Interpolation Filter (802.11n/ac/ax/be)
The parameter applies filtering to the equalizer to reduce signal noise or smooth frequency response.
Equalization values are computed separately for each subcarrier. Since, typically, there is noise on each subcarrier that is independent of noise on other subcarriers, we can average neighboring subcarriers to reduce the noise power in the equalizer response, and consequently, reduce the EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. in the demodulation measurement. This assumes that the true transmitter frequency response is smooth enough, which is usually (though not always) a valid assumption.
However, there is a trade-off: equalizer smoothing makes the equalizer less able to track real frequency-domain roughness in the transmitter, sometimes making EVM worse on signals with low noise or signals with more roughness in the transmitter frequency response.
There are three choices:
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No interpolation filter is applied.
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Equalizer smoothing is applied using a simple triangular-weighted moving average, with special handling at the ends of the channel response to avoid biasing the end points.
The triangular weights applied to the subcarrier and adjacent subcarriers are as follows: 1 2 ... (N-1)/2 ... 2 1
Example: for a smoothing value of 5, to following is a calculation of the smoothed value of subcarrier n's equalizer coefficient eq'n:
w = [1 2 3 2 1]
eq'n = 1/sum(w) * w * [ eq-2 eq-1 eq0 eq1 eq2]'
Near the endpoints, where there are not enough equalizer values to apply the specified smoothing value, the smoothing algorithm makes the smoothing window narrower in order to keep the window symmetrical. For example, the smoothed coefficient eq'1 is calculated from eq0, eq1, and eq2 regardless of the smoothing value.
-
Wiener filtering is applied...
See Also
