How to Enable Massive Access Under Mobility and Heterogeneity in Future IoT Networks: A Survey

Abstract

Grant-free and contention-based access is the foundation of massive Internet of Things connectivity in 5G-Advanced and 6G, yet the device population it must serve is becoming simultaneously dense, heterogeneous across capability tiers, and mobile. This survey examines a specific and under-served question: whether contention-based access can sustain its reliability and latency targets when a multi-tier heterogeneous population is genuinely mobile. We introduce a two-dimensional taxonomy that classifies massive-access designs by the methodology through which the access decision is produced, from signature design and sparse recovery through optimization to learned and generative control, and by the way mobility enters the model, from absent to explicit within the access attempt. The literature is reviewed as the five clusters this taxonomy induces, namely access foundations, heterogeneous and multi-cell service coexistence, the management of high mobility, AI-native and computation-oriented access, and 3GPP standardization with the 6G vision, and each reviewed scheme is assessed against a common set of criteria: access type, multi-tier heterogeneity, explicit mobility, multi-cell operation, traffic realism, and the signaling and coordination cost it incurs. A cross-comparison of the methodological families shows that advancement in methodology has been paid for in the treatment of mobility, and three standardized deployment scenarios make the resulting shortfall concrete. The synthesis identifies a consistently under-addressed combination, the explicit mobility of a multi-tier contending population in contention-based access, and develops AI-driven cooperative control, with its implementation challenges and evaluation metrics, as the direction it opens.