## 1 Introduction  
The conversion of a Terrestrial Trunked Radio (TETRA) network to a Long Term Evolution (LTE) network in public protection and disaster relief (PPDR) systems is a possible solution to provide future services requiring high radio interface throughput and allowing broadband PPDR (BB-PPDR) radio communications [1]. In Report 218, The Electronic Communication Committee (ECC) proposed a roadmap until 2025 for the transition to broadband communication in PPDR systems [2]. This roadmap envisages coexisting TETRA and LTE networks for several years until the LTE system has all the functionalities of the PPDR systems. Therefore, the introduction of LTE for PPDR should complement, not replace, existing TETRA networks, which will continue to be the best choice for short-term mission-critical voice service.  
In addition, PPDR users cannot leave their current TETRA systems until a new mobile broadband network is built that is able to provide radioelectric coverage equal to or better than the radioelectric coverage currently provided by TETRA systems. Therefore, before any broadband solution can replace the current TETRA systems, the LTE network should meet all radioelectric coverage requirements currently satisfied by the existing network.  
The most plausible future scenarios to deliver the increasingly data-intensive applications demanded by PPDR agencies are expected to rely on the use of both dedicated and commercial LTE networks [3]. For rural areas, the base stations of a radio communication system are located in the mountains, and the radioelectric coverage has 'dark zones' (without radioelectric coverage), usually in forests, mountains, rivers, etc. In these zones, operators are not interested in deploying new base stations to offer broadband communications. However, in urban environments, operators have already deployed commercial broadband networks, that could be used by PPDR agencies for broadband communications. Therefore, the possibility of a hybrid model (dedicated network for rural areas and commercial networks for urban areas) could be an interesting solution for BB-PPDR radio communications.  
More than 114 countries around the world have deployed TETRA networks (at the regional or national level) in the past years according to The Critical Communications Association (TCCA) [4]. Now, the users of these networks are considering how to proceed to achieve a BB-PPDR network in the coming years. Therefore, PPDR agencies are interested in analyses of the many issues (cost analysis of the resources needed, the expected radioelectric coverage, quality of service, etc.) involved in planning the transition of a TETRA network to an LTE network.  
A framework for the modeling and planning of an LTE public safety broadband network has been presented by [PERSON] et al. [5]. The study was carried out on a nationwide scale, dividing the total area into 20 \(\times\) 20 km squares and establishing a minimum percentage of radioelectric coverage in each square.  
In this study, several radio planning considerations in the TETRA to LTE migration for PPDR networks are offered. In this case, different from [PERSON] et al.'s work [5], the total area is divided into geographical areas of interest that are defined as administrative divisions (region, municipal areas, natural parks, etc.). To illustrate this proposal, a radioelectric coverage analysis in a rural area with an existing TETRA network that has to transition to an LTE system has been carried out in a real scenario by means of both simulations and measurements. The analysis was performed by taking advantage of the potential of a radio planning tool [6] based on a geographic information system (GIS) [7], and the measurements were performed and used to tune propagation models. The proposed methodology can help PPDR agencies efficiently estimate the cost of transitioning a TETRA network to an LTE network.