# Resource-Efficient Multicast URLLC Service in 5G Systems
## 1 Introduction
Thus, the considered multicast solutions may not be suitable for mURLLC. In other work, many URLLC-aware transmission parameters selection [21; 22; 23; 24] and scheduling [25; 26; 27; 28] algorithms have been designed for unicast traffic. An open question is how to adapt them for the multicast case. The paper aims to fill this research gap. It shows which approaches to provide mURLLC require changing the paradigm and which approaches can be inherited from existing ones. Table 1 summarizes these findings, which are discussed in detail in the following sections. An arrow with the label \"new\" means that the paper proposes a modification of a solution to mURLLC. An arrow without the label \"new\" means that the solution or a specific subset of solutions that is defined in the paper can be applied to mURLLC. The sign \"X\" means that the solutions proposed for other areas are inefficient for mURLLC. While analyzing the algorithms, we pay much attention to the computational complexity because mURLLC requires data delivery to multiple UEs and the decisions needs to be made within a short time. Thus, only a limited set of algorithms with low complexity can be applied for mURLLC. The contributions of the paper are as follows:  
1. We review various existing transmission parameter selection algorithms developed for multicast broadband traffic and unicast URLLC traffic and determine how to adapt them for mURLLC;
2. We carry out extensive performance evaluation and comparison of various algorithms under the same conditions using link-level and system-level simulations;
3. Based on extensive simulation results, we provide a set of recommendations on how to configure the new multicast mechanisms and determine the algorithms providing mURLLC with low resource consumption, which, in turn, increases system capacity. Note that some recommendations contradict those for multicast broadband traffic and unicast URLLC traffic. For example, several works [29; 30] report that feedback-based multicast HARQ retransmissions are inefficient for broadband traffic because they slightly reduce the downlink resource consumption while significantly increasing the uplink resource consumption. In contrast, we show that some feedback schemes do significantly reduce the overall resource consumption under strict reliability constraint. Another example of a non-trivial recommendation is related to the resource allocation algorithm. In contrast to the existing works [25; 26] that recommend using the Frequency-Selective (FS) scheduler (i.e., allocate resource blocks taking into account their quality) for unicast URLLC traffic, we show that the gain of an FS scheduler for a large multicast group is below 5% with respect to a non-FS scheduler while the complexity increases by 40%. Thus, the type of scheduler (FS or non-FS) should be selected depending on the multicast group size. The rest of the paper is organized as follows. We describe the considered scenario and formulate the problem in Section 2. In Section 3, we analyze various transmission parameter selection algorithms for multicast traffic and show how to adapt them to mURLLC. We evaluate their performance in Section 4. Section 5 concludes the paper with recommendations on providing resource-efficient mURLLC.