Receiver Algorithm Design And Implementation Of Ambient Backscatter Communication Systems

The technology of ambient backscatter communication(AmBC)transmits information by ambient signals,so it does not need extra spectrum and can save spectrum resource.Meanwhile,the backscatter device is suitable for Internet of Things(IoT)due to its low cost and low power consumption.The backscatter device modulates its information on the ambient signals based on the changing of its impedance which forms different reflection state.The signals received by the receiver include the direct-link signal and the backscatter-link signal.Usually,the power of the backscatter-link signal is several orders of magnitude lower than that of the direct-link signal,so it is difficult for the receiver to recover the symbol of the backscatter device.Therefore,the design of the receiver is an important issue in the AmBC system.There are many kinds of wireless signals in the environment.In this thesis,the design of the receiver algorithm of the AmBC system when different signals are used as the radio-frequency(RF)source is studied.For the case where the sine wave is the RF source,the frame structure of the backscatter device and the signal processing modules of the receiver are designed based on the system model and the receiver design in the existing literature.The software-defined radio platform is used to implement a backscatter communication system,and the performance of the system is measured.The measurement results verify the feasibility of the system.When the RF source is a two-state amplitude shift keying(2ASK)signal,this thesis designs a coding scheme for the RF-source symbols,so that the power of the RF-source signal is not 0 within half a symbol period of the backscatter device.It solves the problem that when the RF-source symbol is 0,the receiver cannot recover the symbols of the backscatter device.Then this thesis proposes a joint maximum likelihood detector and a simple two-step detection algorithm.The simulation results show that for the detection of the RF-source symbols,the performance of the above two detection algorithms is better than or close to the bit error rate(BER)performance of the 2ASK signal,and that for the detection of the symbols of the backscatter device,the performance of the above two detection algorithms is very close.Most of the existing commercial communication systems use OFDM(Orthogonal Frequency Division Multiplexing)technology,therefore this thesis studies further the problem of receiver design of the AmBC system when the OFDM signal is the RF-source signal.In the scenario where a backscatter device is used,this thesis proposes three detection algorithms based on interference cancellation because of the fact that the power of the backscatter-link signal is several orders of magnitude lower than that of the directlink signal.The simulation results show that with the above detection algorithms,an error floor will appear in the BER curves of the RF-source symbols,which will decrease as the relative signal-to-noise ratio(SNR)decreases and that the BER of the backscatter device symbols is superior to that of the algorithm of direct energy detection.In the scenario where multiple backscatter devices are used,this thesis uses the ideas of time division multiple access(TDMA)and cyclic prefix(CP)to design the signal format of the pilot symbol 1 of the backscatter device and the signal format of the OFDM system.When the pilot symbol 0 of the backscatter device is transmitted,two design schemes of the related OFDM pilot symbol are given in this thesis,and corresponding detection algorithms are proposed.The simulation results show that when scheme one is used,the BER curves of the direct link and the backscatter link will have an error floor,while scheme two does not.Meanwhile,increasing the number of the backscatter device will degrade the performance of the direct link and the backscatter link,because the number of subcarriers used in the process of detection is decreased.