Research On Self-Interference Suppression Schemes For Ambient Backscatter Communication

Ambient Backscatter Communication(ABC)technology,which uses Radio Frequency(RF)signal as carrier,can realize wireless communication at microwatt level,and is one of the most promising solutions for the Internet of Things(IoT)with low power consumption.However,due to the complexity of the source of the environmental RF signal,the strength of the signal is much higher than that of the backscattered signal,which causes serious selfinterference to the backscattered signal transmitted in the same channel.Self-interference makes it difficult for the receiver to demodulate the backscattered signal from the ambient RF signal,which seriously restricts the transmission rate and communication distance of the system.Existing self-interference suppression schemes have problems such as high-power consumption of backscattering circuit modules or excessive utilization of spectrum resources,which limit the development of self-interference suppression techniques.Therefore,this thesis mainly focuses on the self-interference suppression of environmental backscattering communication:(1)Aiming at the backscattering communication scene in remote environment,a selfinterference suppression scheme based on the slight frequency shift of the LoRa signal is proposed.In the proposed scheme,there is a frequency interval between the backscattered signal and the original RF signal.According to the characteristic that each LoRa signal in the Chirp Spread Spectrum(CSS)technology is completely orthogonal,the backscattered signal after frequency shift is nearly orthogonal to the RF signal through theoretical derivation,so as to suppress the self-interference.Furthermore,a single side band frequency shift transmission backscattering circuit is designed by using a low power consumption power splitter/combiner and a quarter wave transmission line.The circuit eliminates the interference of the redundant side bands and improves the backscattering signal intensity.Based on the proposed scheme,a demodulation algorithm based on frequency offset compensation is designed at the receiver to recover the weak backscattered signal.Based on the above scheme,the backscatter tag hardware prototype is used to evaluate the performance of the system in remote scenarios.Experimental results show that the proposed self-interference suppression scheme not only reduces the self-interference of environmental RF signals,but also achieves the transmission distance of nearly 350 m at microwatt level power consumption.(2)A self-interference suppression scheme is proposed for the backscattering communication scenarios in the near range environment,in which the whole frequency shift of Wi-Fi signal is used to transmit the backscattering signal in the protective interval band.In this scheme,the backscattered information is transmitted by the protective interval of WiFi signals,which improve the overall spectrum utilization of the system.The double side band frequency shift transmission backscattering circuit is designed by using the multiimpedance network for eliminating high harmonics and the Field Programmable Gate Array(FPGA)with low power consumption.The backscattering signal frequency is shifted to the center frequency of each subcarrier,and then the sidelobe interference is reduced.According to the characteristics of the signal after frequency shift,a correlative demodulation algorithm is designed at the receiver,and the Least Square(LS)method is used to estimate the channel,so as to reduce the bit error rate at the receiver.Based on the above scheme,this thesis uses the hardware prototype of backscatter tag to evaluate the performance of the system in a short range scenario.The experimental results show that the proposed self-interference suppression scheme can achieve a transmission rate of about 10 kbps.