# Low-Cost Sensor Based on SDR Platforms for TETRA Signals Monitoring
## 1 Introduction  
Nowadays, the electromagnetic spectrum has become increasingly crowded, especially due to the development of mobile communications. Thus, it is noticed that channels with different bandwidths are used, from narrowband to wideband. The 4G generation of mobile communications uses variable channel bandwidths, from 1.4 to 20 MHz, while IEEE 802.11n/ac/ax wireless local area networks, to improve data throughput, extend channel bandwidths up to 160 MHz. In addition, channel resource access techniques and digital modulation methods create electromagnetic field variability in both amplitude and time. The efficient use of resources requires real-time knowledge of the electromagnetic spectrum. In this context, a different approach to monitoring the electromagnetic spectrum is needed. International Telecommunication Union (ITU) has issued several recommendations regarding electromagnetic spectrum monitoring equipment [1]. Thus, due to the large variability in time, it is recommended that the measurements be performed in real time and the analysis of the electromagnetic spectrum include not only an amplitude analysis but also a phase analysis of the spectral components [1]. Monitoring strategies for the electromagnetic spectrum have been reviewed in several countries [2; 3; 4]. Thus, in the USA, starting in 2013, the National Telecommunications and Information Administration (NTIA) and the National Institute of Standards and Technology (NIST) initiated new electromagnetic spectrum monitoring capabilities [2; 5]. By implementing these capabilities, a specific infrastructure for electromagnetic spectrum monitoring has been developed andthe results of electromagnetic field measurements can be easily accessed in real time using Internet facilities. To this end, a pilot program involving industry entities, academia and government agencies was developed, with the objective of generating a database called the Measured Spectrum Occupancy Database (MSOD). Some recent studies have shown that electromagnetic spectrum monitoring suffers from two essential limitations [6; 7]: lack or reduced scalability and lack of supporting applications to be operated directly by the monitoring sensor. The electromagnetic monitoring is the responsibility of government agencies, which usually use expensive monitoring stations. This makes scalability limited or even nonexistent. The monitoring stations are usually composed of high-performance spectrum analyzers that already have various functions implemented. This creates a relatively closed environment for further processing of the recorded data. A low-cost alternative to deploying a spectrum monitoring station was recently offered by Software Defined Radio (SDR) technology. The requirements for large-scale broadband real-time monitoring are successfully met [7]. Due to the relatively low price of SDR platforms, the number of sensors in the entire architecture of the monitoring network may become much higher. Additionally, their small size and flexibility offer the possibility of placing them in different locations. In this way, the qualitative and quantitative descriptors of the electromagnetic environment are substantially improved. Several electromagnetic spectrum monitoring solutions have recently been proposed--for example, the Microsoft Spectrum Observatories ([[https://www.google.com/get/spectrumadabase/](https://www.google.com/get/spectrumadabase/)]([https://www.google.com/get/spectrumadabase/](https://www.google.com/get/spectrumadabase/)) (accessed on 24 March 2021)). A recent project is the ElectroSense project ([[https://electrosense.org/](https://electrosense.org/)]([https://electrosense.org/](https://electrosense.org/)) (accessed on 24 March 2021)), which is the first initiative that aims to exploit the facilities offered by low-cost programmable spectrum sensors, such as SDR sensors [8; 9; 10]. The adequacy of SDR platform parameters for real-time and accurate monitoring is an important factor in the design of the monitoring station.