# C-V2X Centralized Resource Allocation with Spectrum Re-Partitioning in Highway Scenario
## 1 Introduction  
Cellular Vehicle-to-Everything (C-V2X) communication is a new technology that has been added to the LTE cellular system to support intelligent transportation systems [1]. 3 GPP has introduced two modes of communication in Release 14 to exchange information between vehicles on the road, to increase self-driving car safety, to improve situational awareness and travel comfort, and to decrease traffic congestion [2]. Each vehicle should broadcast a Cooperative Awareness Message (CAM) periodically to inform the surrounding vehicles about its status [3]. C-V2X Mode 3 is a kind of communication in which the cellular infrastructure supports centralized resource allocation for vehicles located inside the cellular coverage area. The cellular infrastructure is responsible for selecting, re-selecting, scheduling, and managing the radio resources used for distribution of awareness messages transmitted by vehicles [4, 5, 6]. The C-V2X communication of vehicles operating in Mode 4 applies decentralized resource allocation and is mainly used outside the cellular coverage. The vehicles operating in Mode 4 are responsible for autonomous selection, re-selection, scheduling, and managing the resource allocation for broadcasting CAM messages [7]. Single-Carrier Frequency Division Multiple Access (SC-FDMA) has been selected as the C-V2X communication technique [8]. The resource blocks in SC-FDMA have been divided into groups to form sub-channels in the frequency domain, while two consecutivetime slots form a subframe in the time domain (see Figure 1). Each vehicle can use one sub-channel to broadcast a CAM message periodically [9]. The transmitted packet consists of two parts in adjacent or non-adjacent configuration [10]. The first part, containing the physical sidelink control channel (PSCCH), is placed in two resource blocks in each sub-channel to help the receiver to decode the second part that consists of a variable number of resource blocks, constituting the physical sidelink shared channel (PSSCH). When transmitting CAM, these resource blocks carry data related to speed, acceleration, deceleration, vehicle type, and direction. The number of resource blocks in the second part depends on the Modulation and Coding Scheme (MCS) used to broadcast these data [11]. Because of high mobility and high density of vehicles, 3 GPP has increased the number of DeModulation Reference Signals (DMRSs) in the sub-frame structure from two in conventional Device-to-Device (D2D) communication to four in C-V2X, to handle the Doppler effect, especially for those vehicles that move with high speed up to 250 kmph. The Tx-Rx turnaround symbol is added at the end of a sub-frame to better track the channel at high vehicle speeds [12]. In the case of Mode 4, 3 GPP in Release 14 [6] proposed a Re-selection Counter (RC) mechanism to generate a random number defining the periodicity of resource re-selection. The range of RC is selected and related to the CAM interval as follows [13]: [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15  
vehicles in Mode 3 should notify the infrastructure about the type, size, and broadcasting cycle of CAMs to reserve the fitting sub-channels. The probability of correct reception of messages with properly selected receiving parameters and the required C-V2X communication range ensuring communication with high reliability are 3 GPP use cases [19] discussed in this paper. The main objective of the research reported in this paper is to avoid the drawbacks in C-V2X Mode 3 observed for such reasons as: overlapping in CAM's broadcasts area, high signalling cost and signal latency between cellular infrastructure, and mitigating the interference signals that can negatively impact the packet reception ratio of vehicles in C-V2X Mode 3. In this paper, cooperation among Road Side Units (RSUs), a part of the cellular infrastructure, is proposed. We have investigated application of the RC in centralized re-selection resources of C-V2X Mode 3.