Comparative analysis of noise tolerance of quasi-coherent phase shift keying signal demodulators

The noise tolerance of quasi-coherent PSK signal demodulators employing phase-locked loops to filter the reference oscillation is analyzed comparatively for the Siforof scheme, the Costas scheme, the reversible Karshin demodulator, the remodulator-comparator scheme and the adaptive demodulator scheme. The phase of reference oscillation in these quasi-coherent demodulators represents a random process, so that the phase error between the received and references signals is also random. It is found that increasing the initial frequency detuning reduces the noise tolerance of quasi-coherent PSK reception. The reversible and remodulation-comparator schemes have the poorest noise tolerance because of their steady-state phase error, which is twice that of the other schemes. When the frequency instability and amplitude of the received signal are high, it is best to use the adaptive demodulated structure. The formulas derived can be used to select the structure for a quasi-coherent PSK signal demodulator according to the required systemwide indicators of the digital transmission system, and to assess its noise tolerance during the design phase.