Demodulation schemes requiring the carrier phase are termed coherent . Those that do not need the phase are termed incoherent . It describes demodulation schemes that are sensitive only to the power in the signal. With ASK, the power is either present, or it is not. With wide-band FSK, the power is either present at one frequency, or the other.
Incoherent modulation is inexpensive but has poorer performance.
Coherent demodulation requires more complex circuitry, but has better performance. In ASK incoherent demodulation, the signal is passed to an envelope detector . A decision is made as to whether the signal is present or not. Envelope detection is the simplest and cheapest method of demodulation. In optical communications, phase modulation is technically very difficult, and ASK is the only option.