# Jam Mitigation for Autonomous Convoys via Behavior-Based Robotics
## 2 Background
### Jammers  
Jamming is an effective and simple radio interference attack that can be used against wireless networks. In its most basic form, jammers emit radio frequency signals to fill a  
Figure 1: High-level architecture for a follower vehicle in an autonomous ground vehicle convoy. Figure 2: Common leader-follower communication topologies, adapted from [21]: (**a**) predecessor following; (**b**) predecessor-leader following; (**c**) bidirectional; (**d**) bidirectional-leader. wireless channel to interfere with the sending and receiving of wireless communications. The jammer can work to either prevent the source from transmitting packets or disrupt the receiver from properly recognizing and processing legitimate packets. The end result in either case is the interference of the physical transmission and reception of wireless communications [10]. Different classifications of jammers utilize different techniques in jamming wireless networks. The five most commonly described techniques in literature are as follows in Table 2.  
Despite the relative simplicity of basic jammers, such as constant jamming and random jamming, jamming attacks are difficult to defend against. Conventional security mechanisms are ineffective, because attackers can disrupt the entire medium of communications itself rather than having to exploit the traditional areas of confidentiality, authentication, and integrity [10]. Additionally, the deployment of jammers has a low barrier to entry. All radio operations on a wireless communications channel can be completely disrupted by a single device emitting noise at high power, regardless of what security measures are built into the underlying communications protocols. In addition, Software Defined Radios that can be used to launch jamming attacks are becoming increasingly affordable and easy to use [9]. In response to the vulnerabilities presented by jamming, several efforts throughout the years have proposed different anti-jamming methods. While many novel anti-jamming methods exist, the primary mechanisms to mitigate jamming can be broadly classified into larger groupings, with two prominent groupings being filtering and Spread Spectrum (SS) techniques. In filtering techniques, the radio receiver attempts to filter out the jammed signal by various means so that the intended signal can be properly parsed. One method of doing this used beamforming, in which a beamformer performed spatial filtering of spatial samples collected from propagating wave fields [23]. It then separated signals with overlapping frequency content that were delivered from different spatial locations. In a different filtering approach, an adaptive Gaussian filter was used to combat jamming caused by narrowband interference with gaussian white noise and pulsed noise [24]. The adaptive filter used optimal time-frequency localization and variable notch depth in conjunction with a fast Fourier-transform-based correlation to filter both continuous and time-varying narrowband interface. In addition to filtering techniques, SS techniques are often used to combat jamming. With SS techniques, a narrowband information signal uses data-independent, random sequences to spread the signal over a wide band of frequencies, in hopes that the jammer is unable to disrupt the entire frequency band [25]. The receiver can then correlate the signal it received with a copy of the random sequence to decode the data that was meant to be delivered. The two most commonly used SS techniques are direct sequence SS (DSSS) and frequency hopping SS (FHSS) [26]. In DSSS systems, the information signal is spread throughout the entire available bandwidth at the same time. This spreading of the signal causes the energy present at each particular frequency of the bandwidth to be very low and mistaken as noise by jammers that are actively listening for signals to jam. The intended receiver can then reconstruct the signal that it received from