  Integrated sensing and communication (ISAC) base stations can provide
communication and wide range sensing information for vehicles via downlink (DL)
transmission, thus enhancing vehicle driving safety. One major challenge for
realizing high performance communication and sensing is how to deal with the DL
mutual interference among adjacent ISAC base stations, which includes not only
communication related interference, but also radar sensing related
interference. In this paper, we establish a DL mutual interference model of
adjacent ISAC base stations, and analyze the relationship for mutual
interference channels between communications and radar sensing. To improve the
sensing and communication performance, we propose a collaborative precoding
design for coordinated adjacent base stations to mitigate the mutual
interference under the transmit power constraint and constant modulus
constraint, which is formulated as a non-convex optimization problem. We first
relax the problem into a convex programming by omitting the rank constraint,
and propose a joint optimization algorithm to solve the problem. We furthermore
propose a sequential optimization algorithm, which divides the collaborative
precoding design problem into four subproblems and finds the optimum via a
gradient descent algorithm. Finally, we evaluate the collaborative precoding
design algorithms by considering sensing and communication performance via
numerical results.
