Fundamental CRB-Rate Tradeoff in Multi-antenna Multicast Channel with ISAC

Abstract

With technical advancements, how to simultaneously support common data broadcasting and sensing is a new problem towards the next-generation multiple access (NGMA). This paper studies the multi-antenna multicast channel with integrated sensing and communication (ISAC), in which a multi-antenna base station (BS) sends common messages to a set of single-antenna communication users (CUs) and simultaneously estimates the parameters of an extended target via radar sensing. Under this setup, we investigate the fundamental performance tradeoff between the achievable rate for communication and the estimation Cramér-Rao bound (CRB) for sensing. First, we derive the optimal transmit covariance in semi-closed form to maximize the achievable rate while ensuring the CRB constraint subject to a maximum transmit power constraint at the BS, and accordingly characterize the outer bound of the so-called CRB-rate (C-R) region. It is shown that the optimal transmit covariance should be of full rank, consisting of both information-carrying and dedicated sensing signals in general. Next, we consider a practical joint information and sensing beamforming design, and propose an efficient approach to optimize the joint beamforming for balancing the C-R tradeoff. Numerical results are presented to show the C-R region achieved by the optimal transmit covariance and the joint beamforming, as compared to the benchmark scheme with isotropic transmission.