## 1 Introduction  
With the ongoing increase of data traffic as well as the emergence of new services with high speed and reliability requirements, Mobile Network Operators (MNO) are actively trialing and deploying fifth generation (5G) networks in an effort to support new vertical-driven use cases and enhanced user experiences [1]. 5G New Radio (5G NR) deployments are progressively introduced on top of existing legacy technologies, such as Long-Term Evolution (LTE), leading to heterogeneous deployments with multiple radio access technologies (RATs), different cell types, such as macrocells, indoor and outdoor small cells, and operating in a wide range of spectrum bands (e.g., sub-6 GHz bands used by all RATs and millimeter wave (mmW) bands used by 5G New Radio). Network densification has been a mainstay of LTE networks, and the need for small cells will be even more critical in 5G and beyond systems due to operation in higher spectrum bands and the need to support traffic densities that are two to three orders of magnitude higher than in LTE [2]. The general industry consensus is that 5G will drive hyperdense deployments with site densities in excess of 150 sites/km\({}^{2}\) in urban and selected indoor scenarios [3]. In this context, multi-connectivity (MC) has emerged as a key technology for supporting simultaneous access via LTE and 5G networks [4], and is also expected to play a key role in further network evolutions towards the sixth generation (6G). The central idea behind the MC concept is that User Equipment (UE) has connectivity with different nodes of the Radio Access Network (RAN) at the same time, e.g., eNodeBs (eNB) operating with LTE and/or gNodeBs (gNB) operating with 5G NR [5]. There is one master node (MN) responsible for the radio-access control plane and one, or in the general case multiple, secondary node(s) (SN), that provide additional user-plane links. In this way, the UE can be provided with radio resources from distinct eNBs/gNBs, which has benefits in terms of additional capacity for better supporting the data rate and reliability requirements of 5G and beyond systems. At the same time, this brings the challenge of properly managing the MC operation through the configuration of the traffic sent through each node. This allows exploiting those situations where MC is more beneficial in front of other situations in which MC may not be the best option depending on the load in the involved nodes or the propagation conditions experienced by the UE. In the Third Generation Partnership Project (3 GPP), MC is specified through the Multi-Radio Dual Connectivity (MR-DC) feature defined in [6] that considers different options depending on the technology used by the MN and by the SN, and on the core network technology (i.e., 5G core or Evolved Packet Core (EPC)). The operation of MR-DC is built upon the use of three different types of radio bearers [6], namely the Master Cell Group (MCG) bearers in which data is transmitted through the MN, the Secondary Cell Group (SCG) bearers in which data is transmitted through the SN, and the Split bearers, in which data is split between the SN and the MN at the Packet Data Convergence Protocol (PDCP) layer of the radio interface protocol stack. The MR-DC feature between LTE and 5G is widely used nowadays in the current 5G Non-StandAlone (NSA) deployments that have enabled a quick introduction of 5G NR making use of the EPC core network of the legacy LTE systems [5]. The interest in MC is also reflected in some 3 GPP Release 17 study items that have addressed the MR-DC with multiple cells operating in different bands. One example is [7], which considers up to four bands in LTE and two bands in 5G NR, one for the sub-6 GHz Frequency Range 1 (FRI) and the other for the mmWave Frequency Range 2 (FR2).