## 1 Introduction  
The rapid advancements in communication technologies have opened new avenues for improving ECSs. Such systems play a pivotal role during crises and natural catastrophes, especially when conventional communication channels fail. While various solutions have been proposed in the literature, the integration of open-source technologies with private communication infrastructures remains an area ripe for exploration. This paper aims to bridge this gap and contribute to the scientific understanding of ECSs' potential and limitations, particularly in environments where conventional channels might be compromised. Therefore, our primary research hypothesis is to introduce a low-cost, open-source experimental communication platform based on hybrid network architecture that reliably supports emergency scenarios where traditional communication channels are ineffective or unavailable. These networks demand flexible, realistic, and scalable experimental platforms supported by simulation tools. In this direction, low-cost communication platforms have been developed, using an ExpressMIMO2 PCI Express (PCIe) board [1], but this option does not incorporate hybrid network scalability and voice transmission over a private mobile network. As far as network scalability is concerned, we have presented our work previously in [2]. Thus, in this article, we present the creation of a non-commercial, independent experimental communication platform for cellular mobile systems using SDR devices instead of ExpressMIMO2 PCI Express (PCIe) boards, which can make calls via a private network over LTE, called voice over LTE (VoLTE). To build the on-premises mobile base station, we have used open-source OAI Public License V2.1. software and state-of-the-art equipment. We have integrated the required operating code of a private 4G-LTE network into a portable experimental-based station that uses a general-purpose universal software radio peripheral (USRP) software-defined radio (SDR) device. We have also installed an Asterisk server, which uses open-source private branch exchange (PBX) Mizudroid V.4.0.2 software for voice transmission. The OMNeT++ simulation tool helped us to configure the network parameters and therefore to safely select our network devices and deployment topology. Although a wireless sensor network can be reliably based on an LPWAN topology, voice communications require a mobile voice and data transmission system. In case of a disaster, an emergency communication network can help users, support the disaster relief center, achieve on-site data transmission, and enable other activities to support emergency rescue personnel [3]. To describe and test our ECS platform, we first consider an experimental scenario and two hypothetical search and rescue scenarios (Section 1). After that, we provide a summary of recent work on SD-WAN-based ECSs and experimental platforms (Section 2), then present the system's features (Section 3) and end up with the simulation of a private VoLTE network (Section 4), before concluding with the experimental implementation (Section 5) and discussion of our future research (Section 6). Search and Rescue ScenarioIn the following scenario, we consider a forest area where the regular communication network is out of cover. A lost hiker wanders through this area, while a search and rescue team, consisting of two equal subteams, is trying to find him. One or two portable private 4G base stations are placed inside the rescue area, near a forest road, providing a local 4G/LTE network for up to 10 users equipped with mobile devices in which a pre-registered proprietary network SIM card is installed. To create the above network, we have implemented two sub scenarios: a simulation and an experimental one, as seen in Table 1. The lost hiker is not able to communicate with the private mobile network. Every portable base station can be mounted on a vehicle, with antennas installed on a telescopic mast of variable length, achieving optimal network coverage.