Digital Self-interference Cancellation Technique In Co-time Co-frequency Full-Duplex Radios

The concept of Co-time Co-frequency Full Duplex(CCFD)is put forward to relieve the increasingly tense relationship between the rapid growth of the business needs and the limited spectrum resources through changing the wireless communication technology.Theoretically,it can transmit and receive signals at the same time and at the same frequency,which would make the spectral efficiency of the existing communication systems be doubled.However,due to the existence of strong self-interference(SI),and the specific solution is still in the optimization process,CCFD was only listed as a potential technique in the white paper of 5G.Currently,the research on SI cancellation can be mainly divided into three domains: antenna cancellation,analog cancellation and digital cancellation.And recent years,digital cancellation has been paid close attention to because of its high flexibility and powerful ability in signal processing.This paper focuses on how to optimize the technique of digital SI cancellation,and realized the suppression of SI together with antenna attenuation and analog cancellation in CCFD system.Though the existing SI cancellation methods can already attenuate some SI to guarantee the basic communication of simple CCFD systems,it's impossible to realize the complete SI elimination in practical full-duplex system by only classical linear SI cancellation.This is mainly affected by the RF impairments in the transceiver chains,such as: nonlinear distortions,IQ imbalance,oscillator phase noise,quantization noise and Gaussian noise,etc.Firstly,the nonlinear effect of Power Amplifier(PA)is taken into consideration,and the nonlinear characteristics with wideband input signals is analyzed,then the Parallel Hammerstein model is chosen to approximate PA.Afterwards,the unknown channel response is estimated off-line according to the Least Square(LS)criterion based on the Single-input Single-output(SISO)CCFD system.The simulation results show that compared with linear SI cancellation,the proposed scheme can obtain higher interference cancellation ratio(ICR)in digital domain.Secondly,a new digital adaptive cancellation scheme is proposed for the scenario where the channel is variable.Considering the PA non-linearity,it ensured the accuracy of the channel estimation by tracking the channel variety in real time.And during the estimation period,the Least Mean Square(LMS)algorithm is adopted.The performance of this cancellation method is also varied and analyzed at last.Finally,the detrimental effects introduced by both the IQ imbalance and the transmitter nonlinearity in CCFD system is researched.On the basis of these,a joint digital cancellation scheme is proposed,in which not only the effect of IQ imbalance and PA non-linearity individually,but also the comprehensive function of them are taken into account.And a widely linear model is used to simulate the non-ideal quadrature modulator in transceiver chains.The results show that the proposed canceller can eliminate more compared with other cancellation schemes,and the overall SI attenuation can reach 108 d B together with antenna separation and analog cancellation.