## 2 Downlink Connection Landscape of the Mobile Operators in Bucharest  
Using a radio spectrum analyser consisting of Nestor software and TSMA 6 hardware [13], interesting results and an overview of the network operators' availability in the radio monitoring point were obtained. An omnidirectional antenna for monitoring the downlink connections was used and it can be observed that the radio spectrum allocated to mobile network operators was very crowded. Nestor software based on TSMA 6 hardware is a very useful tool for network operators to analyse their radio coverage and make field measurements of the services they provide. After these measurements are completed, operators analyse the data obtained and, if necessary, make modifications or implement new cells to improve the coverage and service quality for subscribers. As depicted in Figure 1, all Romanian network operators had a radio presence at the monitoring point, providing different technologies across several frequency bands. In Romania, network operators have made significant changes, such as improving the spectral efficiency by deploying LTE technologies in lower bands, like 800/900 MHz and 2100 MHz. Figure 1 shows interesting measurements that are characteristic of downlink connections: GSM technology is represented in green, and it is provided by all Romanian operators in the 900 MHz band, typically in channels 1-124. UMTS technology is indicated in blue, and it can be observed that network operators have significantly reduced their radio presence in providing UMTS services. The  
Figure 1: Analysis of spectrum usage by mobile network operators at the radio monitoring point, as obtained with the Nestor monitoring platform. trend in Romania is to exclude this technology from future networks to make room for 5G SA. * LTE and LTE-M technologies are shown in purple. They are provided by all Romanian operators across different frequency bands, each with various bandwidths. Table 1 presents the frequency bands and technologies provided in Romania by Digi Mobil and are leased from the authorities. These are identified by the radio frequency channel and the central frequency. For our experiment, it was very important to know the radio spectrum allocation for these frequency bands and technologies. However, the most important aspect was to conduct radio measurements at the specific point of interest to be aware of the availability of the network. Based on the data in Table 1 and the radio measurements conducted, we could conclude that Digi Mobil allocates frequency bands and technologies based on the specific areas they aim to cover. For rural areas that require signal coverage, Digi Mobil tend to utilise technologies in lower frequency bands. Conversely, in urban areas, the cell density is increased, and higher frequency bands are used to deliver quality services. In the following sections, we focus on presenting our experiments and radio measurements on services provided by Digi Mobil. Although we conducted measurements on all available Romanian network operators, we established that the results obtained in the laboratory, which depended on the network operator's radio configuration strength, are worth presenting to the scientific community. The lab setup for the measurements of the radio spectrum allocated to Romanian network operators consisted of the following:  
TSMA 6 was the hardware platform for running the software Nestor produced by Rohde & Schwarz; its capabilities and parameters setup used in our measurement campaign are detailed in the following. For all the measurements, we used an omnidirectional antenna without gain. From the settings, we selected scanning all available bands (700 MHz, 800 MHz, 900 MHz, 1800 MHz, 2100 MHz and 2600 MHz) and technologies (GSM, UMTS and LTE) that network operators provided in the area (we know that in other areas/continents, mobile technologies are provided in different bands than in Europe).