Researches On Self-interference Cancellation Technology Of Full-duplex Radio Frequency Front-end

Full duplex is a two-way communication technology that simultaneously transmits and receives in the same frequency band.Compared with traditional half-duplex systems,this technology can theoretically double the spectrum efficiency and make full use of spectrum resources.However,the system's own transmission signal will cause serious selfinterference on the receiving link,so it is difficult to correctly recover the received signal from the target.Therefore,effect suppression and elimination of the self-interference signal at the receiver is the biggest challenge in full-duplex communication.The self-interference signals are mainly generated by the leakage of energy at the antenna end,mutual coupling,etc.,so self-interference suppression at the radio frequency domain is the most direct and effective way.This thesis focuses on the self-interference problem of the radio frequency end of the full-duplex communication system,and in-depth research has been carried out on the coupling between channels,the self-interference suppression and elimination methods of the antenna end.The main content of the thesis can be summarized as follows.1.Investigation on the coupling between SIW adjacent channels in millimeter wave systems: SIW transmission lines are often used for circuit design in millimeter wave frequency bands,but due to their incomplete shielding structure,energy leakage and coupling between channels may still increase the full duplex system self-interference.Therefore,a circuit analysis method based on the equivalent model of the transmission line is proposed.For the structure where two adjacent SIWs share a common row of metallic cylinders,according to the physical structure size,the coupling coefficient form one channel to the other channel is directly derived.In order to validate the proposed calculation method,three SIW couplers with different coupling coefficient were designed.The transmission coefficient and coupling coefficient of the simulated results show very good agreements with the theoretical calculation results,which proves the accuracy of the method.2.Investigation on self-interference suppression based on active quasi-circulator:For a full-duplex communication system with a common antenna for both transmitter and receiver,the radio frequency self-interference suppression mainly relies on the port isolation of the circulator or quasi-circulator.Therefore,by combining the unidirectional amplification and high power capacity characteristics of Ga N HEMT,an active quasicirculator operating at 28 GHz is designed based on the path cancellation method.Two selfinterference paths are constructed in the quasi-circulator.By controlling the transconductance of the transistors,the two self-interference signals are equal in amplitude and out of phase,so that the transmitted signal are superposed and cancellated at the receiving port,and the self-interference is realized.Experimental verification shows that the proposed active quasi-circulator has the advantages of simple circuit structure,high isolation,low insertion loss,and high power capacity.3.Investigation on the method of self-interference elimination with separate transmitting and receiving antennas: For a full-duplex communication system with separate transmitting and receiving antennas,(a)Focusing on the self-interference suppression of cross-polarized antennas,a slot-coupled microstrip patch antenna with broadband self-interference cancellation performance is proposed.By introducing an auxiliary port and loading a reflective terminal,an indirect coupling path is constructed to further cancel the remaining interference of the cross-polarized antenna,thereby obtaining higher port isolation in a wider frequency band.(b)Based on the theory of dual-antenna decoupling network,a design method of frequency-tunable decoupling network is proposed.This method is applied to the frequency tunable monopole antenna and realize the simultaneous change of transmission zero with antenna resonate frequency.This design method has no restriction on the antenna structure itself,and can be designed independently of the antenna,which has universal versatility.4.Investigation on broadband millimeter wave dielectric integrated selfencapsulated full-duplex antenna: In order to meet the broadband and high isolation requirements of 5G millimeter-wave full-duplex communication,two structures are proposed.(a)The self-packaged broadband differential-fed dipole antenna.It shows an impedance bandwidth of up to 58% and achieves a stable gain and radiation pattern.(b)The broadband high isolation dual antenna structure suitable for full-duplex communication.By loading decoupling branches between the two antennas,the spatial coupling electric field is suppressed,and the broadband isolation of the transmitting and receiving antennas is realized,which provides a self-packaged antenna design solution for broadband millimeter wave fullduplex communication.