Research On Detection Technology Of Multi-antenna Backscatter Communication Systems

As an important part of mobile communication,the main idea of Internet of Things is to connect things together to form a network,which can effectively improve the efficiency of production and life.In the white paper published in 2021,IMT-2030(6G)Promotion group proposed the vision of "all things intelligent,digital twin",and the number of Internet of Things devices will up to 100 billion in the future.How to achieve ultra low-power and low-cost Internet of Things communication has become a key issue.Backscatter communication technology has attracted wide attention because it can use the radio frequency signal sent by other devices as a carrier to transmit its own information.It has the features of low power and low cost,and can be used as one of the solutions of low power Internet of Things.By adjusting the load impedance of the antenna,the backscatter device can alter the amplitude and phase of the incident signal.Through this process,the information of the backscatter device is carried on the incident signal,so the backscatter device don’t generate an RF carrier.The detection of backscattered signals is one of the key challenges in backscatter communication systems.First,the limited energy of backscatter devices limits the number of pilot symbols that can be transmitted.Second,the characteristics of backscatter channels are different from traditional point-to-point channels.In addition,in the backscatter communication system,the receiver not only receives the signal from the backscatter device,but also receives the interference signal from the radio frequency source.The backscattered signal undergoes two-fold channel fading and its signal strength is weak,so the detection of the weak backscattered signal will be affected by the strong interference signal.When considering ambient backscatter communication systems,the detection of backscattered signals is more challenging due to the unknown RF source signal.In this thesis,the problems of signal detection under two different backscatter communication systems are studied,aiming at the problems encountered in each system,effective detection scheme are provided.Firstly,the signal detection problem in bistatic backscatter communication system is studied in this thesis.The backscattered signal detection in the system is affected by direct link interference signal and phase noise,so the detection method based on interference rejection combining is proposed in this thesis.In this method,the interference signal is roughly estimated and eliminated,and then a combining coefficient is designed to combine the received signal,so as to suppress the residual interference in the signal.Finally,the memory of the line code is used to combine the successive two-bit signal for detection.Considering the phase rotation caused by phase noise,the detection scheme based on clustering is proposed.Simulation results show that the proposed detection method based on interference rejection combining can effectively boost the detection performance.Secondly,the signal detection problem in the ambient backscatter communication system is studied in this thesis.The scenarios of single device and multiple devices are considered respectively.For the scenario containing single device,based on the difference of the eigenvalue of the received signals when the backscatter device sends different symbols,the eigenvalue-based detector is designed and the optimal detection threshold which can minimize the bit-error-rate is derived.Simulation results show that the proposed detector has better detection performance under low signal-to-noise ratio.For the scenario containing multiple devices,an noncoherent parallel detection method is proposed which can recover the symbols of all backscatter devices without estimating channel state information.Firstly,a low-complexity expectation maximization algorithm is used to cluster the received signals,and then the mapping relationship between each cluster and the symbol combination of backscatter device is obtained by using the state flipping in the waveform of the backscatter device,and then the signals of each device are recovered.The simulation results show that compared with the benchmark methods,the proposed method has better bit-error-rate performance.