Receiver gain planning for spread spectrum/low SNR system

Hi everone,

I was wondering how one defines the required gain for a receiver when the signal SNR is negative, which is the case for example for spread spectrum coms.

My current understanding is that knowing the system bandwidth, one can determine the noise power at the system input. Then knowing the ADC full scale range, the gain can be determined as the gain required to bring the noise power at the input of the receiver up to the full-scale range. Is it the right approach?

Then what about the required gain range? The signal power may vary but the thermal noise power stays constant. Does it mean that, if we neglect interferers and the thermal noise is considered constant, the gain can be constant an no VGA is required. Please comment...

Regards
Ghost Tweaker