Key Techniques Of Broadband Adaptive Optics Self-interference Cancellation In In- Band Full-duplex Communication

With the continuous development of the Internet and mobile communication technologies,new mobile Internet applications such as cloud services,virtual reality,and intelligent driving are constantly emerging,and the demand for transmission capacity and transmission rate of wireless communication systems is also increasing.In-band full-duplex(IBFD)technology supports simultaneous transmission and reception of wireless signals in the same frequency band.Compared with frequency-division duplex(FDD)and timedivision duplex(TDD),IBFD can double the spectrum efficiency as well as achieve high efficiency and flexible wireless network access or relay.It is regarded as the most potential development area in the future wireless communication field.In-band self-interference cancellation(SIC)is the most critical technology for implementing IBFD communication,including the elimination in spatial transmission domain,analog domain and digital domain.In the analog domain SIC,the electrical selfinterference cancellation(ESIC)is affected by the inherent conditions of the RF microwave device,which limits the SIC bandwidth.Compared with the ESIC technology,the opticsbased SIC(OSIC)technology can break through the limits of bandwidth and the working frequency band of ESIC,and meet the requirements of large bandwidth,large capacity,and long-distance transmission of future IBFD wireless communication systems.This thesis mainly focuses on the key technologies of broadband OSIC in IBFD.The total research work can be divided into the following aspects:1.Theory and broadband implementation of OSIC systemIn order to study the OSIC principle deeply and implement the self-interference cancellation better,we model the OSIC systems of the two main architectures and derive their SIC implementation principles,and analyze the main affecting factors of SIC depth and bandwidth.For each architecture,a new OSIC solution is proposed and experimental validation is conducted.Based on the OSIC theory of incoherent light source,an experimental study of DMLbased OSIC system is proposed.In the non-coherent light source OSIC system,the optical tunable delay line and optical attenuator are used to achieve precise adjustment of the delay and amplitude of the reference signal.The theory proves that the high-precision adjustment of amplitude and time delay is the main reason for increasing the cancellation depth and increasing the self-interference cancellation bandwidth.In the DML-based OSIC experiment,more than-33 dB SIC depth is achieved over a 400-MHz bandwidth in 900-MHz and 2.4-GHz bands,and more than-27 dB SIC depth is achieved over the total 6-GHz band,which enables high-bandwidth self-interference cancellation in a common frequency band using low-cost devices.Based on the OSIC theory of coherent light source,the theoretical and experimental research on the OSIC system based on DD-MZM is proposed.In the OSIC system of the coherent light source with DD-MZM as an example,the amplitude and time delay of reference signal are adjusted in the electric domain,and the coherence of the light source enhances the optical coupling precision,so that the self-interference signal is better eliminated by the reference signal.The DD-MZM-based OSIC system has the characteristics of simple experimental structure and long-distance transmission of useful signals,and achieves more than-27 dB self-interference cancellation depth covering a broadband range up to 25 GHz.2.Broadbandization technology of OSIC for high frequency IBFD transmissionThe high frequency IBFD demonstration of OFDM RF signals is conducted,and the main parameters of the OSIC system are analyzed.In the demonstration,we find and analyze the main factors that limit the expansion of IBFD to the full frequency band of the antenna,and propose two solutions.The demonstrated IBFD transmission relies on an EML-based OSIC system to eliminate self-interference signals.In the case of wireless transmission using a horn antenna,the experimental results show-30 dB cancellation depth covering the 100-MHz bandwidth and IBFD transmission performance of different bandwidth OFDM signals in the 11.15-GHz band,and obtain about 52.2-dB SFDR,which proves that the EML-based OSIC system has good linearity for broadband IBFD communication.In the experiment,the channel response between the two antennas is proved to be a key factor to affect the performance of the OSIC.In this case,two targeted OSIC broadband solutions are proposed.One is the OSIC broadbandization scheme based on time domain predistortion.A solution for limiting the OSIC bandwidth caused by the mismatch between the wireless selfinterference channel and the wired reference channel response is proposed for the first time.The reference signal is pre-distorted in the time domain before sent to the transmission channel.The response mismatch of the reference channel and the SI channel is compensated in baseband.The full-band IBFD of the wideband antenna has been experimentally verified for the first time,which greatly improves the working bandwidth of the IBFD system and improves the utilization of the antenna frequency resources.The other is the DSP-assisted post-processing OSIC broadbandization solution.A DSP post-processing auxiliary SIC broadband technology is proposed,and the DSP postprocessing system is designed to eliminate residual self-interference caused by channel response mismatch.The experimental results show that a-10 dB suppression ratio can be obtained by DSP technology under the condition that the wideband depth of OSIC is about-20 dB.3.Research on adaptive technology in OSIC systemIn order to adapt to the time-varying self-interference wireless channel,the OSIC system parameters have to be quickly adjusted to completely align the reference signal with the self-interference signal in amplitude,phase and content.The adaptive feedback control scheme and adaptive equalization scheme for delay,gain and multipath are proposed and studied,respectively.For the adaptive delay and gain feedback control technology,an unconstrained optimization model is established for the delay and amplitude alignment of the reference channel and the self-interference channel.And the direct method of unconstrained optimization model is employed due to OSIC system's characteristics.Based on several classical unconstrained optimization direct algorithms,a new direct algorithm is proposed and designed towards the characteristics of OSIC system.The sample time is greatly reduced,and the adaptive speed is significantly improved.For the adaptive frequency domain equalization technique of time-varying multipath channel,an OSIC scheme based on OFDM frequency domain equalization is proposed for the characteristics of time-varying multipath self-interference channel,which compensates the shortcomings in several reference multipath OSIC cancellation schemes.With only one sample and feedback,the use of OFDM frequency domain equalization to match the reference signal with the multipath self-interference signal is accurate and rapid,which achieves more complete elimination of multipath self-interference signals.After the above three parts of the research,this thesis realizes the high frequency band,wide bandwidth and adaptive optical self-interference cancellation in in-band full-duplex communication,and proposes several innovative solutions.As the first author,relevant research achievements include 5 SCI papers published in the mainstream journals in the field of optics,and 2 EI papers in international conferences,as well as several journal and conference papers published in collaboration.