I’m not sure I follow how phase noise is related to an interfering channel emitting meaningful radiation 100MHz out of band. If all of these transmitters are compliant then it’s a non-issue. I’ve also never seen a transmitter where phase noise was more of a factor than IF in the context to out of band emissions.

Could you elaborate on how phase noise would cause an issue in this scenario?

N.B. Most of my experience is in optical systems, my wireless knowledge is limited.