arXiv:2610.29364v1 Announce Type: new Abstract: In this paper, we develop a dual-function waveform design framework for multiple-input multiple-output integrated sensing and communication (MIMO-ISAC) systems operating in congested electromagnetic environments. In such environments, the dual-function waveform must coexist with legacy systems, such as localized positioning sensors, maritime radar networks, and satellite communication links, that occupy specific frequency bands and angular sectors. To enable precision coexistence, we incorporate localized angular-spectral region constraints that suppress beamformed energy only within the angle-frequency regions occupied by each legacy system, while preserving the spatial degrees of freedom (DoFs) available outside those regions. The resulting waveform design problem is formulated as a nonconvex optimization problem and solved using a low-complexity majorization--minimization alternating direction method of multipliers (MM--ADMM) algorithm. Simulation results demonstrate that the proposed framework provides precise legacy-system protection while maintaining robust radar sensing and multi-user communication performance.