Digital Self-Interference Cancellation With Variable Fractional Delay FIR Filter for Full-Duplex Radios

In full-duplex radios, delay alignment errors result in random mismatch between the self-interference (SI) signal and the reconstructed cancellation signal, leading to the performance degradation of the SI cancellation. In this letter, a variable fractional delay (VFD) FIR filter with more precise delay alignment is exploited to suppress the SI in digital domain. The closed-form expressions of the VFD FIR filter coefficients are reformulated by minimizing the residual SI power. Based on the obtained SI cancellation capability, the influence of the filter order on the SI cancellation is then characterized. Simulations show that the deployment of the VFD FIR filter significantly enhances the SI cancellation performance. Specifically, when the SI signal bandwidths are 0.5 and 10 MHz, the cancellation capability of employing the VFD FIR filter based on the proposed coefficients outperforms the VFD FIR filter under the LS criterion by 6.5 and 3.7 dB, respectively, which presents a potential contribution to enhance the SI cancellation performance.