## 2 Related Work
Latency requirements may also impose restrictions on the number of replicas that can be transmitted, and consequently on the reliability levels that may be achieved. In this context, several recent contributions have analyzed slight modifications to the \(K\)-repetitions scheme. For example, [16] proposed adaptively configuring the number of replicas transmitted based on the channel conditions. The objective is to utilize the radio resources efficiently by avoiding unnecessary retransmissions when the channel quality is good. A similar objective is sought in [17] where authors propose conditions to stop the transmission of replicas. Other interesting proposals in 3 GPP standardization working groups include: the transmission of replicas within mini-slots (to reduce the latency) [18], the possibility for transmitting replicas across the slot border, or the concept of periodicity boundary [19]. These studies propose interesting variants of the \(K\)-repetitions scheme. However, it is yet unknownwhether 5G NR grant-free scheduling with \(K\)-repetitions and shared resources can really support URLLC communications with strict reliability and latency requirements under the presence of aperiodic or sporadic traffic. This traffic is critical in many verticals, for example in Industry 4.0. In this context, this study conducts an in-depth evaluation of 5G NR grant-free scheduling with \(K\)-repetitions and shared resources in the presence of aperiodic or sporadic traffic. The study identifies the reliability and latency levels that can be achieved with 5G NR grant-free scheduling, and identifies its current limitations. The study analyzes the impact of the number of UEs in the network, the number of reserved radio resources, and the number of replicas \(K\). The study also analyzes for the first time the impact of self-collisions. The conducted analysis helps to identify the reliability and latency levels that can be achieved based on network deployments and configuration options for 5G NR grant-free scheduling. It should be noted that 3 GPP standards define the possibility of utilizing grant-free scheduling and transmitting \(K\) replicas, but do not define a specific scheme to be implemented. This study is based on the implementation of 5G NR grant-free scheduling with \(K\) replicas and shared resources proposed in [15]. This implementation is chosen because it has been specifically designed to guarantee stringent URLLC latency and reliability requirements. To this aim, the implementation transmits original packets and all of the replicas using grant-free scheduling on shared radio resources. A different approach is proposed in [13] where dedicated resources are used to transmit the original packets, and shared resources are used for the following replicas. This approach can increase the delay compared to [15] if grant-based scheduling is utilized to allocate the dedicated resources. The efficient utilization of resources could also be compromised if dedicated resources were reserved for each UE when supporting applications with aperiodic traffic. The implementation of 5G NR grant-free scheduling with \(K\)-repetitions and shared resources proposed in [15] is therefore better suited to support URLLC applications with aperiodic or sporadic traffic.