Introduction

In a WLAN radio a single oscillator is typically used to derive clocks for sampling and modulation. The oscillators in the transmitter and receiver radios do not run at the exact same frequency. Due to this mismatch, the sampling instants at the receiver shift relative to the transmitter. Therefore, a sample rate offset (SRO) exists between receiver and transmitter. Similarly, a carrier frequency offset (CFO) exists between the receiver and transmitter due to mismatched carrier frequencies. The inclusion of pilot subcarriers in the IEEE® 802.11™ standard allows for tracking and correction of SRO and CFO impairments.

In OFDM systems SRO manifests itself as a subcarrier and symbol dependent phase rotation and inter-carrier interference (ICI) [ 1 ]. When the SRO is large, and a packet is long, subcarriers far away from DC will experience a substantial impairment. CFO manifests itself as ICI and a symbol dependent phase rotation common to all subcarriers. The phase rotation on subcarriers from one OFDM symbol to the next due to these impairments is illustrated below. is the phase error, is the subcarrier index, is the number of subcarriers. is the SRO, is the carrier frequency offset, is the period of the symbol, is the phase error gradient (PEG), and is the common phase error (CPE). The PEG and CPE can be used to estimate SRO and residual CFO.

In this example, an IEEE 802.11ac™ VHT waveform is generated with fixed SRO and CFO impairments [ 2 ]. The impaired waveform is synchronized and the data portion demodulated and decoded with and without joint timing and phase tracking to correct for SRO and CFO. The equalized constellation with and without tracking is shown to demonstrate the effectiveness of tracking.

Generate a Baseband Waveform

A VHT configuration object is created to parameterize the transmission. Because the impairments are easier to visualize with fewer OFDM symbols, a data payload with only 500 bytes is transmitted using 16-QAM modulation.

% Create a VHT configuration
cfgVHT = wlanVHTConfig;
cfgVHT.ChannelBandwidth = 'CBW20';
cfgVHT.NumTransmitAntennas = 1;
cfgVHT.NumSpaceTimeStreams = 1;
cfgVHT.MCS = 4;          % 16-QAM and 3/4 coding rate
cfgVHT.APEPLength = 500; % Bytes

% Create a random PSDU
s = rng(10); % Seed the random number generator
psdu = randi([0 1],cfgVHT.PSDULength*8,1,'int8');

% Generate a VHT packet
tx = wlanWaveformGenerator(psdu,cfgVHT);

Model Impairments

Sample rate offset between the transmitter and receiver is modeled by resampling the transmitted waveform. The resample function can be used to model a limited range of large sample rate offsets. In this example a -100 parts per million (PPM) sample rate offset is modeled.

% Model sample rate offset
p = 1e4;   % Interpolation factor
q = 1e4-1; % Decimation factor
sro = (1-p/q)*1e6;
disp('Impairments:');
disp(['  Sample rate offset (SRO): ' num2str(sro,'%.1f') ' PPM']);

% Resample the waveform, appending zeros to allow for filter delay
N = 100; % Size of filter used for resampling
rx = resample([tx; zeros(N,cfgVHT.NumTransmitAntennas)],p,q,N);

Impairments:
  Sample rate offset (SRO): -100.0 PPM

Residual carrier frequency offset is added to the waveform using the helper function helperFrequencyOffset. In this example we assume the same oscillator is used for sampling and modulation, therefore the CFO will be a function of the SRO and carrier frequency.

fc = 5.25e9;         % Carrier frequency, Hertz
cfo = (sro*1e-6)*fc; % Carrier frequency offset, Hertz
disp(['  Carrier frequency offset (CFO): ' num2str(cfo,'%.1f') ' Hz']);

fs = wlanSampleRate(cfgVHT);           % Baseband sample rate
rx = helperFrequencyOffset(rx,fs,cfo); % Add frequency offset

  Carrier frequency offset (CFO): -525052.5 Hz

Noise is added to the waveform with 30 dBW variance.

awgnChannel = comm.AWGNChannel('NoiseMethod','Variance','Variance',10^(-30/10));
rx = awgnChannel(rx);

Front-End Synchronization and Receiver Processing

The following processing steps occur to synchronize the packet, in preparation for recovering the data field.

% Generate field indices
ind = wlanFieldIndices(cfgVHT);

% Packet detection
tOff = wlanPacketDetect(rx,cfgVHT.ChannelBandwidth);

% Coarse frequency offset correction
lstf = rx(tOff+(ind.LSTF(1):ind.LSTF(2)),:);
coarseCFOEst = wlanCoarseCFOEstimate(lstf,cfgVHT.ChannelBandwidth);
rx = helperFrequencyOffset(rx,fs,-coarseCFOEst);

% Symbol timing synchronization
nonhtPreamble = rx(tOff+(ind.LSTF(1):ind.LSIG(2)),:);
symOff = wlanSymbolTimingEstimate(nonhtPreamble,cfgVHT.ChannelBandwidth);
tOff = tOff+symOff;

% Fine frequency offset correction
lltf = rx(tOff+(ind.LLTF(1):ind.LLTF(2)),:);
fineCFOEst = wlanFineCFOEstimate(lltf,cfgVHT.ChannelBandwidth);
rx = helperFrequencyOffset(rx,fs,-fineCFOEst);

% Noise power estimation
lltf = rx(tOff+(ind.LLTF(1):ind.LLTF(2)),:);
lltfDemod = wlanLLTFDemodulate(lltf,cfgVHT);
noiseEst = helperNoiseEstimate(lltfDemod,cfgVHT.ChannelBandwidth,cfgVHT.NumSpaceTimeStreams);

% Channel estimation
vhtltf = rx(tOff+(ind.VHTLTF(1):ind.VHTLTF(2)),:);
vhtltfDemod = wlanVHTLTFDemodulate(vhtltf,cfgVHT);
chanEst = wlanVHTLTFChannelEstimate(vhtltfDemod,cfgVHT);

Recovery Without Sample Rate Offset or Residual CFO Tracking

The coarse and fine frequency offset estimation and correction removes the majority of CFO, but residual CFO remains due to the presence of impairments in the waveform. This must be tracked and corrected by the receiver.

disp('Front-end impairment correction:');
frontEndCFOEst = coarseCFOEst+fineCFOEst;
disp(['  Estimated CFO: ' num2str(frontEndCFOEst,'%.1f') ' Hz']);
residualCFO = cfo-frontEndCFOEst;
disp(['  Residual CFO after initial correction: ' num2str(residualCFO,'%.1f') ' Hz']);

Front-end impairment correction:
  Estimated CFO: -524764.3 Hz
  Residual CFO after initial correction: -288.2 Hz

The helper function trackingVHTDataRecover recovers the VHT data field with optional pilot tracking to correct for timing and phase errors due to SRO and CFO. Pilot tracking is controlled using the helper object trackingRecoveryConfig.

The data field is first recovered without pilot tracking. The data field is extracted from the waveform using the start and end sample indices of the field at the baseband rate. If the receiver sampling rate is higher than the transmitter rate, the receiver requires more samples than the transmitter produces. To allow for this, Ne additional samples are extracted from the waveform and passed to the recovery function. The maximum number of additional samples required is a function of the expected SRO, the baseband sampling rate, and the maximum packet duration.

% Recovery configuration with pilot tracking disabled
cfgRec = trackingRecoveryConfig;
cfgRec.PilotTracking = 'None';

% Extract data field with Ne additional samples to allow for negative SRO
maxDuration = 5.484e-3; % Maximum packet duration in seconds
maxSRO = 120;           % PPM
Ne = ceil(fs*maxDuration*maxSRO*1e-6); % Number of extra samples
dataInd = tOff+(ind.VHTData(1):ind.VHTData(2)+Ne);
dataInd = dataInd(dataInd<=length(rx)); % Only use indices within waveform
data = rx(dataInd,:);

% Perform demodulation and decoding
[rxPSDUNoTrack,~,eqSymNoTrack] = trackingVHTDataRecover(data,chanEst,noiseEst,cfgVHT,cfgRec);

The equalized constellation is plotted which shows a rotation of all constellation points caused by residual CFO, and a spreading of constellation points due to SRO. Despite the modest AWGN added to the waveform, the impairments cause bit errors within the decoded PSDU.

ConstNoTrack = comm.ConstellationDiagram;
ConstNoTrack.Title = 'Equalized symbols with no pilot tracking';
ConstNoTrack.ReferenceConstellation = wlanReferenceSymbols(cfgVHT);
ConstNoTrack(eqSymNoTrack(:));

[~,berNoTrack] = biterr(rxPSDUNoTrack,psdu);
disp('Bit error rate:');
disp(['  Without tracking: ' num2str(berNoTrack)]);

Bit error rate:
  Without tracking: 0.086558

Recovery With Sample Rate Offset Tracking and Residual CFO Tracking

Now the data field is recovered with joint timing and phase pilot tracking to correct for SRO and residual CFO.

The tracking algorithm used in this example estimates absolute values of and per OFDM symbol, and applies a per subcarrier and symbol phase correction to the demodulated symbols to reverse the phase errors caused by SRO and CFO. The phase error between each received pilot subcarrier and the expected value is calculated per symbol and averaged over PilotTrackingWindow OFDM symbols. From this, least-square estimates of and are calculated per symbol. These estimates are used to apply a phase correction to each symbol and subcarrier [ 3, 4 ].

% Recovery configuration with pilot tracking enabled
cfgRec = trackingRecoveryConfig;
cfgRec.PilotTracking = 'Joint'; % Joint timing and phase tracking
cfgRec.PilotTrackingWindow = 9; % Averaging window in OFDM symbols

% Perform demodulation and decoding
[rxPSDU,~,eqSymTrack,cpe,peg] = trackingVHTDataRecover(data,chanEst,noiseEst,cfgVHT,cfgRec);

The equalized constellation is plotted which shows a clear 16-QAM constellation with no spreading or rotation. There are no bit errors.

ConstTrack = comm.ConstellationDiagram;
ConstTrack.Title = 'Equalized symbols with joint pilot tracking';
ConstTrack.ReferenceConstellation = wlanReferenceSymbols(cfgVHT);
ConstTrack(eqSymTrack(:));

[~,berTrack] = biterr(rxPSDU,psdu);
disp(['  With tracking: ' num2str(berTrack)]);

  With tracking: 0

The function trackingVHTDataRecover returns measurements from which the residual CFO, and SRO can be estimated:

The SRO and residual CFO are estimated from these measurements using a linear least-square fit of the rate of change. The measurements are plotted using the helper function trackingPlotSROCFOEstimates.

[residualCFOEst,sroEst] = trackingPlotSROCFOEstimates(cpe,peg,cfgVHT);

% Display estimated SRO, residual CFO and total CFO
fprintf('Tracked impairments:\n');
fprintf('  Estimated residual CFO: %3.1f Hz (%.1f Hz error)\n', ...
    residualCFOEst,residualCFOEst-residualCFO);
fprintf('  Estimated SRO: %3.1f PPM (%.1f PPM error)\n',sroEst,sroEst-sro);
cfoEst = frontEndCFOEst+residualCFOEst; % Initial + tracked CFO estimate
fprintf('Estimated CFO (initial + tracked): %.1f Hz (%.1f Hz error)\n',cfoEst,cfoEst-cfo);

rng(s); % Restore the state of the random number generator

Tracked impairments:
  Estimated residual CFO: -260.5 Hz (27.8 Hz error)
  Estimated SRO: -101.2 PPM (-1.2 PPM error)
Estimated CFO (initial + tracked): -525024.8 Hz (27.8 Hz error)

Conclusion

The example shows how sample rate and carrier frequency offsets can be tracked and corrected for while recovering the data field of a WLAN waveform.

Data field recovery functions with joint pilot tracking for VHT, HT-MF and non-HT formats are provided, along with an object to configure the recovery algorithms:

Appendix

This example uses the following helper functions:

Selected Bibliography