Key Technologies On Spatial-Domain Self- Interference Cancellation In Co-Time Co-Frequency Full-Duplex

Co-time co-frequency full-duplex(CCFD)wireless communications simultaneously transmit and receive electromagnetic wave signals over the same frequency resource.Compared with half-duplex(HD)wireless communications that receive and transmit signals in either time domain or frequency domain,CCFD wireless communications can receive and transmit signals in both time and frequency domains,and thus the spectral efficiency can be doubled in theory.Since CCFD wireless communications transmit and receive signals at the same time and frequency,the transmitter would generate strong selfinterference(SI)to the receiver.Therefore,the precondition of realizing CCFD wireless communications is to effectively suppress strong SI.SI suppression techniques can be roughly divided into three domains: spatial domain,radio frequency(RF)domain and digital domain.Spatial SI suppression techniques reduce the SI power before entering the receiver by exploring the spatial characteristics of the antennas.This paper studies spatial SI suppression techniques,containing four aspects as follows:First,considering the scenario where the local transmit and receive antennas are separated,the reference SI signal is introduced by the auxiliary receive antenna and the SI suppression performance of the auxiliary receiving antenna is studied.The local transceiver with a transmit antenna,a primary receive antenna and an auxiliary receive antenna serves a single-antenna remote transceiver.The optimal weighting coefficient for the auxiliary receive antenna is given,and the SI suppression ratio and signal-tointerference-plus-noise-ratio(SINR)after SI suppression are derived.Compared with the single-antenna scheme that the reference SI signal is introduced by direct RF coupling,the SI suppression of the auxiliary receive antenna would change the received power of the desired signal.The analysis and simulation results show that the desired channel correlation coefficient significantly affects the SINR of the auxiliary receive antenna after SI suppression: when the optimal weighting coefficient is 1 and the desired channel correlation coefficient is 1,the SINR after SI suppression is about quadruple than singleantenna scheme;when the optimal weighting coefficient is 1 and the desired channel correlation coefficient is-1,the SINR after SI suppression is about zero.Second,an SI suppression algorithm for maximizing the SINR of the local receiver is proposed.The local transceiver explores multiple transmit and multiple receive antennas to serve a single far-field transceiver.An optimized algorithm that maximizes the SINR of the local receiver is given.The algorithm is divided into two steps: the closedform expression of the local receiver beamforming vector is derived by maximizing the local receiver SNR;and the local transmitter beamforming vector is given by minimizing the SI.The simulation results demonstrate that the SI suppression values of the proposed algorithm are 65.5 dB and 89.8 dB when the signal-to-noise ratio(SNR)of the local transmitter is 30.0 dB and 60.0 d B,respectively.Compared with singular value decomposition(SVD),the improvements of SI suppression are 25.5 dB and 19.8 dB respectively.Third,considering the scenario that the local transceiver with multiple transmit antenna and one receive antenna serves several remote transceivers,the near-field spatial SI suppression problem under far-field desired signal transmission is studied.By relaxing the initial problem,two methods are given,i.e.,least square(LS)and 1-norm relaxation linear programming.The simulation results show that when the direction of the local receiver antenna is different from that of the remote transceiver,the receiving of the desired signal at the far-field transceiver almost does not affect the SI suppression.When the directions of the local receiver antenna and the far-field transceiver are in the same direction,the SI suppression performance is significantly reduced.This result shows that there is a clear trade-off between SI suppression and the reception of the desired signal at the far-field transceiver especially when the local receiver antenna and the far-field transceiver are in the same direction.Fourth,considering the scenario where multiple multicast groups are served by the local transceiver with multiple transmit antenna and one receive antenna,SI and multicast co-channel interference are considered and the far-field multicast beamforming problem with near-field spatial SI suppression is studied.Firstly,under the specific SI suppression value,an optimization problem of maximizing the minimum received SINR of far-field transceivers is established,which is non-convex and NP-hard.Secondly,this problem is converted to convex problem by SDR relaxation and solved by the modern interior point method.Finally,beamforming vectors are recovered by Gaussian randomization,linear programming power adjustment with an outer loop search.The simulation results show that SI channel using spherical wave model can effectively suppress SI under multicast signal transmission,even if the directions of the local receiver antenna in the near-field and the far-fied transceiver are the same.In this paper,we study the SI suppression of the auxiliary receive antenna scheme,the co-optimization problem of SI suppression and desired signal reception,and the optimization of near-field SI suppression and far-field receiver beamforming problem.The research results can improve the spectral effectiveness in CCFD wireless communications.