Fundamental Limits for ISAC: Information and Communication Theoretic Perspective

The integrated sensing and communication (ISAC) paradigm, where sensing and communication functions are synergistically implemented by possibly sharing the same frequency band and hardware, has emerged as one of key technologies in future wireless systems. While substantial efforts have been devoted to investigate various techniques under different scenarios and system architectures, the fundamental limits of such integrated systems remain generally unknown. In this chapter, we aim to present some recent progress from the information theoretical perspective. First, we provide a simple information-theoretic model for single-user channel and present the fundamental tradeoff between communication and sensing. Albeit simplified, the proposed information-theoretical model captures the relevant scenario where a transmitter, equipped with a radar receiver (monostatic sensing), wishes to use the backscattered signal for sensing while sending data via a common waveform. Then, we extend our study to more realistic setups ranging from imperfect channel state information at receiver, correlated channel states to two-user broadcast and multiple access channels. In the presence of multiple users, the full characterization of the tradeoff region is challenging even without sensing. Therefore, we focus our attention to highlight novel coding strategies that suitably balance between different operating points on the communication and sensing. Finally, we discuss some open problems and future research directions, such as more general modeling and tighter bounds for memoryless ISAC channels, information-theoretical bounds for block-varying/Markov ISAC channels, the interaction between information theory and artificial intelligence (AI) in ISAC.