Integrated Sensing and Communications: A Mutual Information-Based Framework

Integrated sensing and communications (ISAC) is capable of circumventing the limitations of existing frequency-division sensing and communications (FDSAC) techniques. Hence, it has recently attracted significant attention. This article proposes a novel framework for ISAC from a mutual information (MI) perspective. Based on the proposed framework, the sensing performance and the communication performance are evaluated by the sensing MI and the communication MI, respectively. Under this framework, the sensing and communication (S&C) performance metrics, i.e., the S&C MI, have similar physical and mathematical properties as well as the same unit of measurement, which could facilitate theoretical analyses and waveform design. This framework defines ISAC's fundamental performance limits, which serves as an alternative candidate for evaluating the S&C performance tradeoffs. Based on this framework, the S&C performance of downlink and uplink ISAC systems is investigated and compared with that of FDSAC systems. Numerical results are provided to demonstrate the superiority of ISAC over conventional FDSAC designs. Finally, promising open research directions are provided in the context of MI-based ISAC.