# Multi-User Scheduling for 6G V2X Ultra-Massive MIMO System
## 1 Introduction
In this investigation, a low computational complexity sub-optimal user scheduling scheme based on matrix Gauss elimination method was combined with maximum norm antenna selection to maximize users sum rate and reduce complexity, under the constraints based on the number of maximum service users and the maximum transmission power. The proposed algorithm gained much improved sum rate performance over the semi-orthogonal user scheduling algorithm and had lower complexity compared to the exhaustive search algorithm. Aiming at the maximum minimum fairness precoding problem under the antenna power constraint, a suboptimal precoding algorithm was proposed by decoupling the problem into a precoding direction vector design problem and a power allocation problem in [13]. The signal-to-leakage-and-noise ratio (SLNR) metric was considered in the precoding direction vector design to properly mitigate the inter-group interference. Moreover, aiming at the maximum minimum fairness criterion, the power allocation procedure can guarantee fairness between users. However, the above-noted designs of the user scheduling algorithms were based on static user channels, and did not consider the real situation of channel dynamic changes. In view of the above problems, we propose a 6G-oriented new multi-user scheduling algorithm based on block diagonalization (BD) precoding for ultra-massive MIMO systems. The main work of this paper is as follows:  
1. Based on the high-speed movement of 6G V2X and the characteristics of millimeter waves, the time-varying geometric channel model is adopted. Doppler frequency shift is generated due to the high-speed moving characteristics of the vehicle. The channel is more suitable for the research scenario in this paper, because the Doppler frequency shift is introduced into the channel model and combined with the conventional millimeter wave channel. 2. On the basis of the time-varying channel model, for the BD precoding algorithm's sensitivity to channel correlation, the Pearson coefficient between users and the user noise enhancement factor are jointly considered, and the Pearson coefficient after matrix vectorization is used to measure the channel correlation between users The scheduling factor that can measure the quality of the channel is designed according to the user noise enhancement factor. It satisfies the minimum user interference while ensuring the channel quality of the scheduled users. This algorithm overcomes the operational problems of using many matrix decompositions in the scheduling algorithm, effectively reduces the system error rate, and improves the system throughput rate. The purpose of this paper is to propose a multi-user scheduling algorithm for the ultra-massive MIMO system in the 6G V2X scenario. Section 2 introduces the 6G V2X ultra-massive MIMO system model based on multi-user scheduling and the channel model suitable for car networking scenarios. Section 3 introduces BD precoding and provides a multi-user scheduling algorithm based on BD precoding. The proposed algorithm is simulated and analyzed in Section 4. The paper concludes with some final remarks in Section 5.