A DSP DPLL demodulator with sequence estimator for CP-FSK signals in the presence of large Doppler shift

An analogue phase locked loop (APLL) has been well known as an FM demodulator with threshold extension capability. This property leads to the feasibility of using APLL as a low BER detector for digital FM. An APLL also has a frequency tracking ability for the input center frequency variation. We propose a detection scheme of binary continuous phase FSK (CP-FSK) signals, using a digital signal processing type digital PLL (DSP DPLL) equivalent to the APLL. We have also adopted a sequence estimation scheme to mitigate the intersymbol interference (ISI) caused by the finite loop bandwidth of the DSP DPLL. Through computer simulations, we have shown that the proposed DSP DPLL demodulator with the sequence estimator exhibits very low BER characteristics in the presence of large Doppler shift on flat Rician fading channels. This scheme is useful for detecting CP-FSK signals from LEO satellites.