A DSP DPLL demodulator with sequence estimation for MSK signal in the presence of Doppler shift on Rician fading channel

A phase locked loop (PLL) is well known as an FM demodulator with threshold extension capability. This property can also be useful for low BER detection. A PLL also has frequency tracking ability for input center frequency variation. The author proposes a detection scheme for an MSK signal, using a digital signal processing type digital PLL (DSP DPLL) demodulator. The author also adopts a sequence estimation scheme to mitigate the inter-symbol interference (ISI) caused by the finite bandwidth of the DSP DPLL. Through computer simulations, the author shows that the proposed DSP DPLL demodulator with the sequence estimator exhibits very low BER characteristics in the presence of time-varying Doppler shift on frequency non-selective Rician fading channels. This scheme will be useful for detecting an MSK signal with large Doppler shift from, for example, LEO satellites.