An all-digital coherent AFSK demodulator for CubeSat applications

Abstract

Audio frequency-shift keying (AFSK) is a widely adopted modulation scheme for CubeSat systems due to its favorable bandwidth efficiency and implementation simplicity. However, coherent detection usually is avoided because synchronization impairments, caused by intense line-of-sight (LOS) dynamics inherent in low Earth orbit (LEO), may significantly degrade the bit error rate (BER). This paper presents a new all-digital coherent AFSK demodulator based on a Kalman filter (KF) for carrier phase and timing delay synchronization and the Viterbi algorithm for bit detection. The Viterbi algorithm is employed for maximum likelihood sequence detection, and the detected bit statistics are fed back to the KF to estimate phase shift, Doppler frequency shift, and Doppler drift induced by the LOS dynamics. Original mathematical analyses are derived to provide a theoretical foundation for the proposed demodulator's operation, specifically addressing its synchronization accuracy in dynamic LEO environments. The proposed demodulator is evaluated considering an additive white Gaussian noise channel with real CubeSat orbits. The performance results obtained through computer simulations demonstrate that the proposed model can withstand such scenarios with a gain of 5 dB in terms of BER compared to the conventional noncoherent AFSK demodulator. The KF performance is assessed using a moving root-mean-square error (MRMSE) statistic and the trace of its state error covariance matrix estimate.