## 2 State-of-the-Art Overview and Problem Formulation
### Problem Formulation  
Based on the HetNets structure that is featured with the MR-DC (multi-connectivity [5]), the main objectives of our work are to experimentally assess the feasibility of performing an efficient handover scheme, i.e., the LHO in a multi-radio environment, to evaluate the benefits of such handover over the conventional RSSI-based approaches, and to understand the key factors of maintaining such improvements. Essentially, the LHO benefits from the advantages of both cmWave and mmWave APs within the HetNets that we summarize in Table 1. In particular, the throughput of the robot is augmented by connecting it with the mmWave AP as long as the robot is in the LoS state w.r.t. the mmWave AP. Meanwhile, more robust connectivity can be ensured by handing over to the cmWave AP before the robot loses the LoS connection w.r.t. the mmWave AP. Therefore, the key performance indicators of the LHO are the reliability of the achieved location awareness and the corresponding positioning accuracy.  
To illustrate our technical context, a conceptual figure of the principle pf the proposed handover scheme is developed in Figure 1, where a robot (i.e., a mobile vehicle) is moving within an industrial multi-radio environment while performing certain tasks, such as cargo transportation or video surveillance. To ensure the service quality of the tasks that require wireless connectivity, our objective is to enhance the link robustness while augmenting the throughput throughout the whole robot trajectory. As illustrated by Figure 1, the robot is communicating with the mmWave AP, wherever there is LoS radio connection between them (with a potential horizontal handover, i.e., a switch between the networks of the same RAT in the light blue region), and switches to WiFi (i.e., a vertical handover) before entering the NLoS region of the mmWave AP (see the light green region) based on the available location awareness of the robot, i.e., LHO.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**RAT** & **Advantages** & **Disadvantages** \\ \hline cmWave & High robustness to blockage; & Low throughput due to limited \\ (WiFi) & low diffraction loss & bandwidth \\ mmWave & High throughput at LoS owing & Low robustness to blockage; \\ (WiGig) & to large bandwidth & high diffraction loss \\ \hline \hline \end{tabular}
\end{table}
Table 1: Advantage and disadvantage of different RATs.  
Even though a related topic was investigated in several past works as discussed in Section 2, most of the existing papers report enhanced communication performance by assuming ideal location information for all the UEs/vehicles within the network. In our work, the location information is not assumed to be known perfectly, but rather is estimated via the positioning solutions. Exploiting a laboratory setup, we implement and evaluate the positioning algorithms as well as the LHO scheme utilizing a multi-RAT robotic platform. From the network-level perspective, all data are transferred by the evolved packet core (EPC) with 3 GPP RAT (mmWave) stream routed via the serving gateway (S-GW) and packet data network gateway (PGW) to the Internet, represented by the packet data network (PDN). The non-3 GPP data is served in the same manner utilizing the evolved packet data gateway (ePDG). At the same time, inter-networking is ensured by the mobility management entity (MME), which communicates with both the home subscriber server (HSS) and the authentication, authorization, and accounting (AAA) unit. Further details on the handover procedure implementation between 3 GPP and non-3 GPP networks are given in [44].