RFSYSTEM,
One very easy and clean expalnation starting from natural noise which is -174 dBm/SQR Hz
For 1 Mhz bandwith is -174+10 log B=-114dBm minimum bandwith for 8 MHz bandwith OFDM TV is 10 MHz it is +10 dB which will give -104 dBm. for good receiving we have to S/N=12 dB + F(dB) of receiver .it is around -104+12+1=-91dbm that mean that we need spectral denasity of noise better than 90 dBc/sqr Hz to not see oscillator influence.We have in OFDM 6k carriers that mean that this noise have to be 1kHz from carrier.
GL XTASA