NR Uplink LBT Category Selection for Unlicensed Spectrum

The LBT procedure is segmented into multiple categories (CAT-1, CAT-2, CAT-4) with different complexity levels and timing requirements. CAT-1 provides immediate transmission opportunity, CAT-2 provides deferred opportunity with 25us delay, and CAT-4 provides further deferred opportunity with random backoff. This segmentation allows the system to balance between fair coexistence (using CAT-4 when needed) and minimizing delays (using CAT-1 or CAT-2 when possible).

The LBT category is dynamically selected based on the time gap between DL signal reception and UL transmission start. When the gap is small (≤25us), CAT-1 or CAT-2 is used to minimize delay. When the gap is larger, CAT-4 is used to ensure fair coexistence. This dynamic adaptation allows the system to optimize the trade-off between reliability and time loss in real-time.

The transmission timing is segmented into deterministic parts (DL signal reception time, fixed offsets like 25us) and a stochastic part (random backoff for CAT-4). By clearly separating these components, the system manages timing uncertainty in a structured way, allowing UEs to calculate expected timing ranges and prepare buffers accordingly, thus reducing the practical impact of the randomness.

The UE performs preliminary actions by preparing transmission buffers and resources in advance based on the received DL signal timing and the indicated LBT category. For CAT-4, the UE prepares for a range of possible transmission times including the random backoff period, allowing it to quickly transmit once the LBT succeeds without additional processing delays.