Reconfigurable Integrated Sensing and Communications (RISAC): Sparse MIMO and Hybrid Beamforming in Far and Near Fields

This article provides a comprehensive investigation into reconfigurable integrated sensing and communications (RISAC), an emerging paradigm designed to maximize the performance–cost tradeoff by exploiting the inherent spatial sparsity of multiple-input multiple-output (MIMO) arrays. Deviating from conventional static ISAC, RISAC adopts a cognitive “perception–action” cycle, enabling the adaptive reconfiguration of array geometries and beamforming weights in response to dynamic environmental feedback. Central to RISAC is the exploitation of two intertwined layers of degrees of freedom (DoFs): element-space sparsity via sparse MIMO array design and beam-space sparsity via hybrid beamforming (HBF). We further demonstrate that by synergistically co-designing these coupled DoFs, sparse MIMO HBF can rival the performance of fully digital systems when accounting for practical mutual coupling, at a significantly reduced hardware cost. This work analyzed both far-and near-field propagation regimes across various ISAC frameworks, including radar-centric, communication-centric, and joint co-design. In addition to downlink co-design, we also examine uplink coexistence via shared waveform design. Finally, this article outlines promising research directions, positioning RISAC as a critical evolution toward adaptive, hardware-efficient, and dual-functional systems.