Multiple Frequency Shift Chirp Modulation And The Interference Suppression Algorithm

As a low-power wide area network(LPWAN)technology,LoRa(Long Range)working in the unauthorized ISM band has been widely used in the internet of things(IoT)and has a broad development prospect.However,LoRa has several limitations such as limited bit rate,collision and susceptibility to interference,which makes it unable to adapt to the ever-changing requirements of the IoT scenario.In this paper,based on the existing LoRa modulation,the multiple frequency shift chirp modulation(MFSCM)based on LoRa signal multiplexing is proposed to meet the development trend of high rate and large capacity of IoT.Firstly,the LoRa modulation and demodulation model is built,and the characteristics of LoRa signal in time domain,frequency domain and fractional domain are analyzed.The time domain waveform of LoRa signal is similar to that of chirp signal,and the spectrum is scattered with Fresnel integral characteristics.The fractional Fourier transform(FRFT)of LoRa signal has an optimal rotation angle,and the energy is concentrated in the fractional domain of the optimal rotation angle.Secondly,the principle of MFSCM design and the mathematical model of MFSCM are given.After the modulation and demodulation process of MFSCM,the feasibility,nominal bit rate,signal-to-noise ratio and peak-to-average ratio of MFSCM are analyzed in detail.It is found that when the lengths of information bits are the same,MFSCM has the same performance as LoRa,but MFSCM has more modes to choose,and MFSCM can achieve much higher bit rate than LoRa by adjusting the length of address bits and information bits.By the analogy with OFDM system,the fractional domain implementation of MFSCM based on fractional Fourier transform is proposed due to the characteristic of LoRa energy concentration in fractional domain.Then,the LoRa WAN network protocol and data collision problems are studied.Considering that when multiple LoRa signals in MFSCM are assigned to different users,multiple users can transmit information simultaneously,the multi-access design of MFSCM is proposed,which can significantly reduce the probability of data collision and increase the user capacity.Finally,the effects of different types of interference on the performance of MFSCM are analyzed,and the interference are divided into narrowband interference,chirp interference with different chirp rate and chirp interference with the same chirp rate.It is found that when the signal to interference ratio(SIR)is large,only the chirp interference with the same chirp rate has a great effect on the performance of MFSCM.The other two interferences have significant effects on the performance of MFSCM only when SIR is small enough.For different types of interference,the interference suppression or interference avoidance algorithms are presented respectively.