Research On Theory And Techniques Of Radio Frequency Interference Cancellation Systems

With rapid evolution of wireless communication and sensing technologies,wireless transceiving systems are becoming more and more complex and diverse,and self-interference in wireless receivers due to inadequate TX-to-RX isolation is becoming a common problem.Self-interference need to be suppressed,as it may cause receiving sensitivity degradation and even receiver saturation.RF interference cancellation is a widely applicable signal suppressing method that is especially suitable for self-interference suppression.Research on RF interference cancellation is of great significance for the development of wireless communication and detection systems.The focus of this work is on the theoretical model of analogue least mean square loop based RF interference cancellation systems and the influence of RF interference cancellation on receiving noise of wireless receivers.High performance reflected power cancellers are designed and fabricated based on the proposed models.And the theoretical models are verified through experiments.The main work in this dissertation is as follows:1.A linear time invariant model for 1st order ALMS loop based reflected power canceller(RPC),which is utilized to accurately investigate the cancellation performance and stability of RPC under carrier leakage excitation,is presented.A linear time invariant(LTI)model for RPCs under single carrier leakage excitation is built based on the integral equation of the RPC,upon which an LTI model for RPCs under linear frequency modulation leakage excitation is built.The amplitude frequency response,operating bandwidth and loop stability of the RPC is investigated based on the proposed LTI model.The influence of non-idealities of practical circuits on loop stability and cancellation error is investigated.The proposed models are of great significance to accurately analyzing the key performance of RPCs.2.A noise model of the RPC,which offers theoretical basis for analyzing and calculating the influence of the cancellation ratio,linearity and noise performance of the RPC on receiving sensitivity,is proposed.The influence machnisms of the RF canceller and its controller on receiving sensitivity is investigated according to different signal type and power level of interference and the controlling method in specific applications.A noise model,which is utilized to investigate the influence of the noise performance of the internal circuits of the RPC,loop parameters and cancellation results on the output noise of the RPC,is then presented based on the LTI model.It is concluded that when carrier leakage is sufficiently suppressed the main contributor to output noise of the RPC is the vector modulator,which indicates that vector modulators with low additional noise is of crucial importance to the implementation of high performance RPCs.3.Based on the proposed theoretical models,design guidelines of the RPC are derived,aiming at improving cancellation performance,loop stability and noise performance of the RPC.The influence of loop parameter design,loop gain distribution,characteristics of internal circuit components and devices on loop stability,cancellation and noise performances are thoroughly discussed.Typical circuit implementation of RPC is discussed according to RPC performance requirements,circuit complexity,costs and performance of state-of-the-art microwave devices.An improved RPC based reflection modulation is then proposed.The proposed RPC based on reflection type modulators exhibits much more compact circuit structures while offers comparable adaptive cancellation performance and noise performance when comparing with traditional RPCs based on transmission type modulators,so it is suited for applications including miniature CW radars and RFID readers where system volume is confined.4.Based on the proposed design guidelines,two UHF band RPC prototypes,which respectively utilizing a transmission type vector modulator and a reflection type vector modulator,are designed and fabricated.The UHF band prototypes exhibit high stability,wide operating bandwidth and high power handling capacity.The typical input referred receiving noise PSD is-156 d Bm/Hz@100k Hz offset in the UHF band prototype with transmission type vector modulator,which is the lowest among the reported RF carrier cancellers at UHF band.Experiment results demonstrate that cancellation results in practice can be accurately predicted using the proposed LTI model.In order to demonstrate the validity of the LTI model at microwave band,experiments are conducted based on a K band RPC.The key circuits and loop parameters are ajusted in order to improve cancellation performance of the K band RPC.The experiment results are in agreement with that predicted using the LTI model,which further verifies the validity of LTI model at microwave frequency.5.The residual modulation noise derivation mechanism in PIN diode based control circuits is discussed,and the factors affecting the residual modulation noise level is discussed.The residual modulation noise of modulators with different circuit configurations are compared,and certain circuit designing methods to lower the residual noise of the modulator is discussed.Residual modulation noise of transmission type and reflection type vector modulators utilizing three different model of commercial PIN diodes are measured.General laws of residual noise of vector modulator varying with its operating state are summarized.The noise performance between transmission type vector modulator is compared with that of reflection type modulator,which verifies the theoretical analysis.This work lay the foundation for the implementation of low residual noise vector modulators and high performance RF cancellation systems.6.The convergence and gradient noise of ALMS loops are investigated referring the small step size statistical learning theory for analyzing discrete time LMS algorithm,under the assumption that the interference and the reference vector are jointly Gaussian,and their spectral bandwidth is much high than the loop bandwidth.It is concluded that,the mean square value of cancellation error due to gradient noise increases as the autocorrelation of the reference vector,the optimal cancellation error and the loop bandwidth increasing.Finally,cancellation of narrowband random interference in ALMS loop based canceller is demonstrated using two 1st order ALMS loop based RPCs which operating at UHF band and K band respectively.The experiment results show that the UHF band RPC and K band RPC are stable when excited by narrowband random interferences.Considerable cancellation performance is observed.