Combining OFDM-MFSK and Chirp Spread Spectrum Modulation for Applications in Low-Power Wide-Area Networks

Abstract

Chirp spread spectrum (CSS) has attracted a lot of interest in the last few years, especially in the field of low-power wide-area networks (LPWAN). One of the most commonly employed LPWANs solution is LoRaWAN, which has LoRa as its physical layer. The concept of LoRa is to use the combination of CSS and M-ary frequency shift keying (M-FSK) for transmission. LoRa is known to be highly robust and well-suited for low-complexity implementation but the weak aspect is in its low spectral efficiency. Orthogonal frequency division multiplexing M-FSK (OFDM-MFSK) is a technique which combine M-FSK with OFDM. For this method, the transmission bandwidth is divided to several parallel M-FSK transmission, i.e. more than one sub-carrier can be active at a time, for improving the spectral efficiency. The reception of OFDM-MFSK can be done non-coherently, therefore it holds many similar design principles as LoRa. The problematic scenarios for OFDM-MFSK are for the case with multipath-induced frequency selective channel and for the channel with presence of strong narrowband interference. We propose the combination of OFDM-MFSK and CSS in this paper. The weakness and strength of the two techniques complement each other well. The implementation of this combination is simple, since CSS can be viewed as special case of direct sequence spread spectrum and only chirp signal multiplication at the transmitter and the receiver are needed. Simulation results show that the proposed method provide improvement compared to OFDM-MFSK in both multipath-induced frequency selective channel and for the channel with presence of strong narrowband interference.