Doppler Radar with In-Band Full Duplex Radios

The use of in-band full duplex (IBFD) is a promising improvement over classical TDD or FDD communication schemes. To enable IBFD radios, the electrical balance duplexer (EBD) has been proposed to suppress the direct self-interference (SI) at the RF stage. The remaining SI is typically assumed to be canceled further in the digital domain. In this paper, we show that the non-zero Doppler frequencies can be extracted from the residual SI, giving information about the speed of objects in the environment. As a result, an IBFD radio can be seen as a monostatic Doppler radar which is affected by the communication signal. This paper presents a detailed performance analysis of the different sources of interference affecting the Doppler radar. The performance is also evaluated using a radar-enhanced IBFD prototype consisting of a SDR module and an EBD designed to achieve up to 55 dB Tx-Rx isolation in the 1.74 GHz RF band. A measurable Doppler component is created by moving a 15 dBsm cone at known velocities at 0. 5-1.3 m from the prototype. For an EBD SI rejection of 45 dB, speeds in the range of 200 to 800 mm/s are detected with high accuracy.