# Multi-User Scheduling for 6G V2X Ultra-Massive MIMO System
## 1 Introduction  
Vehicles to Everything (V2X) is a large interactive network composed of vehicle information such as vehicle location and speed, which includes four types of communication: vehicle-to-vehicle (V2V), vehicle-to-infrastructure (V2I), vehicle-to-network (V2N), and vehicle-to-pedestrian (V2P). V2X establishes a platform for intelligent information interaction between people, vehicles, and roads, and finally realizes the deep integration of people, vehicles, and the environment [1]. At present, automatic driving vehicle terminals are equipped with many sensors, which can assist the vehicle to obtain environmental information, expand the identification range of existing sensors by using the external information received by V2X communication, support safer and natural automatic driving, and realize intelligent driving function. With the development of 6G networks, 6G V2X is an effective technology that can meet the needs of automatic driving and can support some conditions for automatic driving [2, 3]. Ultra-massive MIMO technology is proposed to support higher capacity communication in 6G networks. It is similar to the concept of 5G massive MIMO and further increases the scale of transmitting antennas. In theory, more antennas lead to exponential growth of system capacity, which can further improve energy efficiency and greatly increase the throughput of wireless communication systems. Ultra-massive MIMO technology can well meet the needs of 6G high transmission rates and highly reliable communication [4, 5, 6, 7]. However, ultra-massive MIMO technology needs to face new problems brought by the sharp increase in the number of antennas, densification, and beam directionality, such as limited system resources and reliable communication of dynamic users [8]. Therefore, it is of great significance for 6G V2X communication to designan algorithm that can solve the problem of limited system resources in an ultra-massive MIMO system. In recent years, much research work has been devoted to improving the transmission capability of communication systems. A scheme in which the antenna selection of the base station and the user scheduling at the receiver side are performed simultaneously was proposed in [9]. This scheme employs an algorithm based on semi-orthogonal user selection for user scheduling. The main goal of the combination of antenna selection and user scheduling algorithms is to maximize the sum-rate capacity of the system; the selected group of antennas is used to transmit data to the best user set selected by the user scheduling algorithm. In the 6G V2X scenario, the channel is time-varying, so the feasibility of this method is low. A novel cyclic user grouping technology to reduce the complexity of the system was proposed in [10], using three different combinations of user selection schemes and semi-orthogonal user selection based on the norm-based antenna scheduling method to achieve higher system capacity. The existing techniques in the development of massive MIMO specifically concentrating on energy and spectral efficiency was analyzed and the challenges in the system design were discussed in [11]. A detailed investigation of the existing energy efficiency of massive MIMO system design and its preliminary and state-of-the-art technologies was conducted to explore energy-saving MIMO systems for 5G communications. It was pointed out that in multi-user and multi-cell environments, the existence of inter-user and inter-cell interference complicates the design of energy-saving MIMO systems. The idea of using space resources to maximize energy efficiency while suppressing interference is worth studying. Antenna selection and user scheduling were studied in a down-link multi-user MIMO system, and the aims were to maximize users sum rate and reduce complexity in [12].