Integrated Sensing and Communications in FDD MIMO Without CSI Feedback: Towards FDD MIMO ISAC

In monostatic integrated sensing and communications (ISAC) systems, frequency division duplex (FDD) is often the preferred choice because, in time division duplex (TDD), reflected echo signals can cause uplink interference. However, implementing FDD in MIMO systems presents challenges due to the overhead associated with channel state information (CSI) acquisition, which hinders the realization of FDD ISAC systems. Therefore, addressing the CSI acquisition problem in FDD systems is crucial for enabling efficient FDD MIMO ISAC operations. In this paper, we propose a new MIMO transmission mechanism that reconstructs downlink CSI based on uplink sounding signals by leveraging partial reciprocity in frequency-invariant channel parameters. Building upon this, we optimize an ISAC precoder that not only fulfills given sensing tasks but also provides robust spectral efficiency for communication users.