation Configuration
The waveform generator wlanWaveformGenerator can be configured to generate one or more packets and add an idle time between each packet. In this example four packets with a 20 microsecond idle period will be created.
numPackets = 4; % Generate 4 packets idleTime = 20e-6; % 20 microseconds idle period after packet
The PSDU transmitted in each packet is scrambled using a random seed for each packet. This is accomplished by specifying a vector of scrambler initialization seeds. The valid range of the seed is between 1 and 127 inclusive.
% Initialize the scrambler with a random integer for each packet scramblerInitialization = randi([1 127],numPackets,1);
Create a PSDU Containing a MAC Header, Body and FCS for Each Packet
For an IEEE 802.11ac data transmission the MAC frame is termed a MAC protocol data unit (MPDU), the MAC header is termed the MPDU header, and the frame body is an aggregated MAC service data unit (A-MSDU). One or more MPDUs are delimited, padded and aggregated to create an aggregated MPDU (A-MPDU). The A-MPDU is delimited and padded to form the physical layer service data unit (PSDU) which is coded and modulated to create the transmitted packet. This process of encapsulation is shown in the following diagram:
In this example a PSDU is created containing a single MPDU for each packet. The MPDU consists of an MPDU header containing no content, random frame body and valid FCS. The helper function vhtWaveformMACHeader.m creates a MPDU header with no content. A CRC is used to generate the FCS for the A-MPDU. The 32 bit long FCS is appended to the MPDU header and frame body.
The helper function vhtWaveformGeneratePSDU.m pads and delimits the MPDU to form a PSDU ready for physical layer processing and transmission by wlanWaveformGenerator as specified in [ 1 ]. The MPDU bits are delimited and form an A-MPDU containing a single MPDU. The length of this A-MPDU in bytes is the APEP Length, and the appropriate vhtCfg.APEPLength property of the VHT configuration object is set accordingly. The A-MPDU is then padded to form a PSDU of the required length, ready for physical layer processing. A PSDU is generated for each packet and is concatenated into a vector data for transmission with wlanWaveformGenerator. The processing to create the concatenated PSDU bits data is shown in the diagram below:
% Generate FCS and form MPDU, IEEE Std 802.11TM-2012 Section 8.2.4 crcGenerator = comm.CRCGenerator([32 26 23 22 16 12 11 10 8 7 5 4 2 1 0]); crcGenerator.InitialConditions = 1; crcGenerator.DirectMethod = true; crcGenerator.FinalXOR = 1; data = ; for i=1:numPackets % Generate a payload for the frame body, a random A-MSDU of 4048 octets amsdu = randi([0 1],4048*8,1); mpduHeader = vhtWaveformMACHeader; % Create MPDU with header, body and FCS mpdu = crcGenerator([mpduHeader; amsdu]); % Delimit and pad the MPDU to form the PSDU for a single packet [psdu,vhtCfg] = vhtWaveformGeneratePSDU(mpdu,vhtCfg); % Concatenate packet PSDUs for waveform generation data = [data; psdu]; %#ok<AGROW> end
Generate a Baseband Waveform
The concatenated PSDU bits for all packets, data, are passed as an argument to wlanWaveformGenerator along with the VHT packet configuration object vhtCfg. This configures the waveform generator to synthesize an 802.11ac VHT waveform. To generate 802.11n™ HT or other format waveforms different configuration objects are used such as those created by wlanHTConfig or wlanNonHTConfig. The waveform generator is additionally configured using name-value pairs to generate multiple packets with a specified idle time between packets, and initial scrambler states.
% Generate baseband VHT packets txWaveform = wlanWaveformGenerator(data,vhtCfg, ... 'NumPackets',numPackets,'IdleTime',idleTime, ... 'ScramblerInitialization',scramblerInitialization); fs = wlanSampleRate(vhtCfg); disp(['Baseband sampling rate: ' num2str(fs/1e6) ' Msps']);
Baseband sampling rate: 160 Msps
The magnitude of the baseband waveform is displayed below. Note the number of packets and idle time configured.
figure; plot(abs(txWaveform)); xlabel('Sample index'); ylabel('Magnitude'); title('Baseband IEEE 802.11ac Waveform'); legend('Transmit antenna 1');
The frequency spectrum of the generated time domain waveform, txWaveform, can be viewed using the DSP System Toolbox™ spectrum analyzer. As expected, the 160 MHz signal bandwidth is clearly visible at baseband.
spectrumAnalyzer = dsp.SpectrumAnalyzer; spectrumAnalyzer.SampleRate = fs; spectrumAnalyzer.SpectrumType = 'Power density'; spectrumAnalyzer.RBWSource = 'Property'; spectrumAnalyzer.RBW = 100e3; spectrumAnalyzer.FrequencySpan = 'Span and center frequency'; spectrumAnalyzer.Span = fs; spectrumAnalyzer.SpectralAverages = 10; spectrumAnalyzer.YLabel = 'PSD'; spectrumAnalyzer.YLimits = [-80 -40]; spectrumAnalyzer.Title = 'Baseband IEEE 802.11ac Waveform'; spectrumAnalyzer(txWaveform);
Generate an Over-the-Air Signal Using an RF Signal Generator
The baseband waveform created by WLAN System Toolbox can now be downloaded to a signal generator to perform receiver tests. Instrument Control Toolbox™ is used to generate an RF signal with a center frequency of 5.25 GHz RF using the Keysight Technologies N5172B signal generator.
% Control whether to download the waveform to the waveform generator playOverTheAir = false; % Download the baseband IQ waveform to the instrument. Generate the RF % signal at a center frequency of 5.25 GHz and output power of -10 dBm. if playOverTheAir fc = 5.25e9; %#ok<UNRCH> % Center frequency power = -10; % Output power hDownloadAndPlayWaveformUsingN5172B('192.168.0.1',txWaveform, ... fs,fc,power); end
This example uses the following helper functions:
- 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.