# A Review of Counter-UAS Technologies for Cooperative Defensive Teams of Drones
## 4 Neutralization Systems
### Electronic Neutralizers
#### 4.1.1 Radio Frequency Jamming
Similarly, if the target communication system uses direct sequence spread spectrum (DSSS), the spreading properties of the signal to be attacked can be used to transmit a jamming signal possessing a high correlation with respect to the original one in order to increase the bit error rate of the communication to neutralize. Therefore, smart jamming is both effective, as it is calibrated precisely on the target signal to be disturbed, and efficient in power, as it operates only in correspondence with the target signal (in time and frequency). In any case, if no prior knowledge of the communication system to be neutralized is known, an analysis of the relevant signal must be carried out in order to identify its characteristics and weak points. Consequently, SDR technologies are well suited to the implementation of smart jamming; thanks to their flexibility, they allow for both the analysis of the target signal and the reproduction of an ad hoc signal to be used as an interfering signal ([75,76]). The performance of the above jamming techniques can be assessed with respect to most available communications on commercial drones, i.e., the communications based on spread spectrum, like the transmission systems ACCST (Advanced Continuous Channel Shifting Technology), based on FHSS, and FASST (Futuba Advanced Spread Spectrum Technology), based on FHSS with the addition of Gaussian filtering and DSSS applied on the data. Furthermore, in order to consider even drones equipped with Wi-Fi connection, the IEEE 802.11b standard, based on DSSS, and the 802.11g standard, based on orthogonal frequency division multiplexing (OFDM), can be assessed. The listed systems cannot complete the panorama of implementable communications, but offer an early coverage of the transmission protocols (with reference to the physical layer) typically used by drones. The experimental evaluation of tone, sweep and smart jamming with respect to ACCST and FASST is shown in [75]. The results showed that smart jamming is significantly more efficient than tone and sweep jamming--the tone jammer can successfully jam a single channel of the link but is not sufficient to terminate the remote control link, and the sweep jammer requires relatively high jammer-to-signal ratios (JSRs) to completely prevent the communication, whereas smart achieves successful jamming at relatively low JSRs but requires significant knowledge about the targeted system. In the same work, noise jamming was also evaluated (with respect to an FHSS/DSSS hybrid system by means of simulations),resulting in the techniques which require a higher JSR to prevent the communication. When considering the impact that the jamming signals have on WLAN devices, a comparison between noise and tone jamming is shown in [77]. If the jamming signal is stronger than the desired signal, the 802.11g system cannot operate in any mode unlike the 802.11b system, which uses lower rate DSSS modes, however, at the certain packet error ratio (PER) the 802.11g system can offer higher data rates than the 802.11b system under wideband jamming. When tone jamming is used, the performance of the 802.11g system depends highly on the jamming frequency. In DSSS systems, the jamming frequency is not as important a factor as in OFDM, but higher JSRs are required to increase the PER with respect to the noise jamming. An experimental comparison between sweep and smart jamming for the WLAN case is shown again in [75]; in the most extreme cases, the sweep jammer halted the WLAN communication, while the protocol-aware jammer solely limited its maximum throughput. Neutralizers based on RF jamming techniques, also known as jammers, can be integrated on static, mobile and/or portable ground platforms (examples are illustrated in [78; 79; 80]). It is also possible to integrate them on aerial platforms, like mini drones.