wlanLLTFNoiseEstimate

Noise estimation using L-LTF or S1G-LTF1

Since R2023a

Syntax

noiseEst = wlanLLTFNoiseEstimate(rxSym)

Description

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 function performs this estimate assuming a resistance of 1 ohm.

Examples

Estimate Noise Power Using L-LTF

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);
end

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

disp(10*log10(mean(noiseEst)))

  -10.0690

Input Arguments

rxSym — Demodulated L-LTF or S1G-LTF1 OFDM symbols

3-D array

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

Note

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

noiseEst — Mean noise power estimate

positive real scalar

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

Data Types: single | double

More About

L-LTF

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. This diagram¹ shows the L-LTF 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 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 320	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

Extended Capabilities

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

Version History

Introduced in R2023a