## 3 Challenges in Resource Allocation of Sharing Network
This ensures an enhanced user experience for users allocated to those frequency bands. Spatial Domain Challenges: The spatial domain challenges arise due to differences in product forms (e.g., the number of antennas) for FDD and TDD frequency bands among different operators. For instance, TDD usually utilizes massive MIMO antenna arrays, leading to a larger spatial dimension, while FDD mainly employs 4T4R or 8T8R configurations with a smaller spatial dimension. After co-construction and sharing, the coexistence of wide and narrow beams increases the complexity of spatial beam combinations. Differences in rank streams and Multi-User Multiple Input Multiple Output (MU-MIMO) pairing layers in the spatial channel space result in variations in user experiences. Moreover, users of different  
Figure 1: Multi-user scheduling for different operators. Figure 2: Uplink and downlink experiences in different frequency bands and coverage areas. operators are not evenly distributed on different beams, suppressing the further release of spatial MU pairing capability. As depicted in Figure 3, Operator A has more users, and due to user clustering, the TDD beam loads of Operator A are imbalanced. At the same time, Operator B has fewer users, leading to lower FDD beam loads. Without coordinated spatial resource scheduling between Operator A and Operator B, the idle resources of Operator B cannot be shared, resulting in wasted spectrum resources for Operator B. Therefore, in the spatial domain, it is crucial to holistically allocate beam resources to users. This involves balancing the user loads across different beams for operators, activating more users to achieve MU-MIMO, and efficiently utilizing the available spatial resources. Furthermore, the allocation of time, frequency, and spatial resources needs to cooperate with each other. As shown in Figure 4, the network allocates appropriate time, frequency, and spatial resources based on the volume and type of service for different users. For instance, For low-latency services like UE0, a shorter time-domain symbol resource can be allocated, and according to the volume of the user's service, dynamically allocating suitable frequency-domain Resource Blocks (RB) resources. For high-data-rate services, such as enhanced Mobile Broadband (eMBB) for UE1, a larger allocation of time-domain symbols, frequency-domain RBs, and spatial resources can be provided. This approach ensures the specific requirements of each user, leading to an optimized allocation of resources in the time, frequency, and spatial domains. In summary, after implementing a co-construction and sharing network, there may be challenges, such as user imbalances in the time domain, significant variations in user experience across different frequency bands in the frequency domain, and uneven distribution of users in the case of spatial multi-dimensional combinations. Therefore, it is necessary  
Figure 4: Cooperative resources allocation of time, frequency and spatial domains. Figure 3: Unbalanced load distribution on distinct beams of different operators. to achieve a coordinated allocation of time, frequency, and spatial resources. Existing methods predominantly focus on the resource allocation of certain domains and rarely consider the resource allocation of all domains globally. Therefore, we need to explore the collaborative resource allocation method that maximizes the user experience and guides the optimal allocation of time, frequency, and spatial resources in the case of co-construction and sharing so as to enhance network performance.