Resource Allocation for 6G Mission-Critical Systems: A Survey on Enabling Technologies, Techniques, and Future Directions

Sixth-generation (6G) networks are expected to deliver transformative performance gains that fundamentally reshape resource allocation (RA) for mission-critical systems (MCS). Yet, to the best of our knowledge, no existing survey jointly addresses 6G enabling technologies, MCS-specific constraints, and RA techniques as a unified design space. This survey closes that gap by formalizing the MCS-RA problem as a hard-constrained, multi-objective optimization, establishing its NP-hardness as distinct from the non-convexity of the constraint geometry, and systematically mapping MCS domain requirements to actionable RA design constraints across the Internet of Healthcare (IoH), Internet of Vehicles (IoV), and Industrial Internet of Things (IIoT). We propose Integrated Ubiquitous Communication (iUcom), a conceptual service class at the three-way intersection of enhanced Ultra-Reliable and Low-Latency Communications, Ultra-massive Machine-Type Communications, and Ultra-enhanced Mobile Broadband, to describe MCS applications that require ultra-reliability, ultra-broadband, and massive connectivity simultaneously. We show its joint feasibility set to be non-empty at the link and capacity level under achievable 6G parameters, leave its schedulability open, and identify Artificial Intelligence (AI)-native resource orchestration as the route toward realizing it. We further construct an implication matrix relating 6G enabling technologies to their RA opportunities, challenges, and domain applicability, and present a layered taxonomy with a domain-specific technique selection guide. We provide structured case studies for the three domains that trace RA pathways from problem formulation through technology integration, technique selection, and performance analysis. We conclude by setting out open research problems, including safe exploration under hard constraints, AI-based RA certification, and zero-touch self-configuring MCS networks.