Preemption in 5G NR Downlink Dynamic Scheduling

Fig. 1: Observe that device-B is pre-empting device and device-A is pre-empted device. Device-B has taken over some of the resources within the slot that were meant for device-A (Source: E. Dahlman et al., ”5G NR”).

In 5G NR, downlink preemption means that a device (e.g. device-B) uses the resources scheduled for another device (e.g. device-A). Device-A is pre-empted device (i.e. its resources are hijacked by device-B) and device-B is the pre-empting device (the hijacker of resources). Usually, preemption occurs for only a few symbols within the transmission time interval i.e. slot (See Fig. 1 above).

This preemption (hijacking) of a few OFDM symbols by device-B is in good faith, i.e. it may need to transmit time-critical information in the downlink and hence it captures a few OFDM symbols from the slot scheduled for device-A. However, the preempted transmission on device-A gets corrupted and require retransmission. Device-A is notified of the preemption (i.e. symbol hijacking) by gNB.

Preemption is the new feature in 5G NR and was not present in LTE. It is highly useful for URLLC scenarios, especially when the number of users is quite large and devices may experience a lack of resources. Preemption allows transmission of high-priority data (in the downlink) without waiting for the beginning of the new slot (which is otherwise the minimum possible scheduling unit in 5G NR).

Handling of Preemption

There are several possibilities to handle this in NR. One approach is to rely on…