Grantfree

Grant-free refers to uplink transmission schemes in which user equipment transmits data without waiting for an explicit scheduling grant from the network. It aims to reduce latency and signaling overhead for sporadic or small-payload transmissions, common in massive machine-type communication (mMTC) and ultra-reliable low-latency communication (URLLC) use cases in 5G and beyond.

Two main variants exist: configured grant-free, where the network preconfigures resources for reuse and may provide limited HARQ feedback, and unconfigured (fully grant-free), where the UE selects resources without prior authorization and HARQ feedback is typically absent or limited.

Operations rely on preambles, preconfigured resources, or code-domain access to identify users. To mitigate collisions, networks may use resource partitioning, repetition, semi-persistent scheduling, or enhanced receiver processing. In URLLC and mMTC, the balance between reliability, latency, and energy efficiency guides design choices.

Applications include IoT sensors, smart metering, and other devices that generate small, irregular transmissions. The main challenges are collision risk, variable quality-of-service, and delivery reliability under high device density. Ongoing research explores more efficient random access protocols, coding strategies, and hybrid schemes combining grant-free and scheduled access.

See also: configured grant, 5G NR, mMTC, URLLC, random access, NOMA.