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Research on Multipath Reception and Soft Decision Algorithms for Frequency Shift Chirp Modulation

Frequency Shift Chirp Modulation (FSCM) is the physical layer modulation technique adopted by LoRaWAN-one of the Low Power Wide Area Network (LPWAN) technology. The frequency sweeping feature of the chirp signal in FSCM provides better anti-fading performance than FSK, but when suffering severe multipath interference, the performance of the LoRa system deteriorates greatly. This paper focuses on the reception of FSCM under the multipath channel, compares the performance of two multipath receiving methods: frequency domain equalization and preamble symbol estimation, and proposes FSCM soft-decision demodulation algorithms. Simulation and comparison are performed in the FSCM system combined with coding and interleaving. The results indicate that the FSCM system with soft-decision demodulation has better performance than hard-decision, and the soft-decision algorithms provide bit likelihood information for applying advanced codes (e.g., LDPC codes) to get better performance.