## 2 Related Work
In [26], a DRL-based V2V communication distributed resource allocation mechanism that can be used in both unicast and broadcast scenarios was developed to learn how to meet V2V latency constraints while minimizing the interference to V2I communication. In [27], a DRL-based mode selection and resource allocation method are proposed to maximize the total capacity of V2I users while ensuring the delay and reliability requirements of V2V users. In these schemes, each V2V pair independently updates its strategy at different times to maximize its reward. However, when two or more V2V pairs update their policies at the same time, it becomes a seemingly unstable environment. In [28], an effective transfer actor-critic learning approach was proposed to ensure the ultra-reliable and low-latency communication requirements of the V2V links, but this work can only be applied to low-dimensional state-action mapping. Using a distributed scheme can significantly reduce signaling overhead, but distributed DRL may lead to local optimization. Based on the research work analysis of the above-mentioned mode 3 and mode 4 resource allocation schemes, how to design an effective V2V resource allocation framework is the key to improving the data rate of V2I communication and meeting the requirements of V2V communication delay. The analysis of the research work in [23; 24; 25; 26; 27; 28] shows that DRL-based resource allocation with delay as the constraint is more common in the mode 4 scheme. However, due to the high signaling overhead, there are few studies on the mode 3 solution and primarily focus on solving the rate problem between V2I users and V2V users, without considering the delay requirements of V2V users. Compared with mode 4, the advantage of mode 3 is that the BS collects the CSI of all vehicles in the coverage area and can allocate spectrum resources more effectively. However, in a high-speed moving vehicle environment, it is a challenge for the BS to obtain complete CSI. Based on the above considerations, this paper proposes a DRL-based V2V resource allocation framework in the underlay of mode 3. In this framework, the BS uses double DQN for resource allocation. In particular, to reduce the overhead of CSI feedback, in the framework of this article, the V2V links only feedback partial CSI, and the BS only optimizes the resource allocation strategy based on the feedback information. The main contributions of this work are summarized in the following aspects:  
1. In view of the low latency requirements of V2V communication, a DRL-based resource allocation framework is proposed in the underlay method of mode 3, which aims to meet the V2V links latency requirements with low signaling overhead while increasing the V2I link throughput. It is novel to consider the delay requirement of V2V links in the DRL-based mode 3 spectrum allocation scheme. 2. To reduce the signaling overhead of BS acquiring CSI in the resource allocation scheme of mode 3, in this design, each V2V link only needs to feed back interference links information and delay information to the BS, instead of feeding back complete CSI signaling. The BS only optimizes the resource allocation strategy based on the feedback information, which significantly reduces the CSI feedback overhead. 3. The simulation results show that, compared with other solutions, the proposed solution can not only meet the low latency requirements of the V2V links but also increase the throughput of the V2I links, and the proposed partial CSI has comparable performance compared with the complete CSI. The rest of the paper is organized as follows. The system model is introduced in Section 3. Section 4 presents the DRL framework to solve the problem formulation of spectrum resource allocation. The simulation results and analysis are shown in Section 5. Section 6 is the conclusion.