detect a signal in a noisy enviroment

to detect a signal in a noisy enviroment, setting a received power level threshold doesnt always work, because noise power often varies, for example in a quiet enviroment, the noise power is -100dbm and we think its ok to set a threshold at -90 dbm and anything above -90dbm is a signal, it works until we moved to an enviroment that is abit more noisy, say -80dbm noise and -70 dbm signal, then we would falsely report that there's always having a signal present.

what does really matter is the signal to noise ratio.

we have to normalize the receiver gain such that the output of receiver amplifier chain is always closed to saturated, in the above example we should normalize the received signal to 0dbm so the noise become -10 dbm. And the demodulator should always try its best to perform demodulation, once the  received signal to noise ratio is above a certain level, the demodulator would giving correct output data, by verifing the checksum of demodulated data, we can report with confident that a signal is presented.