Research On Key Technologies Of Digital Self-interference Suppression In Co-Time Co-Frequency Full-Duplex Systems

The co-time co-frequency full-duplex(CCFD) system use the same frequency to transmit and receive signal at the same time. When compared with the traditional frequency division duplexing(FDD) and time division duplexing(TDD) systems, it can achieve a double frequency spectral efficiency in ideal case. It is one of the most effective measures to improve capacity in the next generation communication system.In practice, the implementation of a CCFD system needs to solve the co-frequency self-interference. The self-interference suppression measures common used include antenna suppression, analog suppression and digital suppression. Among them, digital suppression is used as the last suppression measure. The residual self-interference after digital suppression will be the final residual self-interference. Therefore, digital suppression plays an important role in the final suppression performance. This paper focuses on high performance and low cost digital self-interference suppression. The main contributions are as follows.First, symbol error rate(SER) expression related to interference-to-signal ratio(ISR) and analog-to-digital convertor(ADC) digits in digital self-interference suppression system is deduced. By analyzing the influence of ISR and ADC digits on the real judgment threshold, the symbol error rate of quadrature amplitude modulation(QAM) digital self-interference suppression system is attained. This result can provide a basis for the setting of analog self-interference suppression requirement to limit ISR and the selection of ADC digits in the engineering. As a special case, it needs 12-bit ADC to achieve the 510- SER bound when bit signal-to-noise ratio(SNR) is 14 d B, ISR is under 40 dB, and 16-QAM is used.Second, a digital self-interference suppression scheme based on accurate measurement of nonlinear self-interference by feedback is proposed. By analyzing the suppression performance, feedback aided scheme is proved to be better than the non-feedback scheme because it knows the nonlinearity of transmitting channel. Feedback aided scheme achieves 27 dB higher self-interference suppression than non-feedback scheme when the phase noise variance is 20.0075 rad and the time delay difference between feedback and receiving channel is 30 ns. Then a digital self-interference suppression scheme based on accurate measurement of nonlinear self-interference by feedback is proposed. Such a scheme can decrease the requirement of feedback ADC digits without suppression performance loss. As an example, the feedback requires only 8-bit ADC to receive nonlinear self-interference component to ensure suppression performance while it uses 12-bit ADC to receive full self-interference with linear and nonlinear components.Third, a precoding self-interference suppression scheme applicable for single antenna full-duplex system is proposed. The existing self-interference suppression schemes based on precoding are applicable only for multi output full-duplex system. The scheme proposed here extends the range of precoding self-interference suppression application. By introducing a digital pre-cancelling signal to cancel the self-interference path with more time delay based on precoding in the transmitting channel, self-interference is suppressed in such a scheme. It can achieve 20 dB suppression for the target path when the estimation has 5% magnitude error.Fourth, a full-duplex MIMO self-interference suppression scheme with crosstalk compensation is proposed. The nonlinear component of self-interference is suppressed by the compensation of crosstalk and digital pre-distortion before transmission when crosstalk and nonlinear distortion exist in transmitting channels. Therefore, it needs only linear reconstruction of self-interference for suppression, where the complexity can be reduced. For a 20 MHz bandwidth LTE full-duplex 2×2 MIMO, such a scheme can provide 35 dB digital self-interference suppression.The research results on CCFD digital self-interference suppression in this paper can be used as important references in full-duplex transceiver system level design, ADC selection, and digital self-interference suppression algorithm design.