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To obtain Tx-Rx isolation in an in-band full-duplex (IBFD) transceiver, the electrical-balance duplexer (EBD) can be utilized to suppress the self-interference (SI). Although the EBD can provide more than 50 dB cancellation of the SI directly at RF, it cannot fully overcome strong signals reflecting off of nearby objects. As a result, the environmental reflections could still limit the performance of IBFD communication and have to be attenuated in an extra cancellation step, e.g. in digital baseband using a digital SI canceler. While mitigating the reflections is a challenging dimension of this technology, it opens a new opportunity to perform environmental sensing. In this paper, an IBFD transceiver architecture is introduced to enable such context-aware communication functionality. We investigate how the EBD's SI rejection property influences the performance of the proposed communication device which also functions as a Doppler radar. The simulation result demonstrates that the proposed architecture can produce high-resolution Doppler when the EBD provides > 20 dB Tx-Rx isolation. Concerning the constraints dictated by the application, this paper finally suggests a radar-communication trade-off.Index Terms-In-band full duplex, self-interference cancellation, Doppler radar, wireless sensing.

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.

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