Fine estimate of carrier frequency offset
Create 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 Hz frequency offset.
pfOffset = comm.PhaseFrequencyOffset('SampleRate',20e6,'FrequencyOffset',2); rxSig = pfOffset(txSig);
Extract the L-LTF and estimate the frequency offset.
ind = wlanFieldIndices(nht,'L-LTF'); rxlltf = rxSig(ind(1):ind(2),:); freqOffsetEst = wlanFineCFOEstimate(rxlltf,'CBW20')
freqOffsetEst = 2.0000
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-LTF.
vht = wlanVHTConfig; txltf = wlanLLTF(vht);
Set the sample rate to correspond to the default bandwidth of the VHT configuration object.
fs = 80e6;
Create TGac and thermal noise channel objects. Set the noise figure of the AWGN channel to 10 dB.
tgacChan = wlanTGacChannel('SampleRate',fs, ... 'ChannelBandwidth',vht.ChannelBandwidth, ... 'DelayProfile','Model-C','LargeScaleFadingEffect','Pathloss'); noise = comm.ThermalNoise('SampleRate',fs, ... 'NoiseMethod','Noise figure', ... 'NoiseFigure',10);
Pass the L-LTF through the noisy TGac channel.
rxltfNoNoise = tgacChan(txltf); rxltf = noise(rxltfNoNoise);
Create a phase and frequency offset object and introduce a 25 Hz frequency offset.
pfoffset = comm.PhaseFrequencyOffset('SampleRate',fs,'FrequencyOffsetSource','Input port'); rxltf = pfoffset(rxltf,25);
Perform a fine estimate the frequency offset using a correlation offset of 0.6. Your results may differ slightly.
fOffsetEst = wlanFineCFOEstimate(rxltf,vht.ChannelBandwidth,0.6)
fOffsetEst = 28.0773
Correct for the estimated frequency offset.
rxltfCorr = pfoffset(rxltf,-fOffsetEst);
Estimate the frequency offset of the corrected signal.
fOffsetEstCorr = wlanFineCFOEstimate(rxltfCorr,vht.ChannelBandwidth,0.6)
fOffsetEstCorr = -3.1286e-13
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 = 2.0221e+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 = -11.0795
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 = -2.0048e-11
The CFO is zero.
rxSig— Received signal
Received signal containing an L-LTF, specified as an NS-by-NR matrix. NS is the number of samples in the L-LTF and NR is the number of receive antennas.
If the number of samples in
rxSig is greater
than the number of samples in the L-LTF, the trailing samples are
not used to estimate the carrier frequency offset.
Complex Number Support: Yes
cbw— Channel bandwidth
Channel bandwidth in MHz, specified as
corrOffset— Correlation offset
Correlation offset as a fraction of the L-LTF cyclic prefix, specified as a real scalar from 0 to 1. The duration of the short training symbol varies with bandwidth. For more information, see L-LTF.
fOffset— Frequency offset
Frequency offset in Hz, returned as a real scalar.
The legacy long training field (L-LTF) is the second field in the 802.11™ OFDM PLCP legacy preamble. The L-LTF is a component of VHT, HT, and non-HT PPDUs.
Channel estimation, fine frequency offset estimation, and fine symbol timing offset estimation rely on the L-LTF.
The L-LTF is composed of a cyclic prefix (CP) followed by two identical long training symbols (C1 and C2). The CP consists of the second half of the long training symbol.
The L-LTF duration varies with channel bandwidth.
|Channel Bandwidth (MHz)||Subcarrier Frequency Spacing, ΔF (kHz)||Fast Fourier Transform (FFT) Period (TFFT = 1 / ΔF)||Cyclic Prefix or Training Symbol Guard Interval (GI2) Duration (TGI2 = TFFT / 2)||L-LTF Duration (TLONG = TGI2 + 2 × TFFT)|
|20, 40, 80, and 160||312.5||3.2 μs||1.6 μs||8 μs|
|10||156.25||6.4 μs||3.2 μs||16 μs|
|5||78.125||12.8 μs||6.4 μs||32 μs|
 IEEE Std 802.11™-2016 (Revision of IEEE Std 802.11-2012). “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications.” IEEE Standard for Information technology — Telecommunications and information exchange between systems — Local and metropolitan area networks — Specific requirements.
 Li, Jian. “Carrier Frequency Offset Estimation for OFDM-Based WLANs.” IEEE Signal Processing Letters. Vol. 8, Issue 3, Mar 2001, pp. 80–82.
 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.
 Perahia, E., and R. Stacey. Next Generation Wireless LANs: 802.11n and 802.11ac. 2nd Edition. United Kingdom: Cambridge University Press, 2013.
 IEEE® Std 802.11-2012 Adapted and reprinted with permission from IEEE. Copyright IEEE 2012. All rights reserved.