Seamless exchange of cooperative awareness messages (CAMs) and prompt wide-area propagation of emergency warnings among neighbor vehicles are essential to improve safety of vehicles in intelligent transportation systems (ITS). This paper proposes enhanced C-V2X mode-3 operations with i) efficient multiple overlapped cells with orthogonal RF resource managements, ii) smart handover across overlapped cells to minimize the signaling overhead, and iii) prompt wide-area propagation of emergency warnings across multiple cells with collaborations of eNodeBs(eNBs). In the proposed scheme, scalable overlapped cells are configured with 3 ∼ 4 orthogonal sub-channels for up to 400 vehicles per eNodeB. The eNB continuously monitors the CAM broadcasting by vehicles and assigns resources to the vehicles with balanced RF channel usages to mitigate half-duplex problem. The smart handover across overlapped cells guarantees seamless CAM deliveries even at the boundaries of cells and reduces delay by avoiding the re-registration process of vehicles at boundary of each cell. It also provides prompt propagation of wide-area emergency warnings based on collaborations of eNBs. The operations of the proposed scheme have been implemented and analyzed with NS-3 simulation, and measured results of CAM delivery rates are more than 99.4 percent in highly congested metropolitan areas. Also, the prompt propagation of emergency warnings across multiple cells are provided within 200 msec.

To enhance safety in intelligent transportation systems (ITS), seamless exchange of cooperative awareness messages (CAMs) and prompt wide-area broadcasting of decentralized environmental notification messages (DENMs) among neighbor vehicles are essential. This paper proposes an enhanced Cellular vehicles-to-everything (C-V2X) mode-3 networking with i) scalable overlapped eNodeBs (eNBs) with orthogonal radio frequency (RF) resource management with minimized RF interference, ii) smart handover across overlapped eNBs to minimize the signaling overhead, and iii) prompt wide-area propagation of DENM across neighbor eNBs with collaborations of eNBs. In the proposed scheme, scalable overlapped eNBs are configured with 3 ~ 4 orthogonal sub-channels supporting up to 400 vehicles per eNB. The eNB continuously monitors the CAM broadcasting by vehicles and assigns resources to the vehicles with balanced RF channel usages to mitigate half-duplex problem. The smart handover across overlapped eNBs guarantees seamless CAM deliveries even at the boundaries of eNBs and reduces delay by avoiding the re-registration process of vehicles at boundary of each eNB. The proposed system also supports efficient delivery of emergency warnings which are collected by vehicles or public warning system (PWS) and delivered as DENMs through system information block (SIB). The DENMs are promptly propagated to neighbor eNBs and broadcasted based on collaborations of eNBs. The operations of the proposed scheme have been implemented in NS-3 simulation and measured performance results are analyzed in detail. The CAM delivery rates are more than 99.8 percent in highly congested metropolitan areas, and the prompt propagation of DENMs across multiple eNBs is provided within 100 msec.

Cellular vehicles-to-everything (C-V2X) is an essential technology to enhance road safety, traffic management and smart mobility. C-V2X Mode 4 uses sensing based semi-persistent scheduling (SB-SPS) to mitigate packet collisions, that consists of several mechanisms (i.e., random selection of resources, probabilistic reselection, and monitoring resource utilization pattern). The packet collision probability, however, is still non-negligible, and the CAM (cooperative awareness message) delivery ratio (CDR) is far from target requirements for high reliability and low latency applications. Especially, when the number of vehicles increases beyond 300, the packet collision probability increases in the environment of congested metropolitan area. In this paper, we analyze the performance of C-V2X Mode 4 in congested metropolitan areas, and propose a balanced resource allocation scheme that is providing a remarkable enhanced performance, considering the half-duplex communication mechanism in C-V2X Mode 4. The proposed method allocates resources in distributed manner with balanced usage of subframes, instead of random selection of resources in resource map, to mitigate CAM packet losses by half-duplex communications. From the performance analysis with simulation, we confirmed that the proposed scheme provided 97% of CDR for 500 vehicles, while the existing scheme provided only 87% for the same condition.

The number of connected vehicles are increasing rapidly in recent few years, and meeting the requirements of vehicular networks (i.e., high reliability and low latency) is challenging. The 3rdgeneration partnership project (3GPP) Mode 4 of the newly designed cellular vehicles-to-everything (C-V2X) which is specially designed for vehicle-to-vehicle (V2V) communication uses semi-persistent scheduling (SPS). SPS contains mechanisms of random selection of resources, probabilistic reselection, and monitoring resource utilization pattern to avoid packet collision. However, the collision probability especially in congested scenarios is still a challenge that makes its performance far from the expected results. In order to increase the efficiency of C-V2X, we propose cognitive collision resolution mechanism which is an additive of the standard SPS mechanism, and further we analyze the impact of different parameters on system performance. The proposed algorithm firstly senses the available resources through candidate sensing window, then chooses the candidate resource FSecondly it broadcasts the reselection counter value to reduce the collisions in congested scenarios. Result performance obtained is better than the existing mechanisms; it shows that packet reception ratio increases considerably with different number of vehicles and different values of resource reselection probability.