# 5G V2X System-Level Architecture of 5 GCAR Project
## 4 Detailed Analysis of Selected Features
### Redundant Mode PC5 and Uu  
The support of safety-critical automotive applications imposes strict reliability requirements on V2X communications, which for specific use cases needs to be as high as 99.999%. In order to achieve this degree of reliability, both the Uu and PC5 links can simultaneously be used to provide one degree of redundancy [33]. As illustrated in Figure 12, data flows are duplicated and transmitted both on the PC5 and on the Uu links. Since this procedure makes intensive use of resources, its use shall be constrained to specific use cases and scenarios, and limited in space and time, based on specific operator's policies. Figure 11: (**a**) Pre-attach to operators and (**b**) force roaming procedures. In this section, we discuss and compare four different implementation solutions for the redundant scheduler, differentiated by the layer at which the duplication/deduplication of flows is performed. Specifically, the scheduler may operate at the facilities layer, transport layer, RRC layer, and medium access control (MAC) layer. Among the different solutions proposed, the implementation of the redundant scheduler at the facilities layer is arguably the easiest to implement and to test, and by design it limits the redundancy feature to only a specific subset of applications. Furthermore, the duplication is RAT-agnostic, meaning that flows may be routed over no matter which air interface is available. However, this implementation requires all UEs involved in the communication to implement the ETSI ITS stack [34], and the receiving UEs to be capable of receiving on both interfaces. Being located in a higher layer of the stack, this solution enables no interaction with the lowest layers, not enabling optimizations (such as temporarily stopping the duplication) when one or more links are saturated. When implemented at the transport layer, the redundant scheduler may rely on already available protocols, such as Multi-Path TCP [35] and quick UDP connections [36]. For this reason, as opposed to the facilities layer implementation, this scheduler may also be made available to other non-vehicular applications running on the same UE, in case of need. On the downside, multipath transport protocols are currently only designed for IP traffic, effectively excluding non-IP flows, which are prominent in V2X. Furthermore, due to its location in the higher layers of the protocol stack, it prevents an optimized use of the duplication when links are saturated, similarly to the facilities layer implementation. The RRC sub-layer represents another convenient location to implement the redundant scheduler, due to the availability of already standardized components. For instance, RRC is already defined for Release 14 semi-persistent scheduling (SPS), which is defined both for the Uu and the PC5 interfaces. Furthermore, Mode 3 sidelink already enables cross-carrier scheduling, which represents a solid base to define an extension designed to support the Uu + PC5 mode. However, it is difficult to estimate the entity of the modifications to the RRC sub-layer necessary to support the redundant scheduler. Furthermore, it requires the gNB and the RSU to be tightly interconnected, to enable efficient simultaneous usage of both interfaces for a V2I or V2N2I service. Finally, a redundant scheduler located at the MAC layer may also be based on a concept formalized in Release 14, notably the V2X concurrent inter-band configuration for UU and PC5 communications; the MAC entity implementing the redundant scheduler may be integrated in a similar way to the uplink carrier aggregation. As opposed to higher-layer implementations, the MAC scheduler may control the duplication, and temporally suspend it, in case of channel congestion. On the downside, however, this is the most complex solution to implement: in this case, its location on a lower layer makes it difficult for the scheduler to enable/disable the redundancy for specific flows unless further dedicated cross-layer mechanisms are applied.