# Dynamic Packet Duplication for Industrial URLLC
## 2 Background
### 5G Multi-Connectivity Overview  
Multi-connectivity in 5G New Radio (NR) inherits from the Long Term Evolution (LTE) Dual Connectivity (DC) concept. LTE DC was first specified in Release 12 [29] and allows UE to simultaneously send/receive data from different evolved NodeBs (eNBs). The data split is performed at the Packet Data Convergence Protocol (PDCP) layer of the transmitting eNB. At the receiving side, the information is decoded from lower layers, and it is combined at the PDCP layer on the receiver. This process allows boosting the throughput [30]. In Release 15, multi-RAT DC was specified for DC operation with NR and LTE nodes [31]. Not only data split but also PD at the PDCP layer is introduced. PD allows the same packet to be transmitted by different nodes, thus, improving the reliability. The nodes are commonly known as the Master Node (MN) and Secondary Node (SN) and are interconnected via a Xn interface. MN is in charge of activating/deactivating PD via Radio Resource Control (RRC) signaling [12]. If the different links are spatially uncorrelated, transmitting the duplicate packet can compensate poor channel conditions. This is very important in industrial scenarios, where the fundamental problems are interference and multipath propagation, due to the presence of concrete walls and large metallic machinery and structures. Moreover, NR-NR DC for standalone deployments was standardized in Release 16 [31], in which a UE is connected to one gNB that acts as a MN and another gNB that acts as a SN. In this paper, NR-NR DC with the PD approach is assumed for the downlink direction. Packet Duplication for URLLC  
PD is a multi-connectivity solution that improves reliability by increasing redundancy of the transmission. When PD is activated, the PDCP entity in the MN is responsible for PD, whereas the PDCP entity in the receiver is responsible for detecting and removing duplicated packets. The PDCP entity duplicates the packet data unit (PDU) to avoid twice performing functions, such as ciphering, header compression, integrity protection etc. This PDCP PDU has the same sequence number in both. Then, the packet is forwarded by MN to the SN via Xn-U interface for transmission to the UE. The packet will undergo through independent Radio Link Control (RLC), Medium Access Control (MAC) and physical layer processing at each gNB. This implies that the packet can be transmitted at different time intervals and over different frequency resources and that physical transmission aspects, such as beamforming, MCS, ACK/NACK signaling and the Hybrid Automatic Repeat Request (HARQ) mechanism, are independent. On the receiver side, multiple copies of the packet are received, and the UE will forward the first successfully received packet to the higher layers and remove duplicated packets received later, based on the PDCP sequence number. Figure 1 shows a 5G NR-NR PD scheme for downlink transmission. As PD improves reliability, it becomes a suitable mechanism for URLLC, which demands a higher reliability and low latency. Not only to improve reliability but also to reduce latency, as independent transmissions are performed by two different gNBs, as mentioned before. In this case, latency reduction will be dependent on the best link. Reliability and latency for URLLC are dependent on each other, that is, high reliability is  
Figure 1: A downlink packet duplication scheme in a NR-NR DC scenario. consequential only if packets are received within the latency constraint. As a consequence, PD may be used to improve both the reliability and latency. Nevertheless, PD operates at the cost of wasting resources on SN. Thus, it is not efficient to always send packets duplicated, as there could be situations where the conditions are fulfilled by the primary node. For that reason, it is important to provide a dynamic mechanism for MN that controls the activation/deactivation of PD for URLLC devices, in order to reduce the resource costs at SN, which may affect other UEs attached on SN.