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Noise estimation using L-LTF and S1G-LTF1

Since R2023a



noiseEst = wlanLLTFNoiseEstimate(rxSym) estimates the mean noise power, in watts, using the demodulated OFDM symbols from the legacy long training field (L-LTF) or S1G long training field (S1G-LTF1). The estimation is performed assuming a resistance of 1 ohm.


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Create a configuration object for a high-throughput packet. Set an initial signal-to-noise ratio of 10 dB.

cfg = wlanHTConfig;
snr = 10;

Extract the OFDM information of the L-LTF in the configuration. Change the signal-to-noise ratio to remove the noise contribution from the null subcarriers.

ofdmInfo = wlanHTOFDMInfo("L-LTF",cfg);
snrValue = snr-10*log10(ofdmInfo.FFTLength/ofdmInfo.NumTones);

Extract the field indices that correspond to the L-LTF.

indLLTF = wlanFieldIndices(cfg,"L-LTF");

Generate a time-domain waveform for the configuration.

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

Perform the noise estimate for 100 separate noise realizations.

noiseEst = zeros(1,100);
for i = 1:100
rx = awgn(tx,snrValue);
rxLLTF = rx(indLLTF(1):indLLTF(2));
rxSym = wlanLLTFDemodulate(rxLLTF,cfg);
noiseEst(i) = wlanLLTFNoiseEstimate(rxSym);

Display the average value of the noise estimate, in dB.


Input Arguments

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Demodulated L-LTF or S1G-LTF1 OFDM symbols, specified as an NST-by-2-by-NR array. NST is the number of occupied subcarriers. NR is the number of receive antennas. The two columns of the array correspond to demodulated L-LTF or S1G-LTF1 OFDM symbols.


The function does not support noise estimation using the S1G-LTF1 field for the S1G 1 MHz mode.

Data Types: single | double
Complex Number Support: Yes

Output Arguments

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Mean noise power estimate in watts, returned as a positive real scalar.

Data Types: single | double

More About

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The L-LTF is the second field in the 802.11™ OFDM PLCP legacy preamble. The L-LTF is a component of EHT, HE, VHT, HT, and non-HT PPDUs.

The L-LTF, second in the legacy preamble

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 cyclic prefix followed by the two long training symbols in the L-LTF

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, 160, and 320312.53.2 μs1.6 μs8 μs
10156.256.4 μs3.2 μs16 μs
578.12512.8 μs6.4 μs32 μs

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Version History

Introduced in R2023a

See Also