## 3 Intelligent Jamming  
To disrupt communications through jamming, it is sufficient to block either the downlink or uplink frequencies of the smartphones. Uplink and downlink are fundamental concepts in telecommunications that refer to the direction of data transmission within a communication system, especially in wireless networks [12]. In a wireless communication system, like a cellular network, the uplink and downlink operate on separate frequency bands to facilitate simultaneous two-way communication. This frequency division ensures non-interference between uplink and downlink transmissions to promote efficient and balanced communication between users and the network infrastructure. The principle of jamming involves emitting interfering signals on the same frequencies as the target device's communication signals but with greater power and the same bandwidth. This interference overwhelms the target system, rendering it unable to effectively transmit or receive information. The suitable scenarios for the experiments we took into consideration were the following [5]:  
* Frequency matching: jamming devices are designed to transmit signals on the same frequency bands or channels used by the target system. * Continuous jamming: Jamming can be continuous, where a constant jamming signal is transmitted, or it can be pulsed, where jamming signals are emitted intermittently. Pulsed jamming can make it more challenging for the target system to adapt or counteract the interference. * Partial-band jamming (PBJ): jamming can target specific frequencies or frequency bands, while broadband jamming covers a wider range of frequencies. Figure 5: Smartphone registered in GSM 900 MHz band. The choice depends on the goal and the capabilities of the jamming device. In all our test experiments, we used frequency-match jamming, continuous jamming and partial-band jamming. Partial-band jamming is a strategy that disrupts a specific portion of the total bandwidth. This is achieved by transmitting additive white Gaussian noise (AWGN) within that specific band section. When the jamming power remains constant, the efficiency of this approach is directly related to the proportion between the bandwidth used for jamming and the bandwidth of the original signal. Jamming can be used for illegal purposes, such as disrupting commercial communication systems, GPS signals or even Wi-Fi networks. To mitigate the effects of jamming, organisations and governments employ techniques like frequency hopping, spread-spectrum communication and encryption to make it more challenging for jammers to interfere with their systems. Additionally, they invest in signal-analysis and jamming-detection technologies to identify and counter jamming attempts. With the increasing prominence of SDR technology, jammers have gained greater adaptability and simplicity in usage, becoming conveniently available for online purchase at reasonably affordable costs. However, the improper use of such devices poses a significant societal challenge, as it undermines the reliability and safety of critical wireless communications. Furthermore, there is a potential economic impact that harms the earnings of mobile service providers in the event of a large-scale attack. In this study, we conducted experiments on jamming in downlink bands based on the sensibility of the mobile phone, signal noise and strength of the signal level of the network; interesting results were obtained. Intelligent jamming methods consist of analysing the availability of the mobile network operators at a specific point of interest. To achieve this, a Python code was developed and the SDR Hack-RF Sweep was used to measure the amplitude of the uplink frequency. Based on these measurements, a jamming signal was emitted on the corresponding downlink frequency [12]. Within the context of this study, based on the theoretical considerations regarding the LTE telecommunications technologies presented above, a jamming system was tested and interesting results were obtained under laboratory conditions.