# Exploring ISAC: Information-Theoretic Insights
## 8 Conclusions and Future Research Direction  
In this work, we revisited several models of integrated sensing and communication (ISAC) and information-theoretic results on their fundamental performance limits and the tradeoffs between sensing and communication. These results emphasize the dual role of signals in estimating channel characteristics and enabling communication. We began by analyzing a point-to-point communication setup, where a simple modification of the traditional telecommunication framework allows integrated sensing at the transmitter or receiver. We then reviewed the extended models for network scenarios such as the broadcast channel, multiple-access channels, interference channels, and device-to-device communication. As we have seen, in these network scenarios, the backscattered feedback signals not only enable sensing at the Txs but can also be leveraged for collaborative transmissions of sensing and communication data in future blocks. This improves both the communication and sensing performance metrics and allows new improved tradeoffs and improved overall efficiency. To fully exploit the concept of collaborative communication and sensing in these network ISAC scenarios, advanced coding schemes involving joint source-channel coding schemes are needed. While a large number of results has already been obtained on ISAC systems with distortion constraints, important problems remain open, in particular, single-letter or numerical solutions for channels with memory as well as improved coding schemes and matching converse results for network ISAC systems. Emerging ISAC systems often face security constraints and the transmitted data or properties of the sensing targets have to be kept secure from external or internal eavesdroppers. Information theorists have studied such ISAC systems with secrecy constraints and determined bounds on the fundamental performance limits, with a focus on secure ISAC coding schemes. We presented these secure ISAC coding schemes and the corresponding securely achievable rate-distortion tuples. Results for different setups were reported: the first setup only requires that part of the data be kept secret, while the second setup imposes the more stringent constraint that in addition to the data, part of the sensing information should also remain unknown to an external eavesdropper. Various future research directions are still open on the information-theoretic framework of secure ISAC, starting with improved achievability and converse results, channels with memory, multi-user systems, and other security constraints regarding the information the eavesdropper obtains about the state sequence. In addition to the mentioned ISAC scenarios where sensing performance is measured in distortion, this overview article has also considered a fundamentally different ISAC model where the sensing task consists of a detection/hypothesis testing problem. In this setup, the relevant property of the sensing target is characterized by a single finite-valued parameter and the goal of the sensing task is to correctly guess this parameter. The main focus here was on the tradeoff between the achievable data rates and the exponential decay rate of the detection-error exponents. Full characterizations of the set of achievable rate-exponent tuples were obtained under the assumption that the Tx produces its channel inputs in a non-adaptive way. Only preliminary results are available under adaptive coding, thus leaving an interesting field of future research directions. Additional possible directions for future research include also network scenarios or setups with memory. Furthermore, key research topics include the role of reconfigurable intelligent surfaces (RISs), high-frequency systems (Terahertz and beyond), computational paradigms, and holographic technologies in ISAC systems. Additionally, practical wireless propagation aspects, such as near-field effects, require focused attention. Relevant recent works addressing these challenges include [118; 119; 120; 121; 122; 123; 124].