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Variations in attenuation and tx-power figures

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Weaver:

XGS_Is_On:
SNR margin has to be dependent on bits per bin. SNRM is the margin between required SNR for a particular modulation order and, hence, bits per symbol in the bin and the actual SNR in that frequency range.

Using QPSK / 2 bits per symbol requires an SNR of about 12 dB, using 64 QAM / 6 bits per symbol needs 24 dB. With an SNR in that bin's frequency of 27 dB you've a margin of 3 dB if 64 QAM is being used but 15 if using QPSK.

It's this lowering of modulation order and, indeed, bins that don't have good enough SNR for QPSK not being used, that is why speeds slow in the presence of noise, all other things being equal. It's also why interleave is used: the coding gain from FEC alongside increased resistance to impulse nose improves the bit error rate for a given RF SNR allowing more bits to be packed in without increasing error rate.

Weaver:

XGS_Is_On:
I've no idea how you would choose those values either, Sir. The Tx power can probably be ditched and the attenuation given how much of your line is overhead will vary somewhat.

Weaver:

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