Coherent and noncoherent operation choice.

Different processors may see an operation and assume different sequences of values (this is considered noncoherent behavior)

"Coherent versus noncoherent Bloch oscillation".

In noncoherent STC the receiver does not know the channel impairments but knows the statistics of the channel.

The performance degradation is a result of noncoherent transmission - in this case it refers to the fact that tracking of the phase is completely ignored.

There are modulation types (like frequency shift keying) that can be demodulated without carrier recovery (noncoherent demodulation) but this method is generally worse.

However, two audio tones can be sent simultaneously on a conventional, constant-envelope FM RF carrier, but the noncoherent detection of the FM signal at the receiver would destroy any signal-to-noise ratio advantage the multitone scheme might have.