## 2 Related Work  
Within the context of C-V2X, radio resource management is an actively studied topic. Recent works in this topic are classified into network-controlled and autonomous approaches. For the former, [PERSON] et al. [22] transformed the latency and reliability constraints of C-V2X into optimization constraints that are computable using only the slowing varying channel state information. They also formulated a problem to optimize the performance of both vehicular UEs and cellular UEs, and proposed an algorithm to solve it. [PERSON] et al. [23] considered the resource allocation problem under Mode 3. When there is conflict in the allocation between two overlapping groups of vehicles as in an intersection or merging highways, some vehicles cannot receive each other's information. They designed algorithms with different complexity and performance to solve the problem. [PERSON] et al. [24] considered the problem of reusing the same resource block to maximize the number of concurrent V2V communications. Under eNodeB's control, the proposed algorithm can improve the spectrum efficiency, so that LTE can support a high density of vehicles. [PERSON] et al. [25] considered a network-controlled resource management based on the vehicle position information. They exploited the concept of minimum reuse distance, at which the same resource can be used by a different transmitter without affecting those receivers that are in the awareness range. They showed that the accuracy of the localization affects the error rate. [PERSON] and [PERSON] [26] also proposed location-based scheduling, where the base station scheduler can leverage the optimal scheduling distance to maximize the cell throughput via resource reuse among different vehicles. To achieve the objective, they exploited the relations among cell throughput, reliability, and communication range. [PERSON] et al. [27] proposed a position-based resource allocation scheme that allocates a different frequency and time resources based on vehicle speed, density, direction, and position. They showed that the scheme can improve the packet reception ratio (PRR). [PERSON] et al. [28] proposed how to allocate resource for the out-of-coverage vehicles when they enter such area. They assumed that the network infrastructure makes the prediction as to the movements of the vehicles, and accordingly makes the reservation for them. They showed that the prediction accuracy is important for transmission success. As vehicles can be in locations where there is no infrastructure support, autonomous resource allocation is an indispensable mechanism. Consequently, works are emerging that attempt to evaluate or improve on the 3 GPP C-V2X Mode 4. [PERSON] et al. [29] analyzed through simulation the impact of parameters on its wireless resource allocation performance. They varied five physical-layer (PHY) and three medium-access layer (MAC) parameters and classified them into three groups: those having negligible impact, those affecting the quality of service, and those creating the trade-off relation. In particular, they found that the resource keeping probability creates the trade-off between the PRR and update delay. [PERSON] et al. [30] analyzed the collision probability and average delay in the SPS under the assumption of perfect PHY performance. They found that a flexible resource block selection enables a trade-off between the delay and the collision probability. Specifically, they found that by letting the UEs choose the closest resource block, the delay in the reselection event can be significantly reduced at the cost of the moderately increased collision probability. [PERSON] et al. [31] showed that by separating the control and the corresponding data packets in the time axis and by letting them carry the reservation information for each other, the collisions are significantly reduced.