## 2 Multi-Connectivity Model Formulation
### Proposed O-RAN Based System Architecture for MC Configuration  
In order to enforce in the network, the multi-connectivity configuration \(\mathbf{B_{u}}\) obtained as a result of the above problem, this paper proposes the system architecture depicted in Figure 1. It is based on the O-RAN Alliance reference architecture [25], which complements 3 GPP 5G standards with a foundation of virtualized network elements, white-box hardware and standardized interfaces that fully embrace the core principles of openness and intelligence. One of the principal characteristics of O-RAN architecture is the RAN disaggregation, which splits a 5G NR base station (gNB) into different functional units, namely a Central Unit (CU), a Distributed Unit (DU), and a Radio Unit (RU) (called O-CU, O-DU, and O-RU in O-RAN specifications). The O-CU can be further split into two logical components, one for the Control Plane (CP), and one for the User Plane (UP). The O-RANarchitecture also considers the use of LTE technology with so-called O-eNB nodes. The O-CU hosts the upper layers of the radio interface protocol stack. These include the PDCP layer that splits the traffic in case of multi-connectivity, in addition to the Radio Resource Control (RRC) and the Service Data Adaptation Protocol (SDAP) layers for control plane and user plane, respectively, that are on top of PDCP. In turn the O-DU hosts the lower layers of the protocol stack, namely the Radio Link Control (RLC), the Medium Access Control (MAC), which hosts the scheduler in charge of allocating the PRBs to the different UEs, and the upper parts of the Physical (PHY) layer (e.g., channel coding, modulation). Finally, the O-RU hosts the lower parts of the PHY layer (e.g., Inverse Fast Fourier Transformation (IFFT) for Orthogonal Frequency Division Multiple Access (OFDMA) transmission) and the Radio Frequency (RF) functions. The multi-connectivity situation illustrated in Figure 1 considers the downlink traffic transmitted to a UE served by two cells of 5G NR RAT \(m=1\). The cell \(n=1\) is at the MN while the cell \(n=2\) is at the SN. The multi-connectivity configuration for the different users is determined by an MC controller, which can be hosted at the near-real time RAN Intelligent Controller (near-RT RIC) of the O-RAN architecture. The near-RT RIC is deployed at the edge of the network, is able to operate control loops with a periodicity between 10 ms and 1 s, and it can interact with the DUs and CUs in the RAN. A relevant characteristic of the near-RT RIC is that it supports the execution of third-party applications, referred to as xApps [26]. Then, the MC controller can be implemented as one of these xApps. The inputs of the MC controller are different measurements (further explained in next section) from the RAT/cells, as shown in Figure 1. These measurements can be sent to the near-RT RIC through the E2 interface. At the near-RT RIC, they are sent to the xApp MC controller where they are collected and processed in order to be adapted to the format required by the DQN agent that constitutes the core of the MC controller, as is explained in Section 3. The output of the MC controller is the configuration \(\mathbf{B}_{u}=\{\beta_{m,n}\}\) with the weights \(\beta_{m,n}\) to be configured at the PDCP layer of the MN, which resides in the O-CU. This configuration can be established via the E2 interface that supports enabling, disabling or modifying the dual connectivity process [27]. Based on this configuration, the PDCP splits the traffic between the MN and the SN, and delivers the part of the traffic of the SN via the Xn interface that interconnects the MN and the SN. Then, the MAC scheduler at each O-DU allocates the necessary amount of bandwidth resources \(\gamma(\beta_{m,n})\) to the UE to transmit the fraction of traffic \(\beta_{m,n}\) corresponding to the cell. The specific design of the  
Figure 1: O-RAN-based architectural components for supporting the MC configuration.