# A Packet Collision Reduction Based on Reselection for LTE V2X Mode 4
## 1 Introduction  
With the advance of Intelligent Transport Systems (ITS), Vehicle to Everything (V2X) communication has been attracting attention. V2X can be roughly divided into wireless LAN-based Dedicated Short Range Communication (DSRC) and cellular-based Cellular V2X (C-V2X). The former has been standardized as IEEE 802.11p [1] in the United States in 2010 by extending the IEEE 802.11 standard [2]. In Japan, ARIB STD-T109 [3] has been standardized by the Association of Radio Industries and Businesses (ARIB) in 2012 after formulating the experimental guidelines, ITS FORUM RC-006 [4]. On the other hand, C-V2X has been standardized as Long Term Evolution (LTE) V2X based on existing LTE by Third Generation Partnership (3 GPP) release 14 in 2017. LTE V2X supports the uplink/downlink communication which corresponds to Vehicle to Network (V2N) and the sidelink communication, which corresponds to V2V, V2I, and V2P, as shown in Figure 1. Ref. [5] analyzes scenarios of the C-V2X and gives a survey of the related articles. Modes 3 and 4 are available as ways to use radio resources in the sidelink communication [6, 7]. In Mode 3, an evolved Node B (eNB) allocates the resources to User Equipment (UE) under network coverage. In Mode 4, the UEs autonomously select the resources without the need for control by the eNB. The UEs communicate with each other by sensing-based Semi-Persistent Scheduling (SPS) for LTE V2X Mode 4. For autonomous resource selection, packet collisions are reduced by selecting resources that are not expected to be used by other UEs from reservation information and a measurement of the received power. However, because the resource selection depends on past information, the resource newly selected by one UE cannot be detected by other UEs until transmission on the resource. Therefore, the packet collision may occur because the UEs that select a resource at the same time cannot estimate the resources selected by each other. Regarding an evaluation of the received power, the resource released at the reselection cannot be effectively utilized because the evaluated value is smoothed without information about the resource reselection of other UEs. Basic performance evaluation of Mode 4 is verified in a highway scenario [8; 9; 10; 11] and a urban scenario [12; 13]. The evaluation of congestion control and the impact of parameters are presented in [14; 15] and [16; 17; 18]. In addition to these works, analytical models are provided in [19; 20]. On the other hand, methods to reduce the packet collisions are proposed in [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31]. In [21; 22; 23], each of the UEs notifies other UEs of resources virtually reserved by the UE in advance. In [24], the UEs adjust the timing of reselection with other UEs. In [25], the hidden terminal problem is mitigated by sending additional acknowledgments and feeding this information back to the transmitter. As a similar approach to Mode 3, a cluster-based resource allocation has been proposed in [26]. However, these methods are not preferable from the viewpoint of compatibility with the standard because they require additional information and the format of control information needs to be changed. Geo-based resource allocation methods corresponding to the position of the UEs on a road have been proposed in [27; 28; 29], and can avoid packet collisions with the UEs located on Non-Line of Sight (NLOS). Even so, it is necessary to know a correspondence between the UE position and the resource pool in advance, and it is difficult to achieve a high degree of efficiency unless all UEs follow the same rule. In [30], although the UEs use alternately two different resources to reduce consecutive packet collisions, it is difficult to estimate the resources used by other UEs. A nonlinear averaging method has been proposed in [31], which assigns higher priority to the latest information in evaluating the received power to effectively utilize the resources released by other UEs.