  This paper studies a communication-centric integrated sensing and
communication (ISAC) system, where a multi-antenna base station (BS)
simultaneously performs downlink communication and target detection. A novel
target detection and information transmission protocol is proposed, where the
BS executes the channel estimation and beamforming successively and meanwhile
jointly exploits the pilot sequences in the channel estimation stage and user
information in the transmission stage to assist target detection. We
investigate the joint design of pilot matrix, training duration, and transmit
beamforming to maximize the probability of target detection, subject to the
minimum achievable rate required by the user. However, designing the optimal
pilot matrix is rather challenging since there is no closed-form expression of
the detection probability with respect to the pilot matrix. To tackle this
difficulty, we resort to designing the pilot matrix based on the
information-theoretic criterion to maximize the mutual information (MI) between
the received observations and BS-target channel coefficients for target
detection. We first derive the optimal pilot matrix for both channel estimation
and target detection, and then propose an unified pilot matrix structure to
balance minimizing the channel estimation error (MSE) and maximizing MI. Based
on the proposed structure, a low-complexity successive refinement algorithm is
proposed. Simulation results demonstrate that the proposed pilot matrix
structure can well balance the MSE-MI and the Rate-MI tradeoffs, and show the
significant region improvement of our proposed design as compared to other
benchmark schemes. Furthermore, it is unveiled that as the communication
channel is more correlated, the Rate-MI region can be further enlarged.
