wifi: speed up SpectrumWifiPhy::StartRx

Gabriel Ferreirarequested to merge
Gabrielcarvfer/ns-3-dev:spectrumWifiPhyBandPower into master

While investigating why wifi-primary-channels test suite took so long, I have found that RfFilter+multiply+integral used in the SpectrumWifiPhy::StartRx was the source of the issue.

Instead of creating a filter for each band, multiplying to the channel PSD and then calculating the integral over the entire filtered channel, we can simply calculate the integral of the specified band, saving up allocs, multiplications and sums.

Also noticed that the SpectrumValue::Integral is generic enough for non-uniformly sized bands (bw=bit->fh-bit->fl), but we have uniformly sized bands, which means the number of multiplications can be further reduced.

The result is a drastic reduction of runtime for the TAKES_FOREVER wifi-primary-channels test suite from ~12h down to ~2h.

Edited by Gabriel Ferreira

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