Appendix B: spectral efficiency of cable and satellite transmissions with DVB parameters
The bandwidth occupied by an AM modulated signal is twice the bandwidth of the modulating signal, due to the two lateral bands generated by the modulation process. In the case of a digital signal of symbol period T which is Nyquist filtered with a roll-off α, the bandwidth will be:
BW = (1 + α) × 1/T
From this formula, we can now calculate the capacity of the channel and the spectral efficiency for various hypothetical cases:
With a roll-off α = 0.15, a European channel of bandwidth BW = 8 MHz can accept a maximum symbol rate Fs = 1/T = BW / (1 + α) = 8/1.15 ≅ 7MHz. A commonly used value is 6.875MHz.
The usable order of QAM will depend on the network ...