Fundamental Limits on Integrated Sensing and Communications Frameworks: An IR-UWB Case

The integrated sensing and communication (ISAC) where both sensing and communication may share resources from multiple domains, has become a key point in future 6G wireless networks. One fundamental problem to ISAC is to discuss the essential tradeoff between sensing and communications. In this paper, we try to exploit this issue using the estimation theory, since transmitted symbols and channel characterizations are essentially unknown parameters to be jointly estimated. The Fisher information matrix (FIM) and Cramer Rao lower bound (CRB) are both used to give the ISAC performance benchmarks. The impulse radio ultra wideband (IR-UWB) signals are adopted for the case study. We first give the general models on the UWB ISAC system. We then compare the CRBs of sensing only, to ISAC systems, to illustrate the information coupling between sensing and communications. After proper decoupling operations such as pilots, we can see the inevitable performance degradation in ISAC, where uncertainties are essentially introduced by transmitted symbols. Furthermore, we also perform the energy optimization between pilots and data bits, to verify the tradeoff between sensing and communications.