Sorry, that probably wasn't too helpful!
For digital broadcasting to work, you need a reception bitrate error of approx 1 in 1000 or better. Below that and the error correction system can cope, above and it can't.
For orthogonal carrier systems, as the number of carriers "tends towards" infinity, in a Gaussian ("white noise") channel, this errror rate requires a carrier(s) to noise power of at least 17dB, a "magic number"! (below that, the bit error rate very rapidly increases as carrier power falls).
For COFDM, either 2k or 8k carriers is "close enough" to infinity for this rule to apply.
For COFDM, channel noise is approximately equal to measured variance, and average carrier power approximately equal to measured modal value.
(with COFDM Rayleigh scattering - radio wave reflection from nearby buildings - some carriers cancel out while others reinforce and strengthen. The strongest carrier gives the modal value. Although SNR improves on reinforced carriers, it worsens on cancelled out ones, so the overall SNR remains roughly the same).
This is only true under "best" reception conditions, and only the Setpal receiver (Novapal tuner) seems to almost manage it. If there's any i.f. channel noise - ie insufficient image and spurious rejection - then the received SNR via aerial will have to be better.
Also, the receiver's local oscillator frequency must be very exact, otherwise the carriers are no longer exactly orthogonal, and some further SNR advantage is lost.