On Beamforming Design for ISAC Transceiver in Presence of Self-Interference

To support object detection and tracking as one of the main services of a mono-static integrated sensing and communications (ISAC), an ISAC node needs to operate in full-duplex mode, and therefore suffers from strong self-interference (SI) mainly due to mutual coupling (MC) between its antenna arrays. This paper studies beamforming (BF) designs at transmitter (Tx) and/or receiver (Rx) sides of ISAC node to support both multibeam transmissions for simultaneous communications and sensing, and also manage SI cancellation (SIC) over signaling bandwidth and suppress specific spatial reception angles. Provided that ISAC node has estimated SI channel only over a few frequency indices, a null-space projection (NSP) based BF designs at Tx-only, Rx-only, and joint Tx-Rx are studied to see which one is preferred from hardware-efficiency perspective. Moreover, a system level analysis with a proposed sensing frame structure is discussed to support reliable and periodic sensing of a region by choosing proper parameters while minimizing communications performance loss. Performance of SIC and effect of number of its constraints are discussed through numerical results. It is shown than NSP-based Tx-BF is sufficient for SIC and one can relax RxBF by only utilizing simple phase-shifters. Moreover, importance of having precise estimation of MC, and performance of object detection with and without SIC are discussed.