  Traditional discrete-array-based systems fail to exploit interactions between
closely spaced antennas, resulting in inadequate utilization of the aperture
resource. In this paper, we propose a holographic intelligence surface (HIS)
assisted integrated sensing and communication (HISAC) system, wherein both the
transmitter and receiver are fabricated using a continuous-aperture array. A
continuous-discrete transformation of the HIS pattern based on the Fourier
transform is proposed, converting the continuous pattern design into a discrete
beamforming design. We formulate a joint transmit-receive beamforming
optimization problem for the HISAC system, aiming to balance the performance of
multi-target sensing while fulfilling the performance requirement of multi-user
communication. To solve the non-convex problem with coupled variables, an
alternating optimization-based algorithm is proposed to optimize the HISAC
transmit-receive beamforming in an alternate manner. Specifically, the transmit
beamforming design is solved by decoupling into a series of
feasibility-checking sub-problems while the receive beamforming is determined
by the Rayleigh quotient-based method. Simulation results demonstrate the
superiority of the proposed HISAC system over traditional discrete-array-based
ISAC systems, achieving significantly higher sensing performance while
guaranteeing predetermined communication performance.
