Viterbi Soft Demodulation Algorithm for CPFSK Systems

Continuous Phase Frequency Shift Keying (CPFSK) is a phase-continuous constant-envelope modulation scheme. In modern communication systems, Turbo codes or LDPC codes are commonly used as error control codes to combat the effects of channel noise and fading. In the process of channel decoding, soft demodulation information is required to reduce the bit error rate(BER). This paper proposes a soft output calculation method for CPFSK signals based on the Viterbi demodulation algorithm. The method utilizes the difference of cumulative correlation values of the signal to characterize the log-likelihood ratio (LLR) information of the information symbols based on the derived results from conditional probability theory. These LLR values are then used as soft decision outputs. Simulation results demonstrate that, with almost no increase in algorithm complexity, this method achieves a performance improvement of approximately 3dB compared to hard decision demodulation.