# 5G V2X System-Level Architecture of 5 GCAR Project
## 3 Overall 5 GCAR Architecture Design
### Network Procedures
Furthermore, the work in [16] derives the coverage probabilities of small cell and macro cell BSs for all users in 5G small cell networks, respectively. The approach in [16] can be tailored for SM-Zone and extended for resource configuration selection among multiple RSUs belonging to the same SM-Zone. Another network procedure improvement introduced by 5 GCAR is related to the facilitation of the establishment of unicast communication via sidelink (SL). Currently, unicast communication over SL may be realized by using L1 broadcast SL [9] and using ad-hoc solutions for obtaining unicast features on higher layer--e.g., non-access-stratum or application layer functionalities allowing a UE to discard a message if addressed to a different UE. In this option, the setup of the unicast communication session over SL relies also on non-access-stratum or application-layer signalling, including the discovery phase of the involved UEs, procedures which involve side effects in terms of time consumption thus reducing overall efficiency. In addition, due to using L1 broadcast SL, UEs non-involved in the unicast communication but which are in proximity of the two unicast UEs may need to process and then discard the unintended unicast communications. The utilization of L1 unicast SL, instead of L1 broadcast SL with unicast features at higher layers, may overcome the limitations discussed above under the assumption that each of involved UEs obtain a locally unique L1 ID, allowing that a Tx UE of the L1 unicast SL may schedule an L1 unicast SL transmission addressed to only a targeted Rx UE by indicating the locally unique L1 ID of the targeted Rx UE. Given that the assignment of such locally unique IDs needs to be guaranteed, in practice this relies on assistance from a serving network, given that other solutions such as using a globally unique UE ID such as international mobile subscriber identity (IMSI) or application ID as L1 ID involve high L1 signalling overhead as well as security implications. In this regard, 5 GCAR evaluated the introduction of some network procedures to be included in RAN for facilitating and speeding up the setup of an SL unicast communication (i.e., network-driven SL unicast setup). In particular, the following was considered:  
* The serving RAN may indicate cell-specific support for assisting the L1 unicast SL in the signalling of system information, signalling which may include allocation of L1 ID and SL resources for L1 unicast SL with constraints such as the maximum number of unicast SLs per an SL communication session. * In case the initiator UE is in the network coverage, the UE decides whether to use L1 unicast SL and performs a request towards the serving RAN, a request which may include also a request for an SL resource allocation. The response from RAN may provide a resource allocation or, otherwise, an indication that requested network assistance, for the time being, cannot be provided. * As a fallback solution, if the initiator UE is out of coverage of the RAN is not able to assist the setup of L1 unicast SL, L1 broadcast SL is used.