  Different from conventional radar, the cellular network in the integrated
sensing and communication (ISAC) system enables collaborative sensing by
multiple sensing nodes, e.g., base stations (BSs). However, existing works
normally assume designated BSs as the sensing nodes, and thus can't fully
exploit the macro-diversity gain. In the paper, we propose a joint BS
selection, user association, and beamforming design to tackle this problem. The
total transmit power is minimized while guaranteeing the communication and
sensing performance measured by the signal-to-interference-plus-noise ratio
(SINR) for the communication users and the Cramer-Rao lower bound (CRLB) for
location estimation, respectively. An alternating optimization (AO)-based
algorithm is developed to solve the non-convex problem. Simulation results
validate the effectiveness of the proposed algorithm and unveil the benefits
brought by collaborative sensing and BS selection.
