Quadrature down-conversion based analog self-interference cancellation for continuous wave radars

For continuous wave radars, self-interference signals coupled from the local transmitter are significantly stronger than the desired echoes, such that the receiver will become saturated. To cancel the self-interference, this paper proposes a novel scheme based on a simple quadrature down-conversion operation, for which two frequency-swept carriers with a 90-degree phase-difference are adopted in both transmitter and receiver to realize a high- sensitive reception of echoes as well as high isolation between the transmitter and the receiver. A search algorithm is proposed to minimize the power of the residual self-interference. Since both the desired echoes and the self-interference are replicas of the transmitted signal, the desired echoes may be mistaken for self-interference signals and attenuated significantly. Thus, this paper analyzes the impact of the proposed scheme on the desired echoes and develops a method to avoid the potential attenuation of the desired echoes. In addition, this paper derives a closed-form expression for the self-interference cancellation performance. Extensive simulations are performed to verify the proposed method.