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wlanCoarseCFOEstimate

Coarse estimate of carrier frequency offset

Syntax

fOffset = wlanCoarseCFOEstimate(rxSig,cbw)
fOffset = wlanCoarseCFOEstimate(rxSig,cbw,corrOffset)

Description

example

fOffset = wlanCoarseCFOEstimate(rxSig,cbw) returns a coarse estimate of the carrier frequency offset (CFO) given received time-domain L-STF[1] samples and channel bandwidth.

example

fOffset = wlanCoarseCFOEstimate(rxSig,cbw,corrOffset) returns a coarse estimate given correlation offset, corrOffset.

Examples

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Create a non-HT configuration object.

nht = wlanNonHTConfig;

Generate a non-HT waveform.

txSig = wlanWaveformGenerator([1;0;0;1],nht);

Create a phase and frequency offset object and introduce a 2 kHz frequency offset.

pfOffset = comm.PhaseFrequencyOffset('SampleRate',20e6,'FrequencyOffset',2000);
rxSig = pfOffset(txSig);

Extract the L-STF.

ind = wlanFieldIndices(nht,'L-STF');
rxLSTF = rxSig(ind(1):ind(2),:);

Estimate the frequency offset from the L-STF.

freqOffsetEst = wlanCoarseCFOEstimate(rxLSTF,'CBW20')
freqOffsetEst =

   2.0000e+03

Estimate the frequency offset for a VHT signal passing through a noisy, TGac channel. Correct for the frequency offset.

Create a VHT configuration object and create the L-STF.

vht = wlanVHTConfig;
txstf = wlanLSTF(vht);

Set the channel bandwidth and sample rate.

cbw = 'CBW80';
fs = 80e6;

Create TGac and thermal noise channel objects. Set the delay profile of the TGac channel to 'Model-C'. Set the noise figure of the thermal noise channel to 9 dB.

tgacChan = wlanTGacChannel('SampleRate',fs,'ChannelBandwidth',cbw, ...
    'DelayProfile','Model-C','LargeScaleFadingEffect','Pathloss');

noise = comm.ThermalNoise('SampleRate',fs,'NoiseMethod','Noise figure', ...
    'NoiseFigure',9);

Pass the L-STF through the noisy TGac channel.

rxstfNoNoise = tgacChan(txstf);
rxstf = noise(rxstfNoNoise);

Create a phase and frequency offset object and introduce a 750 Hz frequency offset.

pfOffset = comm.PhaseFrequencyOffset('SampleRate',fs, ...
    'FrequencyOffsetSource','Input port');
rxstf = pfOffset(rxstf,750);

For the model-C delay profile, the RMS delay spread is 30 ns, which is 3/8 of the 80 ns short training symbol duration. As such, set the correlation offset to 0.375.

corrOffset = 0.375;

Estimate the frequency offset. Your results may differ slightly.

fOffsetEst = wlanCoarseCFOEstimate(rxstf,cbw,corrOffset)
fOffsetEst =

  749.8770

The estimate is very close to the introduced CFO of 750 Hz.

Change the delay profile to 'Model-E', which has an RMS delay spread of 100 ns.

release(tgacChan)
tgacChan.DelayProfile = 'Model-E';

Pass the transmitted signal through the modified channel and apply the 750 Hz CFO.

rxstfNoNoise = tgacChan(txstf);
rxstf = noise(rxstfNoNoise);
rxstf = pfOffset(rxstf,750);

Estimate the frequency offset.

fOffsetEst = wlanCoarseCFOEstimate(rxstf,cbw,corrOffset)
fOffsetEst =

  682.9147

The estimate is inaccurate because the RMS delay spread is greater than the duration of the training symbol.

Set the correlation offset to the maximum value of 1 and estimate the CFO.

corrOffset = 1;
fOffsetEst = wlanCoarseCFOEstimate(rxstf,cbw,corrOffset)
fOffsetEst =

  747.8501

The estimate is accurate because the autocorrelation does not use the first training symbol. The channel delay renders this symbol useless.

Correct for the estimated frequency offset.

rxstfCorrected = pfOffset(rxstf,-fOffsetEst);

Estimate the frequency offset of the corrected signal.

fOffsetEstCorr = wlanCoarseCFOEstimate(rxstfCorrected,cbw,corrOffset)
fOffsetEstCorr =

  -3.2763e-11

The corrected signal has negligible frequency offset.

Estimate and correct for a significant carrier frequency offset in two steps. Estimate the frequency offset after all corrections have been made.

Set the channel bandwidth and the corresponding sample rate.

cbw = 'CBW40';
fs = 40e6;

Coarse Frequency Correction

Generate an HT format configuration object.

cfg = wlanHTConfig('ChannelBandwidth',cbw);

Generate the transmit waveform.

txSig = wlanWaveformGenerator([1;0;0;1],cfg);

Create TGn and thermal noise channel objects. Set the noise figure of the receiver to 9 dB.

tgnChan = wlanTGnChannel('SampleRate',fs,'DelayProfile','Model-D', ...
    'LargeScaleFadingEffect','Pathloss and shadowing');
noise = comm.ThermalNoise('SampleRate',fs, ...
    'NoiseMethod','Noise figure', ...
    'NoiseFigure',9);

Pass the waveform through the TGn channel and add noise.

rxSigNoNoise = tgnChan(txSig);
rxSig = noise(rxSigNoNoise);

Create a phase and frequency offset object to introduce a carrier frequency offset. Introduce a 2 kHz frequency offset.

pfOffset = comm.PhaseFrequencyOffset('SampleRate',fs,'FrequencyOffsetSource','Input port');
rxSig = pfOffset(rxSig,2e3);

Extract the L-STF signal for coarse frequency offset estimation.

istf = wlanFieldIndices(cfg,'L-STF');
rxstf = rxSig(istf(1):istf(2),:);

Perform a coarse estimate of the frequency offset. Your results may differ.

foffset1 = wlanCoarseCFOEstimate(rxstf,cbw)
foffset1 =

   2.0003e+03

Correct for the estimated offset.

rxSigCorr1 = pfOffset(rxSig,-foffset1);

Fine Frequency Correction

Extract the L-LTF signal for fine offset estimation.

iltf = wlanFieldIndices(cfg,'L-LTF');
rxltf1 = rxSigCorr1(iltf(1):iltf(2),:);

Perform a fine estimate of the corrected signal.

foffset2 = wlanFineCFOEstimate(rxltf1,cbw)
foffset2 =

    6.5375

The corrected signal offset is reduced from 2000 Hz to approximately 7 Hz.

Correct for the remaining offset.

rxSigCorr2 = pfOffset(rxSigCorr1,-foffset2);

Determine the frequency offset of the twice corrected signal.

rxltf2 = rxSigCorr2(iltf(1):iltf(2),:);
deltaFreq = wlanFineCFOEstimate(rxltf2,cbw)
deltaFreq =

  -1.2890e-12

The CFO is zero.

Input Arguments

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Received signal containing an L-STF, specified as an NS-by-NR matrix. NS is the number of samples in the L-STF and NR is the number of receive antennas.

Note

If the number of samples in rxSig is greater than the number of samples in the L-STF, the trailing samples are not used to estimate the carrier frequency offset.

Data Types: double
Complex Number Support: Yes

Channel bandwidth in MHz, specified as: 'CBW5', 'CBW10', 'CBW20', 'CBW40', 'CBW80', or 'CBW160'.

Data Types: char | string

Correlation offset as a fraction of a short training symbol, specified as a real scalar from 0 to 1. The duration of the short training symbol varies with bandwidth. For more information, see L-STF.

Data Types: double

Output Arguments

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Frequency offset in Hz, returned as a real scalar.

Data Types: double

More About

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L-STF

The legacy short training field (L-STF) is the first field of the 802.11™ OFDM PLCP legacy preamble. The L-STF is a component of VHT, HT, and non-HT PPDUs.

The L-STF duration varies with channel bandwidth.

Channel Bandwidth (MHz)Subcarrier Frequency Spacing, ΔF (kHz)Fast Fourier Transform (FFT) Period (TFFT = 1 / ΔF)L-STF Duration (TSHORT = 10 × TFFT / 4)
20, 40, 80, and 160312.53.2 μs8 μs
10156.256.4 μs16 μs
578.12512.8 μs32 μs

Because the sequence has good correlation properties, it is used for start-of-packet detection, for coarse frequency correction, and for setting the AGC. The sequence uses 12 of the 52 subcarriers that are available per 20 MHz channel bandwidth segment. For 5 MHz, 10 MHz, and 20 MHz bandwidths, the number of channel bandwidths segments is 1.

References

[1] IEEE Std 802.11™-2012 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] Li, Jian. “Carrier Frequency Offset Estimation for OFDM-Based WLANs.” IEEE Signal Processing Letters. Vol. 8, Issue 3, Mar 2001, pp. 80–82.

[3] Moose, P. H. “A technique for orthogonal frequency division multiplexing frequency offset correction.” IEEE Transactions on Communications. Vol. 42, Issue 10, Oct 1994, pp. 2908–2914.

[4] Perahia, E. and R. Stacey. Next Generation Wireless LANs: 802.11n and 802.11ac. 2nd Edition. United Kingdom: Cambridge University Press, 2013.

Extended Capabilities

Introduced in R2015b


[1] IEEE® Std 802.11-2012 Adapted and reprinted with permission from IEEE. Copyright IEEE 2012. All rights reserved.

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