## 2 Introduction
In [7], the authors studied two types of jammers, namely, the random signal jammer and the phase shift signal jammer, which were tested against the 5G signal. In [8], the authors proposed a prototype for a cascaded detector-jammer that can block a radio signal for 2G, 3G and 4G networks within a small distance range to be used for protected areas like prisons, examination rooms, confidential facilities and military bases. The authors of [9] presented an analytical procedure for evaluating the anti-jamming capacity of an underwater acoustic direct sequence spread spectrum (DSSS) communication system under different types of jammers (single tone, partial band and wideband) and compared the theoretical results with the ones obtained based on simulations. In [10], the authors mathematically evaluated and simulation-tested the performances of a deep-reinforcement-learning-based anti-jamming method for different types of jammers and designed a jamming algorithm that was found to be effective with the anti-jamming algorithm to prove its limitations. In [11], the authors proposed a reinforcement-learning-based jamming algorithm and its universal software radio peripheral (USRP) implementation showed that it was effective for 5/6G communications systems that involved the orthogonal frequency-division multiplexing (OFDMA) technique. As one can see, some of the abovementioned research covered only partial aspects of the issues. Some analysed the vulnerability of mobile communication technologies to jamming, while other solutions performed long-term jamming in all downlink (DL) bands of all mobile communication bands (e.g., jamming systems for explosive improvised devices). Also, when searching the literature, we found few results regarding the practical implementation of USRP-based narrowband jamming signals based on monitoring the downlink connection, and no results for the specific communication landscape offered in Bucharest. The originality of this study consisted of the fact that it involved, at first, an analysis of the spectral domain used by each mobile communication operator, and based on this, the mobile service operator that could potentially be selected by a terminal that offered the best network parameters was chosen, and the jamming was performed only on the DL frequency of the selected service. The authors choose not to use broadband jamming for the entire DL frequency range, but only on that particular DL channel of the service with significant parameters selected by a user in a limited time interval. Within this study, several objectives were aimed towards:  
1. To determine whether downlink jamming using the SDR Hack-RF can be performed in two different scenarios: when the position of the receiver was neighbouring the cell and when it was at the edge of the cell by considering two different values for the strength of the received signal. 2. To evaluate the possibility of using reactive jamming on the downlink connection when the receiver was at the edge of the cell, to evaluate the technical limitation when the receiver was neighbouring the cell and to identify a solution for intelligent jamming. 3. To develop an intelligent jamming system using commercial equipment that can be exploited in indoor conditions to react to a downlink signal when an amplitude of the monitored uplink signal above a predetermined threshold is detected. Based on the abovementioned objectives, the main contributions of this study are as follows:  
1. This study proposed a practical approach to reveal the ease of jamming by a malicious person of communications carried by mobile network operators under indoor laboratory conditions [12]. 2. The robustness and applicability of the proposed practical method were verified through extensive simulations and rigorous testing procedures. The results indicate that the intelligent jamming system's ability to withstand dynamic changes in power provided by the network operators was limited.